From TWI.co.uk http://www.twi.co.uk/technical-knowl...alloys-part-1/
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 (heat affected zone) with the latter being the more frequent. Both weld metal and HAZ cracking are generally the result of contamination by grease, oil, dirt, etc left behind following inadequate cleaning;. [...] .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. [...G]as purity and the efficiency of the gas shield must be as good as possible. 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.

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.

INCOLOY alloy 800 has good weldability by all welding processes. Material to be welded must be thoroughly clean, and the proper joint designs must be used. [...]
For (TIG) gas-tungsten-arc, gas-metal-arc, and submerged-arc welding, INCONEL Filler Metal 82 is recommended. Alloy 625 has been extensively used for welding dissimilar joints in austenitic and duplex steels. Use of this filler metal has caused problems and has been discontinued. Alloy 59 or C22 filler metals has replaced Alloy 625 as the filler of choice.

For additional information, refer to the Special Metals publication “Joining” on the Special Metals website at www.specialmetals.com
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