Microstructural evolution and mechanical properties of 9Cr18 steel after thixoforging and heat treatment

The thixoforging of 9Cr18 steel was conducted in a designed set-up. A functionally graded material (FGM) with a hard surface and tough center was fabricated. The microstructural evolution inside and outside the solid/liquid boundary was investigated, respectively. The relationship between microstructure and mechanical properties was clarified. The results showed that phase transformation of solid particles was mainly determined by the alloying elements. Meta-austenite was retained after thixoforging. Martensite transformation might occur during certain kinds of subsequent heat treatment, which would be harmful to the inner strength due to the high hardness and carbide precipitation. The wear-resisting skin layer outside the solid/liquid boundary could be retained after heat treatment and the FGM structure improved the inner mechanical properties. The optimal heat treatment strategy for 9Cr18 thixoforging specimen was to temper at 550 °C for 2 h. The inner area exhibited a strong work hardening behavior and the ultra-compressive strength could reach 4680 MPa with the compressive strain of 53.2%. The high strength was attributed to the fact that the austenite structure was retained and the brittle eutectic structure was extruded outside the solid/liquid boundary.