Abstract
The diffusion of carbon atoms from martensite to retained austenite (RA) is controlled by the carbon partitioning kinetics when the quenching and partitioning (Q&P) process is conducted. The RA is divided into film-like and blocky ones in morphology. This research aims to study the influence of the morphology of RA on the kinetics of carbon partitioning mainly by developing a numerical simulation. A one-step Q&P process was modeled at the partitioning temperature of 330–292 °C, with a partitioning time ranging from 10−6 to 5 × 103 s. The finite element method was employed to solve the carbon diffusion equation. A thermomechanical simulator Gleeble-3500 was used to conduct the corresponding Q&P heat treatment, and the RA was examined by X-ray diffraction. The results show that the film-like RA will be enriched in carbon within a short time at first, followed by a decrease in carbon concentration due to the massive absorption of carbon by blocky RA, leading the stable film-like RA to become unstable again. The end of the kinetics of carbon partitioning was the concentration determined by the constrained carbon equilibrium (CCE) model, provided that the CCE condition was employed in this study. It took quite a long time (thousands of seconds) to complete the carbon partitioning globally, which was influenced by the partitioning temperature.
Highlights
Quenching and partitioning (Q&P) is a promising technique to produce steels with excellent mechanical performance [1,2]
The partitioning of carbon from martensite to untransformed austenite during the partitioning process was simulated by employing the finite element method with the governing equation based on the chemical potential of carbon in austenite and martensite
From the results presented in this study, the following conclusions can be drawn:
Summary
Quenching and partitioning (Q&P) is a promising technique to produce steels with excellent mechanical performance [1,2]. The Q&P steel is first quenched to a temperature below the martensite start (Ms ) temperature to obtain a desired volume fraction of martensite. The steel is held at or above this temperature for carbon atoms diffusing from the martensite to untransformed austenite [3]. The untransformed carbon-enriched austenite will remain stable after the secondary quenching to room temperature [4]. The retained austenite (RA) will transform to martensite when subject to deformation [5], enhancing the ductility and toughness of the Q&P steel [6,7]. Obtaining substantial stable RA at room temperature is essential to achieve high strength without compromising the toughness [8,9,10]
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