Effect of Quenching and Tempering Temperatures on Microstructure and Properties of Ultrahigh Strength Cast Steel

Low‐alloy cast steel with ultrahigh strength is produced by a combination of normalizing and reheating, followed by water quenching and high‐temperature tempering. The cast steel quenched at 1000 °C for 30 min and tempered at 550 °C for 120 min exhibits superior tensile ductility. More precisely, the tensile strength is 1683.5 MPa, yield strength is 1635.3 MPa, elongation is 12.0%, and microhardness is 500.3 HV. The microstructure is composed of martensite lath + ferrite + M 23 C 6 carbide. The strengthening mechanisms of the steel are mainly precipitation, grain refinement, and dislocation strengthening. When the quenching temperature is increased from 950 to 1000 °C, the M 3 C carbides precipitated between the martensite lath are transformed from short rod‐like to spherical shapes, which improves the plasticity of the steel. When the tempering temperature is increased from 550 to 630 °C, the number and size of the carbides also increase. The M 23 C 6 carbides interact with the dislocations. M 23 C 6 carbides play a crucial role in the precipitation strengthening and efficiently improve the strength of the steel. The fracture is changed from a mixed ductile and brittle fracture to a ductile fracture.