Effect of heat treatment on the structural transformations and properties of high-nitrogen chromium steels

We established the dependences of the phase composition of high-chromium (18% Cr) steels on the content of nitrogen. As the nitrogen content increases from 0.4 to 1.2% the proportion of untransformed austenite increases and that of martensite decreases respectively. This changes the structural class of the steels from martensitic (at 0.4% N) to austenitic (at 1.2%).

In heating of a quenched steel with 18% Cr and 0.4–1.2% N we determined two temperature ranges of structural transformations that correspond to the segregation of chromium nitrides from the initial martensite (A _s−A _f) and from austenite (B _s−B _f).

With the growth in the chromium concentration from 15 to 24% at 1–1.3% N quenching yields a stable austenite structure preserved even after tempering at 700°C. After heating the quenched steel to a temperature corresponding to the pointB _f, the segregation of nitrides from the austenite causes the formation of martensite upon cooling and hence the growth in hardness.

Tempering of steels with an initial structure of martensite or austenite + martensite is accompanied with growth in the hardness due to dispersion hardening of martensite. The hardening is maximum (650–690HV) at a tempering temperature of 500–600°C. The highest level of hardening (649HV) has been observed in steel Kh18A4 quenched from 1200°C and tempered at 600°C.