Investigation of the effect of the changed initial microstructue on the mechanical properties of 41Cr4 steel by pre-heat treatment

In this study, the effects of the pre-heat treatments which applied before the conventional quenching and tempering process on changes in the initial microstructure and on the final microstructure and mechanical properties of 41Cr4 steel were investigated. Double austenitisation heat treatment requires twice austenitizing and quenching before tempering. Due to the application of pre-quenching from high temperature austenitization in first step, second austenitization step is started from a martensitic microstructure which containing refined carbides lead to reduced the prior austenite grain size carbide size in the final microstructure. In the pre-spheroidizing heat treatment, the conventional quenching and tempering process is started from the spheroidized ferrite/perlite microstructure. In the quench and tempering process starting from the spheroidized ferrite/perlite microstructure, the prior austenite grain sizes are reduced and there are coarse and spherical carbides in the final microstructure. Conventional quench and tempering treatment with similar parameters conducted after the pre-quenching and pre-spheroidizing processes. In the study, quench and tempering treatment starting from the ferritic-pearlitic microstructure was accepted as a reference. The effects of the pre-heat treatments on the prior grain size and the carbide size in the final microstructure were investigated. In addition, the effects of the tempering temperature parameter in the double austenitizing heat treatment were examined. Evolution in mechanical properties depending on the developments in the final microstructure investigated in terms of hardness, strength, toughness, wear resistance and hardenability properties. It was determined that the pre-quenching and pre-spheroidization heat treatments provided a similar reduction in the prior austenite grain size. It has been observed that small sized carbides in the microstructure are dispersed at a higher frequency with the double austenitization heat treatment compared to the conventional quench and tempering process. Spherical and large sized carbides dispersion observed at a higher frequency in the microstructure after pre-spheroidization heat treatment. It was found that the quench and tempering treatment after the pre-heat treatments which applied under similar tempering temperature conditions did not cause a significant change in the Vickers hardness values, while the high tempering temperature in the double austenitizing process affected the hardness in a decreasing way. Double austenitization and pre-spheroidization heat treatments significantly increased the tensile and yield strengths of 41Cr4 steel, but caused a slight decrease in ductility. It was observed that the changes observed in the prior austenite grain sizes and carbide sizes did not occur at the extend that would affect the Charpy impact energy in 41Cr4 steel. However, it was determined that the double austenitizing heat treatment applied under high tempering temperature conditions greatly increased the Charpy impact energy. When the wear ball volume and friction coefficients are considered, similar effects are observed, but the most advanced tribological properties are seen after the double austenitization heat treatment applied at low tempering temperature. Similar hardenability characteristics were observed in all applied heat treatment conditions.