In-situ synchrotron X-ray diffraction during quenching and tempering of SAE 52100 steel

The quenching and tempering process of SAE 52100 bearing steel was continuously monitored via in-situ dilatometry and high-energy X-ray diffraction, mapping the evolution of the constituent phases and lattice distortions and their direct correlation with observed dimensional changes of the material. During quenching from an austenitisation temperature of 860–50 °C, there was a continuous increase in lattice distortion in the parent austenite. Below the martensite start temperature of M_S = 209 ± 15.0 °C, the c - lattice parameter and lattice distortion of the formed martensite initially decreased, then subsequently presented an increasing trend. An increase in sample length was detected only at martensite fractions ≥ 18 vol%, and occurred simultaneously with an increase in the c_α’/a_α’ tetragonality ratio. An increase in sample length was detected during isothermal holding at 50 °C with a reduction in the martensite c_α’ parameter, potentially due to the expulsion of carbon. Tempering promoted the loss of martensite tetragonality at 283 ± 7.80 °C. At a tempering temperature of 340 °C, the austenite retained after quenching transformed completely within 1.5 min. The following reduction in sample length and expulsion of carbon from martensite is correlated to the relaxation of lattice distortion in the martensite-bainitic ferrite matrix.