Abstract
Abstract In this paper, the effect of chemical inhomogeneity on the isothermal bainite formation is investigated in medium-carbon low-silicon spring steel by dilatometry and microscopy. The analysis of the microstructure at different times during transformation shows that chemical segregation of substitutional alloying elements resulting from casting strongly affects the bainite formation by retarding the transformation kinetics and limiting the maximum achievable bainite fraction. During holding at temperatures close to and above the martensite start temperature, a homogeneous lower bainitic microstructure can be eventually obtained, whereas at higher temperatures, incomplete bainitic reaction is evident. It was also found that at the early stages of the transformation, differences in the bainite formation kinetics, due to local inhomogeneities in Cr and Mn concentration, result in retardation of the growth of bainite in the high Mn and Cr concentration regions. The calculated difference in driving force for nucleation between the enriched and the depleted areas is not by itself sufficient to explain the microstructures obtained and thus significant influence of growth on bainite formation is observed. Particularly, it was calculated and experimentally observed that Cr partitions in the carbides in the high Mn, Cr regions during the isothermal treatment, limiting the transformation kinetics.
Highlights
IN the past decades, the continuous market demand for improving vehicle performance and the rising awareness about CO2 emission and energy consumption created the need to develop new, more energy efficient production processes that include commonly used heat treatments
After isothermal holding at the lowest temperature, the dilatometric signal is linear during the final quench, indicating the absence of transformation in that segment
An isothermal treatment was applied to 51CrV4 steel grade as an alternative to quenching and tempering
Summary
IN the past decades, the continuous market demand for improving vehicle performance and the rising awareness about CO2 emission and energy consumption created the need to develop new, more energy efficient production processes that include commonly used heat treatments. This research is related to the definition of alternative heat treatments to produce automotive springs. Low-alloyed, medium- or high-carbon steels are the most commonly used materials for automotive spring production. Research on spring steels started to concentrate on springs with bainitic microstructures, instead of the commonly used tempered martensitic microstructures. Bainitic microstructure offers in principle significant advantages over the most commonly used tempered martensite. First of all, it offers a fine microstructure with high strength and acceptable ductility. The fully bainitic microstructure has a lower crack growth rate than tempered martensite
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