Mossbauer study of tempering of the stainless martensitic steel alloyed with nitrogen and/or carbon

The effects of alloying with carbon, nitrogen and mixed carbon + nitrogen on the behavior of the retained austenite (RA) during tempering of martensitic Cr15Mol steel and distribution of chromium in the solid solution are studied. Samples of grades containing (in wt. lo) 0.6C, 0.6N, and 0.3C + 0.3N were solution treated at 1100 degrees C for 30 min, water quenched and tempered in the temperature range of 200-650 degrees C. The fraction of RA and its change during tempering were measured by means of transmission Mossbauer spectroscopy. The distribution of the chromium atoms in the quenched and tempered solid solutions was evaluated from integral intensities of the sextets belonging to iron atoms having different chromium neighborhoods. It is shown that substitution of carbon by nitrogen increases the stability of the chromium austenite in respect to martensitic transformation and to tempering. However, the most stable austenite is obtained due to the mixed C + N alloying. Alloying with nitrogen causes a more homogeneous distribution of the chromium atoms in solid solution, which can be characterized as short range atomic ordering while carbon assists clustering of the chromium atoms.