Austenite Grain Ultra-refinement of 42CrMo Steel Induced by Electropulsing Treatment

Abstract

With the increasing demands for steel products with both higher strength and toughness to meet the requirements for weight reduction and security promotion, steels with fine grained microstructures cannot meet the application demands any more, and should be further strengthened without toughness or other property loss, such as fatigue lifetime. Ultragrain refinement is a promising way to strengthen steels without toughness loss. For steels with the final bainite, martensite or tempered martensite structures, microstructure refinement basically depends on the prior austenite grain size reduction. In this study, electropulsing treatment (EPT) was used to refine the austenite grain, because pulse current shows remarkable influence on microstructure of metallic material. Results in this study confirm that the prior austenite grains (PAGs) can be ultra-refined by pulse current. When the steel was quenched from peak temperature less than about 850°C during EPT, the average PAGs size was less than 5μm. With the increasing of peak temperature during EPT, the PAGs became coarsening. But the PAGs in samples quenched from peak temperatures less than 1000° were still finer than that quenched from recommend 850° during conventional heat treatment (CHT). EPT induced grain refinement is related to the thermal effect and athermal effect of high density pulse current. The thermal effect results in high heating rate as high as 7700°C/s. In order to analysis the influences of rapid heating effect and athermal effect of pulse current itself on the grain refinement, J Mat Pro software was used to calculated the PAGs size at the same heating rate as that during EPT, which represented the nucleation rate and grain size only under the influence of thermal effect related to rapid heating. However, the simulated result shown the PAGs cannot be ultra-refined during simplex rapid heating. This reveals that the athermal effect of high density electric current itself plays a key role in the EPT induced austenite grain ultra-refinement.