Abstract
Low-carbon, low-alloy steels undergo auto-tempering and carbon partitioning to austenite during quenching to martensite. The microstructures of two such steels quenched at two cooling rates have been evaluated using electron microscopy to characterise lath and carbide precipitate morphologies, and the results have been compared with theoretical predictions based on the Thermo-Calc modules DICTRA and TC-Prisma. The modelling tools predicted the carbon depletion rates due to diffusion from the bcc martensite laths into austenite and the precipitation of cementite in the ferrite matrix. The predictions showed a satisfactory agreement with the metallographic results, indicating that the Thermo-Calc based software can aid in the design of new low-carbon martensitic steels.
Highlights
Low-carbon martensitic steels have become important structural materials as they provide economical solutions in weight-critical applications by maximising their performance-to-weight ratio and thereby reducing their carbon footprint [1]
The relatively dark regions of the martensite that have transformed at low temperatures are seen to contain very small precipitates throughout
The effect of carbon content, the lath thickness, and cooling rate on the incidence of cementite could be rationalised using the Thermo-Calc Prisma and DICTRA software to predict the rate at which carbon is depleted from the laths during quenching via either cementite precipitation or diffusion to the surrounding austenite
Summary
Low-carbon martensitic steels have become important structural materials as they provide economical solutions in weight-critical applications by maximising their performance-to-weight ratio and thereby reducing their carbon footprint [1]. Such low-carbon martensitic steels are normally produced by fast cooling techniques to achieve the desired displacive martensitic transformation [2]. Due to the supersaturation of carbon in the martensite, there is a large driving force for the carbon to precipitate as a carbide [4,5] This phenomenon is called auto-tempering [2]
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