Welding Inspection & Metallurgy API ICP Self Study Notes

The most common measure of weldability and susceptibility to hot cracking is
the ferrite number of the weld metal. Austenitic welds require a minimum
amount of delta ferrite to resist cracking. The amount of ferrite in the weld
metal is primarily a function of both base metal and weld metal chemistry.
For welds made without filler metal, the base metal chemistry should be
appropriate to produce the small amounts of ferrite that is needed to prevent
cracking. If the base metal chemistry will not allow for ferrite formation, then
filler metal is recommended to produce adequate ferrite in the weld metal.
Welding parameters and practices can also effect ferrite formation. For
example, small amounts of nitrogen absorbed into the weld metal can reduce
ferrite formation. WRC Bulletin 342 contains diagrams that accurately predict
the amount of ferrite present in a weld metal based on the calculation of
nickel and chromium equivalents based on weld metal and base metal
chemistry. A number of resources recommend a minimum of 5% ~ 20% ferrite
to prevent cracking.