Low-temperature martensitic transformation in tool steels in relation to their deep cryogenic treatment

The low-temperature martensitic transformation in steel X153CrMoV12 containing (mass%) 1.55C, 11.90Cr, 0.70V, 0.86Mo is studied using dilatometry, Mossbauer spectroscopy, X-ray diffraction, mechanical spectroscopy and transmission electron microscopy. Some additional measurements were carried out on steel X220CrMoV13-4. It is shown that, in contrast to the widely known absence of martensitic transformation during deep cryogenic treatment, this transformation occurs with isothermal kinetics within the temperature range of 100 down to 170 degrees C with its largest intensity near 150 degrees C. No transformation is observed at 196 degrees C. The remarkable features of the isothermal martensitic transformation are: (i) the plastic deformation, which is explained by the absence of ageing of martensite at low temperatures; and (ii) the abnormally low tetragonality of martensite. In contrast to existing interpretations, the abnormally low c/a ratio is interpreted in terms of the capture of immobile carbon atoms by gliding dislocations during plastic deformation at low temperatures. A recommendation is proposed for optimizing the deep cryogenic treatment of tool steels. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.