- Introduction : Liquid penetrant test method enhances the visibility of surface breaking flaws such as cracks, fissures, crevices and pores. It can be used very successfully regardless of component size and can tolerate complicated part geometry. Penetrant testing is used on metals such as aluminium, magnesium, brass, copper, cast iron, steel, stainless steel, titanium and other common alloys. It can also test other materials, including glazed ceramics, plastics, molded rubber, powdered metal products and glass. Some limitations are, The discontinuity to be detected must be open to the surface and the interior free from foreign materials. The test surface should not be porous. The material under test must not be susceptible to damage from the liquids used for the examination. The test process has temperature limitations [ 10 to 52

0 C ].

[ using special materials, the range is -15 to 2000 C part temperatures ]

Special requirements, Penetrant materials must be designed with a low sulfur and halogen content to avoid harmful effects on the test parts. Stainless steels are especially susceptible to corrosion when exposed to chlorine and Carbon steels to sulfur. Titanium is extremely susceptible to embrittlement when in contact with halogens. High Nickel alloys are also affected by sulpher and halogens. These harmful chemicals can be found in penetrant materials but are limited to 1% by weight of content.

Common penetrant materials attack PVC, making it brittle, which leads to cracking. Liquid oxygen compatible penetrant materials must be used when testing parts that will be in contact with either liquid or gaseous oxygen. Method : A penetrating liquid is applied to the clean and dry test surface and allowed to enter the discontinuity opening over a period of time. The liquid soaks into material flaws that are open at the test surface. After a suitable penetration time, the excess surface penetrant is removed and the test surface is dried. A developer is then applied in a thin uniform coating which acts as blotter and draws some of the entrapped penetrant out of the discontinuities. The penetrant stains the developer and the indication becomes visible. The surface flaw becomes increasingly visible to the eye, because the dye spreads in the developer and effectively broadens the trace. The dye indicator appears either red or blue on a white background for color contrast penetrants or appears Yellow – green or orange - red on a dark violet background, when the surface is illuminated by an ultraviolet lamp for a fluorescent test process. The sensitivity of the test depends on the size of the discontinuity opening and not on the length. Crack width is the more important determinant of a penetrant's ability to detect cracks. Tight narrow cracks, regardless of length, are more difficult for penetrants to find. Fatigue cracks and forging cracks are tighter and require more judicious processing and higher sensitivity penetrants to locate than casting cracks of the same length. The crack limit for this inspection method is approximately 0.5 µm crack width. Detection of surface flaws depends on the general condition and finish of the test surface. The depth of the anticipated defect should be three times deeper than the general surface roughness "valley" depth. Defect detect- ability can be further diminished by normally acceptable surface conditions such as tool marks, scratches, scale, edges, grooves, notches, rough weld surfaces etc, where penetrant is trapped. Although the dye penetrant test is a very easy process, it is also very easy to make mistakes and make the test ineffective. Care must be exercised by the operator in order to perform all the steps of the examination correctly.

Test process

Test Process : Surface cleaning : Careful cleaning of the test surface and inside of the discontinuities is an extremely important step for success of the penetrant inspection because the penetrant method works by allowing a liquid to enter a defect which is open to the surface. If there is dirt in the crack, there is no room for the penetrant to enter it, and the process will not work. Surfaces to be examined are cleaned so that they are free from dirt, scales, oil, grease, paint, rust and corrosion and discontinuities are free inside of water, oil or other contaminants. This is known as pre cleaning. Following cleaning, the parts shall be thoroughly dried. Penetrant application : After drying, the entire surface to be tested must be wetted with a layer of the penetrant and kept wet during the entire dwell time. This can be achieved by immersion, flow - on, spraying or brushing the liquid. The temperature of the test surface should be between 10° C and 52° C. Rubbing the surface with a penetrant soaked cloth is not permitted. Penetrant Dwell : The penetrant is left on the test surface for sufficient time to allow penetration into the discontinuity openings. The time involved depends on the viscosity of the penetrant, temperature of the part surface and the tightness of the discontinuity opening. Penetration time used is generally between 10 and 30 minutes. At low temperatures, longer penetration time shall be used because the viscosity of penetrant increases. This time is known as dwell time. Emulsification : For post emulsification type penetrant, the emulsifier is applied after the completion of the dwell time. Emulsification makes the penetrant water washable. Emulsification time varies between 1 to 3 minutes, depending on the type of emulsifier, the penetrant , the surface condition and actual time shall be determined by practical tests. Removal of excess penetrant : The purpose of removing excess penetrant is to free the surface of penetrant so that the penetrant, which has entered a discontinuity, will be readily visible when it re-emerges onto the surface. After the dwell time, excess surface penetrant is to be removed with a solvent wipe, water spray rinse, or the penetrant has to be emulsified so that it can be washed-off with water rinse. Excess penetrant removal depends on the removal characteristic of the penetrant being used. The removal of excess penetrant is a delicate procedure because it is essential not to remove penetrant from the discontinuities. Penetrant is so easily removed from smooth polished surfaces that special procedures may be required to prevent over - removal. Rough surfaces reduce removability by retaining penetrant in the indentations or recesses by preventing the emulsifier from evenly combining with the surface penetrant. A completely cleaned surface may remove some or all of the penetrant trapped in discontinuities. To make sure that the surface has not been over - washed, the cleaning may allow a very low level penetrant background to remain. A slight shading of penetrant should be visible in the developer layer. During this cleaning operation, the surface has to be checked for residual penetrant using suitable illumination. Developer application : To make the penetrant indication clearly detectable a developing medium is used. Developer may be applied by dusting or dipping for dry powder or spraying or immersion for water base developers. Non aqueous wet developers should be best applied by spraying only. The developer should then be allowed to dwell on the part surface for sufficient time [ usually 7 to 30 minutes ] to permit it to draw penetrant out of any surface flaws to form magnified visible indications of such flaws. This is known as developing time. Longer times may be necessary for tight cracks. Inspection of indications : Inspection is then performed under ordinary light for color contrast penetrants or under ultraviolet illumination for fluorescent penetrants to detect any flaws which may be present.

Post Cleaning : After the completion of examination, the test surface is cleaned to remove developer and penetrant. In some industries, such as nuclear, the failure to post clean a part can have very detrimental results. For parts that will be in contact with liquid oxygen systems, the failure to post clean a part could result in a fire and possible serious injury. If penetrants and developers that contain halogen and sulphur products are used, can have detrimental effects on some metals and must be cleaned according to approved technical data. Penetrant and developer residue tend to absorb moisture and cause corrosion. The removal of dry developer and non aqueous developer is easy. Wet aqueous developer are difficult to remove because they are baked onto the part during the drying process. Soluble developers can be removed by water rinse. The longer the developer remains on a part the harder it is to remove. The use of steam with detergents is probably the most effective of all methods. Some parts, such as those used in liquid oxygen systems, require total penetrant removal after the inspection. Entraped penetrant in discontinuities can be removed most effectively by hot tank solvent cleaning or ultrasonic cleaning. Penetrant inspection normally leaves the part’s surface clean and exposed and thus making it susceptible to corrosion. All efforts should be made to protect the parts surfaces from corrosion effectively after the penetrant inspection.

Properties of Penetrants : The performance of a penetrant is achieved by a combination of controlled physical and chemical properties ; Viscosity Is related to the rate at which a liquid will flow under some applied force. It affects the speed of penetration through the discontinuity opening. A low viscosity liquid is used for faster penetration and also less drag out in immersion. Surface tension Is one of the most important properties which determines the penetrating ability of a liquid. Low surface tension liquids provide better penetration and spreads well on part surface. Wetting ability is another important property which also determines the penetrating ability of a liquid. Ability to wet or spread on the surface is related directly to the contact angle between the liquid and the surface at the point of contact. To have a good wetting ability, the contact angle must be small. Penetrants used for testing have contact angle of 5

0 or less.

Brightness The dye in the penetrant should be highly stable and bright enough to be visible in very thin film. Volatility is the speed with which a liquid evaporates. Penetrant should be non volatile to allow long penetration and inspection time. The penetrant must not dry during the examination period. Flash point is the temperature at which flammable vapor is given off. For safety purpose, a penetrant should have higher or no flash point. Chemical Innertness is the ability of a material not to interact when mixed with or brought into contact with other materials. Penetrant should be as inert and non corrosive as possible towards the materials to be tested. Solubility is the ability of a material to be dissolved into another material. Penetrant must be soluble in order to be easily removed from the surface of the part being examined. Creep is the ability of small amounts of liquid in discontinuities to com back out to form an indication. Tolerance for Contamination is the ability to tolerate small amounts of foreign substances and not affect unfavorably the action of a penetrant. A penetrant is a compound of several ingredients and a little water, acids, detergents and degreaser solvents may upset the balance and cause the penetrant to lose some or all of its important properties. Toxicity, Skin irritation and odor No penetrant should contain poisonous, corrosive or skin irritating material or have an offensive odor. Penetrant properties : Low surface tension and contact angle, capable of smoothly

and evenly spread on the part surface. Low viscosity, capable of entering small discontinuity

openings faster. Non volatile. Easy removal from part surface but not from the

discontinuities. The penetrant should get removed with no dye precipitation.

Capable of emerging from discontinuities after excess penetrant is removed.

High visibility and contrast in small quantities and thin films on part surface.

Does not corrode the test surface. Non Toxic.

Capillary action : The principle of liquid penetrant testing is based on the ability of some liquids to enter a discontinuity opening and then re-emerge from it when the excess penetrant is removed from the surface. Capillary action is the means by which a liquid enters a discontinuity opening. This action is what causes a piece of sponge to absorb liquid. Capillary action is a phenomenon in which water or other liquids will rise above the normal liquid level in a small bore or capillary tube due to the attraction of the molecules in the liquid for each other and for the wall of the tube [ cohesion and adhesion ]. Cohesion is interaction between two surfaces of the same material in contact, makes them cling together [ with two different materials the similar phenomenon is called adhesion ]. According to kinetic theory, cohesion is caused by attraction between particles at the atomic or molecular level. Surface tension, which causes liquids to form spherical droplets, is caused by cohesion. The distance the liquid will rise up the tube of a given diameter and material is a function of three factors; Surface tension, Wetting ability, Tube open or close at the top end. Liquid will rise less in a closed end tube. The practical circumstances, a penetrant encounters during testing is more complex. Cracks, for example are not capillary tubes, but simulate the basic interaction between a liquid and a solid surface, which is responsible for the migration of penetrant into its open space. This same interaction acts again and penetrant emerges from the discontinuity when the excess penetrant is removed from the surface.

When an aqueous developer [ solution or suspension ] is used after water rinse, the test surface is dried after the application of the developer. The water washable penetrant process is very useful for inspection on rough surface, keyways, threads, where penetrant is generally trapped. The water washable color contrast process is also used for inspection of primary materials such as slabs of stainless steel. In general the excess penetrant may be removed by simple low pressure [ with or without air assistance ] water spray. The penetrant may initially gel as it is washed from the surface to resist removal from shallow open defects. This process may be assisted by immersion of components in water, agitated by compressed air or propeller type device. When the form of components is very complicated, immersion in an agitated water bath is preferred. It is very rare that an agitated immersion wash can be successfully used alone to remove water washable penetrants, a water spray is used to complete the process. The temperature of the water should be controlled between 20 and 30

0 C [ 10 to 38

0 C allowable ]. The disadvantage of immersion of component in water after completion of

penetration time is that the water becomes contaminated. Care must be taken to ensure that the water is changed frequently enough to avoid excessive contamination. In a spray system, clean water can be used at each application. Spray washing using large water droplets is preferred for removal of water washable penetrant particularly from castings. Water temperature should be controlled within a range of 20 to 30

0 C. This will ensure uniformity of processing. The

water pressure or combined air / water pressure should not exceed 40 p.s.i. High pressure will wash out penetrant from wider discontinuities. The spray nozzle should be maintained at a distance of 8 -12 inches and at 45

0 to 75

0 angle. On

manual lines no part of the test surface should be sprayed for more than 2 minutes. In automated water wash stations these conditions can be varied to give optimum performance. The services to water wash stations should be equipped with pressure gauges and temperature gauges to monitor the required parameters. Water washable penetrants are more convenient and less expensive to use. In fully manual process, excess penetrant can be removed by wiping with a clean, dry, and lint free cloth or absorbent toweling. The remaining surface penetrant is then be removed with a water or solvent dampened cloth or towel, or very carefully spraying the surface with water using a plastic spray bottle. Water washable penetrants are susceptible to water contamination. A specification requirement is that these penetrants tolerate at least 5% water contamination without gelling, separating, or coagulating. They must also meet the requirement for tank life without separation of emulsifier from the penetrant. The limitations of water washable process, particularly it’s vulnerability to over - removal of penetrant from defects with large opening and inability to detect very tight cracks make the water washable penerant process the least sensitive.

Water Washable Penetrant process : Advantages : Disadvantages : Fast economical test process. Not reliable in finding wide and shallow Good on wide range of defects discontinuities. and rough surface Not as reliable on second or third running Easily washes with water. of parts [ re - test ]. Easily adaptable to small parts. Susceptible to over - washing. Good on rough surfaces, keyways Water contamination is more destructive and threads. to this penetrant. Relatively inexpensive. Requires longer penetration time. Ideal for automation. Affected more by acids and chromates.

Types of penetrants : Penetrants are further categorized by one of the 3 methods used to remove the excess penetrant from the test surface. They are Water Washable Penetrants, Post Emulsified / Post Removable Penetrants and Solvent Removable Penetrants. Water Washable Penetrants contain a built in emulsifier and are self emulsifying, so that they are removable with plain water in a single step rinse process. The built in emulsifier in the penetrant is activated by water and renders the penetrant instantly water soluble. Adding a lipophilic emulsifier to a post emulsifiable penetrant will not produce a satisfactory water washable penetrant. When water washable penetrant is used, it is extremely important to prevent over washing, which can cause the penetrant to be washed out of the discontinuities.

Types of penetrants 1 : Penetrants are generally a chemical solution of highly stable dyes, either visible or fluorescent, in a mixture of surface active agents with a blend of highly refined hydrocarbon distillate. The carrier liquid is practically colorless and transparent. The dye provides a high contrast indication against the background. Petroleum distillates, which are relatively nonvolatile and used to dilute both water washable and non - water washable penetrants, contribute to penetrant performance in several ways. As a diluent, a light petroleum distillate dissolves light organic soils on surfaces and in flaws, and it assists crack penetration with its low surface tension and affinity for metal surfaces. Also, petroleum distillates naturally fluoresce a light blue color and, in a minor way, contribute to penetrant brightness. Petroleum distillates are source of halogens and sulphur. Water based fluorescent penetrants [ using water as diluent which represents at least 50% of a penetrant ] have been developed for easy disposal, previously used as non-certified applications, are now approved. Water based fluorescent penetrants are restricted to water washable method and available in sensitivity level 1 and 2. The penetrants are liquid oxygen compatible, suitable for use as a leak detector and for inspection of plastics. The penetrants are compatible with water based precleaners, e.g., hot alkaline, which for environmental reasons have replaced petroleum solvents and vapor degreasers. Being compatible with the aqueous cleaner, these penetrants have tolerance for rinse water carry over. Also, flaw entrapped water is less likely to impair the inspection process, such as would happen when flaw entrapped water repelled a petroleum based penetrant. Water in water based penetrants will evaporate, and regular, testing with a refractometer is necessary to measure the loss. Periodic water additions are part of the maintenance criteria for these penetrants. Longer penetration time [ 30 mins ] improves sensitivity and dry developers are less suitable for these penetrants. Selection of penetrant type depends on, Sensitivity required. [ tightness of the opening ] Number of articles to be tested. Surface condition of the part to be tested. Configuration of the test specimen. Availability of water, electricity, compressed air and other

equipments. Suitability in environment where the test will be performed. Cost of inspection. Visible Dye : Visible dye or color contrast penetrant inspection makes use of a dye that is visible in ordinary light. These penetrants contain a highly stable bright red or purple dye, so that the indications produce a definite contrast with the white background of the developer powder. The dyes are visible in very thin film. Color contrast penetrant can not achieve the level of sensitivity which is possible with the fluorescent penetrants because of higher viscosity and inability to enter very tight cracks. As a general rule of thumb, visible penetrant examination is roughly equal to fluorescent penetrant examination of level 1 sensitivity.

Discontinuities in the range of 50 in NiCr panels are routinely

detected and under the right conditions and processing 30 discontinuities can be highlighted. The advantage of using a visible penetrant is that it can be used with ordinary shop lighting and from a small portable solvent removable kit at any location. They are specially suitable for field applications and where darkening the inspection area is impractical.

Visible penetrants are less vulnerable to contamination from cleaning fluid that can significantly reduce the brightness of a fluorescent indication. Visible penetrants are used in maintenance, repair and in manufacturing processes where a visible indication helps in locating the area for subsequent repair work, such as large rough castings. These penetrants are also used for through leak testing of heat exchangers and tanks, where penetrant is applied to one side and developer to the other. Effect of Temperature: Penetrants are normally developed for use from about 10 to 52

0 C.

However they can be used as low as 40 C but

penetration time must be increased by 2 to 3 times. At low temperature, penetrant becomes highly viscous and sluggish. Ideal operating temperatures for normal use of penetrants lie between 10

0 C and 38

0 C.

Penetrant testing at higher part temperature has positive advantage. The molecular movement of a liquid is directly related to the liquid's absolute temperature. The higher the temperature, the greater the movement and viscosity decreases. Thus, a penetrant, in contact with a heated part, will be less viscous and fast penetrate into flaws by displacing any gas or liquid from the interior of the discontinuity. Heat vaporizes discontinuity entrapped solvents and moisture, which might otherwise interfere with penetration. Heat liquefies heavy oils and waxes present from previous processing, facilitating penetrant displacement. Thus, a heated penetrant will more effectively fill a crack. In addition, when penetrants are heated, dwell time may be reduced because of faster penetration. During the dwell time, the part temperature will fall and the viscosity of the penetrant will increase. This will resist over removal. Similarly, the developing step is also faster, penetrant molecules are more readily absorbed by the developer layer, and the developer dries faster. When inspecting heated surfaces with water washable penetrants, it would be prudent to wipe the surface with a solvent or cleaner resistant to high temperatures, since boiling water may not rinse excess penetrant satisfactorily. High temp certified penetrant materials shall be used.

Penetrants for high temp applications: Com Pent Clnr Devl Temp

0F

S K017 K019 D-350 200 - 350 MR MR68H MR91H MR70H 122 - 356 A P303A 150 - 350 ML VP302 R502 D702 150 - 350 S Sherwin inc, MR MR Chemie, A Ardrox inc ML Met - L - Check,

Types of penetrants 2 : Fluorescent Dye [ Type I ] : Fluorescent penetrant uses minerals or chemical compounds which emit visible light when exposed to ultraviolet light. Fluorescent penetrant’s response is maximum when exposed to 365 nm wavelength ultraviolet light. The dye absorbs this energy and emits between 520 nm to 620 nm wavelengths which is visible as brilliant yellow-green. Orange - red type penetrants are also available. The efficiency of fluorescent dyes in converting UV light to visible light may be reduced by prolonged exposure to UV light. Elevated temperature can reduce the fluorescence of penetrant in open tanks. Fluorescent penetrants are designed for different sensitivity levels ; Level 1/2 Ultra low, for castings and rough surfaces. Level 1 Low sensitivity 50μm NiCr crack panels. Level 2 Normal sensitivity 40μm, general purpose use. Level 3 High sensitivity 20 - 30μm. Level 4 Ultra high sensitivity 10μm extremely critical use. The actual certifications for sensitivity level ½ to 4, involves the use of a series of titanium and nickel alloy panels containing very small, laboratory generated fatigue cracks. Fluorescent penetrants are more sensitive than visible dye because they have lower viscosity, can penetrate smaller openings, and have excellent visibility. It is easier to detect something glowing in the dark than to observe a small colored area on a white background. Fluorescent indications are many times brighter than their dark surroundings. The brightness of indications depends on the incident UV - light intensity and minimum 1000 micro watts / cm

2 is required at the test surface. Background

white light illumination is to be limited to 20 Lux. Fluorescent penetrants require a developer with lower particle concentration, because the white background provided by the developer is not necessary. Developer is required only for the blotting action. Fluorescent indications are visible in very thin film. For these penetrants, removal of excess penetrant by solvent wiping requires more care and is a problem when a large surface area is to be cleaned. Water washable type will reduce this problem. For fluorescent penetrant testing, it is very important to ensure that the part surface is completely free from nitric, sulphuric, chromic acids, alkaline permanganate, acid chromate solutions, acid ferric solutions, peroxides, persulphates etc. These common cleaners all degrade or kill the fluorescence completely. Dual sensitivity penetrants [ Type III ] : These penetrant contains a combination of red visible dye and orange fluorescent dye, such as ‘By Lux’ from Sherwin. The test object is first viewed under white light for red dye indications followed by ultraviolet light in a darkened area for orange indications. These penetrants provide two levels of inspection sensitivity. One advantage is that flaws located during UV examination are marked by visible indications for repeat examination or repair work. Another advantage is that flaws found in one mode can be verified in the second mode. However, the brightness of the visible red color and the fluorescent color are less than the individual visible dye and fluorescent penetrants. Important : Fluorescent penetrant should never be used on surfaces which have been processed previously with color contrast penetrant. In many cases residues of dyes persist in a defect, the fluorescent process suffers not only from the residue reducing the amount able to get into the defect, residues of color dyes will compete for light and effectively kill the fluorescence of any entrapped fluorescent penetrant in subsequent tests. 1% visible penetrant can stop the fluorescence. If fluorescent inspection is required, then the part shall be cleaned thoroughly to remove any color penetrant residue from the interior of the discontinuities. Applying a reversible penetrant developer, which contains a fluorescent dye, reacts with,

and thus is quenched by, the red dye. When the surface is viewed under UV illumination, the residual spots of red dye can be detected, which stands out as dark spots against a fluorescent background. Once the removal of red penetrant dye has been completed and verified by this procedure, the developer is removed by rinsing with water. Fluorescent Brightness : is the amount of visible light given off when a fluorescent dye is exposed to UV light. It depends on ; * the thickness of the penetrant film. * the intensity of the UV light. * amount of fluorescent dye and its capability to absorb UV light. * the efficiency of the dye in converting the released electrons to visible light. Penetrants were examined by placing a drop of used and new penetrant next to each other on a paper towel for a visual comparison. Most penetrant specifications require replacement when the brightness drops by more than 10%. Under test situations, the human eye may detect a change of about 15% or more in brightness. Fluorescent brightness measurement can be performed using a approved Photo fluorometer such as NDT- Italiana, S – 291. The instrument is available from NDT-Italiana, FAX 011-39 -39 -647799.

Note : The general rule is to use the lowest sensitivity fluorescent penetrant that reveals the discontinuities of interest.

Use of Penetrants : If there is dirt in the crack, there is no room for the penetrant to enter it, and the process will not work. The part shall be cleaned to open the discontinuities. Following cleaning, the part, including the interior of the discontinuities shall be thoroughly dried. Penetrant application : After drying, The temperature of the test surface should be between 10°C and 52°C. The entire area to be tested must be wetted with a layer of the penetrant and allowed to dwell. If Penetrant pulls back in droplets, re-cleaning is necessary. The penetrant must not dry during the dwell time, if it does, it will require rewetting with penetrant or the part shall be reprocessed. The method of penetrant application has little effect on the inspection sensitivity, but electrostatic spraying method has been reported to produce slightly better results than other methods of applications. Penetrant can be applied by immersion, flow - on, spraying or brushing. Brushing applies minimum penetrant and is recommended for a small area on a large part and applying penetrant in a confined space. Aerosol sprays, pneumatic, pump - based or electrostatic sprayers are useful for large parts that cannot be immersed or for spot - checking of components in the field. For immersion application, the part shall be dipped and allowed to drain outside the penetrant container. It is not recommended to leave the part immersed in the penetrant during the dwell time. This increases drag out and contaminates the emulsifier rapidly in post emulsification processes. Penetrant Dwell : The penetrant is left on the test surface for sufficient time to allow penetration into the discontinuity openings. The time involved depends on the viscosity of the penetrant, temperature of the part surface and the tightness of the discontinuities to be detected. Wrought products require longer penetration time than cast products. Penetration time used is generally between 10 and 30 minutes. At low temperatures, longer penetration time shall be used because of the increased viscosity of the penetrant. Penetration time is known as dwell time. Developer application : To make the penetrant indication clearly detectable a developing medium is used. The developer blots the penetrant out of the flaws, makes the indication larger and easier to see. Developer may be applied by dusting or dipping for dry powder or spraying or immersion for water base developers. Non aqueous wet developers should be applied by spraying only. The developer should then be allowed to dwell on the part surface for sufficient time [ usually 7 to 30 minutes ] to permit it to draw penetrant out of any surface flaws to form visible indications of such flaws. This is known as developing time. Longer times may be necessary for tight cracks. Developer provides a white background for color contrast penetrants.

Developers : Normally a developer is used to complete the penetrant process. Developers are fine white powders applied as dry or with a liquid carrier to form a thin uniform coating on the test surface. Developer acts like a sponge with very fine random capillary paths. When the penetrant contacts the developer, it spreads through these paths by capillary action [ known as blotting or reverse capillary action ]. The penetrant gradually diffuses into the developer and indications become larger than the discontinuity opening. The white color of the developer provides contrast with color contrast penetrants and also provides a darker background for fluorescent indications. It should be noted that the developer itself does not produce indications but simply absorbs the penetrant already present below it and makes it more visible. Developer should be applied as soon as possible or within 30 minutes after the removal of the excess penetrant. Dry developers [ Form A ] : [ blends of chemically inert white powders ] Dry developers are a non toxic fluffy absorbent white powder that is used mostly with florescent penetrant process. These dry powders do not normally provide sufficient white background for contrast with color contrast penetrants. This may be achieved by use of electrostatic spray application but is not always controlled easily. Dry powders must be used on dry surface only. Dry developers are the least sensitive. The dry powders should be used in a closed container or, if sprayed, inside a booth with good extraction. Where the throughput of components is low, components can be placed on a grid in a tank and powder applied using a scoop or from a pear shaped rubber puffer. Excess powder is then removed by gentle air blast, shaking or tapping. Dry powder may also be applied by brushing on rough test surfaces. Dry powder is best applied by placing the component in a dust storm cabinet which may be of the tunnel type or top loading. The cabinet is sealed to be dust proof. Most cabinets work on a preset cycle so that once started the cabinet cannot be opened until the cycle is complete. The dry powder developer is then agitated with dry and oil free compressed air, which blows the powder into a cloud around the component to coat the surface. It is advisable to incorporate an " extract and return " system with the cabinets, since the fine particles take a very long time to settle from the air. Dust chambers should be fitted with heaters to maintain a dry inside atmosphere. Installing a fan at the bottom of the container to blow the powder eliminates most of the problems associated with the use of compressed air. Dry powders can be applied after charging them in a electrostatic spray system. Some dry powders are of a chemical composition which does not become sufficiently charged and this point should be checked before attempting to use this method of application. Dry developers should cling to dry metallic surfaces in a fine film of dust. The adherence of the powders should not be excessive, as the amount of black light available to energize indications will be reduced. In automatic processing systems, dry powders are applied by passing the part through a conveyor in a fluidized bed tunnel system. This application relies on producing a controlled air / powder mixture which passes over the components and coat the surfaces. Extremely fine dry developer powders are available for application by dipping the dried part into it. This process has the disadvantage that the developer gradually becomes contaminated by fluorescent flecks and produces false indications. For purpose of storage and applications, the dry powders should not be hygroscopic and they should remain dry. If they pickup moisture when stored in areas of high humidity, they agglomerate or lump up and lose their ability to flow and dust the surface adequately. The developer should be checked daily for caking and contamination. Dry powder developers can dry the skin and irritate the eyes and the respiratory system. Use of rubber gloves and respirators is desirable.

Developer properties : Good blotting action. Must be easily wetted

by penetrant and draw the maximum amount of penetrant from the defect.

Fine grained. Show indications from small amount of

penetrant. Mask out interfering penetrant and part

surface background. To form a thin and uniform coating. Non fluorescent. Easily cleaned from the surface. Non flammable. Non toxic and non corrosive.

Developer selection : Select wet developer in preference to dry

on very smooth test surfaces. Select dry developer in preference to wet

on very rough test surfaces. Cleaning and re inspecting a rough surface is difficult, if a wet developer was used for a prior inspection.

Wet developers can not be used reliably where part configuration accumulates developer at certain locations such as threads. Excess developer can mask out indications. Dry developer can be used.

Solvent suspended developers are very effective for flaws with narrow opening, but are not satisfactory for finding wide shallow defects.

Dust Storm Cabinet : dry developer is blown inside the closed cabinet, while settling, the powders coat the part surface.

A developer coat which is just thick enough to mask the metal surface is about right. If the coat is too thick, the penetrant might not blot through to the surface. If it is too thin, it might not blot well enough. Spraying should be performed from a distance of 8 to 10 inches from the test surface. Holding the can too close to the surface will result in excessive developer being applied every time. Starting the spray path from one side of the part, moving the spray onto and across the part, and finishing the spray pass beyond the part, is the preferred technique. Additional spray passes are required to ensure full, but minimum coverage. This method results in even coat without puddles that can result when the spray path is begun by aiming directly at the part. The solvent shall not be allowed to accumulate on the test surface or the penetrant will be diluted and the indications will become diffused and weak. It is wise to test the aerosol can for its uniform spraying ability before using it on the test part.

Aqueous [ Wet ] developers : These developers are used in penetrant inspection stations. Aqueous developer uses water as the carrier of the developer particles. The developer is used as a solution [ Form B ] or as suspension [ Form C ]. Water based developer includes a wetting agent, corrosion inhibition system and dispersing medium. These developers are applied directly on a water washed surface and after drying a solvent washed surface. Parts are dried immediately after excess developer has drained from the component. These developers are less sensitive when compared to non - aqueous developers. Water Soluble developers [ Form B ] : [ Blends of white crystalline powders to be dissolved in water ] Supplied as soluble white powders to be dissolved in water at a concentration recommended by the manufacturer [ 40 - 60 g / lit ] to prepare a transparent working solution. The prepared bath is completely soluble and does not require agitation. If the solutions are stored in open tanks, precipitate may form due to loss of water by evaporation. The concentration of the solution is monitored by measuring the specific gravity with a hydrometer. Water soluble developers are not recommended for use with water washable penetrants because of the potential to wash the penetrant from within the flaw unless the developer application is very carefully controlled. Water solution developers can be applied by immersion followed by drainage in a stainless steel tank. The developer may be applied by flow-on, brush or a wet spray. When spray application is used, it is important, that the spray is wet and the droplets coalesce to form a continuous layer on the test surface. The part surface must be dried actively after the developer application to form an uniform layer. The dried developer film is water soluble and can be removed after inspection by simple water rinsing. Soluble developers contain inorganic compounds which ionizes in the solution. Dipping different metals in contact with each other and use of uncoated basket produce galvanic corrosion. The contaminated developer solution becomes gray to dark gray in color. Water Suspended developers [ Form C ] : [ Blends of white inert powders and surfactants for suspension in water ] supplied as insoluble white powders to be suspended in water at a concentration recommended by the manufacturer [ around 70 g / lit ] to give a working preparation. Water suspension developers may be applied by immersion in an agitated bath or by spraying. The immersion tank should be of stainless steel with mechanical stirrer to prevent settling. Air agitation should be avoided since the developers contain wetting agents and corrosion inhibitors which may become degraded by chemical reaction with the oxygen or carbon dioxide in the air. If the developer is to be applied by spraying, the design and operation of the spray apparatus must allow continuous agitation of the reservoir of developer and the application must allow the suspension to arrive wet at the test surface. Foaming must be avoided because when foam bubbles break they leave holes in the developer coating. The amount of powder in suspension must be carefully maintained. Too much or too little developer on the test surface can seriously affect sensitivity. It is essential that the developer bath is monitored by checking the specific gravity and the wetting characteristics. Water suspension developer is applied before drying, the developing time decreases because the heat from the drying operation helps penetrant back to the surface opening and the developer film being already in place, the developing action begins at once. Non Aqueous Suspension developers [ Form D ] : [ Chemically inert white pigments suspended in volatile liquid carriers ] These consist of developer particles suspended in a volatile organic solvent. The use of a quick drying solvent as carrier makes non aqueous wet developer the most sensitive for detection of minute cracks. The solvent combines with the penetrant in the cracks, and the result is a less viscous liquid which bleeds into the developer fairly quickly as the solvent evaporates. Non aqueous suspension developers must be applied by spraying on a dried part surface. Spraying may be by aerosol can or conventional spray gun. Since volatile solvents are used in these developers, efficient extraction facility must be provided in the inspection area. The suspended particles separate on standing [ settles to the bottom of the container ] and so must be agitated before use to an homogeneous mixture. When aerosol cans are used, this is done by shaking the can. For a spray gun application, the reservoir must be equipped with an efficient agitator. Alternative methods of application, such as immersion, flow-on or brushing on will cause loss of sensitivity. However, when spraying is a problem brushing is recommended. .

the test surface. Metal working lubricants, protective oils, abrasives, metal debris, heat treatment scales and surface conditions in welds such as slag, spatter, anti spatter chemical compounds are all must be removed as well. A good cleaning procedure will remove all contamination from the part and not leave any residue that may interfere with the inspection process. The banning of ozone depleting hydrocarbons has resulted in shifts to alternative cleaning processes. The cleaning processes are often optimized by their surface cleaning capabilities, but the materials and processing methods may not be adequate for [ or may even interfere with ] sensitivity of defect detection. Removal of the cleaning medium from the interior of the exposed discontinuities is necessary otherwise penetrant entry may be reduced or prevented. Surface contaminants may cause retention of penetrant on the surface and give rise to background which may obscure actual discontinuity indications or produce false indications. Water is the most common contaminant causing penetrant to gel or separation. Hot rinsing combined with extended periods of oven drying may be necessary to completely remove water from the test parts. For critical applications, re -qualification of the system is necessary when major changes are made in the precleaning processes. The following surface preparation [ pre cleaning ] methods are recommended, Mechanical preparation : The initial step in precleaning is often mechanical removal of loose surface materials. Processes such as milling, polishing, or abrasive blasting effectively remove surface contamination, but can smear or peen the metal surface and thus reduce or prevent access of the penetrant to the discontinuity. It is well recognized that machining, honing, lapping, hand sanding, hand scraping, shot peening, grit blasting, tumble deburring, and peening operations can cause a small amount of the material to smear on the surface of some materials. If the parts have been machined, sanded, or blasted prior to the penetrant inspection, it is possible that a thin layer of metal may have smeared across the surface and closed off defects. This layer of metal smearing must be removed before penetrant inspection. Mechanical processes of sufficient intensity to cold work the metal surface are not recommended for softer metals, such as aluminium, titanium, magnesium, beryllium. Detergent Cleaning : Initial pre-cleaning is for removal of dirt and soil. Secondary pre-cleaning is for removal of residual chemical or oily films after paint striping, acid or alkaline cleaning. Detergent cleaning is accomplished by scrubbing with a soft bristle brush and a detergent cleaner that is non corrosive to the material being cleaned. Some domestic soaps and commercial detergents can clog flaw cavities and reduce the wettability of the metal surface, thus, reducing the sensitivity of the penetrant. Steam Cleaning : Is performed with a heated degreasing or detergent solution applied in a high pressure spray. It is particularly adapted to the cleaning of large articles and assemblies which cannot be handled easily. Cleans cutting oils, polishing compounds, grease, chips and deposits from EDM. Steam cleaning, can also cause metal smearing in softer materials. Vapor Degreasing : For removing grease and oil from surface and in crevices, a vapor degreaser with a solvent cleaning solution is used. In vapor degreasing, methyl chloroform [ 1,1,1 – trichloroethane ] was the accepted solvent until it was found to deplete the ozone layer and its manufacture was prohibited. Solvent Cleaning : Residue - free solvents are used manually for removing cleaning solution residues, Grease, oil, waxes and sealants, paints and other organic matters. This is not always adequate if solid soils are imbedded in crevices. Solvents may be applied by flushing the test surface with a spray, a wipe on and wipe off technique or immersed in a dip tank. It is the most common method for pre and post cleaning and must be performed in a well ventilated area due to the toxicity of the vapors. Some solvents may be flammable. Acid Cleaning : Solutions of acids or their salts are used to remove rust, scale, corrosion products, and dry shop contamination. Strong solutions for removing heavy scale, mild solutions for light scale, weak [ etching ] solutions for removing lightly smeared metal. Acid entrapment from a penetrant etch can have disastrous effects on the penetrant inspection. Aqueous Alkaline cleaners : Alkaline cleaners, often containing silicates, are essentially powders dissolved in water and typically used at more than the ambient temperature. Alkaline cleaners have been introduced to substitute the use of chlorinated solvents for cleaning various types of soils. There are even means to convert vapor degreasers to alkali cleaning stations. Hot alkaline and acid cleaners are used for removal of corrosion, rust and scales. Alkaline cleaners are also used for cleaning braze stopoff, oil, grease, polishing material and carbon deposit and gross metal removal in aluminum. They are usually applied by dipping or brushing and allowed to act on the surface for a certain period of time and rinsed off. The manufacturer’s using instructions must be closely adhered to or this method could damage the part. An acid or alkaline cleaning is always followed by a detergent washing or solvent flushing to totally free the surface of residual chemicals. It has been found that some alkaline cleaners can be detrimental to the penetrant inspection process if they have silicates in concentrations above 0.5%. Evaporated, water-based alkali cleaners may leave silicate, or other residue, entrapped in flaws to inhibit penetrant entering those flaws. Sodium metasilicate, sodium silicate, and related compounds can adhere to the surface of the part and form a coating that prevents penetrant entry in to the cracks. Molten salt baths : are used in removing heavy, tightly - held scale and oxide from low alloy steels, nickel, and cobalt –base alloys, and some types of stainless steel. They cannot be used on aluminum, magnesium and titanium alloys.

Precleaning of test surfaces : Adequate pre cleaning of work surface is essential for liquid penetrant examination processes to operate successfully. Contaminants prevent the penetrant from wetting the surface properly and block the opening of the discontinuity cavity. Discontinuities must be opened to the surface and internally cleaned to allow the penetrant to enter it. All contaminants including grease, carbon deposit, dirt, varnish, wax, oils, oxides, corrosion and water must be removed from the test surface and the discontinuity cavity. Mill scales, paint, plating or corrosion inhibitor coatings seal the defects and must be removed from

Cleaning of test surface 2 : Paint Removal : The paint film must be completely removed to expose the surface of the examination area. Paint removers are chemical bond release agents or solvent strippers for the removal of paint and corrosion inhibitor coatings. They are applied by dipping or brushing and work well with heat to accelerate the removal process. The paint strippers is removed and the surface is cleaned by detergents or solvents. Ultrasonic Cleaning : is performed on large number of small parts by immersion or dipping the part in a ultrasonic cleaning tank with almost any kind of cleaning solution. Solvents and alcohol work well with ultrasonic agitation. It is the most effective method for opening up defects by removing contaminants imbedded in the discontinuity cavity. Ultrasonic cleaning has been found to be effective in removing penetrant entrapped in discontinuities. Tight flaws require 2 hours of cleaning. For geometrically complex components, huge benefits result from combining the chemical action of a solvent with ultrasonic agitation. Ultrasonic cleaning is based on the principle of producing cavitations in a liquid solvent in which the component to be cleaned is immersed. Large quantities of tiny bubbles are created in the solvent [ cavitations ] by high intensity compression waves. The collapsing pressure of the bubbles forces them into crevices where the fluid penetrates between contamination and the work piece surface. This action forces the contaminant off the work piece leaving it totally clean. The frequency of ultrasonic vibration is 30 to 40kHz although higher frequencies may sometimes be used. Cleaners are available as static solvents or ultrasonic types. Etching : This process uses acids or alkaline solutions to dissolve smeared metal from the surface. Etching is performed after any operation that could mechanically remove material from the surface, such as sand blasting, machining, grinding or power brushing, to assure that the cracks are opened to the surface. These mechanical operations tend to smear and close the discontinuity openings. Etching on steel is accomplished using 5 to 20% of acid solutions or 20% solution of ammonium per sulphate. The solution is applied for a specific period of time, neutralized and rinsed. The etching time is depends on the amount of material that is to be removed. Etching usually requires a separate certification because of the inherent danger to the test operator and the component being used. Acid entrapment from etching can have disastrous effects on the penetrant inspection. Sodium hydroxide caustic often used to etch aluminum parts does not affect penetrants but can reduce the brightness of indications. Careful cleaning of both acid and caustic etches before penetrant inspection is highly recommended. Drying the Part’s surface : Components that require the use of cleaning agents for the pre cleaning process must be dried completely before penetrant application. When water based cleaners are used during pre cleaning, the final step is a wash with clean water and the parts are thoroughly dried. Water or cleaning liquid residue will hinder entrance of penetrant into discontinuities. The normal way of removing water from cleaned parts before penetrant application is to heat them in a temperature controlled re - circulating air oven for one hour at around 125

0 C. An alternative is

to immerse the components in a dewatering fluid which is then itself removed by vapor degreasing. The Part surfaces are to be dried again after removal of excess penetrant and the stage at which surfaces are dried depends on the type of developer to be used. When dry powders or non aqueous solvent suspension developer is used, the surfaces must be completely dry before application. When water soluble or water suspended developer is used, the parts are dried after the application of the developer. Drying temperatures of 60 - 80°C [ 140 - 176°F ] is normal for oven drying. Ovens should be thermostatically controlled with supply of clean dry re - circulating air and should be capable of drying the parts in 10 minutes. Care should be taken so as not to raise the surface temperature of the part above 60

0 C. In an oven, drying of small and

large parts together is not recommended because of dissimilar drying time. Excessive drying can damage the part as well as evaporation of the penetrant, which can impair the sensitivity of the inspection.

Drying components before penetrant application may require heating them to 120

0 C for at least

1 hour in an oven operating at ambient pressure. The component must cool down to 40

0 C before the application of the penetrant. A

vacuum drying oven can dry the component within 10 minutes. Vacuum drying ovens can maintain a working pressure as low as 10 milli bars absolute and this can be achieved within 2 minutes. At this pressure, water boils at a temperature below 5

0 C. However, atmospheric water vapor will

condense on work pieces removed from a drying oven operating at this temperature. Vacuum drying ovens equipped with external heating elements in their base and wall to maintain a working temperature at nominal 40

0 C can reduce drying times to less than 10

minutes. The residual temperature of approximately 40

0 C is achieved after drying,

which is ideal for penetrant application. The ovens are smaller in size and can accommodate a single, relatively large work piece, or many smaller components. With recommended care, the oven provides a long and efficient service enhancing the drying process after precleaning.

Detergent

cleaning.

Solvent removable penetrant process : Penetrants which cannot be removed directly with water can be removed by use of organic solvents. The solvents used are quite versatile and dries quickly without a residue and a developer can be applied on the dry surface. Normally, the same type of solvent is used for pre cleaning and for removing excess penetrant from the test surface. Typical applications involve in situ inspection or testing of a specific location on the component when it is not practical to remove the penetrant using water, which would involve subsequent drying of the component. Three types of solvent removers are used : Volatile hydrocarbon distillates. Halogeneted hydrocarbon solvents [ non flammable ]. Volatile aliphatic alcohols. The flammable cleaners are potential fire hazards but are free from halogens, while the non flammable cleaners contain halogens, which render them unsuitable for some applications. Solvent removable penetrant may be applied to the test surface by spraying, dipping, flooding or brushing. For a small test area, brushing controls applied penetrant and removal of excess penetrant is easier. Excess penetrant removal is performed by wiping the part surface in one direction only, with a clean and lint free cloth or absorbent paper. The proper procedure is to make a single pass, then fold the cloth to expose a clean surface for each succeeding wipe. This will remove most of the penetrant. The remaining traces are then removed by wiping with a new cloth or paper lightly wetted with the solvent remover. After each pass, the surface of the cloth is examined. If there is more than a trace of penetrant on the cloth, the cloth is folded to expose a clean surface, remoistened with the solvent, and the surface is wiped again. The process is repeated until there is little or no trace of penetrant on the cloth. Cleaning should be done quickly, using a minimum of cleaner. If the operation is prolonged or if excessive amount of the cleaner is used, some penetrant may be removed from discontinuities. The surface is then viewed under suitable light to ensure that the excess penetrant is removed. For application of developer, the surface is dried by evaporation or wiping. Non flammable, volatile solvents can be used in the vapor phase in automatic processing stations [ like vapor degreasing ]. The solvent should be redistilled frequently to avoid heavy contamination with penetrant, since the penetrant will start to co-distil with the solvent and cause heavy background.

Solvent removable penetrant process : Advantages : Disadvantages : No water required. Not as good as water- wash on No electricity for color contrast penetrants. keyways, threads and rough surfaces. Spray can supplies fresh uncontaminated Less sensitive for wide shallow materials. defects. Can be re-run. [ repeat test ] Materials are more costly. Less susceptible to over-wash Less production than water wash. of penetrant, if used carefully. Greater hazard of toxicity or Ideal for large items. flammability for cleaners [ solvents ]

Note : The volatile organic solvents used for penetrant removal are the most aggressive and great care must be taken in their use to prevent over-removal. For manual processing, the solvent remover must be used by wiping with a cloth. Use of a solvent remover by spraying or flooding will results in over wash or dilution of the penetrant and the indications will become diffused and weak. The optimum sensitivity is achieved when the brightest indications appear against a thin developer contrasting background. When normal cleaning cannot sufficiently remove penetrant from as-welded surfaces, machined parts with sharp inside corners, or parts with depressions or pits, such areas can be further cleaned with a commercially available cotton swab or with a cotton swab on a toothpick wetted with the solvent.

Hydrophilic removers are applied by immersion in an aqueous solution. The concentration for immersion varies between the range of 2.5% to 10%, 20% and up to 30 % depending on brands and conditions of agitation. The manufacturer of the emulsifier can provide the proper information concerning this concentration. Concentrations higher than recommended by the emulsifier manufacturer or the qualified percentage is prohibited. The immersion time varies from 20 seconds to 5 minutes depending on the penetrant, emulsifier concentration, surface roughness and agitation. The surface active agent in the remover combines with a small quantity of penetrant from the surface and prevents the penetrant from recombining with the remaining surface penetrant. A slight agitation is necessary to remove the colloidal suspension of penetrant – emulsifier from the surface and to expose fresh penetrant. The preferred method of agitation is mechanical. Agitation using compressed air can cause heavy foaming and may introduce contaminants. When application is by immersion, hydrophilic emulsifiers malfunction as a result of penetrant accumulation. Contamination increases when part geometry includes cavities, or areas which entrap penetrant to be carried over to the emulsifier tank. Emulsifier with lower concentration gets contaminated faster. The 30% concentration, if used, tolerates some three times more penetrant contamination as the 10% concentration does. Everything being equal, a 30% concentration lasts three times longer than a 10% concentration, and is economical because of less frequent disposals. Hydrophilic emulsifiers are infested by fungus and algae and the tanks must remain covered when not in active use. 30% concentration has more resistance to this infestation. Hydrophilic remover solutions are also applied by spray or as foam. The concentration of the solution tend to be much lower up to 5%. However, higher concentration may be used depending on the manufacturers recommendation. Immediately following the remover spraying, a freshwater rinse of the entire part is required to stop the act ion of any remover remaining on the surface of the part. Hydrophilic removers minimize background fluorescence on part surfaces as well as bleed out of excess penetrant from hollow parts. The concentration of the detergent solution is critical to the success of the process and must be controlled. Water loss in hydrophilic emulsifiers results in more concentrated and more active solutions, a cause for over-emulsification with resultant failure of the penetrant process. To monitor the concentration, the refractive index of the working solution is generally measured and compared against graphs generated by measuring the refractive index of solutions of known concentrations, using a Refractometer [ an optical device with a reading scale ]. A drop of the working solution is placed on the proper spot on the refractometer and a reading is made on the scale and then converted into percentage by using a table or a graph applicable for the particular remover and supplied by the emulsifier manufacturer. The graph may be generated in the workstation using prepared solutions of the standard remover from 50% of the working strength to 50 % above. A minimum of 5 solutions of different strengths should be prepared for this purpose.

Post Emulsification Penetrant process : Advantages : Disadvantages : Good on wide, shallow defects. Additional steps required in process Washes well after emulsification. for application of emulsifier. Short penetration time. Materials are more costly. Good production, especially on large parts. Not as good as water – wash on Normally not affected by acids or chromates keyways, threads and rough surfaces Suitable for smooth surface. Usually needs UV system.

Post Emulsification penetrants : Hydrophilic emulsifiers or removers are water based viscous liquids and contain blends of non ionic detergents, coupling agents, corrosion inhibitors and dyes. These emulsifiers are usually supplied as colored concentrates to be diluted in water to prepare the working solution. Hydrophilic emulsifiers function by displacing excess penetrant by detergent action. The force of the water spray or the agitating action in the dip tanks provides the necessary scrubbing action while the detergent displaces the film of penetrant from the part surface. These emulsifiers are slower acting than the oily lipophilic emulsifier; therefore, it is easier to control the cleaning process. Because it is incompatible with water, pre rinsing prior to application of remover is recommended. The purpose of this step is to drive off physically a large proportion of the surface penetrant so reducing the quantity of penetrant to be removed by the detergent solution to a very small amount. Pre rinsing also leaves an even layer of surface penetrant. Immersion technique tend to be unsuitable for this pre - rinse operation.

There are two types of emulsifiers used in the removal processes ; They are Lipophilic emulsifiers and Hydrophilic removers [ detergents ] Lipophilic emulsifiers [ Method B ] are oil based and contain blends of surfactants, esters and high boiling point hydrocarbon distillates. They are colored other than green to allow easy identification. These emulsifiers are used as supplied, and function by mechanical and chemical action. The commonest method for application of lipophilic emulsifiers is direct immersion followed by drainage. After the emulsifier has coated the surface of the part, mechanical action starts to removes some of the excess penetrant as the mixture drains from the part. During the emulsification time, the emulsifier diffuses into the remaining penetrant film on the surface of the part and render it water washable. The emulsifier is fast acting, thus making the emulsification [ or contact time ] very critical. The emulsifier continues to act as long as it is in contact with the penetrant, therefore, the rinse operation should take place quickly to avoid over - emulsification. The component may be immersed rapidly in a large volume of water to stop the action of the emulsifying agent. On removal from the water, the wash is completed by spray rinsing. Spray application followed by drainage has also been used successfully. Brush application of lipophilic emulsifiers is prohibited, because this would mechanically mix the emulsifier into the penetrant resulting in non uniform emulsification. Flowing the emulsifier on the component surface is not recommended because it may not be possible to cover the component surfaces rapidly enough to ensure uniform emulsification. It is essential that complex shaped components are rotated during the drainage stage so that the various surfaces receive similar processing. The three properties of lipophilic emulsifiers that control the washing characteristics are activity, viscosity, and water tolerance. Specific contact times for lipophilic emulsifiers should be established for each application. The contact time can vary between 60 and 180 seconds depending on the type of the emulsifying agent, the penetrant in use and the surface condition of the components. Lipophilic emulsifiers are used at temperatures between 15 and 25

0 C. Lipophylic emulsifier

are miscible with penetrants in all concentrations. However, if the concentration of penetrant contamination in the emulsifier becomes too great, the mixture will not function affectively as a remover. A specification requirement is that, lipophilic emulsifiers be capable of tolerating 20% penetrant contamination without a reduction in performance. The emulsifier is to be replaced when its cleaning action is less than that of new material. Since lipophilic emulsifiers areoil - based, they have a limited tolerance for water. When the tolerance level is reached, the emulsifier starts to thicken and will eventually form a gel as more water is added. Specification requires that lipophilic emulsifiers be formulated to function adequately with at least 5% water contamination and that lipophilic emulsifiers be replaced when the water concentration reaches the limit.

Post Emulsification Penetrant process : Advantages : Disadvantages : Good on wide, shallow defects. Additional steps required in process Washes well after emulsification. for application of emulsifier. Short penetration time. Materials are more costly. Good production, especially on large parts. Not as good as water – wash on Normally not affected by acids or chromates keyways, threads and rough surfaces Emulsification time is critical.

Post Emulsification penetrants : These penetrants are not directly water washable and the danger of over- washing the penetrant from the discontinuities is reduced. An emulsifying agent is used that makes the surface penetrant soluble in water so that the excess penetrant can be removed by water rinse and then a developer can be applied. These penetrants have better penetrating ability than water washable penetrants and can detect minute flaws. These processes are widely used with fluorescent penetrants but much less with color contrast penetrant. Emulsifiers are liquids used to render the excess penetrant water washable. The manufacturers carefully formulate the emulsifiers depending upon the penetrant to be used. Penetrants and emulsifiers are used as a system, and emulsifier from one manufacturer may not perform adequately on a different manufacturer’s penetrant.

Ultraviolet light : Ultraviolet radiation is a band of wavelengths within the electromagnetic spectrum and below visible [ white ] light. Ultraviolet wavelengths are too short to be seen by the human eye. The boundary between visible light [ white ] and UV [ black ] light is a wavelength of 400 nm [ nano meters ] or [ 4000 Angstroms ]. UV - spectrum is divided into three ranges : UV - A [ 400 - 300 nm ] - Long Wave. UV - B [ 300 - 280 nm ] - Medium Wave. UV - C [ 280 - 180 nm ] - Short Wave. Ultraviolet A is not considered harmful to the human eye. Ultraviolet B can cause sunburn as well as be harmful to the eyes. Ultraviolet C is used for germicidal purposes and is harmful to the eyes and the body. Daylight viewing is considered to be a range of 540 through 570 nm, with the average being 555 nm. The fluorescent materials used in NDT for both the magnetic and penetrant inspections are selected to provide maximum fluorescent properties with UV - A excitation in the 365 nm wavelength. In fact these materials normally fluoresce under excitation of any wavelength, from 320 - 400 nm. The fluorescence, which is reflected back is in the range of 540 - 620 nm and is visible as yellow-green to the human eye. Any excessive white light in the inspection area [ especially those in the green / yellow wavelengths ] will decrease the contrast of indication to background, for this reason, specifications limit the ambient white light level to 20 lux maximum. Black light sources are normally high pressure mercury arc bulbs and low pressure mercury vapor discharge tubes. Low pressure mercury vapor light sources have a deep purple coating on the inside surface of the bulb to filter out undesired UV wavelengths and blocks the visible light generated by the bulb. Mercury vapor arc [ HID ] lamps used for NDT have only two lines of UV - A energy in significant intensities. One of these bands is at 365 - nanometer wavelength. New micro power gas discharge technology [ MPXL ], is recognized to create a much higher contrast of fluorescent indication to background, thus improving the operator's visual acuity. MPXL spot lamps produce in excess of 90,000 micro watts / cm

2

from 12 inch distance, and allow inspection to be performed in areas where previously it would have been impossible. The light intensity of ultraviolet lamps is measured with a suitable UV light meter. Most of the UV intensity measurement meters for NDT usage are designed for use on mercury vapor light sources at 365 nanometers. Fluorescence : is the emission of electromagnetic radiation by a substance as the result of the absorption of electromagnetic or corpuscular radiation of greater energy than that of the fluorescent radiation. Fluorescence is characterized by the fact that it occurs only so long as the stimulating radiation is maintained. UV light filter : Ultraviolet lamps produce both the harmful ultraviolet radiations and visible light. The deep purple filter in the lamp allows only the wavelengths between 320 and 400 nanometers to pass through and eliminates the harmful ultraviolet

radiations. The dark color of the filter also cuts off sufficient visible [ white ] light. UV radiations below 320 nm can burn the retina of the eyes and also the skin of the exposed person. Examiners must protect themselves from the harmful UV light by ensuring that the filter used in the UV lamp is in perfect condition. Any cracked or damaged filter shall be immediately replaced or the Lamp shall be rejected. A lamp with a damaged filter must not be used. Looking directly at the lamp with a proper filter will not cause any harm but may cause temporary blurring of vision.

Standard / Test samples 1: Quench cracked aluminium block : These blocks are made of ASTM B 209, SB 211 or 2024 - T3 aluminum approximately 2" wide X 3" long X 3/8’’ thick pieces. The blocks are heated and quenched to produce an overall circular crack pattern. A groove is machined in the middle of the block to separate the cracks into two zones. Since the cracks are uncontrolled, the blocks are used for comparing the performance of penetrants only and not for absolute evaluations. At the center of the face, an area approximately 1 inch in diameter shall be marked with 950

0 F temperature indicating crayon or paint. The

marked area shall be heated with a blow torch to a temperature between 950

0 and 975

0 F. The area should be heated within 4 minutes

to produce a temperature difference within the block. The block is then immediately quenched in cold water which produces a network of relatively tight and uniform crack pattern on the face. The block shall then be dried by heating to approximately 300

0 F.

It is because of tight crack pattern, that color contrast penetrants are not normally checked with this block. These penetrants are not as sensitive as fluorescent penetrants. The cracking will be symmetrical on either side of the separation groove but the depth and the width of the cracks produced is uncontrolled and unknown. For the color contrast penetrants comparison only, the 950

0 F cracked block is heated a second time to 800

0 F and quenched

again to open the cracks wider. New and used penetrants or different brands of penetrants can be processed and then compared side by side. A penetrant in use can be checked for its crack detection ability and indicating brightness against fresh penetrant samples. One half of the block is processed with the used penetrant and the other half with the fresh penetrant. The whole plate is then processed to completion in the relevant way. The indications from each half can be tested under identical illumination on the same alloy with the same surface condition, resulting in a true comparison of relative sensitivity. The slight potential for error with this device arises with the requirement that the discontinuities in both halves be as identical as possible. If cracks detected on the used penetrant side is not as complete as the fresh penetrant side, the used penetrant is considered contaminated and should be discarded. If the brightness of the used penetrant indications appear to be below 90 % of the brightness of the fresh penetrant indications, the used penetrant is discarded. Use of penetrant outside their normal operating temperature range [ 16

0 to 52

0 C ] can be qualified using these blocks. Two separate

blocks are processed. One block is heated or cooled to the proposed examination temperature and maintained at that temperature throughout the processing cycle. The other block is processed within the normal operational temperature of the penetrant. If the indications obtained under the proposed conditions are essentially the same as obtained with the normal conditions, the proposed technique shall be considered to be qualified for use. These blocks can be reused by careful cleaning. A 30 minutes soak in isopropyl alcohol clears the induced cracks of residual penetrant. System performance check / KDS Panels : System performance checks involve processing a test specimen with known defects to determine if the process will reveal discontinuities of the size required. The purpose of the known defect Standard is not to verify the sensitivity level of the penterant, but to monitor the performance of the entire penetrant examination system. Further, The specimen must be processed following the same procedure used to process production parts. A system performance check is typically required daily, at the reactivation of a system after maintenance or repairs, or any time the system is suspected of being out of control. As with penetrant inspection, results are directly dependent on the skill of the operator, therefore each operator should process a panel. Known Discontinuity Standards [ KDS panels ] : The panels come in pairs having known discontinuities in known locations. Two KDS panels are produced simultaneously from a single sheet of stainless steel, so they are precisely matched as to plating thickness, crack size, and surface roughness. The center section is grit blasted to specification roughness. This rough section is used to compare washability of different types of penetrant.

The two outer edges have 25 mm wide strips of brittle chrome plating of 0.1 mm thickness. Five controlled induced cracks of varying dimensions are produced in pairs. The sheet is sheared into two equal sections and they are virtual twins. Both the sections are processed for side by side comparison.

Standard / Test samples 2: KDS panels permit the monitoring of in use penetrants as well as side by side comparison to new, unused penetrant. They are rugged and can withstand daily system processing. A simple 30 min soak in isopropyl alcohol clears the induced cracks of residual penetrant. Cracked nickel chromium plated panels : The nickel chromium test panels are ideal penetrant sensitivity comparators. The panels are available with crack of 10, 20, 30, 50 microns and come in matched pairs. The size refers to the depth of the nickel plating. Matched pairs enables studies of the effects of process changes as compared to a standard process or comparing used material vs fresh material. A thick brass panel measuring 1-1/2" x 4", plated with brittle nickel, on one side. The nickel plating is then plated with a very thin layer of chromium for protection. The panel is stress loaded on one side, which produces fine lateral cracks through the plating. These cracks are then used for the penetrant system performance check. Two panels containing identical or nearly identical crack patterns are used to perform the checks. The depth of the plating can be controlled, and panels are available with plating thickness of 10µm [ 0.0004" ]. Nickel chromium panels have been sectioned and micro-photographed. The photographs reveal crack width to be 1/20th of the depth. So, the width of the cracks in the 10µm plated panel would be 0.5µm [ 0.00002" ]. Under laboratory conditions, high sensitivity penetrants both water washable and non water-washable find the flaw lines in these 10µm panels. So, theoretically, the penetrant that can show cracks in a 10µm nickel chromium sensitivity panel, is expected to find a flaw measuring 0.5µm x 0.5µm x 10µm. Apart from length, another measure of concern should be flaw width, since a penetrant's ability to enter a crack depends more on a crack’s width than its length. For example, cracks in forgings are inherently tighter and more difficult to reveal than cracks of equal length in castings or extrusions. Cracks under compression, squeezed tight and having virtually no width, are often impossible to show with penetrant inspection. Limitations : A problem with test pieces is that once a penetrant process has shown all the discontinuities in a test piece no other process can appear more sensitive. Similarly if the discontinuities are difficult to show and can be found only by use of the most sensitive process all other processes appear equally ineffective. The size of the reference discontinuities itself increase with use. The size of the indications must be recorded [ photograph / transfer lacquer replica ] when the block is first used and compared with the indications obtained with subsequent uses.

Important : Several precautions are necessary when using the penetrant test panels. The panels must be thoroughly cleaned as quickly as possible after each use. Penetrant residues when left in discontinuities become very viscous and extremely difficult to remove. In cases of severe contamination by previous residues, the PSM5 and NiCr test panels can be boiled in 5 to 20% detergent solution for 30 minutes. This is followed, after cooling, by a water rinse then a rinse with acetone. In between uses they should be kept in dry acetone [ flammable ] or 1,1,1, trichloroethane. If water is allowed to contaminate either of the solvents, the discontinuities can become corroded. Solvents are best kept dry by use of molecular sieves or silica gel. Some specifications limit the number of times that a test panel can be reused.

The left picture shows crack comparisions on Ni-Cr panels and the right shows washability test for two different penetrants.

KDS Indications

Cross-section of Ni-Cr plated panel.

Evaluation of indications 1: Evaluation is the process of deciding the severity of the condition after the discontinuity indications have been interpreted. Evaluation leads to the decision as to whether the part must be rejected, to be repaired or directly accepted for use. In liquid penetrant testing, discontinuities are judged on the size of the indication and not on the actual size of the discontinuity opening visible at the surface. The indication enlarges with time and it is better to evaluate the indications at the minimum development time allowed by the procedure. Recommended development time as per ASME Sec V is minimum 7 to maximum 30 minutes. Evaluation should begin with an over all examination of the test surface to determine that the work piece has been properly processed and is in satisfactory condition for inspection. Usually, the indications are classified as linear or rounded. Linear type is an indication with a length greater than three

times the width. It will appear as a continuous, straight or jagged line. These indications represents more harmful discontinuities. Cracks, seams, deep scratch, undercut, lack of fusion at surface, rolling and forging laps, cold shut etc produce linear indications. The same flaws may show up as an intermittent line indicating that the flaw may be partially closed at the surface.

A rounded indication is defined as a circular or elliptical and with a length of three times or less than its width. Porosity or a cavity at surface produces such indications.

Intermittent line indications are caused by linear discontinuities if they have been affected by previous processing steps or in service use or they are partially subsurface.

Color contrast penetrant indications : Indications will appear in red against white background at the locations of the discontinuities. Depth or size of the internal cavity may be co-related with the brightness of color, speed of bleeding and dye spread. Usually, a crack, deep scratch or similar opening will show up as a solid line. Tight crack or a lap show as a broken line. Gross porosity will produce large rounded indications. Fine porosity is indicated by red dots. Porous material will produce randomly distributed tiny red dots. Crater crack may produce round indication because they tend to trap large amount of penetrant. Indications with a light pink color may indicate excessive cleaning. Diffused or weak indications are difficult to interpret and the part should be reprocessed. In many cases they turn out to be false indications caused by improper processing. Fluorescent indications : These indications are viewed under ultraviolet light in a darkened area. The indication glows brilliant yellow green or orange red against violet background of the developer. Porosity will show as glowing spots. Cracks will show as fluorescent lines. Where a large discontinuity has trapped a quantity of penetrant, the indication will spread wide on the surface. Experience in the use of this method allows conclusion to be drawn from the extent of this spread with respect to the relative size of the discontinuities. Poorly processed part will have broad fluorescent patches. Acceptance standard for welds : ASME Sec VIII Appendix 7 a. relevant indications : indications with major dimensions greater than 1 / 16 in. [ 1.6 mm ]. b. linear indications : an indication having a length greater than three times the width. c. rounded indications : an indication of circular or elliptical shape with the length equal to or less than three times the width. An indication in excess of the limits specified below is not acceptable : a. Relevant linear indications; b. Relevant rounded indications greater than 3 / 16 in. [ 4.8 mm ] c. Four or more relevant rounded indications in a line separated by 1 / 16 in. (1.6 mm) or less [ edge – to – edge ].

Evaluation of indications : Penetrant inspection provides indirect indication of flaws by showing zones of penetrant accumulation. Penetrant may be accumulated on the part, due to presence of flaws, part configuration or improper processing. It cannot always be determined at first glance whether an indication is real, false or non relevant. A real indication is caused by a flaw such as a crack,

porosity or surface breaking defects. Non relevant and false indications, are the indications, that may be interpreted erroneously as a flaw.

Non relevant are true indications, which occurs as a result of the component geometry or configuration such as keyways, splines, nut and bolt threads, press fit parts, riveted, assembled part or very rough surfaces such as a weld bead. Tool marks, loose scales and scratches may also produce non relevant indications.

A False indication is an accumulation of penetrant, not caused by a discontinuity in the work piece. It may appear as a result of improper handling or poor processing during the inspection process. This may appear in the form of excessive penetrant background or streaking from insufficient or improper penetrant removal. Common causes are, penetrant contaminated developer, penetrant residue on inspection table, dirt, lint, or finger prints from handling during the test process. For fluorescent penetrants, inadequate cleaning of oil and grease, can also be a source of false indications.

Verification of Flaw Indication : Swabbing the indication with a very fast drying solvent dampened, cotton tipped applicator, re-application of developer and witnessing the re-appearance of an indication can also greatly assist the interpretation process. An acceptance or rejection decision should be made before this test. The ability of an indication to re-appear is called persistence. This will alert the inspector to the fact that the discontinuity has depth or volume. Swabbing an indication is an useful technique to verify false or retest a non relevant indication. However, swabbing should not be done until the indication has been evaluated for size. If allowed by the specific procedure, indications may be evaluated by wiping the indication with a fast drying solvent-dampened swab, allowing the area to dry, and redeveloping. Redevelopment time shall be as long as the original development time, except non aqueous redevelopment time shall be 3 min minimum. If no indication reappears, the original indication is considered false. This procedure may be performed twice for any given original indication. Spraying non aqueous developer onto a cotton swab and wiping, removes the indication, cleans the surface and reapplies developer in one step. Whether or not the flaw indication re appears after the solvent wipe depends on ; 1. how volatile the solvent is, 2. how much solvent is applied, 3. which applicator is used, 4. finger pressure exerted, 5. developer used …… powder or non aqueous, 6. developing time before verification, 7. developing time after verification, 8. number of wipes.

Inspection : Development time : After application of the developer, it must be allowed to work. If the size of the discontinuity opening is small, the penetrant bleeds out slowly into the developer. Dry powder developer rely upon capillary action, which is slower. The

developer should be left on the surface for a minimum of ten minutes before inspection of indications.

A water base [ aqueous ] developer is to be dried by circulating hot air or heating in a temperature controlled oven. The minimum development time is 5 minutes, after the developer layer becomes dry.

A minimum of 5 minutes developing is also needed for non aqueous developer. This time is taken from the point when the developer layer appears to be dry.

Watching the indication form immediately after developer application and awareness of the type of defects that may be encountered beforehand is extremely helpful. The indication is not the same size as the discontinuity opening, the developer will magnify the discontinuity size to make it more visible to the eye for easier detection. Penetrants continue to bleed out into the developer and indications become larger over a period of time. Wise usage of minimum and maximum development time is therefore highly recommended. Gross and large discontinuities will begin to form indication immediately. Tight and extremely small discontinuities will take long time to form. For evaluation of indications for accept / reject condition, a development time of 7 to 30 minutes is generally used after the developer layer becomes dry. Lighting : All penetrant examinations must be performed under recommended light levels, as written in the test procedure. To see faint indications with color contrast penetrants, adequate white light is required. Inspection should be carried out in good white light of at least 500 lux at the examination surface. Inspection of fine detail may require up to 2000 lux. In natural daylight, penetrant indications appear deep red against the white developer background. Under strip light [ tube lamps ], indications appear somewhat darker than in the natural daylight. For fluorescent penetrant inspection the test surface must be examined under ultraviolet [ black light ] of peak wavelength around 365 nm and intensity of at least 1000 micro watts / cm

2 at the examination surface

with a minimum of 6 inches distance between the test surface and the light source. Inspection must be carried out in a darkened area where the ambient white light level should not exceed 20 lux. The interior of the inspection booths should be painted mat black to avoid distracting reflections. Brightness of indication depends on the intensity of incident ultraviolet light and the darkness of the surrounding. If fluorescent inspection is to be performed under normal lighting condition, then the minimum intensity should be 3000 micro watts / cm

2 at the examination

surface. The output of UV lamp falls with time, the effective output also falls as bulbs, reflectors, or filters become dirty. Once a week the optical parts of the lamp should be cleaned with a dry cloth when the lamp is cold. The UV light must be checked for presence of cracks in the filter. A lamp with cracked filter must not be used. UV light intensity is to be checked when first used each day for required intensity with a calibrated UV light meter, in the darkened room condition and after allowing a 5 minutes warm up period. Fluorescent indications appear brilliant yellow-green or orange-red depending on the type of the dye in the penetrant. The darker the area of inspection, the more brilliantly the indications will glow. Dark adaptation for fluorescent inspection : When evaluating fluorescent indications, the examiners must allow their eyes to adjust when first entering the darkened inspection area. Normally, 5 minutes are enough time for the eyes to adapt to the darkened condition. Inspection should not start until the examiner's eyes have adapted to the darkened conditions. Dark adaptation time can be shortened by wearing suitable red eye glasses in white light areas. Note : Photosensitive eyeglasses should not be used during inspection because these glasses darken in the presence of UV radiation. Examiners performing inspection under ultraviolet light should consider wearing yellowish or special ultraviolet filter goggles to reduce eyestrain.