Kinetics of Carbon Enrichment in Austenite during Partitioning Stage Studied via In-Situ Synchrotron XRD

Abstract

The present study reveals the microstructural evolution and corresponding mechanisms occurring during different stages of quenching and partitioning (Q&P) conducted on 0.6C-1.5Si steel using in-situ High Energy X-Ray Diffraction (HEXRD) and high-resolution dilatometry methods. The results support that the symmetry of ferrite is not cubic when first formed since it is fully supersaturated with carbon at the early stages of partitioning. Moreover, by increasing partitioning temperature, the dominant carbon source for austenite enrichment changes from ongoing bainitic ferrite transformation during the partitioning stage to initial martensite formed in the quenching stage. At low partitioning temperatures, a bimodal distribution of low- and high-carbon austenite, 0.6 and 1.9 wt.% carbon, is detected. At higher temperatures, a better distribution of carbon occurs, approaching full homogenization. An initial martensite content of around 11.5 wt.% after partitioning at 280 °C via bainitic ferrite transformation results in higher carbon enrichment of austenite and increased retained austenite amount by approximately 4% in comparison with partitioning at 500 °C. In comparison with austempering heat treatment with no prior martensite, the presence of initial martensite in the Q&P microstructure accelerates the subsequent low-temperature bainitic transformation.

Keywords: advanced high strength steels; high-carbon steel; high-resolution dilatometry; in-situ synchrotron XRD; martensitic/bainitic phase transformation; quenching and partitioning (Q&P).

Figure 1

Heat treatments (austempering, direct quenching, and quenching & partitioning) were applied in this study. FQ and SQ represent the first and second quench, respectively.

Figure 2

The evolution of the austenite lattice parameter, assuming that the alloy is fully austenitic in the temperature range. The mean slope is used to measure the thermal expansion parameter.

Figure 3

Dilation and XRD evolutions of the samples during the first quench to 165 °C: (a) relative change in length vs. temperature; (b) development of the different intensities of the lattice planes in austenite and martensite (colors from red to dark green represent highest to lowest intensities, respectively).

Figure 3

Dilation and XRD evolutions of the samples during the first quench to 165 °C: (a) relative change in length vs. temperature; (b) development of the different intensities of the lattice planes in austenite and martensite (colors from red to dark green represent highest to lowest intensities, respectively).

Figure 4

(a) Diffusivity of carbon into austenite, change in austenite carbon content as a function of $\frac{\mathsf{\Delta }{a}_{\gamma }}{A}$, percentage of bainitic ferrite and carbon depletion from bct phase during heating from the quenching temperature (165 °C) to the partitioning temperatures; (b) Full width at half maximum peak values (FWHM) evolution of two austenite peaks (202 and 200) showing heterogeneity during reheating stage.

Figure 5

Microstructural evolution of bainitic ferrite and austenite measured by XRD analysis during partitioning at (a) 280 °C, (b) 400 °C, and (c) 500 °C. (For interpretation of the colors in this figure legend, see the online version of this article).

Figure 6

X-ray diffraction line profile of austenite ${\gamma }_{200}$ during partitioning at (a) 280 °C and (b) 500 °C, showing bimodal and homogenous austenite phase compositions, respectively.

Figure 7

(a) Effect of partitioning treatment on martensite start temperature (M_s); (b) Effect of M_s temperature on retained austenite fraction.

Figure 8

Optical micrograph of the specimen partitioned at 280 °C for 900 s, etched with LePera etchant [58]. (For interpretation of the colors in this figure legend, see the online version of this article).

Figure 9

Evolution of ferrite 211 peak during isothermal holding at 280 °C for two states: (a) with partial quenching (Q&P treatment) and (b) without partial quenching (austempering treatment). In figure (a), intensity is logarithmic and colors from green to dark yellow show the lowest (1.70) and the highest (2.20) intensity, respectively. (For interpretation of the colors in this figure legend, see the online version of this article).