Precipitates in Tempered Stainless Martensitic Steels Alloyed with Nitrogen, Carbon or Both

Abstract

The precipitation in martensitic steels alloyed with (wt. %) ∼15Cr, ∼IMo, 0.62N or 0.6 %C or 0.29C+0.35N is studied after tempering in the range of 100-650°C by means of transmission electron microscopy (TEM), Mossbauer spectroscopy (MS), internal friction (IF) and dilatometry. It is shown that the substitution of carbon by nitrogen shifts the precipitation towards higher temperatures and expands the formation of stable interstitial phases over a broad temperature range. TEM studies reveal the precipitation of orthorhombic ξ-nitride and hexagonal e-nitride in the nitrogen martensite while cementite and e-nitride are formed in the nitrogen+carbon martensite. A higher mobility of dislocations in the nitrogen martensite was observed by means of IF. The fraction of retained austenite (RA) in as-quenched martensite is increased in the order: C→N→C+N, and the stability of RA against decomposition during tempering is changed in the same order. According to MS data, the distribution of chromium atoms is characterised by a tendency to clustering in the carbon martensite and to short range ordering in the nitrogen and nitrogen+carbon martensites. The different precipitation behaviour of martensites during tempering is explained by the nitrogen effect on iateratomic bonding and short range atomic order in solid solutions.