Effect of Ultra-Fast Heat Treatment on the Subsequent Formation

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
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.
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