Welding of nickel alloys - Part 1Job KnowledgeNickel is a relatively simple metal. It is face centred cubic and undergoes no phase changes as it cools from melting point to room temperature; similar to a stainless steel. Nickel and its alloys cannot therefore be hardened by quenching so cooling rates are less important than with, say, carbon steel and preheating if the ambient temperature is above 5°C is rarely required. Nickel and its alloys are used in a very wide range of applications - from high temperature oxidation and creep resistance service to aggressive corrosive environments and very low temperature cryogenic applications. Nickel may be used in a commercially pure form but is more often combined with other elements to produce two families of alloys - solid solution strengthened alloys and precipitation hardened alloys. Typical compositions of some of the more common alloys are given in the Table.

Table. Typical composition and properties of some of the more common alloys

Alloy designation

Alloy type

Typical chemical composition %Mechanical properties

    Ni CrMo

Fe Nb Al TiOthers

0.2% proof, MPa

UTS, MPa

El, %

Alloy 200 CP 99.2 - - 0.2 - - - Mn 0.3 148 452 45Monel® 400 SS 68 - - 1.75 - - - Cu 33 235 562 38Monel® K500 PH 65 - - 1.25 - 2.95 0.55 Cu 32 795 1100 18Alloy 600 SS 75 15.5 - 8.5 - - - - 305 670 40

Alloy 617 SS 46 22 9 0.75 - 1.25 0.45

Co 12.5B 0.004

345 725 60

Alloy 625 SS 64 22 8 2.75 3.65 0.25 0.25 - 472 920 45

Alloy 718 PH 52 19 3Rem

5.2 0.5 0.95 - 1100 1420 18

Alloy 800 SS 32 22 - 42 - 0.45 0.45 - 290 605 42

Alloy 825 SS 42 21.5 3 28 - 0.1 0.9Cu 2.25

330 715 39

Alloy C276 SS 55 15.5 16 5.5 - - - W 3.75 345 795 60

Nimonic® PE16 PH 44 16.7 3.3 29 - 1.2 1.2B 0.004Zr 0.03

450 825 28

All the conventional welding processes can be used to weld nickel and its alloys and matching welding consumables are available. As mentioned above, nickel and its alloys are similar in many respects to the austenitic stainless steels; welding procedures are likewise also similar.

Nickel, however, has a coefficient of thermal expansion less than that of stainless steel so distortion and distortion control measures are similar to those of carbon steel.

The most serious cracking problem with nickel alloys is hot cracking in either the weld metal or close to the fusion line in the HAZ with the latter being the more frequent. The main source of this problem is sulphur but phosphorus, lead, bismuth and boron also contribute. Both weld metal and HAZ cracking are generally the result of contamination by grease, oil, dirt, etc left behind following inadequate cleaning; excess sulphur in the parent or weld filler metals causing a problem is a rare event. Machining or vigorous stainless steel wire brushing followed by thorough degreasing with a suitable solvent is necessary prior to welding, with the welding taking place within about eight hours to reduce the risk of contamination. Any heat treatment must be carried out using sulphur-free fuel or by using electric furnaces. Components that have been in service and require weld repair may need to be ground or machined prior to degreasing to remove any contaminants that have become embedded in the surface in or adjacent to the weld repair area. Remember that if mechanical wire brushing is carried out AFTER the degreasing operation or during welding the compressed air from air powered tools contains both moisture and oil and the cleaned surfaces may be therefore be re-contaminated.

Porosity can be a problem with the nickel alloys, the main culprit being nitrogen. As little as 0.025% nitrogen will form pores in the solidifying weld metal. Quite light draughts are capable of disrupting the gas shield and atmospheric contamination will occur resulting in porosity. Care must be taken to ensure that the weld area is sufficiently protected and this is particularly relevant in site welding applications. With the gas shielded processes, gas purity and the efficiency of the gas shield must be as good as possible. Gas hoses should be checked for damage and leaks at regular intervals and, with the TIG process, as large a ceramic shroud as possible should be used together with a gas lens. It goes without saying that gas purging of the root is essential when depositing a TIG root pass.

A small amount of hydrogen (up to 10%) added to the argon shield gas has been found to reduce the problem. Start and finish porosity is a problem when MMA welding. The weld start should be carried out by welding back over the arc strike position, remelting any porosity that has formed due to the poor gas shielding at the start of the weld. Care also needs to be taken at the weld end, with the arc length reduced and travel speed increased slightly to reduce weld pool size.

Oxygen is also a cause of porosity in certain circumstances when it combines with carbon in the weld pool to form carbon monoxide. Consumable manufacturers generally overcome this problem by ensuring that sufficient deoxidants (primarily manganese, aluminium and titanium) are present in the filler metal.

One feature of nickel alloys that is often encountered is the formation on the surface of the weld pool of a viscous and adherent scum. This can be difficult to remove and can result in inclusions and lack of inter-run fusion if not removed prior to depositing the next pass. Wire brushing is frequently not sufficient to remove this layer and it then becomes necessary to grind the weld surface.

The weld pool, in addition to this surface film, is also sluggish and does not flow freely as with a carbon or stainless steel. This may result in a lumpy and very convex weld bead and a poor toe blend unless the welder manipulates the weld pool to avoid such defects. Although stringer beads may be used, a slight weave to assist the weld metal to wet the side walls of the preparation is beneficial. In addition, weld preparations must be sufficiently wide to enable the welder to control and direct the weld pool; an included angle of 70 to 80° is recommended for V butt welds.

A U preparation included angle of 30 to 40° is acceptable and, though more expensive to machine than a V preparation, may be cheaper overall as the amount of filler wire required can be reduced, depending on material thickness. Addition of hydrogen to the shield gas (up to 10%H in argon) in TIG welding also has been found to be beneficial in reducing the weld pool surface tension.

A further characteristic of nickel alloys is that the amount of penetration is less than with a carbon or stainless steel. Increasing the welding current will not increase penetration. The implication of this is that the root face thickness in single sided full penetration welds should be less than with a stainless steel. It is recommended that the thickness of the root face should not be greater than 1.5mm in a zero gap TIG butt weld. Removable backing strips are very useful to control root bead shape. These can be made from copper, stainless steel or a nickel alloy. Carbon or low alloy steel backing strips should be avoided.

Although weldability of nickel and its alloys is generally good the composition, metallurgical structure and its heat treatment and/or service history all affect its response to welding. Wrought, fine grained components have better weldability than cast items as these often have significant amounts of segregation. Coarse grains may lead to micro-fissuring in the HAZ thus high heat input is best avoided. All the alloys are best welded in the annealed or solution treated condition and this applies particularly to the precipitation hardenable alloys such as Inconel 718.

Welding of nickel alloys - Part 2

Job Knowledge

In Part 1 the importance of cleanliness, particularly the removal of all sulphur containing compounds, was mentioned. With respect to defect free welding of nickel and its alloys this cannot be over-emphasised. 

As well as sulphur, however, there are several other substances that can lead to embrittlement of the nickel alloys when they are exposed to high temperatures. Amongst these are lead, phosphorus, boron and bismuth.

These may be present in oils, grease, cutting fluids, paints, marker pen inks, temperature indicating crayons, etc; it may not be possible to avoid using these during fabrication so it is essential that these are removed if the component is to be welded, heat treated or is to enter high temperature service.

Fuel gases frequently contain sulphur and it may be necessary to use radiant gas heaters or electrical elements for local heating or in heat treatment furnaces.

Nickel alloys can be welded using all the conventional arc welding and power beam processes, the commonest processes being TIG or MIG with pure argon, argon/hydrogen or argon/helium mixtures as shield gases and MMA where basic flux coatings provide the best properties.

However, if argon/helium mixes are used it is only when there is more than 40% helium that any significant benefits with respect to penetration and improved fusion will be noticed. Submerged arc welding is restricted to welding solid solution alloys using basic fluxes. Matching welding consumables are available for most of the nickel alloys. See Job Knowledge 22 for recommendations for a range of alloys.

Slag from MMA welding and particularly submerged arc welding can be difficult to remove from the nickel alloys and often needs to be ground between runs to remove it completely. It is also often necessary to grind the surface of each run when welding with the gas shielded processes to remove oxide scabbing, wire brushing simply polishing these oxides.

Failure to remove slag or oxide scabs will result not only in weld metal inclusions but also reduce corrosion resistance if left on exposed surfaces. Total welding times can therefore be substantially longer than the equivalent joint in stainless or carbon steel and welders need to be fully acquainted with these differences when converting from welding steels to nickel alloys.

Comments regarding the recommended weld preparations were included in Part 1. Although the weld preparations are similar to those used for steel it is worth considering the use of double V or U type preparations at thicknesses less than would be considered with steels. The additional cost of the preparation is offset by savings in consumable costs (nickel being an expensive metal) and welding time.

The majority of nickel alloys are best welded in the annealed or solution treated condition, particularly if the alloys have been cold worked. As mentioned in Part 1, preheat is not required except to remove condensation or if the ambient temperature is below about 5°C when a moderate preheat of 40-50°C is recommended.

Interpass temperature should not be allowed to rise above 250°C although some alloy suppliers recommend an interpass as low as 100°C for certain alloys such as Alloy C276.

Remember the potential hot crack problems if thermal crayons are used to measure this temperature! For most alloys heat input should be controlled to moderate levels (say 2kJ/mm maximum) to limit grain growth and HAZ size although for some Alloys 718, C22, and C276 for example, a maximum heat input of 1kJ/mm is recommended.

Conversely if too fast a travel speed is used in an attempt to maintain a low heat input this can result in a narrow weld bead sensitive to centre line cracking. Adequate testing during welding procedure development should be used to optimise the range of acceptable welding parameters.

The solid solution alloys such as Alloy 200 or 625 do not require post weld heat treatment to maintain corrosion resistance but may be subject to PWHT either to reduce the risk of stress corrosion cracking if the alloy is to be used in caustic soda service or in contact with fluoro-silicates or to provide dimensional stability.

A typical stress relief treatment would be 700°C for ½ an hour for Alloy 200; 790°C for four hours for the higher chromium content alloys such as Alloy 600 or 625.

The nickel-molybdenum alloys are identified with the prefix B eg B1, B2, etc. and are used in reducing environments, such as hydrogen chloride gas and sulphuric, acetic and phosphoric acids. Alloy B2 is the most frequently encountered alloy and matching filler metals are available. Unlike Alloy B1, Alloy B2 does not form grain boundary carbide precipitates in the weld heat affected zone, so it may be used in most applications in the as-welded condition.

Alloy 400, a 70Ni-30Cu alloy, has good corrosion resistance when exposed to hydrofluoric acid, strong alkaline solutions and sea water.

A matching filler metal, Alloy 190, is available but this can become anodic in salt solutions, leading to galvanic corrosion and it is recommended that one of the Ni-Cr alloy fillers such as Alloy 600 or 625 is used in this environment.

The age hardened alloy K-500 does not have a matching filler metal and is generally welded using the Alloy 190 filler, the reduction in strength being taken into account during the design phase.

Precipitation hardened alloys are best welded in the solution treated condition; welding these alloys in the age hardened condition is likely to result in HAZ cracking.

The ageing process in the alloys is sufficiently sluggish that the components can be welded in the solution treated condition and then aged at around 750°C without the mechanical properties being degraded.

A solution treatment of the welded item followed by ageing will provide the highest tensile strength.

The sensitivity of the age hardened alloy to cracking causes problems when attempts are made to repair items, particularly when these have been in high temperature service and additional precipitation on the grain boundaries has occurred.

Little can be done to overcome this problem apart from a full solution heat treatment but this is often not possible with a fully fabricated component. If repair is to be attempted, small weld beads and controlled low heat input welds are recommended.

If the design permits, a low strength filler metal, eg Alloy 200 or 600, may be used to reduce the risk. Buttering the faces of the repair weld preparation, sometimes combined with a peening operation, has been successful.

Many of the nickel alloy filler metals have been used for making dissimilar metal joints with excellent results; dilution when welding joints between ferritic, stainless and duplex steels being less important than when using a type 309 stainless steel filler.

Nickel also has a coefficient of thermal expansion between that of ferritic and austenitic steels and therefore suffers less from thermal fatigue when high temperature plant is thermally cycled. Alloy 625 has been a popular choice, the weld tensile strength matching or exceeding that of the parent metal.

There are limitations to this approach, and caution needs to be exercised when selecting a suitable filler.

For example, Alloy 625 has been extensively used for welding dissimilar joints in austenitic and duplex steels.

Use of this filler metal has resulted in the formation of niobium rich precipitates adjacent to the fusion line and has been discontinued. Alloy 59 or C22 filler metals has replaced Alloy 625 as the filler of choice.