Novakova

The authors established the influence of preliminary heat treatment (annealing at 950°C, 0.5 hrs, cooled with furnace + tempering at the interval of 500-700°C) on hardness of the nitrided layer of steel 38H2MYUA (nitriding at 500°C, 30 h). Similar results were obtained in a model of carbon-free alloy Fe-4% Cr. By the method of nuclear gamma resonance spectroscopy (the Mossbauer effect) on the model alloy it was established that Fe-Cr clusters are formed during the cooling process starting from 950°C and the subsequent tempering. Clustering affects the size and number of particles of chromium nitride formed during subsequent nitriding at 500°C. Depending on the heat treatment before nitriding, Fe-Cr clusters are characterized by different filling of chromium atoms of the first and second coordination spheres of iron. According to the Mössbauer spectra after annealing at 950°C and tempering at 550°C, during the following 10 hrs before nitriding in the structure of the solid solution clusters are formed, and they correspond to the crystalline structure of  the σ-phase Fe-Cr system: Frank-Kasper polyhedra with 14 vertices.