Diffusion behavior of carbon and its hardening effect on plasma carburized M50NiL steel: Influences of treatment temperature and duration.

Diffusion behavior of carbon and its hardening effect on plasma carburized M50NiL steel were investigated systematically. The plasma carburizing was carried out in a mixture of gaseous acetone (30%) and hydrogen (70%) at temperatures of 400, 450, 500 and 550 °C for durations of 4, 8 and 12 h, respectively. Influences of treatment temperature and duration on properties of the carburized layer were investigated with optical microscope, atomic force microscope, X-ray diffractometer, micro- and nano-indenters, and pin-on-disc tribometer. Results of the tests indicate that cementite is the main phase in the surface layer of specimens carburized at 400 and 450 °C, while that in those carburized at 500 and 550 °C is mainly the carbon expanded martensite. The specimens with cementite in the surface layer show lower surface roughness, higher surface hardness and thus lower wear rate. The diffusion behavior of carbon and resultant microstructure evolution mechanism were studied, indicating that plasma carburizing is a diffusion-controlled process. Activation energies of carbon diffusion into martensite and cementite were calculated, which are 64.5 kJ·mol − 1 and 120.7 kJ·mol − 1 , respectively. The information on the activation energies provides basic data for tuning and predicting the surface properties of plasma carburized M50NiL steel. [ABSTRACT FROM AUTHOR]