Effect of Ultra-Fast Heat Treatment on the Subsequent Formation of Mixed Martensitic/Bainitic Microstructure with Carbides in a CrMo Medium Carbon Steel

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.

The Effect of Ultra-Fast Heating on the Microstructure, Grain Size and Texture Evolution of a Commercial Low-C, Medium-Mn DP Steel

“…From the EDXS analysis, it is seen that alloying elements such as manganese and chromium are concentrated in the cementite of the microstructure. This is in accordance with the results of Papaefthymiou et al [9] who predicted the accumulation of these elements in cementite by simulation using Thermo-calc and Dictra software. It is seen from the EDX line scans in Figure 5c,d that the calculated mass % intensity for Cr and Mn was higher in the cementite lamellae and spheres compared to that of the adjacent ferritic matrix.…”

“…In Figure 6e,f, the grain average image quality map is shown in grayscale with different misorientation angles included. According to [9,13], misorientation angles between 17-47 • (black) correspond to ferrite, 48-55 • (red) correspond to bainite and 56-65 • (blue) correspond to martensite. From the supporting charts in Figure 6g,h, bainitic grains were also expected in the microstructure.…”

“…This heating treatment was first introduced by Cola et al and Lolla et al [5][6][7][8] and since it has been thoroughly studied over the last years. Papaefthymiou et al [9] have shown that the UFH of medium carbon CrMo steels results in a multiphase fine microstructure consisting of martensite, bainite, undissolved cementite and retained austenite with grain average diameter less than 2 µm. 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 transform after quenching to mixtures of bainite and martensite, while finely dispersed carbides remain partly dissolved or in their initial condition [10].…”

“…Therefore, depending on the heating rate and the peak temperature, different fractions of recovered ferrite are expected in the microstructure. Concerning the grain size, according to Papaefthymiou et al [9], significant heterogeneity is expected in the size of the prior austenite grains (PAGs). As the γ to α transformation takes place in the PAG boundaries depending on their orientation relationship [28], smaller PAGs will lead to increased fraction of grain boundaries and thus increased possible nucleation sites for the γ to α (martensite/bainite) transformation.…”

Low and Medium Carbon Advanced High-Strength Forging Steels for Automotive Applications

“…The retained austenite suppresses the crack initiation or the void formation and subsequent void coalescence during impact tests via the plastic relaxation of localized stress concentration and an increase in carbon-enriched hard martensite fraction resulting from the strain-induced transformation [32,33]. Ultra-fast heating before hot-forging is also effective for grain refining [67]. Sugimoto et al [63,68] report that a low carbon TBF steel achieves the excellent balance of mechanical properties as shown in Figure 8.…”