AJAX REPRINT No. 182

An Introduction to

SALT BATHHEAT TREATING

by Q. D. Mehrkam

V. P. Research 8 Development

AJAX ELECTRIC COMPANY

Philadelphia, Pa.

Reprinted from TOOLING & PRODUCTION Magazine, June & July, 1967

F i g u r e 1

R a t e o f h e a r i n g c u r v e s h o w s t h e t i m e r e q u i r e d t o h e a t a p a r t o f g i v e n d i m e n s i o n s .

Part I

Salt bath heat t reat ing

b y Q . D . M e h r k a m

V. P. Research & DevelopmentAJAX ELECTRIC CO.

Philadelphia, Pa.

WHEN WORK is fully immersedinto a salt bath furnace, no aircan contact it. Steel scaling, oxi-dation and decarburization can beavoided. It is common to processlow, medium, and high-carbonsteels in the same medium withoutsurface carburization or decarburi-zation. This is particularly advan-tageous for a variety of batch-typeheat treating work. A thick saltfilm adhering to the surface servesas protect ion which continueswhen the work is removed andtransferred (in the case of hard-

ening) to the quench operation.

Conduction heating. When thework is immersed, heat is trans-ferred by direct contact with aheating medium of high capacity.This is 4 to 6 times faster thanpossible in radiation or convectiontype methods. For example, thetime required to heat a part ofgiven dimensions, at 1500°F,* therate of heating is approximately

*All temperatures in this discussion areFahrenheit.

Immersing metals, ceramics, or plastics in a

molten salt makes possible unique characteristics.

5 min/in of thickness or ¼ the timerequired in radiation type equip-ment, see Fig. 1.

Automatic preheat. Upon immer-sion into molten salt, a frozen saltcocoon forms around the part.Layer thickness and the time re-quired to melt it depend upon sizeand shape of the part, bath oper-ating temperature and salt meltingpoint. As a result, the work is sub-jected to an automatic preheatwhich prevents excessive heatshock and distortion damage ofcracking. Furthermore, heat istransferred to the work over itsentire surface uniformly. One sidewill not heat faster than the other.As the part gradually heats to thesalt’s melting point, the cocoonmelts leaving only the molten saltcontacting the work. It is impossi-ble for the work to be higher orlower than the bath’s temperaturecontrol.

Buoyancy. An important propertyin molten salt is molten saltweight; it may be 2 to 3 times thatof water. Thus the piece’s im-mersed weight is appreciably de-creased 25 to 35 per cent of theoriginal. This helps- hold workdistortion to absolute minimum.

Furnace development

About 30 years ago, the first im-portant salt bath furnace installa-tion was made in this country.Since then they’ve gained widerecognition and the number hasgrown-not only in number but insize and capacity. The 5000 in usetoday range in size to 43 ft L x 20ft D x 8 ft W. In use, the unit con-tains 300,000 lbs of salt. Manyoperate with connected loads over2500 km.

The furnace has a ceramic ormetal container filled with moltensalt. The work can be immersedfor either heating or cooling. The

bath’s composition consists of oneor more salts, nitrates, chlorides,carbonates, cyanides, caustics, oradditives in small amounts. Theadditives can be sulphates, fluo-rides, etc.

Heat is transferred betweenwork and molten salt by conduc-tion. The salt lends itself to eitherheating or cooling within a 300o

to 2400o range, as required. Com-patible salt mixtures are selectedwith proper operating ranges andoften are formulated to meet spe-cific temp-range requirements.

Operating principle

The electrode furnace generatesdirect heat in salt by using its re-sistance to current passage. Salts,while insulators in the solid state,are excellent high-resistance con-ductors in the molten state. Poten-tial is applied to the molten saltby use of heavy electrode bars.These are connected to the sec-ondary of special multiple voltageair cooled transformers. Electrodesare located in a recessed area ofthe bath and cause salt circulationby electromagnetic forces whenenergized. A strong magnetic fieldis created between electrodes whenlocated close together.

Using Maxwell’s Law, this fieldwill cause salt particles betweenelectrodes to move downward ina whirlpool fashion. This electro-dynamic circulation assures abso-lu te t empera ture un i f o rmi tythrough the bath and eliminatesdanger of undesirable differentials.

F i g u r e 2

A j a x T y p e H F u r n a c e h a s a m e t a l

p o t w i t h e l e c t r o d e s e n t e r i n g f r o m

t h e t o p . T h i s t y p e f u r n a c e i s p r e -

f e r r e d f o r c y a n i d e h a r d e n i n g , c a r -

bur iz ing , desca l ing , desanding , and

c lean ing .

Design features

Furnace size and shape can bealmost anything desired. One de-sign uses a metal pot for temper-atures up to 1750°. With thisarrangement, e l e c t r o d e s m u s tenter from the top and are usuallylocated along the back wall free ofthe work area, see Fig. 2. All ex-ternal walls are heavily insulatedand radiation losses from surfacesare minimized by a suitable cover.The metal pot in this type furnaceis used whenever salts containappreciable amounts of cyanides,carbonates, and caustic soda, or forlow temperature applications usingnitrates and nitrites.

A different type furnace is pre-ferred for higher temperatureapplications in the 1200o to 2400o

range. The pot is of interlockingceramic tile. Electrodes are re-placed without costly downtime.The tile is merely raised, the oldelectrodes removed, the new orrebuilt electrodes installed, and thetile replaced.

A third type furnace, radicallydifferent from those above, is pri-marily intended for cooling ratherthan heating. The isothermalquench furnace is for martemper-ing, austempering and isothermalheat treating operations (of whichthere are many hundreds). This

F i g u r e 3

C a r b u r i z i n g c u r v e s h o w s t i m e p e n e t r a t i o n n e e d e d t o g e t d e s i r e d c a r e d e p t h

a t v a r i o u s f u r n a c e t e m p e r a t u r e s .

type furnace will always have ametal port heated by either resist-ance heaters or gas fired immer-sion tubes. The external pot sur-face is frequently finned and airunder pressure i s c i r cu la tedaround the outside to extract heat.Both heating and cooling cyclesare automatically controlled tomaintain a constant and uniform±10o temperature.

For quenching, salt must beproperly agitated. This is donewith a propel ler driven pumpwhich forces the molten nitrate-nitrite salt through the quenchheader. Whenever high temper-ature chloride salts are carried by

the work into the nitrate-nitrite, aseparating chamber continuouslyclarifies the bath. Chloride saltssettle to the chamber bottom andare removed periodically. Thistype furnace normally operateswithin 400 to 750o.

By eliminating pumps, separat-ing chamber and cooling chamber,the furnace can also be used forheating applications with in therange of 400 to 1100o. It can beheated by either resistance heatersor gas-fired immersion tubes.

Applications

Liquid carburizing. This method

is used for diffusing carbon andsmall amounts of nitrogen intosteel surfaces. It will alter physi-cal properties of the surface bymaterially increasing strength andwear resistance. Liquid carburiz-ing is done in baths, operating at1450o to 1750o. All carburizingsteel grades, carbon and/or alloy,may be liquid carburized with casedepths of 0.005" to 0.250". A uni-form carburization rate is con-trolled by maintaining sodiumcyanide concentration-the sourceof carbon and nitrogen, see Fig. 3.

Salt samples are removed fromthe bath daily, and are analyzedfor cyanide content. Replenish-ment salt is then added to main-tain cyanide content at the re-quired operating level. Since cy-anide oxidizes in contact with airat these temperatures, the bathoperates with a carbonaceous scumcover which serves two purposes:1) It reduces radiation losses by50 per cent or more-thus savingpower; and, 2) it reduces cyanideoxidation.

It’s interesting to note, a car-burizing bath doesn’t fluctuatechemically-radically or rapidly.’It normally consists of a carbonate,chloride and cyanide mixture. Withcyanide oxidation, carbonate isformed and this requires periodicdilution by adding fresh salt. Pro-prietary salt replenishments areavailable in varying concentra-tions of the above chemicals, tomaintain proper chemical balance.Generally, it is only necessary tomaintain the sodium cyanide oper-ating level.

For example, the carbonate con-centration in a deep case carburiz-ing bath should not exceed 30 to35 per cent with a sodium cyanidecontent of 8 to 12 per cent. Other-wise, carburizing potential or thebath’s ability to do it is adverselyaffected. A higher cyanide con-tent will also account for greaterbreakdown to carbonate. If drag-out losses on the work surface arelow, it may be necessary to bail aportion of the bath each day tomaintain proper chemical balanceby adding fresh salt. As we pointout above, chemical change is notrapid and considerable variationcan exist without affecting carbonpotential.

ure of its effect on the steel sur-face being treated. A more con-clusive examination is made byprocessing steel test coupons, thenexamining the surface hardnessusing micro-hardness or superfi-cial hardness testers or microscop-ically examining a cross section.Besides oxidants, the chloride saltscan be contaminated with othermaterials such as contaminatedwork and fixtures in the bath.This is particularly true if thesurfaces haven’t been properlycleaned. Naturally, any contam-ination carried into the furnacewill cause costly maintenance forremoval.

To maintain acceptable bathneutrality, removing dragout losson the work surface and fresh saltreplenishments are usually suffi-cient. However, where contamina-tion is greater or dragout minimal,a rectification practice may beused. Rectifier selections, eithersolid or gaseous, are available andeach method is selected to meetindividual requirements.

Methyl chloride gas rectificationis used in chloride type salts oper-ating at 1400° to 2000°. Its prin-cipal action is to chemically con-vert oxides to the original chlorideand remove oxygen from the bathas carbon monoxide and carbondioxide. Since by-products aregaseous, there is no residual con-tamination and the bath can berestored to its original composi-tion. However, metallic or non-metallic contaminations are not re-moved by this practice. It may benecessary to resort to use of solidtype rectifiers for more completecontamination removal.

Commonly used solid-type rec-t i f iers are proprietary. Use ofcyanide as the organic Dicyanamidessentially neutralizes bath oxidesby creating a temporary reducingaction. But this is soon dissipated,and additional rectifier must beadded periodically. The main dis-advantage here is that cyanideeventually decomposes into a car-bonate, which is considered con-tamination. As pointed out above,with methyl chloride rectification,the Dicyanamid doesn’t removesolid contamination and if allowedto accumulate beyond a safe limit,other means must be employed.

Rectifiers incorporating siliconcarbide, metallic silicon and silicaare commonly used effectively andwill carry contamination to thefurnace bottom as sludge. This canbe removed by desludging tools.While this rectification method iseffective, it’s a disagreeable task.In a large operation, it could in-volve considerable time for themaintenance crew.

However, a “Chill Block Meth-od” is available, where a heavysteel section is quickly lowered tothe furnace bottom, allowed toremain a few seconds, then imme-diately removed. Sludge attachesto the cocoon forming on the blocksurface and upon removal, thesludge is quickly picked up andremoved. After cleaning, the blockis returned as many times neces-sary to remove sludge completely.This method is particularly effec-tive in furnaces 15 to 20 ft deepand equally successful for thoseonly 2 ft deep but 10 to 20 ft long.

High speed tool steel hardening

This requires the most exactingcontrol of all heat-treat applica-tions. A significant majority ofquality tool manufacturers use saltbath furnace treatment. Any typehigh speed steel can be hardenedwith equal ease and safety merelyby a change in furnace tempera-ture setting. No special provisionis required to adjust bath chemis-try for the hardening of high-chrome high-carbon, moly or tung-sten type steels. By providing closecontrol instruments using eithertime proportioning or stepless con-trol, temperatures can be con-trolled within ±1o.

One customer uses the furnaceoperating at 2200° for treating aSendzimer Roll, see Fig. 4. Thefurnace handles work pieces 12”dia x 108” long. Rolls to 3200 lbseach can be accommodated. Usingthe furnace for preheat, high-heat,quench and draw, the installationenabled him to control distortion,decarburization, maintain uniformhigh-hardness and meet all rigidQC specifications. By quenchingdirectly from the austenitizingtemperature into a bath 1000°,then transferring to a secondquench about 600o, the double salt

bath quench achieved a consist-ently higher uniform hardness lev-el than obtainable by air cooling.

Surface scaling is eliminatedsince there is no contact with airwhile cooling through the oxidiz-ing and scaling temperature range.Since a better surface is obtained,less grinding stock is removedafter hardening - thus reducingfinishing cost.

An all-chloride hardening sys-tem is preferred for high-speedsteel hardening installations. Thepreheating furnace (normally1450° to 1550°F) can be operatedwith either sodium and potassiumchlorides or barium, sodium andpotassium chlorides. The high-temperature furnace at 2000 to2350° usually operates with 100per cent barium chloride. Thepre-heating salt carry-over intothe bath presents no problem be-cause lower melting salts will vol-atilize at higher operating tem-perature. When transferring fromhigh temperature furnace to saltquench, barium chloride is carriedinto the bath at about 1000 to1100o. Since the quench mixtureusually consists of barium, calci-um, sodium and potassium chlo-rides, excess barium settles andmust be periodical ly removed.Metallurgical requirements areeasily met by quenching into thissalt since cooling rate exceeds thatpossible by air.

On steel sections over 5 or 6”,it is desirable to incorporate a 600o

quench using an agitated nitrate-nitrite salt. This further insurescooling of the work from 1000o to600o so that all sections, whetherlight or heavy, will be brought toa lower temperature from whichthe austenitic material can besafely air cooled to room temper-ature. In this case a more uniformsteel transformation from surfaceto center is obtained, with mini-mum thermal and transformation-al stresses. Also, surface oxidationis avoided, and higher hardness isusually obtained. n

Steels carburize at faster rateswith higher temperatures - andthis is another economy factor.With increased temperature-car-bon diffusion rate also increases.Obviously, greater production canbe maintained from a given car-burizing furnace size by raisingoperating temperature; for exam-ple: a case depth 0.070” at 1700o

requires 8 hrs. At 1800°, the samecase depth can be obtained in lessthan 4 hrs. Thus, with an increaseof 100o, capacity has doubled.Higher production rates are possi-ble, obviously; and it is necessaryto design the furnace with ade-quate power to accommodate therequired lbs/hr.

Carburizing, cyaniding and nitrid-ing operations are applied to avariety of work involving use ofeither sodium or potassium cyan-ides as the active agent. For ex-ample, one of our furnaces car-burizes outboard motor crankshafts. A 0.045” case depth is ob-tained at 1700o in 4½ hrs. Thework is then conveyed through thefurnace with a push-pull mecha-nism. Upon reaching the end, it isremoved and allowed to air coolto a subcritical temperature. Thework is then reheated in a neutralbath at 1500o. There is sufficientcyanide salt carry-over into theneutral chloride reheating bath toprotect the surface. The work isthen martempered in a nitrate-nitrite salt at 400° in an agitatedisothermal quench operating withapproximately 1 per cent wateraddition for increased quenchingseverity. The work is then allowedto air cool to room temperatureand then is drawn at about 300o.

After grinding, surface hard-nesses are held within 0.005” run-out. Cyaniding is similarly carriedout in a salt bath furnace providedwith a steel port and over-the-topelectrodes. Operating 15 to 30 percent with cyanide content, bothcarbon and nitrogen are added tothe steel surface and case depth isusually limited to about 0.010”.Generally, operating within the

1400o to 1600° range, the salt is amixture of cyanide, chlorides, andcarbonates. Cyanide hardening isusually applied to obtain a super-ficially high-quality, wear-resist-ant surface for the more light-dutyapplications.

Salt bath nitriding is carried outwithin the 950o to 1050o rangeusing a steel pot container for cy-anide, chloride, carbonate andheated by over-the-top electrodes.Thirty to 60 minute immersion inthe liquid nitriding bath is pre-ferred for some high-speed steelcutting tool applications w h e r emaximum wear resistance is re-quired. Case depth is usual lymeasured in 0.0001”-0.0005” terms.

Nitriding bath modificationshave been made to obtain specialeffects; for example: by bubblingammonia through the bath, ahigher nitrogen potential is ob-tained. This improves case depthand nitrogen content in the steel.Adding sulphur compounds, a sul-furizing treatment avoids seizingand galling. Still another processtakes advantage of bubbling air inthe bath to purposely oxidize thecyanide to a higher cyanate level.This changes the case characteris-

tic so that it is file-hard but flexi-ble. It also permits, in some cases,severe part bending before casefracture.

Neutral hardening

Hardening ferrous alloys in chlo-ride salts without harmful surfaceattack (scaling, pitting, etc.), iscalled “neutral-hardening.” Car-bon, alloy, stainless, hot work andhigh-speed steels are commonlyneutral hardened in furnaces oper-ating over a 1400o to 2350o range.Since the salts provide the imme-diate environment surrounding thework, attention must be given tothe bath’s chemical condition tomaintain neutrality.

As the chloride salt is operatingcontinuously with air contact, it issubject to contamination and ac-counts for increase in salt alka-linity. This is measured either interms of pH or by wet chemicalanalysis in terms of per cent sodi-um oxide or barium oxide. It isgenerally accepted when chloridesalt increases in alkalinity, it in-creasingly tends toward decarbu-rization. However, alkalinity orpH of the bath is not a direct meas-

Figure 4A Sendzimer Rol l being removed

from a salt bath furnace used for

high speed tool hardening.

PART II

Salt bath heat treating

This final part of Mr. Mehrkam’s discussion will coverAnnealing, Brazing, Aluminizing, isothermal Quenching,Martemper and Ausforming, and Cleaning.

by Q. D. MehrkamV. P. Research & Development

AJAX ELECTRIC CO.Philadelphia, Pa.

SALT BATH FURNACES are usedin a wide variety of annealingand solution heat treating appli-cations. We have an installationwhere low carbon steel wire (C-1008), in 36” dia x 30” coils, isannealed in a nitrate type salt at1000o, see Fig. 1. It is done in asteel pot with over-the-top elec-trodes. The furnace is 42” L x48” W with a 48” salt depth. Theconnected load is 160 kw. which iscapacity rated at 1000 lbs/hr. AS

the wire surface enters the salt, ithas an organic lubricant whichoxidizes and burns away. The re-sulting surface exhibits a bluish-black oxide readily removed bymild pickling.

Steels of 0.35 carbon or less aregiven a “process anneal” in a car-bonate-chloride or tenary salt con-sisting of barium, sodium and po-tassium chlorides. This treatmentis applied as a relief to recoveroriginal ductibi l i ty as wire isdrawn to smaller diameter. Coilsare either air cooled or quenched

directly from the bath. The sur-face is scale-free and exhibits onlya slight oxidation responding tomild pickling to prepare for draw-ing lubricants. The process is doneat 1250 to 1300o and cycle time iswithin 20 to 30 min for 200 to300-lb coils.

Salt baths have also been suc-cessfully used to anneal stainlessand nickel-chrome alloy products.Both 300 and 400 series stainlessfittings have been annealed within1550 to 2100o. Total cycle timefalls within the 5 to 30 min range.Work is water-quenched directlyafter removal from the furnace.The surface is slightly oxidized,but it is quickly restored by ashort acid dip.

Maintaining bath chemical bal-ance presents no problem. Whilealkaline earth and other metallicoxides may build up, they are notharmful to low carbon stainlesssteels. However, the baths can berectified in the same manner pre-viously discussed.

When using methyl chloride orcyanide rectification methods, suf-ficient time should elapse after-ward to prevent possible carbonpick-up.

Copper and copper base alloysare successfully annealed in car-bonate - ch l o r ide sa l t s ( 1100 -1500o). Copper-chrome alloy rods,12 ft long, can be suspended ver-

tically for solution annealing inchloride salt. Parts are immersedat 1825o for 30 min, and rods aret h e n q u e n c h e d i n w a t e r . A streated, the surface has only aslight oxidation and is completelyscale-free.

Nitrate salts provide an excel-lent heating medium for solutionheat treatment of aluminum al-loys. High-heat reflectivity prop-erties are no problem since saltheating is by conductivity as saltscontact the surface. Salt heatingis several times faster than airfurnaces. After a few minutes im-mersion time, the work is waterquenched leaving the surface me-tallically clean, while nitrate saltoxidation, by air, causes increasealkalinity in the bath and maycause slight etching. In most ap-plications, this condition is avoidedby replenishing dragout losseswith fresh salt. Otherwise, ‘thesalt can be rectified by adding so-dium or potassium dichromate.

BrazingSalt bath brazing is a limited butattractive method for a number ofjobs. All types of brazing materi-als can be used: silver solder,aluminum, brass and copper. Bothferrous and non-ferrous base ma-terials can be joined or combina-tions thereof which influence joint

Figure 1

L o w c a r b o n s t e e l w i r e i s a n n e a l e d

in th is s tee l pot furnace wi th over -

t h e - t o p e l e c t r o d e s . T h e f u r n a c e

u s e s n i t r a t e s a l t a t 1 0 0 0 F .

F i g u r e 2

T y p i c a l p a r t s t h a t a r e a l u m i n i z e d

for bet ter corros ion res is tance.

design and brazing alloy location,the same for salt bath brazing andother methods. Here brazing alloycan be applied in the form of wire,shim, washers, sheets, etc. And,in joint areas which do not permitpreplacement of preformed mate-rial, paste form brazing alloy canbe applied.

Salts are generally a chloridemixture. but also can be modified

by adding reactive ingredients.While salt immersion excludes aircontact and oxidation, it is neces-sary to provide a fluxing environ-ment to obtain good f low andpenetration of the brazing alloythroughout the joint area.

For silver solder, brass, and cop-per brazing, it is not uncommon toapply a fluoride-borax type fluxto the joint area before immersion.Since these fluxes are usuallywater mixtures, it is essential that

then it is selectively immersed ina chloride salt at 1600o so that thecarbide is exposed above the bath.

The al loy begins melt ing inabout 6 to 8 minutes, and the bit’sshank is thoroughly austenitizedwhile the carbide slots are ade-quately flooded with molten silversolder. The bit is then lifted ver-tically and transferred to a selec-tive isothermal quench where theshank rapidly cools with the car-bide exposed. The alloy is added

F i g u r e 3

P h o t o m i c r o g r a p h s h o w s c r o s s s e c t i o n o f a 5 / 8 ” d i a m e t e r

b o l t . N o t e t h e u n i f o r m i t y o f t h e a l u m i n u m c o a t i n g .

the flux assembly be thoroughlydry before immersion to preventsplattering. Applying flux to thejoint locally creates a temporaryfluxing atmosphere, necessary forsteel surface etching and oxidecontaminant removal, just prior towetting and flow of the brazingalloy. The flux is normally dis-placed by the alloy and dissipatedinto the molten chloride salt. Sincethese fluxes are flushed away fromthe part surface, there is no clean-ing problem.

Carbides are successfully silver-solder brazed to 9” dia mediumalloy steel drill bits. The bit isheld vertical with the carbideend up. And the carbide is gen-erously coated with a brazing flux.The silver solder alloy and carbideis then inserted. The bit is pre-heated to approximately 1000o todrive off moisture from the flux;

continuously to the carbide areauntil the silver solder is complete-ly solidified. This replaces shrink-age cavities that tend to develop.

For brazing ferrous assemblies,the trend has been toward the useof brass alloys, such as 60-copper40-zinc. Compared to silver sol-der, brass is only a fraction of thecost. Also, brass brazing is donein the 1675 to 1750o range. Whenreplacing copper, it eliminatesgrain coarseness and requires onlyhalf the power. Brass alloys ac-commodate a greater variation injoint tolerance without strengthloss; clearances to 0.010” havebeen brass brazed successfully, al-though this is not recommended.

Brass brazing with a 60-40 typewill yield average shear stress val-ues within 42,000 to 46,000 psi.This compares favorably with re-sults usually obtained by copper

brazing. The reason brass brazingis not more widely used with otherheating methods is because ofmelting instability. Zinc is read-ily volatilized, when melted, whenin contact with the atmosphere.When lost from the base alloy, thephysical properties of the remain-ing alloy are completely altered.Dezinctification is avoided andbrass alloy fluidity is easily main-tained in a neutral chloride or car-burizing type salt used to imparta slight carburizing case to thepart’s surface.

In an application using SAE-1010 for bicycle forks, for instance,f lux i s app l i ed by d ipp ing o rspray ing . Alloy rings of 60-40brass are located in contact withthe joints for brazing. This in-stallation indicates what can bedone in a fully mechanized opera-tion using a revolving unit withsix stations through which partspass. They are: 1) Load and un-load, 2) dry, 3) furnace braze, 4)air cool, 5) hot water wash and6) hot water rinse.

The mechanism is operated bya hydraulic cylinder which raisesand lowers the central shaft androtates one position every 3 min.The operator loads 14 assembledforks on each fixture at the firststation, with a 30 second intervalbetween part transfer. In this, thefurnace is maintained at 1700o

temperature.The forks can also be copper

brazed. Here the assembly isfluxed, dried and partially im-mersed in a neutral chloride saltat 2050° for 2 min. Assembliesare then air cooled to about 1500°for 15 to 20 seconds, then waterquenched followed by a hot waterwash. The operation is semi-auto-matic, by a push-button causingthe fixture to be raised and low-ered for a specified time cycle.

An interest ing modif icationcombines carburizing and brassbrazing in one step. A carburizingbath provides an excellent self-fluxing heating medium, simulta-neously giving a superficial carbu-rized surface for wear resistance.A good example is a pair of vise-grip pliers, where all componentparts are SAE-1010. The smalljaw insert is carburized at 1675o,for 30 minutes, in a water soluble

carburizing salt before assembly.After an air-cool to room tem-

perature and a water wash, thejaw and a hairpin clip are pressedinto position. The round bushingand brass alloy ring are insertedat the other end, and the parts arerack loaded. They are then im-mersed in the bath for 20 minutesat 1675o, then quenched in oil,washed and drawn at 500o. Theresulting hardness is Rc 48-51.The brazed joints pass an accept-ance test of 40,000 psi-minimum.The handle portion of the assem-bly has a slight case approximate-ly 0.004”-0.005” which is abrasionresistant and serves well as a finalfinish.

When brazing aluminum, it isnecessary to penetrate the thinoxide coating to obtain a soundbond. This requires an active fluxduring the entire brazing cycle toinsure good flow and alloy pene-tration. The aluminum brazingalloy is particularly vulnerable tooxides or oxygen in the bath; andit is, therefore, essential to pre-clean the assembly thoroughlywith various alkaline and acid be-fore brazing. The function of thebath is to melt the alloy in an en-vironment completely oxidationfree and provide fluxing for sur-face coat removal of the parentmetal.The aluminum dip brazing saltis essentially a chloride mixturewith small percentages of eitheraluminum or sodium fluoride. Thefluoride acts as reducing agent andreacts with any oxides present.T h e b a t h o p e r a t i n g a t 1 0 8 0to 1120o from which the volatileand corrosive fumes are removedby an exhaust system. Two basicprocedures maintain a properbath: since it operates relativelyclose to freezing, it is necessary tomake periodic additions of lithiumchloride for fluidity; and the self-fluxing characteristic is continu-ously depleted so it is necessary tomake periodic fluoride additions.

In a bath with sufficient drag-out, adding fresh salt is usuallysuff ic ient to maintain properchemical balance; but, due to thehydroscopic nature of the alumi-num brazing salt, moisture entersthe bath and becomes a contamina-tion which must be removed. Al-

loy flow and penetration failuresare often indications of water con-tent. The bath is easily rectifiedby immersing an aluminum sheet,where oxygen is picked-up on thesurface and the hydrogen is drivenfrom the bath surface burning oncontact with air. The sheets arere immersed , after cleaning, asneeded for bath dehydration.

Aluminum dip brazing furnacescan be built to meet most dimen-sional requirements. Furnaces 20ft deep and 15 ft long have beenbuilt successfully. They are al-most always heated by immersedelectrodes and have ceramic tilepots.

Aluminizing

Coating the surface of ferrous ma-terial with aluminum gives bettercorrosion resistance. This finishresists many types of atmospheresfrom room temperature to 1500°,see Fig. 2.

To prepare for the process, parts(such as pole line hardware andindustrial fasteners, see photomi-crograph Fig. 3) are descaled in adescaling furnace salt bath, pick-led, preheated in a f lux-typechloride-fluoride salt at 1300° andremain there about 10 to 15 min-utes. During this time the basketscontaining parts are progressivelymoved from right to left, while thebath reacts with the oxides on thepart surfaces so that aluminumwetting-out is precluded. The bas-kets are then quickly transferredto. a pot of molten aluminum at1290° and held there for 1 to 3minutes. During this time partsare agitated so all surfaces arecovered completely with a coatingof from 0.002 to 0.025” thicknessdepending upon dipping time andtemperature.

The bath performs two func-tions: it preheats material to prop-er temperature and chemically re-acts to residual oxides that wouldinterfere with the aluminum wet-ting the surface and diffusing intothe base material. Bath mainte-nance is similar to that for alumi-num brazing and dragout loss re-plenishment is sufficient for chem-ical balance.

Isothermal quenching

In steel hardening, distortion and

F i g u r e 4

M a r t e m p e r i n g t h e 9 - f t . w i n g f l a p

t r a c k i n t h e B o e i n g 7 2 2 j e t l i n e r .

cracking are often associated withwater and oil quenching. How-ever, distortion is considerably re-duced and cracking avoided byquenching in a molten nitrate-ni-trite salt within the 400 to 750o

range. Martempering and austem-pering processes are also successfulfor parts ranging in size fromtypewriter bars to automotivebumpers.

Austempering is usually appliedto carbon steels 3/8“ or less, alloysto 2”, where hardness falls be-tween Rc 35 and 54, and wheretoughness or bendability withoutbreaking is required.

In one installation for austem-pering rotary mower blades ofSAE-1070, they are rack-loaded,austenized in a neutral chloridetype salt at 1550o, and transferredt o t h e b a t h w h e r e t h e y a r equenched at 640o for 27 minutes.The final structure is bainitic andprovides a toughness superior too i l quench and t emper . Sa l tquenching provides a means bywhich heat can be extracted quick-ly to prevent the steel from trans-forming into undesirable high-temperature material. Austenitecan transfer to the desirable bain-itic product by the isothermalquench temperature.

Quenching severity is enhancedby adding water-particularly suc-cessful for borderline harden-abi l i ty applications. Water isadded to the nitrate-nitrite byspraying it on the surface of thesalt then mixing mechanically in-to the bath-water being abouthalf the weight of salt explainingwhy the two must be mechanicallymixed.

Agitation is essential for ex-tracting heat, because it is neces-sary to move the quenching medi-um past the surface of the workbeing quenched. Salt, completelydehydrated, does not have a vaporphase within the normal operat-ing temperature range; howeverby adding water, moisture is driv-en off-which provides faster cool-ing. The addition of water alsoimproves quenching severity sothat heavier steel sections can bequenched satisfactorily. Becauset h e r m a l a n d t r a n s f o r m a t i o nstresses are held to a minimum,

distortion is minimized and crack-ing eliminated.

Martempering is generally forany steel that responds to oi lquench hardening. By martem-pering within the 400 to 750o

range, lower thermal and trans-formation stresses are involved soless distortion is encountered. Asin austempering, water additionresults in improved hardness.

A 9-ft-long wing flap track forthe Boeing 722 jet is a god ex-ample of martempering, Fig. 4.The parts are a modified SAE-4330and 4340 steel held to ±0.0l0” ma-chining tolerances. Both austen-itizing and isothermal quenchingwill handle pieces up to 2000 lbs.The austenitizing furnace operatesat 1600o with a connected load of140 kw, and measures 5 ft dia x16 ft deep. The quench furnaceis rated also at 140 kw, havingworking dimensions of 5 x 7 x 15ft deep.

When parts are removed from anitrate-nitrite salt, a thin salt filmadheres to the surface. The mix-ture is one of the most solubleused in heat treating and will re-spond to a hot water wash andrinse, leaving a bluish-black oxidefinish. Contrasting with oil re-moval by alkaline cleaners, waterwashing for salt removal permitssavings in equipment and mainte-nance. This is in addition to themetallurgical advantages.

Martemper and ausforming

Meeting space-age demands hasrequired new design and fabricat-ing methods, even though manysteels have common fabricatingproblems. Many materials are dif-ficult to form to the desired shapewithout excessive part distortionduring heat treatment. Instead offorming structural members, thenheat treating them, it is possibleby using isothermal quenchingequipment to form and hardensimultaneously. The steel is aus-tenitized at normal temperature,then isothermically quenched in anitrate-nitr ite salt within thesteel’s bainitic range.

While the steel is still austenitic,forming machinery (part of theisothermal equipment) forms thepart. After this operation, partsare removed and air cooled to

room temperature. Dimensions areheld accurately, and the part isfully heat treated ready to drawwithout distortion. This heat-treatand forming method has been suc-cessful for alloy and stainless steelsheet, plate and rod. The processalso improves physical propertiesover conventional treatment.

Another contributing advantageof hot forming is the salt’s lubri-cating effect. Whether the form-ing machinery operates beneaththe quench bath surface or partsare removed from the bath andformed, the salt film adhering tothe surface provides lubrication.In drawing cups from discs, fairlyhigh reductions have been madepossible due to the lubrication ef-fect of salt at 600o.

Cleaning

Maintaining and operating extrud-ing and forming equipment is aproblem that has plagued the plas-tics industry. Dies, plates, screens,etc. become contaminated withplastic materials that resist re-moval by alkali and acid solvents.Hand cleaning was necessary andcostly. Using salt bath cleaningfurnaces in the industry, they nowfind cleaning time reduced fromhours to minutes.

Polymers have been successful-ly removed from steel mold sur-faces by immersing them in anoxidizing type salt. All polymermaterials under go decompositionb y c o m p l e t e r e m o v a l u n l e s sbrought into contact with moltensalt.

Using a caustic-nitrate type saltat 650 to 950°, polymers will de-compose and burn-off as combus-tion products in minutes. Totaltime cycle is usually less than 30minutes for complete organic con-tamination removal, leaving thepart with a bluish-black oxidecoat.

In some cases, a residue resultsfrom material breakdown. Inertsubstances not reacting with thebath will fall to the bottom assludge which must be removedperiodically. There is little chem-ical deterioration in the bath; soadding salt from time to time, toreplace that lost in dragout, isadequate to maintain cleaningefficiency. n

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