For austenitic stainless steels, when the Contract requires it for castings and for deposited weld metal. To avoid hot cracking, weld metal chemistry and casting chemistry is formulated to produce some ferrite in the austenitic matrix. Generally ferrite is limited by the end user in cryogenic service to assure low temperature impact properties of the deposited weld metal and in high temperature applications to mitigate sigma formation (embrittlement).
Another application of ferrite testing of austenitic stainless steels is to assure proper heat treatment of the bends following cold bending operations. Typically used in testing bends in boiler superheaters, rather then hardness testing.
Delta ferrite(expressed in Ferrite No, FN)is required in austenitic s.s weld deposit to prevent microfissuring and subsequent hot cracking of the welds.
A delta ferrite level of at least 3FN will eliminate fissuring in weld metal deposited from austenitic filler metals 308, 308L,316 and 316L. Ferrite level of 4FN is required in E309, 5FN in E318, and 6FN in E347 weld metal to assure freedom from fissures.
If you could purchase this document :- IX-1695-92- (IIW-1166-92,-IX-, Guidance on specifications of ferrite in stainless steel weld metal, you may find this of great help.This document explains "Ferrite in weld metal" under different design scenarios.
Also, delta ferrite is to be avoided for 316/316L in nitric acid service (if you are so inclined to use it in such service) as it accelerates formation of intermetallics such as the sigma phase. These rob chromium from adjacent areas of the SS and allow intergranular attack, akin to sensitized SS.
pradipgoswami, thanks for the hint on that IIW-paper.
Just registered at their website, and these papers are freely available then. Great resource, must say
@ OGMetEngr; isn't 304 a better MoC for most nitric acid services, due to presence of Mo in 316/316L? Im not a metallurgist at all, but this is what I recall from a line class index from a major oil 'n gas supplier.
