Effect of heat treatment process on microstructures and mechanical properties of 9Cr steel

The phase transformation behaviors of 9Cr steel with the change of cooling rates were studied by thermal expansion test. The microstructures and mechanical properties of 9Cr steel with heat treatment at 860 and 1000℃ were observed by OM, SEM, TEM, XRD and measured by tensile test at room temperature. The results show that the ferrite/pearlite, bainite and martensite phase transitions take place in order with increasing the cooling rate. The critical cooling rate of martensite phase transformation is 1.6℃/s. After the heat treatment at 860℃, the microstructures of 9Cr steel are polygonal ferrite and lath martensite/bainite and 4% of retained austenite. After austenitizing at 1000℃, the grain size increases and the ferrite almost disappears. The character of lath becomes obvious. Compared with 9Cr steel austenitized at 860℃, the mechanical properties meet the mechanical property requirement of HL level of sucker rod steel which shows that 9Cr steel has a wide processing window.