PRECIPITATION DURING TEMPERING OF CHROMIUM-RICH IRON-BASED MARTENSITEALLOYED WITH CARBON AND NITROGEN

The structure and properties of iron-based Cr15Mo1 martensite with abo ut 0.3 wt.% C and N each are studied after tempering at 200 to 650 deg rees C by means of hardness measurements, dilatometry, transmission el ectron microscopy (TEM) and Mossbauer spectroscopy. The results are co mpared with the tempering behaviour of Cr15Mo1 (0.6% C) and Cr15Mo1 (0 .6% N) martensites. After quenching from 1100 degrees C the C + N allo y contains a high fraction of retained austenite (RA), a mixture of tw in and dislocation martensites and undissolved coarse cementite partic les. Tempering at 200 degrees C causes precipitation of epsilon-carbid e and epsilon-nitride with hexagonal close packed (HCP) lattices. A de lay of the following carbide/nitride transformations occurs as compare d to the pure nitrogen and carbon martensites. At 500 degrees C epsilo n-carbide transforms into cementite, while cubic CrN nitride is formed from epsilon-nitride. An increase of temperature up to 650 degrees C is accompanied by CrN --> Cr2N transformation, while the cementite par ticles coarsen. Chromium carbonitrides are not observed. An enrichment of cementite in chromium is observed after tempering at 650 degrees C while the chromium content in the coarse cementite globules retained after high temperature solution treatment is nearly equal to that in t he matrix. An evidence for nitrogen-assisted short range atomic orderi ng of the FeCrN and, in particular, FeCr(C + N) solid solutions is der ived from Mossbauer spectra, which means a more homogeneous distributi on of Cr atoms in the nitrogen containing martensites as compared to t hat in the FeCrC martensite. This peculiarity of chromium distribution is supposed to be a reason for a delayed precipitation and small size of the particles in tempered FeCrN and FeCr(C + N) martensites. RA is stable during tempering up to 600 degrees C. A significant splitting of dislocations obviously caused by segregation of the interstitials ( C, N) is observed in the RA tempered at 500 to 600 degrees C.