Investigation of non-uniformity of temperature distribution and phase transformation in spiral bevel gears during carburizing and quenching

Although extensive research has been done on distortion of heat-treated steel parts, our understanding of heat-treated distortion in spiral bevel gears is far from complete. The cooling characteristics and phase transformation of spiral bevel gears during carburizing and quenching and the effect of quench rate on the uniformity of temperature and phase transformation have been investigated using a medium-alloyed Cr–Ni steel. A new thermodynamic-based model of MS (the start temperature of martensitic transformation) has been proposed, which incorporates both the effects of austenite grain size and carbon content gradient to investigate the kinetics of martensitic transformation during quenching. The effect of austenite grain size on MS was considered in the critical driving force calculation model for martensitic transformation, which includes terms for interfacial energy, strain energy and Hall–Petch energy. In addition, a new method was proposed to characterize the non-uniformity of temperature and martensite distribution across the tooth of the spiral bevel gear. The finite element method was utilized to solve the coupled carbon concentration field, temperature field and metallurgical field. Results show that the complex geometry of spiral bevel gears results in a significant degree of non-uniformity of temperature and phase transformation during carburizing and quenching. Higher quenching medium temperature can reduce the non-uniformity of temperature distribution, while the effect on the volume fraction of martensite formed is almost negligible. Research findings presented have the potential to benefit further research on distortions of spiral bevel gears.