The Tempering of Iron–Nitrogen Massive Martensite

Abstract

The first stage of tempering of body-centred cubic iron–nitrogen martensites containing 0·18, 0·35, and 0·51 wt.-%N has been investigated using electrical-resistivity measurements, X-ray and electron diffraction, and thin-film electron microscopy. Transformation of the martensite took place below 200° C (475 K) to ferrite and the body-centred tetragonal nitride, Fe₁₆N₂. Thin-film electron-microscopy studies showed that the nitrides form approximately parallel ribbons, which are initially coherent and are distributed more or less uniformly in the lath substructure of the massive martensite. Electron-diffraction and trace-analysis experiments revealed that the nitrides precipitate with {100}_α′ habit planes in 〈100〉_α′ directions. A dislocation-attraction model due to B.S. Lement and M. Cohen (Acta Met., 1956, 4, 469) has been extended to cover iron–nitrogen alloys and tested using the resistivity data obtained. The expected time exponent of 1/3 was observed for first-stage tempering after nucleation and coherent growth were complete. The model predicts values for the activation energy for the diffusion of nitrogen in martensite that are close to the activation energy for the diffusion of nitrogen in ferrite and also a value for the distribution of nitrides that is in good agreement with experiment.