Weldability of Steels

Steels

Weldability of materials

In arc welding, as the weld metal needs mechanical properties to match the parent metal, the welder must avoid forming defects in the weld. Imperfections are principally caused by:

poor welder technique; insufficient measures to accommodate the material or welding process; high stress in the component.Techniques to avoid imperfections such as lack of fusion and slag inclusions, which result from poor welder techniques, are relatively well known. However, the welder should be aware that the material itself may be susceptible to formation of imperfections caused by the welding process. In the materials section of the Job Knowledge for Welders, guidelines are given on material weldability and precautions to be taken to avoid defects.

Material types

In terms of weldability, commonly used materials can be divided into the following types:

Steels Stainless steels Aluminium and its alloys Nickel and its alloys Copper and its alloys Titanium and its alloys Cast ironFusion welding processes can be used to weld most alloys of these materials, in a wide range of thickness. When imperfections are formed, they will be located in either the weld metal or the parent material immediately adjacent to the weld, called the heat affected zone (HAZ). As chemical composition of the weld metal determines the risk of imperfections, the choice of filler metal may be crucial not only in achieving adequate mechanical properties and corrosion resistance but also in producing a sound weld. However, HAZ imperfections are caused by the adverse effect of the heat generated during welding and can only be avoided by strict adherence to the welding procedure.

This part of the materials section of Job Knowledge for Welders considers the weldability of carbon-manganese (C-Mn) steels and low alloy steels.

Imperfections in welds

Commonly used steels are considered to be readily welded. However, these materials can be at risk from the following types of imperfection:

porosity; solidification cracking; hydrogen cracking; reheat cracking.Other fabrication imperfections are lamellar tearing and liquation cracking but using modern steels and consumables, these types of defects are less likely to arise.

In discussing the main causes of imperfections, guidance is given on procedure and welder techniques for reducing the risk in arc welding.

Porosity

Porosity is formed by entrapment of discrete pockets of gas in the solidifying weld pool. The gas may originate from poor gas shielding, surface contaminants such as rust or grease, or insufficient deoxidants in the parent metal (autogenous weld), electrode or filler wire. A particularly severe form of porosity is 'wormholes', caused by gross surface contamination or welding with damp electrodes.

The presence of manganese and silicon in the parent metal, electrode and filler wire is beneficial as they act as deoxidants combining with entrapped air in the weld pool to form slag. Rimming steels with a high oxygen content, can only be welded satisfactorily with a consumable which adds aluminium to the weld pool.

To obtain sound porosity-free welds, the joint area should be cleaned and degreased before welding. Primer coatings should be removed unless considered suitable for welding by that particular process and procedure. When using gas shielded processes, the material surface demands more rigorous cleaning, such as by degreasing, grinding or machining, followed by final degreasing, and the arc must be protected from draughts.

Solidification cracking

Solidification cracks occur longitudinally as a result of the weld bead having insufficient strength to withstand the contraction stresses within the weld metal. Sulphur, phosphorus, and carbon pick up from the parent metal at high dilution increase the risk of weld metal (solidification) cracking especially in thick section and highly restrained joints. When welding high carbon and sulphur content steels, thin weld beads will be more susceptible to solidification cracking. However, a weld with a large depth to width ratio can also be susceptible. In this case, the centre of the weld, the last part to solidify, will have a high concentration of impurities increasing the risk of cracking.

Solidification cracking is best avoided by careful attention to the choice of consumable, welding parameters and welder technique. To minimise the risk, consumables with low carbon and impurity levels and relatively high manganese and silicon contents are preferred. High current density processes such as submerged-arc and CO 2 , are more likely to induce cracking. The welding parameters must produce an adequate depth to width ratio in butt welds, or throat thickness in fillet welds. High welding speeds also increase the risk as the amount of segregation and weld stresses will increase. The welder should ensure that there is a good joint fit-up so as to avoid bridging wide gaps. Surface contaminants, such as cutting oils, should be removed before welding.

Hydrogen cracking

A characteristic feature of high carbon and low alloy steels is that the HAZ immediately adjacent to the weld hardens on welding with an attendant risk of cold (hydrogen) cracking. Although the risk of cracking is determined by the level of hydrogen produced by the welding process, susceptibility will also depend upon several contributory factors:

material composition (carbon equivalent); section thickness; arc energy (heat) input; degree of restraint.The amount of hydrogen generated is determined by the electrode type and the process. Basic electrodes generate less hydrogen than rutile electrodes (MMA) and the gas shielded processes (MIG and TIG) produce only a small amount of hydrogen in the weld pool. Steel composition and cooling rate determines the HAZ hardness. Chemical composition determines material hardenability, and the higher the carbon and alloy content of the material, the greater the HAZ hardness. Section thickness and arc energy influences the cooling rate and hence, the hardness of the HAZ.

For a given situation therefore, material composition, thickness, joint type, electrode composition and arc energy input, HAZ cracking is prevented by heating the material. Using preheat which reduces the cooling rate, promotes escape of hydrogen and reduces HAZ hardness so preventing a crack-sensitive structure being formed; the recommended levels of preheat for various practical situations are detailed in the appropriate standards e.g. BS EN1011-2:2001. As cracking only occurs at temperatures slightly above ambient, maintaining the temperature of the weld area above the recommended level during fabrication is especially important. If the material is allowed to cool too quickly, cracking can occur up to several hours after welding, often termed 'delayed hydrogen cracking'. After welding, therefore, it is beneficial to maintain the heating for a given period (hold time), depending on the steel thickness, to enable the hydrogen to diffuse from the weld area.

When welding C-Mn structural and pressure vessel steels, the measures which are taken to prevent HAZ cracking will also be adequate to avoid hydrogen cracking in the weld metal. However, with increasing alloying of the weld metal e.g. when welding alloyed or quenched and tempered steels, more stringent precautions may be necessary.

The risk of HAZ cracking is reduced by using a low hydrogen process, low hydrogen electrodes and high arc energy, and by reducing the level of restraint. Practical precautions to avoid hydrogen cracking include drying the electrodes and cleaning the joint faces. When using a gas shielded process, a significant amount of hydrogen can be generated from contaminants on the surface of the components and filler wire so preheat and arc energy requirements should be maintained even for tack welds.

Reheat cracking

Reheat or stress relaxation cracking may occur in the HAZ of thick section components, usually of greater than 50mm thickness. The more likely cause of cracking is embrittlement of the HAZ during high temperature service or stress relief heat treatment.

As a coarse grained HAZ is more susceptible to cracking, low arc energy input welding procedures reduce the risk. Although reheat cracking occurs in sensitive materials, avoidance of high stresses during welding and elimination of local points of stress concentration, e.g. by dressing the weld toes, can reduce the risk.

Weldability of steel groups

PD CEN ISO/TR 15608:2005 identifies a number of steels groups which have similar metallurgical and welding characteristics. The main risks in welding these groups are:

Group 1. Low carbon unalloyed steels, no specific processing requirements, specified minimum yield strength R eH ≤ 460N/mm 2 .

For thin section, unalloyed materials, these are normally readily weldable. However, when welding thicker sections with a flux process, there is a risk of HAZ hydrogen cracking, which will need increased hydrogen control of the consumables or the use of preheat.

Group 2. Thermomechanically treated fine grain steels and cast steels with a specified miniumum yield strength R eH > 360N/mm 2 .

For a given strength level, a thermomechanically processed ( TMCP) steel will have a lower alloy content than a normalised steel, and thus will be more readily weldable with regard to avoidance of HAZ hydrogen cracking and the achievement of maximum hardness limits. However, there is always some degree of softening in the HAZ after welding TMCP steels, and a restriction on the heat input used, so as not to degrade the properties of the joint zone (e.g. ≤2.5kJ/mm limits for 15mm plate).

Group 3. Quenched and tempered steels and precipitation hardened steels (except stainless steels), ReH>360N/mm2

These are weldable, but care must be taken to adhere to established procedures, as these often have high carbon contents, and thus high hardenability, leading to a hard HAZ susceptibility to cracking. As with TMCP steels, there maybe a restriction on heat input or preheat to avoid degradation of the steel properties.

Groups 4, 5 and 6. Chromium-molybdenum and chromium-molybdenum-vanadium creep resisting steels.

These are susceptible to hydrogen cracking, but with appropriate preheat and low hydrogen consumables, with temper bead techniques to minimise cracking, the steels are fairly weldable. Postweld heat treatment is used to improve HAZ toughness in these steels.

Group 7. Ferritic, martensitic or precipitation hardened stainless steels.

When using a filler to produce matching weld metal strength, preheat is needed to avoid HAZ cracking. Postweld heat treatment is essential to restore HAZ toughness.

Group 8. Austenitic stainless steels.

These steels do not generally need preheat, but in order to avoid problems with solidification or liquation cracking upon welding, the consumables should be selected to give weld metal with a low impurity content, or if appropriate, residual ferrite in the weld metal.

Group 9. Nickel alloy steels, Ni≤10%.

These have a similar weldability to Groups 4, 5 & 6.

Group 10. Austenitic ferritic stainless steels (duplex).

In welding these steels, maintaining phase balance in the weld metal and in the HAZ requires careful selection of consumables, the absence of preheat and control of maximum interpass temperature, along with minimum heat input levels, as slow cooling encourages austenite formation in the HAZ.

Group 11. High carbon steels.

These steels will be less weldable owing to their increased carbon content with respect to Group 1. It is likely that care over the choice of consumables and the use of high preheat levels would be needed.

It is important to obtain advice before welding any steels that you do not have experience in.

References

1. BS EN 1011-2:2001 'Welding - recommendations for welding of metallic materials - part 2: Arc welding of ferritic steels' British Standards Institution, March 2001.

2. PD CEN ISO/TR 15608:2005 'Welding - guidelines for a metallic material grouping system' British Standards Institution, October 2005.

Stainless steel


Stainless steels are chosen because of their enhanced corrosion resistance, high temperature oxidation resistance or their strength. The various types of stainless steel are identified and guidance given on welding processes and techniques which can be employed in fabricating stainless steel components without impairing the corrosion, oxidation and mechanical properties of the material or introducing defects into the weld.

Material types

The unique properties of the stainless steels are derived from the addition of alloying elements, principally chromium and nickel, to steel. Typically, more than 10% chromium is required to produce a stainless iron. The four grades of stainless steel have been classified according to their material properties and welding requirements:

Austenitic Ferritic Martensitic Austenitic-ferritic (duplex)The alloy groups are designated largely according to their microstructure. The first three consist of a single phase but the fourth group contains both ferrite and austenite in the microstructure.

As nickel (plus carbon, manganese and nitrogen) promotes austenite and chromium (plus silicon, molybdenum and niobium) encourages ferrite formation, the structure of welds in commercially available stainless steels can be largely predicted on the basis of their chemical composition. The predicted weld metal structure is shown in the Schaeffler diagram in which austenite and ferrite promoting elements are plotted in terms of the nickel and chromium equivalents.

Because of the different microstructures, the alloy groups have both different welding characteristics and susceptibility to defects.

Austenitic stainless steel

Austenitic stainless steels typically have a composition within the range 16-26% chromium (Cr) and 8-22% nickel (Ni). A commonly used alloy for welded fabrications is Type 304 which contains approximately 18%Cr and 10%Ni. These alloys can be readily welded using any of the arc welding processes (TIG, MIG, MMA and SA). As they are non-hardenable on cooling, they exhibit good toughness and there is no need for pre- or post-weld heat treatment.

Avoiding weld imperfections

Although austenitic stainless steel is readily welded, weld metal and HAZ cracking can occur. Weld metal solidification cracking is more likely in fully austenitic structures which are more crack sensitive than those containing a small amount of ferrite. The beneficial effect of ferrite has been attributed largely to its capacity to dissolve harmful impurities which would otherwise form low melting point segregates and interdendritic cracks.

As the presence of 5-10% ferrite in the microstructure is extremely beneficial, the choice of filler material composition is crucial in suppressing the risk of cracking. An indication of the ferrite-austenite balance for different compositions is provided by the Schaeffler diagram. For example, when welding Type 304 stainless steel, a Type 308 filler material which has a slightly different alloy content, is used.

Ferritic stainless steel

Ferritic stainless steels have a Cr content typically within the range 11-28%. Commonly used alloys include the 430 grade, having 16-18% Cr and 407 grade having 10-12% Cr. As these alloys can be considered to be predominantly single phase and non-hardenable, they can be readily fusion welded. However, a coarse grained HAZ will have poor toughness.


The main problem when welding this type of stainless steel is poor HAZ toughness. Excessive grain coarsening can lead to cracking in highly restrained joints and thick section material. When welding thin section material, (less than 6mm) no special precautions are necessary.

In thicker material, it is necessary to employ a low heat input to minimise the width of the grain coarsened zone and an austenitic filler to produce a tougher weld metal. Although preheating will not reduce the grain size, it will reduce the HAZ cooling rate, maintain the weld metal above the ductile-brittle transition temperature and may reduce residual stresses. Preheat temperature should be within the range 50-250 deg. C depending on material composition.

Martensitic stainless steel

The most common martensitic alloys e.g. type 410, have a moderate chromium content, 12-18% Cr, with low Ni but more importantly have a relatively high carbon content. The principal difference compared with welding the austenitic and ferritic grades of stainless steel is the potentially hard HAZ martensitic structure and the matching composition weld metal. The material can be successfully welded providing precautions are taken to avoid cracking in the HAZ, especially in thick section components and highly restrained joints.


High hardness in the HAZ makes this type of stainless steel very prone to hydrogen cracking. The risk of cracking generally increases with the carbon content. Precautions which must be taken to minimise the risk, include:

using low hydrogen process (TIG or MIG) and ensure the flux or flux coated consumable are dried (MMA and SAW) according to the manufacturer's instructions;

preheating to around 200 to 300 deg. C. Actual temperature will depend on welding procedure, chemical composition (especially Cr and C content), section thickness and the amount of hydrogen entering the weld metal;

maintaining the recommended minimum interpass temperature. carrying out post-weld heat treatment, e.g. at 650-750 deg. C. The time and temperature will be determined by

chemical composition.Thin section, low carbon material, typically less than 3mm, can often be welded without preheat, providing that a low hydrogen process is used, the joints have low restraint and attention is paid to cleaning the joint area. Thicker section and higher carbon (> 0.1%) material will probably need preheat and post-weld heat treatment. The post-weld heat treatment should be carried out immediately after welding not only to temper (toughen) the structure but also to enable the hydrogen to diffuse away from the weld metal and HAZ.

Duplex stainless steels

Duplex stainless steels have a two phase structure of almost equal proportions of austenite and ferrite. The composition of the most common duplex steels lies within the range 22-26% Cr, 4-7% Ni and 0-3% Mo normally with a small amount of nitrogen (0.1-0.3%) to stabilise the austenite. Modern duplex steels are readily weldable but the procedure, especially maintaining the heat input range, must be strictly followed to obtain the correct weld metalstructure.


Although most welding processes can be used, low heat input welding procedures are usually avoided. Preheat is not normally required and the maximum interpass temperature must be controlled. Choice of filler is important as it is designed to produce a weld metal structure with a ferrite-austenite balance to match the parent metal. To compensate for nitrogen loss, the filler may be overalloyed with nitrogen or the shielding gas itself may contain a small amount of nitrogen.

Aluminium alloys


Job Knowledge

Aluminium and its alloys are used in fabrications because of their low weight, good corrosion resistance and weldability. Although normally low strength, some of the more complex alloys can have mechanical properties equivalent to steels. The various types of aluminium alloy are identified and guidance is given on fabricating components without impairing corrosion and mechanical properties of the material or introducing imperfections into the weld.

Material types

As pure aluminium is relatively soft, small amounts of alloying elements are added to produce a range of mechanical properties. The alloys are grouped according to the principal alloying elements, specific commercial alloys have a four-digit designation according to the international specifications for wrought alloys or the ISO alpha - numeric system.

The alloys can be further classified according to the means by which the alloying elements develop mechanical properties, non-heat-treatable or heat-treatable alloys.

Non-heat-treatable alloys

Material strength depends on the effect of work hardening and solid solution hardening of alloy elements such as magnesium, and manganese; the alloying elements are mainly found in the 1xxx, 3xxx and 5xxx series of alloys. When welded, these alloys may lose the effects of work hardening which results in softening of the HAZ adjacent to the weld.

Heat-treatable alloys

Material hardness and strength depend on alloy composition and heat treatment (solution heat treatment and quenching followed by either natural or artificial ageing produces a fine dispersion of the alloying constituents). Principal alloying elements are defined in the 2xxx, 6xxx and 7xxx series. Fusion welding redistributes the hardening constituents in the HAZ which locally reduces material strength.

Processes

Most of the wrought grades in the 1xxx, 3xxx, 5xxx, 6xxx and medium strength 7xxx (e.g. 7020) series can be fusion welded using TIG, MIG and oxyfuel processes. The 5xxx series alloys, in particular, have excellent weldability. High strength alloys (e.g. 7010 and 7050) and most of the 2xxx series are not recommended for fusion welding because they are prone to liquation and solidification cracking.

The technique of Friction Stir Welding is particularly suited to aluminium alloys. It is capable of producing sound welds in many alloys, including those heat treatable alloys which are prone to hot cracking during fusion welding.

Filler alloys

Filler metal composition is determined by:

weldability of the parent metal minimum mechanical properties of the weld metal corrosion resistance anodic coating requirementsNominally matching filler metals are often employed for non-heat-treatable alloys. However, for alloy-lean materials and heat-treatable alloys, non-matching fillers are used to prevent solidification cracking.

The choice of filler metal composition for the various weldable alloys is specified in BS EN 1011 Pt 4:2000 for TIG and MIG welding; recommended filler metal compositions for the more commonly used alloys are given in the Table.

Alloy Designation Chemical Designation Classification Filler ApplicationEN AW-1080A EN AW-Al 99.8(A) NHT R-1080A Chemical plantEN AW-3103 EN AW-Al Mn1 NHT R-3103 Buildings, heat exchangersEN AW-4043A EN AW-Al Si5(A) - - Filler wire/rodEN AW-5083 EN AW-Al Mg4.5Mn0.7 NHT R-5556A Ships, rail wagons, bridgesEN AW-5251 EN AW-Al Mg2Mn0.3 NHT R-5356 Road vehicles, marineEN AW-5356 EN AW-Al Mg5Cr(A) - - Filler wire/rodEN AW-5556A EN AW-Al Mg5Mn - - Filer wire/rod

EN AW-6061 EN AW-Al Mg1SiCu HTR-4043AR-5356

Structural, pipes

EN AW-7020 EN AW-Al Zn4.5Mg1 HT R-5556A Structural, transportHT = Heat treatable, NHT = Non Heat treatableImperfections in welds

Aluminium and its alloys can be readily welded providing appropriate precautions are taken. The most likely imperfections in fusion welds are:

porosity cracking poor weld bead profilePorosity

Porosity is often regarded as an inherent feature of MIG welds; typical appearance of finely distributed porosity in a TIG weld is shown in the photograph. The main cause of porosity is absorption of hydrogen in the weld pool which forms discrete pores in the solidifying weld metal. The most common sources of hydrogen are hydrocarbons and moisture from contaminants on the parent material and filler wire surfaces, and water vapour from the shielding gas atmosphere. Even trace levels of hydrogen may exceed the threshold concentration required to nucleate bubbles in the weld pool, aluminium being one of the metals most susceptible to porosity.

To minimise the risk, rigorous cleaning of material surface and filler wire should be carried out. Three cleaning techniques are suitable; mechanical cleaning, solvent degreasing and chemical etch cleaning.

In gas shielded welding, air entrainment should be avoided by making sure there is an efficient gas shield and the arc is protected from draughts. Precautions should also be taken to avoid water vapour pickup from gas lines and welding equipment; it is recommended that the welding system is purged for about an hour before use.

Mechanical cleaning

Wire brushing (stainless steel bristles), scraping or filing can be used to remove surface oxide and contaminants. Degreasing should be carried out before mechanical cleaning.

Solvents

Dipping, spraying or wiping with organic solvents can be used to remove grease, oil, dirt and loose particles.

Chemical etching

A solution of 5% sodium hydroxide can be used for batch cleaning but this should be followed by rinsing in HNO3 and water to remove reaction products on the surface.

Solidification cracks

Cracking occurs in aluminium alloys because of high stresses generated across the weld due to the high thermal expansion (twice that of steel) and the substantial contraction on solidification - typically 5 % more than in equivalent steel welds.

Solidification cracks form in the centre of the weld, usually extending along the centreline during solidification. Solidification cracks also occur in the weld crater at the end of the welding operation. The main causes of solidification cracks are as follows:

incorrect filler wire/parent metal combination incorrect weld geometry welding under high restraint conditionsThe cracking risk can be reduced by using a non-matching, crack-resistant filler (usually from the 4xxx and 5xxx series alloys). The disadvantage is that the resulting weld metal may have a lower strength than the parent metal and not respond to a subsequent heat treatment. The weld bead must be thick enough to withstand contraction stresses. Also, the degree of restraint on the weld can be minimised by using correct edge preparation, accurate joint set up and correct weld sequence.

Liquation cracking

Liquation cracking occurs in the HAZ, when low melting point films are formed at the grain boundaries. These cannot withstand the contraction stresses generated when the weld metal solidifies and cools. Heat treatable alloys, particularly 6xxx and 7xxx series alloys, are more susceptible to this type of cracking.

The risk can be reduced by using a filler metal with a lower melting temperature than the parent metal, for example the 6xxx series alloys are welded with a 4xxx filler metal. However, 4xxx filler metal should not be used to weld high magnesium alloys (such as 5083) as excessive magnesium-silicide may form at the fusion boundary decreasing ductility and increasing crack sensitivity.

Poor weld bead profile

Incorrect welding parameter settings or poor welder technique can introduce weld profile imperfections such as lack of fusion, lack of penetration and undercut. The high thermal conductivity of aluminium and the rapidly solidifying weld pool make these alloys particularly susceptible to profile imperfections.

Nickel and nickel alloys


Nickel and nickel alloys are chosen because of their:

corrosion resistance heat resistance and high temperature properties low temperature propertiesTypes of nickel alloys are identified and guidance is given on welding processes and techniques which can be used in fabricating nickel alloy components without impairing their corrosion or mechanical properties or introducing flaws into the weld.

Material types

The alloys can be grouped according to the principal alloying elements. Although there are National and International designations for the alloys, tradenames such as Inconel and Hastelloy, are more commonly used.

In terms of their weldability, these alloys can be classified according to the means by which the alloying elements develop the mechanical properties, namely solid solution alloysand precipitation hardened alloys. A distinguishing feature of precipitation hardened alloys is that mechanical properties are developed by heat treatment (solution treatment plus ageing) to produce a fine distribution of particles in a nickel-rich matrix.

Solid solution alloys

Solid solution alloys are pure nickel, Ni-Cu alloys and the simpler Fe-Ni-Cr alloys. These alloys are readily fusion welded, normally in the annealed condition. As the heat affected zone (HAZ) does not harden, heat treatment is not usually required after welding.

Precipitation hardening alloys

Precipitation hardening alloys include Ni-Cu-Al-Ti, Ni-Cr-Al-Ti and Ni-Cr-Fe-Nb-Al-Ti. These alloys may susceptible to post-weld heat treatment cracking.

Weldability

Most nickel alloys can be fusion welded using gas shielded processes like TIG or MIG. Of the flux processes, MMA is frequently used but the SAW process is restricted to solid solution alloys and is less widely used.

Solid solution alloys are normally welded in the annealed condition and precipitation hardened alloys in the solution treated condition. Preheating is not necessary unless there is a risk of porosity from moisture condensation. It is recommended that material containing residual stresses be solution-treated before welding to relieve the stresses.

Post-weld heat treatment is not usually needed to restore corrosion resistance but thermal treatment may be required for precipitation hardening or stress relieving purposes to avoid stress corrosion cracking.

Filler alloys

Filler composition normally matches the parent metal. However, most fillers contain a small mount of titanium, aluminium and/or niobium to help minimise the risk of porosity and cracking.

Filler metals for gas shielded processes are covered in BS EN 18274:2004 and in the USA by AWS A5.14. Recommended fillers for selected alloys are given in the table.

Table 1: Filler selection for nickel alloys

Parent Alloy Filler designations Comments

AlloyBS EN

ISO 18274

AWS A5.14Trade names

Pure nickel Nickel 200 Ni 2061 ERNi-1 Nickel 61 Matching filler metal normally contains 3%TiNickel Copper

Alloy 400 Ni 4060 ERNiCu-7 Monel 60Matching filler metal contains additions of Mn, Ti and Al

Nickel Chromium Brightray S Ni 6076 - NC 80/20

Ni-Cr and Ni-Cr-Fe filler metals may be usedNimonic 75 Ni 6076 - NC 80/20Nickel-Chromium-Iron

Alloy 800 Ni 6625ERNiCrMo-3

Inconel 625Thermanit 21/33

Usually welded with Ni-Cr-X alloys, but more nearly matching consumables are available which contain higher C and also Nb

Alloy 600 Ni 6082 ERNiCr-3 Inconel 82 Matching filler metal contains Nb addition

Alloy 718 Ni 7718 ERNiFeCr-2 Inconel 718Matching filler metal is normally used but Alloy 625 is an alternative consumable , if postweld heat treatment is not applied

Nickel-Chromium-Molybdenum

Alloy 625 Ni 6625ERNiCrMo-3

Inconel 625Filler metal is also used widely for cladding and dissimilar welds

Hastelloy C-22 Ni 6022ERNiCrMo-10

Hastelloy C-22

Nickel-Molybdenum

Hastelloy B-2 Ni 1066 ERNiMo-7Hastelloy B-2

Corrosion resistant alloys require matching fillers

Imperfections and degradation

Nickel and its alloys are readily welded but it is essential that the surface is cleaned immediately before welding. The normal method of cleaning is to degrease the surface, remove all surface oxide by machining, grinding or scratch brushing and finally degrease.

Common imperfections found on welding are:

porosity oxide inclusions and lack of inter-run fusion weld metal solidification cracking microfissuringAdditionally, precautions should be taken against post-welding imperfections such as:

post-weld heat treatment cracking stress corrosion crackingPorosity

Porosity can be caused by oxygen and nitrogen from air entrainment and surface oxide or by hydrogen from surface contamination. Careful cleaning of component surfaces and using a filler material containing deoxidants (aluminium and titanium) will reduce the risk.

When using argon in TIG and MIG welding, attention must be paid to shielding efficiency of the weld pool including the use of a gas backing system. In TIG welding, argon-hydrogen gas mixtures tend to produce cleaner welds.

Oxide inclusions and lack of inter-run fusion

As the oxide on the surface of nickel alloys has a much higher melting temperature than the base metal, it may remain solid during welding. Oxide trapped in the weld pool will form inclusions. In multi-run welds, oxide or slag on the surface of the weld bead will not be consumed in the subsequent run and may cause lack of fusion imperfections.

Before welding, surface oxide, particularly if it has been formed at a high temperature, must be removed by machining or abrasive grinding; it is not sufficient to wire brush the surface as this serve only to polish the oxide. During multipass welding, surface oxide and slag must be removed between runs.

Weld metal solidification cracking

Factors which control solidification cracking include alloy, welding process and welding conditions. For example, solidification cracking is a factor which limits the application of submerged arc welding, both with respect to applicable alloys and welding conditions. More generally, this type of cracking leads to restriction of weld shape, welding speed and technique.

Microfissuring

Similar to austenitic stainless steel, nickel alloys are susceptible to formation of liquation cracks in reheated weld metal regions or parent metal HAZ. This type of cracking is controlled by factors outside the control of the welder such as grain size or impurity content. Some alloys are more sensitive than others. For example, some cast superalloys are difficult to weld without inducing liquation cracks.

Post-weld heat treatment cracking

This is also known as strain-age or reheat cracking. It is likely to occur during post-weld ageing of precipitation hardening alloys but can be minimised by pre-weld heat treatment. Solution annealing is commonly used but overageing gives the most resistant condition. Alloy 718 alloy was specifically developed to be resistant to this type of cracking.

Stress corrosion cracking

Welding does not normally make most nickel alloys susceptible to weld metal or HAZ corrosion. However, when Alloy 400 will be in contact with caustic soda, fluosilicates or HF acid, stress corrosion cracking is possible. For such service, thermal stress relief is applied after welding.

Stress corrosion can also occur in Ni-Cr alloys in high temperature water. High chromium filler metal has been developed for welds and overlays in this environment.

Copper and copper alloys


Copper and copper alloys are chosen because of their corrosion resistance and electrical and thermal conductivity.

The various types of copper alloys are identified and guidance is given on processes and techniques which can be used in fabricating copper alloy components with a view to maintaining their corrosion or mechanical properties whilst avoiding the introduction of defects into the welds.

Alloy types

The main categories of copper and copper alloy are listed below:

Table 1. Frequently used copper alloys and recommended filler metals

Alloy type Recommended fillerCoppers (tough pitch, phosphorus deoxidised)

Cu 1897, Cu 1898

Brasses (low Zinc) Cu 6328, Cu 6560Nickel Silvers (20%Zn/15%Ni type) Cu 6328, Cu 6560Silicon Bronze (3%Si) Cu 6560Phosphor Bronze (4.5% to 6%Sn/0.4%P) Cu 5180Aluminium Bronze (<7.8%Al) Cu 6240, Cu 6100Aluminium Bronze (>7.8%Al) Cu 6180, Cu 6328Aluminium Bronze (6%Al/2%Si) Cu 6100Gunmetal (low lead) Cu 5180, Cu 6560, Cu 6180Cupro-Nickel (10%Ni) Cu 7061, Cu 7158Cupro-Nickel (30%Ni) Cu 7158 Pure copper Copper with small alloy additions (less than 5% in total) Brasses e.g. copper-zinc (Cu-Zn) Nickel silvers e.g. copper-zinc-nickel (Cu-Zn-Ni) Bronzes e.g. copper-tin (Cu-Sn) (phosphor bronze alloys also contain phosphorus) Gunmetals e.g. copper-tin-zinc (Cu-Sn-Zn) (some alloys may contain lead) Aluminium bronze e.g. copper-aluminium (Cu-Al) (most alloys also contain iron and many nickel) Cupro-nickels e.g. copper-nickel (Cu-Ni)The most frequently used copper alloys are listed in Table 1, together with a range of welding electrodes for fusion welding as per BS EN 14640:2005. Similar filler wire compositions are given in AWS A5.7/A5.7M:2008 and covered electrodes are specified in A5.6/A5.6M:2007.

It should be mentioned that welding of Nickel Silvers (45%Zn/10%Ni), leaded Gunmetal and high Zinc Brasses (40%Zn) is not recommended.

Copper alloys have quite different welding characteristics due to differences in thermal conductivity. For example copper, due to its high thermal conductivity, may require substantial preheat to counteract the very high heat sink. However, some of the alloys which have a thermal conductivity similar to low carbon steel, such as cupro-nickel alloys, can normally be fusion welded without a preheat.

Copper

Copper is normally supplied in the form of

oxygen bearing, tough pitch copper phosphorus deoxidised copper oxygen-free copperTough pitch copper contains stringers of copper oxide (<0.1% oxygen as Cu2O) which does not impair the mechanical properties of wrought material and it has high electrical conductivity.Oxygen-free and phosphorus deoxidised copper are more easily welded.

TIG and MIG are the preferred welding processes but oxyacetylene and MMA welding can be also used in the repair of tough pitch copper components. Helium and nitrogen-based shielding gases, which have higher arc voltages, can be used as an alternative to argon to counteract the high thermal conductivity of coppers.


During fusion welding of tough pitch copper, the high oxygen content of the alloy often leads to embrittlement in the heat affected zone (HAZ) and weld metal porosity. Phosphorus deoxidised copper is more weldable but residual oxygen can result in porosity in autogenous welds especially in the presence of hydrogen. Porosity can be avoided by using appropriate filler wire containing deoxidants (Al, Mn, Si, P and Ti).

Thin section material can be welded without preheat. However, over 5mm thickness all grades need preheat to produce a fluid weld pool and avoid fusion defects. Thick section components may need a preheat temperature as high as 600 deg.C.

Copper with small alloying additions

Low amounts of sulphur or tellurium can be added to improve machining. However, these grades are normally considered to be unweldable.

Precipitation hardened alloys contain small additions of chromium, zirconium or beryllium. and have superior mechanical properties. Chromium and beryllium coppers may suffer from HAZ cracking unless they are heat treated before welding. When welding beryllium copper, care should be taken to avoid inhaling the welding fumes, which are poisonous.

Brasses (copper-zinc alloys) and nickel silvers

When considering weldability, brasses can be separated into two groups viz. low zinc (up to 20% Zn) and high zinc (30 to 40% Zn). Nickel silvers contain 20 to 45% zinc and nickel to improve strength. The main problem in fusion welding these alloys is the volatilisation of the zinc which results in white fumes of zinc oxide and weld metal porosity. Only low zinc brasses are weldable using fusion welding processes such as TIG and MIG.


To minimise porosity, a zinc-free filler wire should be used, either silicon bronze (Cu 6560) or an aluminium bronze (Cu 6180). High welding speeds will reduce pore size.

TIG and MIG processes are used with argon or an argon-helium mixture but not with nitrogen. Preheat is normally used for low zinc (<20% Zn) to avoid fusion defects due to the high thermal conductivity,. Although preheat is not needed for higher zinc content alloys, slow cooling reduces cracking risk. Post weld heat treatment also helps to reduce the risk of stress corrosion cracking in areas where restraint is high.

Bronzes (tin bronze, phosphor bronze, silicon bronze and gunmetal)

Tin bronzes typically contain between 1% to 10% tin. Phosphor bronze contains up to 0.4% phosphorus. Gunmetal is essentially a tin bronze with up to 5% zinc and it may have lead additions up to 5%. Silicon bronze contains 3% silicon and 1% manganese approximately and it is probably the easiest of the bronzes to weld.


Matching filler compositions are normally employed for welding bronzes. Autogenous welding of phosphor bronzes is not recommended due to weld metal porosity. However, this risk can be reduced by using a filler wire with a higher level of deoxidants. Gunmetal is not considered weldable since it is susceptible to hot cracking.

Aluminium bronze

There are essentially two types of aluminium bronzes; single phase alloys containing between 5 to 10% aluminium, with a small amount of iron or nickel, and more complex, two phase alloys containing up to 12% aluminium and about 5%of iron with specific alloys also containing nickel, manganese and silicon. Gas shielded welding processes are preferred for welding this group of alloys. In TIG welding, the presence of a tenacious, refractory oxide film requires AC(argon), or DC with a helium shielding gas. Due to its low thermal conductivity, a preheat is not normally required except when welding thick section components.


Rigorous cleaning of the material surface is essential, both before and after deposition of each welding pass, to avoid porosity. Single phase alloys can be susceptible to weld metal and HAZ cracking under highly restrained conditions. It is often necessary to use matching filler metals to maintain corrosion resistance but a non-matching, two phase, filler can also reduce the risk of cracking. Two phase alloys are easier to weld. For both types, preheat and interpass temperatures should be controlled carefully to prevent cracking.

Cupro-nickels

Cupro-nickel alloys contain 5 to 30% nickel with specific alloys having additions of iron and manganese; 90/10 and 70/30 (Cu/Ni) alloys are commonly welded grades. These alloys are single phase and generally considered to be weldable using inert gas processes and, to a lesser extent, MMA. A matching filler is normally used. 70/30 (Cu 7158) is often regarded as a 'universal' filler for these alloys. The thermal conductivity of cupro-nickel alloys is similarto low carbon steels, and therefore preheating is not required.


Cupro-nickels do not contain deoxidants, and therefore, autogenous welding is not recommended due to the risk of porosity. Filler metal compositions contain typically 0.2 to 0.5% titanium, to minimise weld metal porosity. Argon shielding gas is normally used for both TIG and MIG but in TIG welding, an argon-hydrogen mixture, with appropriate filler, improves weld pool fluidity and produces a cleaner weld bead. Gas backing (usually argon) is recommended, especially in pipe welding, to produce an oxide-free underbead.

Titanium and titanium alloys


Titanium and its alloys are chosen because of the following properties:

high strength to weight ratio; corrosion resistance; mechanical properties at elevated temperatures.

Titanium is a unique material, as strong as steel but half its weight with excellent corrosion resistance. Traditional applications are in the aerospace and chemical industries. More recently, especially as the cost of titanium has fallen significantly, the alloys are finding greater use in other industry sectors, such as offshore.

The various types of titanium alloys are identified and guidance given on welding processes and techniques employed in fabricating components without impairing their corrosion, oxidation and mechanical properties or introducing defects into the weld.

Material types

Alloy groupings

There are basically three types of alloys distinguished by their microstructure:

Titanium - Commercially pure (98 to 99.5% Ti) or strengthened by small additions of oxygen, nitrogen, carbon and iron. The alloys are readily fusion weldable.

Alpha alloys - These are largely single-phase alloys containing up to 7% aluminium and a small amount (< 0.3%) of oxygen, nitrogen and carbon. The alloys are fusion welded in the annealed condition.

Alpha-beta alloys - These have a characteristic two-phase microstructure formed by the addition of up to 6% aluminium and varying amounts of beta forming constituents - vanadium, chromium and molybdenum. The alloys are readily welded in the annealed condition.

Alloys which contain a large amount of the beta phase, stabilised by elements such as chromium, are not easily welded.

Commonly used alloys are listed in Table 1 with the appropriate ASTM grade, the internationally recognised designation. In industry, the most widely welded titanium alloys are the commercially pure grades and variants of the 6% Al and 4%V alloy.

Table 1: Commonly used titanium alloys and the recommended filler material

ASTM Grade CompositionUTS (min) Mpa

Filler Comments

1 Ti-0.15O 240 ERTi-1 Commercially pure

2 Ti-0.20O 340 ERTi-2 ,,

4 Ti-0.35O 550 ERTi-4 ,,

7 Ti-0.20O -0.2Pd 340 ERTi-7 ,,

9 Ti-3Al-2.5V 615 ERTi-9 Tube components

5 Ti-6Al-4V 900 ERTi-5 'Workhorse' alloy

23 Ti-6Al-4V ELI 900 ERTi-5ELI Low interstitials

25 Ti-6Al-4V-0.06Pd 900 ERTi-25 Corrosion resistant grade

Filler alloys

Titanium and its alloys can be welded using a matching filler composition; compositions are given in The American Welding Society specification AWS A5.16-2004. Recommended filler wires for the commonly used titanium alloys are also given in Table 1.

When welding higher strength titanium alloys, fillers of a lower strength are sometimes used to achieve adequate weld metal ductility. For example, an unalloyed filler ERTi-2 can be used to weld Ti-6Al-4V and Ti-5Al-2.5Sn alloys in order to balance weldability, strength and formability requirements.

Weld imperfections

This material and its alloys are readily fusion welded providing suitable precautions are taken. TIG and plasma processes, with argon or argon-helium shielding gas, are used for welding thin section components, typically <10mm. Autogenous welding can be used for a section thickness of <3mm with TIG, or <6mm with plasma. Pulsed MIG welding using novel coated wires results in very low porosity and spatter.

The most likely imperfections in fusion welds are:

Weld metal porosity Embrittlement Contamination crackingNormally, there is no solidification cracking or hydrogen cracking.

Weld metal porosity

Weld metal porosity is the most frequent weld defect. Porosity arises when gas bubbles are trapped between dendrites during solidification. In titanium, hydrogen from moisture in the arc environment or contamination on the filler and parent metal surface, is the most likely cause of porosity.

It is essential that the joint and surrounding surface areas are cleaned by first degreasing either by steam, solvent, alkaline or vapour degreasing. Any surface oxide should then be removed by pickling (HF-HNO3 solution), light grinding or scratch brushing with a clean, stainless steel wire brush. On no account should an ordinary steel brush be used. After wiping with a lint-free cloth, care should be taken not to touch the surface before welding. When TIG welding thin section components, the joint area should be dry-machined to produce a smooth surface finish.

Embrittlement

Embrittlement can be caused by weld metal contamination by either gas absorption or by dissolving contaminants such as dust (iron particles) on the surface. At temperatures above 500°C, titanium has a very high affinity for oxygen, nitrogen and hydrogen. The weld pool, heat affected zone and cooling weld bead must be protected from oxidation by an inert gas shield (argon or helium).

When oxidation occurs, the thin layer of surface oxide generates an interference colour. The colour can indicate whether the shielding was adequate or an unacceptable degree of contamination has occurred. A silver or straw colour shows satisfactory gas shielding was achieved but for certain service conditions, dark blue may be acceptable. Light blue, grey and white show a higher, usually unacceptable, level of oxygen contamination.

For small components, an efficient gas shield can be achieved by welding in a totally enclosed chamber, filled with the shielding gas. It is recommended that before welding, the arc is struck on a scrap piece of titanium, termed 'titanium-getter', to remove oxygen from the atmosphere; the oxygen level should be reduced to approximately 40ppm before striking the arc on the scrap titanium and <20ppm before welding the actual component.

In tube welding, a fully enclosed head is equally effective in shielding the weld area and is be preferable to orbital welding equipment in which the gas nozzle must be rotated around the tube.

When welding out in the open, the torch is fitted with a trailing shield to protect the hot weld bead whilst cooling. The size and shape of the shield is determined by the joint profile whilst its length will be influenced by welding current and travel speed. It is essential in 'open air' welding that the underside of the joint is protected from oxidation. For straight runs, a grooved bar is used with argon gas blown on to the joint. In tube and pipe welding, normal gas purging techniques are appropriate.

Contamination cracking

If iron particles are present on the component surface, they dissolve in the weld metal reducing corrosion resistance and, at a sufficiently high iron content, causing embrittlement. Iron particles are equally detrimental in the HAZ where local melting of the particles form pockets of titanium - iron eutectic. Microcracking may occur but it is more likely that the iron-rich pockets will become preferential sites for corrosion.

Particular attention should be paid to separating titanium from steel fabrications, preferably by designating a specially reserved clean area. Welders should guard against embedding steel particles into the surface of the material by:

Avoiding steel fabrication operations near titanium components. Covering components to avoid airborne dust particles settling on the surface Not using tools, including wire brushes, previously used for steel Scratch brushing the joint area immediately before welding Not handling the cleaned component with dirty gloves.To avoid corrosion cracking, and minimise the risk of embrittlement through iron contamination, it is best practice to fabricate titanium in a specially reserved clean area.

Further information

Titanium information and technical support

Welding titanium - a guide to best practice

Documents

Weldability of Steels