Evolution of carbides and carbon content in matrix of an ultra-high carbon sintered steel during heat treatment process

DTA, thermal expansion, XRD, and SEM were used to evaluate the effect of quenching temperature on the mechanical properties and microstructure of a novel sintered steel Fe-6Co-1Ni-5Cr-5Mo-1C. Lattice parameters and the mass fraction of carbon dissolved in the matrix of the steel quenched were investigated. It is discovered that the hardness of the steel increases with quenching temperature in the range of 840-900°C and remains constant in the range of 900 to 1100°C. It decreases rapidly when the temperature is higher than 1100°C. The mass fraction of carbon dissolved in the matrix of the steel quenched at 840°C is 0.38, but when the quenching temperature is increased to 1150°C, it increases to 0.98. The carbides formed during sintering are still present at grain boundaries and in the matrix of the steel quenched at low quenching temperatures, such as 840°C. When the quenching temperature is increased to 1150°C, most of the carbides at grain boundaries are dissolved with just a small amount of spherical M₂₃C₆ existing in the matrix of the quenched steel.