Effect of Ultra-Fast Heat Treatment on the Subsequent Formation of Mixed Martensitic/Bainitic Microstructure with Carbides in a CrMo Medium Carbon Steel, Metals 2019, 9, 312; doi: 10.3390/met9030312
Spyros Papaefthymiou, Alexandros Banis, Marianthi Bouzouni, Roumen H. Petrov
Abstract
The current work focuses on complex multiphase microstructures gained in CrMo medium
carbon steel after ultra-fast heat treatment, consisting of heating with heating rate of
300°C/s, 2s soaking at peak temperature and subsequent quenching. In order to better
understand the microstructure evolution and the phenomena that take place during rapid
heating, an ultra-fast heated sample was analyzed and compared with a conventionally
treated sample with a heating rate of 10°C/s and 300s soaking. The initial microstructure of
both samples consisted of ferrite and spheroidized cementite. The conventional heat
treatment results in a fully martensitic microstructure as expected. On the other hand, the
ultra-fast heated sample shows significant heterogeneity in the final microstructure. This is a
result of insufficient time for cementite dissolution, carbon diffusion and chemical
composition homogenization at the austenitization temperature. Its final microstructure
consists of undissolved spheroidized cementite, nano-carbides and martensite laths in a
ferritic matrix. Based on EBSD and TEM analysis, traces of bainitic ferrite are indicated. The
grains and laths sizes observed offer proof that a diffusionless, massive transformation takes
place for the austenite formation and growth instead of a diffusion-controlled
transformation that occurs on a conventional heat treatment.