Dispersion strengthening austenitic stainless steels by nitriding

Abstract

The internal nitridation of thin sections of austenitic Fe−Cr−Ni−Ti alloys containing up to 2 pct Ti was studied over the temperature range 1600° to 2210°F in order to develop a method of strengthening the alloys through the introduction of a dispersoid of stable titanium nitrides. The interparticle spacing (IPS) of the nitrides was found to increase linearly with depth from the external surface; the effects of various parameters on the rate of change of IPS vs depth are presented. The mechanical properties of these alloys at room and elevated temperatures were markedly improved by internally nitriding. Useful mechanical properties were obtained up to 2200°F, with typical properties at 2000°F of 10 to 20 ksi 0.2 pct offset yield strength and 15 to 25 ksi ultimate tensile strength, but section thickness was limited to about 10 mils because of the increase in IPS with depth and the long nitriding times needed for thicker material. In order to produce a small interparticle spacing in a heavier section, internally nitrided 5 mil strip was consolidated by hot roll bonding and evaluated at a 60 mil thickness by tensile and rupture testing at 2000°F. It is demonstrated that the approach taken in this work offers a feasible technique for making a high temperature alloy having useful engineering properties.