Duplex stainless steels are those that are composed
of a mixture of austenite and ferrite, most
typically with each phase in the range of 25 to
75%. The common wrought austenitic stainless
steels, such as types 304 and 316, are usually balanced
to contain approximately 5% ferrite to
facilitate welding, but this level would not be
sufficient for the steel to be classified as a duplex
stainless steel. The common cast stainless steels,
such as CF-8 and CF-8M, are mostly austenite
with some ferrite, perhaps 10 to 15% to aid in
avoiding hot cracking during solidification, but
these steels also would not typically be called
duplex stainless steels. These steels are often
considered to be simple analogs of wrought
alloys with similar compositions; however, they
do not always have the same response to heat
treatment. The corrosion evaluation of these
alloys deserves further study.