Effects of subsequent treatments on the microstructure and mechanical properties of SKD11 tool steel samples processed by multi-stage thixoforging

Abstract

In this study, multi-stage thixoforging and subsequent heat treatments of SKD11 tool steel samples were physically simulated by experiments. The microstructure and mechanical properties of various regions in the multi-stage thixoforged SKD11 samples before and after different heat treatments were investigated. The effects of the heat treatments on the former solid and liquid areas of the multi-stage thixoforged SKD11 sample were quite different, because the contents of alloying elements and microstructural morphologies in these areas were different. Static recrystallization of austenite, precipitation of carbides, growth of austenite, dissolution of carbides, and martensitic transformation occurred sequentially in the former solid areas of the multi-stage thixoforged SKD11 sample during annealing, quenching, and tempering. The values of the residual stress and volume fractions of austenite and martensite in the former solid areas changed significantly by the above microstructural evolution sequence. However, heat treatments slightly affected the volume fraction, distribution, and morphology of the carbides in the former liquid areas of the multi-stage thixoforged SKD11 sample. The mechanical properties of the centre regions of the multi-stage thixoforged SKD11 samples were adjustable during heat treatment because of the low volume fraction (approximately 10%) and dispersed distribution of the carbides in these regions. It is difficult to improve the mechanical properties of the edges of multi-stage thixoforged SKD11 samples by heat treatments owing to the high volume fraction (approximately 45%) and net-shaped morphologies of the carbides in these regions because they were abundant in the former liquid phase.