Strain hardening and plastic flow properties of nitrogen-alloyed Fe-17Cr-(8-10)Mn-5Ni austenitic stainless steels

In this study, the uniform plastic strain behavior of a series of high-nitrogen (0.13 to 1.0 wt% N) austenitic stainless steels was modeled using the modified Ludwik equation proposed by Ludwigson: sigma = K-1 epsilon(n1) + Delta, where Delta = e((K2+n2 epsilon). The following comments can be made regarding the plastic flow behavior: (1) the strain hardening rate increases proportionally with nitrogen; (2) deviation of the flow curves From the Ludwik behavior (i.e. Delta) increases with nitrogen, (3) the strength coefficient (K-1) increases proportionally with nitrogen; (4) increasing nitrogen results in small decreases in the true-strain at ultimate load and therefore has a negative effect on the strain hardening exponent, n(1); (5) true yield stress [exp(K-2)] increases proportionally to nitrogen. correlating with the engineering yield stress. (6) n(2) becomes more negative, but changes only slightly with large increases in nitrogen; and (7) the transition strain epsilon(L) decreases only slightly with increasing nitrogen indicating that higher nitrogen levels; in the materials studied, extend the range whereby the Ludwik relation accurately describes the flow behavior. The effect of nitrogen on the transition strain observed is contradictory to the notion that stacking fault energy decreases with increasing nitrogen. However, the transition strain exhibited by these materials (higher than most f.c.c. materials) is consistent with very low stacking fault energy f.c.c. materials.