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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, 2 s 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 360 s 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.
Highlights
Over the last years the research focus was on both strength and toughness increase of advanced high strength steels (AHSS)
Scope of this paper is to identify, characterize and compare qualitatively, and quantitatively where possible, the microstructures of a chromium molybdenum (CrMo) medium carbon steel formed after conventional heating, isothermal soaking and quenching and the one after heating with 300 ◦ C/s and soaked for maximum of 2 s at the austenitization peak temperature
M23 C6 carbide was not considered to affect the α to γ chemical composition of the phases was determined with Thermocalc® at equilibrium by taking the transformation the results of M23 C6 dissolution were excluded for simplicity
Summary
Over the last years the research focus was on both strength and toughness increase of advanced high strength steels (AHSS). The reason is that the ferrite to austenite phase transformation during ultra-fast heating creates chemically and structurally (morphologically) heterogeneous austenite with very fine grains which transforms after quenching to mixtures of bainite & martensite and finely dispersed carbides [10]. The ultra-fast heat treatment e.g., via induction heating utilizes the effect of the rapid heating to restrict the austenite grain size and its chemical composition prior to quenching In this way, a mixed martensite/bainite microstructure matrix with embedded carbides can be obtained in one step paving the way for increased and more isotropic mechanical properties of the treated steel. Thermocalc and Dictra modelling results are presented and discussed
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