Effects of the inductive hardening process on the martensitic structure of a 50CrMo4 steel

Compared to conventional furnace heat treatment induction hardening offers a range of advantages such as fast production cycles, clean processing, high degrees of automation and reproducibility. However, due to the short austenitizing time, imperfect homogenization of the alloying elements, including carbon can occur, which might lead to unsatisfactory mechanical properties. The aim of the present work was to obtain an inductive heat treating route which results in the least internal stresses and the least distortion of the martensitic structure of a 50CrMo4 steel.

To this end two different prior microstructures (soft-annealed and ferritic pearlitic) were subjected to three different heating rates (1, 10 and 100 K/s) and three different austenitizing temperatures (950, 1000, 1050°C). The resulting martensitic microstructures were investigated and compared to a conventional furnace heat treating sequence, defined through heating at 10 K/s and holding at the austenitizing temperature of 850°C for 20 min. All samples were subsequently quenched in the same manner. The heat treatments were performed in a quenching dilatometer and the martensitic structure of the samples was characterized regarding hardness and microstructure. The analyses included the effect of variable heating rates during austenitization on the resulting martensitic microstructure including packet orientation and size through electron back scattered diffraction (EBSD) as well as lath width and structure through transmission electron microscopy (TEM). The internal stresses were observed via X-ray diffraction.

An optimum combination of heating rate and austenitizing temperature was developed for the chosen steel, which keeps up with the conventional process in regards to homogenization as well as internal stresses and distortion of the martensite.