Stainless steel, precipitation-hardened and cold-drawn wire

S. Paetke and A. R. Waugh

17-7 PH is a stainless steel that in its soft condition is mostly austenitic with some delta-ferrite. The chemical composition of the steel studied here is Fe-16.9Cr-7.02Ni-1.17Al-0.54Si-0.66Mn-0.074C wt%. It can then be heated to a temperature (750°C) where some of the chromium and carbon precipitates as M₂₃C₆, thus causing a substantial amount of the original austenite to transform into martensite. Subsequent tempering at 475°C leads to the formation of ordered NiAl with the resulting strengthening making the material suitable for corrosion resistance springs after cold drawing.

There are some more details available in the proceedings of Solid-solid phase transformations, 1981, p. 769.

Photographs courtesy of Dr Sally Waugh.

As-drawn wire

Stainless steel, precipitation-hardened and cold-drawn wire, metallurgy, physical metallurgy

17-7 PH strainless steel, as drawn in the softened condition. Chemical composition profile as a function of depth, determined using the atom probe.

As-drawn stainless steel wire, field ion image	Corresponding Fe ion image	Corresponding aluminium iron image
Corresponding nickel ion image	Corresponding chromium ion image. The images show essentially that solutes are more or less homogeneously distributed.

After precipitation hardening

Atom probe field-ion micrographs after ageing the steel for 75 min at 475°C. There are early signs of NiAl precipitation together with M₂₃C₆. (additional images)

Atom probe field-ion micrographs after ageing the steel for 750 min at 475°C. The precipitates are larger and there There are early signs of NiAl precipitation together with a large M₂₃C₆ particle. (additional images)