In situ X-ray diffraction investigations of martensitic transformation and austenite modifications during quenching of low alloy steels

In situ X-ray diffraction measurements were performed at the European Synchrotron Radiation Facility in Grenoble, France during quenching of two low alloy steel grades: the ball bearing steel AISI 52100 and the case hardening steel AISI 5120. Variations of the austenitizing temperature were used to achieve varying amounts of carbon in solid solution. Two-dimensional diffraction frames were recorded with a time resolution of 0.8 seconds per measurement during the complete heat treatment cycles. The integrated diffraction frames were analyzed in order to determine the temperature- and time-dependent evolution of phase contents and lattice parameters. The carbon content in solid solution has been determined by analyzing the austenite lattice parameter at the end of the soaking period. The martensitic phase transformation kinetics could be precisely described. With the help of the determined temperature dependent lattice parameters, different effects were investigated during cooling. It appeared that the austenite lattice parameter does not follow only a linear contraction during cooling but that a change of slope occurs at temperatures below Ms. In the case of the ball bearing steel with austenitizing at high temperature, an accelerated decrease takes place, which could be attributed to the generation of high compressive hydrostatic stresses. On the other hand the case hardening steel exhibits a slower decrease of the austenite lattice parameter below Ms. A carbon enrichment of the austenite could be identified, what also explains the (high) amount of 5 Mass.-% retained austenite at room temperature due to a dynamic stabilization. For the ball bearing steel austenitized at low temperature, a competition of both effects takes place.