Heat Treatment of a 56SiCr7 Spring Steel

Abstract

This bachelor’s thesis deals with the heat treatment of 56SiCr7 spring steel with emphasis on the relationship between processing parameters, transformation behaviour, resulting microstructure, and mechanical properties. The work focuses on the effect of quenching and tempering conditions and on the comparison between industrially processed material and laboratory-treated dilatometric specimens. The experimental part included characterization of the as-received material for leaf spring production, proposal of selected quenching and tempering regimes, dilatometric analysis, hardness testing, optical microscopy, transmission electron microscopy, and chemical analysis by EDX in SEM. In addition, cooling simulations were used as a supportive tool for interpreting the expected transformation tendency under different cooling rates. The results showed that the laboratory heat treatment conditions led predominantly to martensitic and tempered martensitic microstructures, while the lower cooling rate increased the tendency to local heterogeneity with the occurrence of Widmanstätten ferrite. The comparison of tempered specimens confirmed the expected effect of tempering temperature on hardness and microstructural appearance. Metallographic and EDX analyses further showed that local chemical heterogeneity, especially silicon enrichment in segregated regions, played an important role in the formation of high-temperature ferrite particles and in the overall transformation behaviour. A clear difference was observed between the industrially processed main part and the laboratory-treated specimens. This indicates that industrial heat treatment data may not always fully represent the actual thermal history of the material. The thesis confirms that proper evaluation of 56SiCr7 spring steel requires a combined interpretation of process, structure, and properties, with attention to local segregation and real heat treatment conditions.