Abstract
In this study, UltraFast Heat Treatment (UFHT) was applied to a soft annealed medium carbon chromium molybdenum steel. The specimens were rapidly heated and subsequently quenched in a dilatometer. The resulting microstructure consists of chromium-enriched cementite and chromium carbides (in sizes between 5–500 nm) within fine (nano-sized) martensitic and bainitic laths. The dissolution of carbides in austenite (γ) during ferrite to austenite phase transformation in conditions of rapid heating were simulated with DICTRA. The results indicate that fine (5 nm) and coarse (200 nm) carbides dissolve only partially, even at peak (austenitization) temperature. Alloying elements, especially chromium (Cr), segregate at austenite/carbide interfaces, retarding the dissolution of carbides and subsequently austenite formation. The sluggish movement of the austenite/carbide interface towards austenite during carbide dissolution was attributed to the partitioning of Cr nearby the interface. Moreover, the undissolved carbides prevent austenite grain growth at peak temperature, resulting in a fine-grained microstructure. Finally, the simulation results suggest that ultrafast heating creates conditions that lead to chemical heterogeneity in austenite and may lead to an extremely refined microstructure consisting of martensite and bainite laths and partially dissolved carbides during quenching.
Highlights
The kinetics of carbide dissolution during austenitization has been thoroughly studied under isothermal conditions [1,2,3,4,5,6,7,8,9]
The purpose of the present work is to study the mechanisms of the phase transformation associated with the carbide dissolution and austenite formation that take place during ultrafast heating, and to correlate them with the formation of mixed microstructures obtained after subsequent quenching
The initial microstructure in the soft–annealed condition consists of spheroidized carbides homogeneously a ferritic matrix size within ranging2–10 within μm 4a)
Summary
The kinetics of carbide dissolution during austenitization has been thoroughly studied under isothermal conditions [1,2,3,4,5,6,7,8,9]. The formation of austenite is a structure sensitive process; the initial microstructure plays a crucial role in the formed austenite. It is well known that in the presence of a ferritic (α)-pearlitic (α − α + Fe3 C) initial microstructure, the austenite (γ) nucleates at ferrite/cementite interface in pearlite. In case of spheroidal cementite carbides, austenite will first nucleate at the junction between a carbide particle and two ferrite grains. The dissolution of carbides continues by carbon diffusion in austenite until either the carbide is dissolved or the austenite achieves its equilibrium carbon content [4,5]
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