Effect of carbon addition on microstructure and properties of boron-containing steel sintered under different atmospheres

High performance sintered steels can be obtained by simultaneous tailoring microstructural feature and improving sintered density. Although sintered hardening is used to produce sintered components with microstructural features providing high tensile strength and hardness, but the performance is limited by low ductility due to the presence of porosity. Near full density can be achieved by liquid phase forming as a result of boron addition to a sintered steel. A liquid formed due to the eutectic reaction of Fe + Fe 2 B, spreads to interparticle spaces leading to densification improvement. Carbon is an indispensable element for high strength sintered steels. It plays important roles in both matrix microstructural development and intergranular liquid phase formation. This work has investigated the sintered Fe-1.5Mo-0.22B- x C steels ( x = 0.1 - 0.4 wt.%) sintered under hydrogen and vacuum atmospheres. It was found that the hydrogen-sintered Fe-1.5Mo-0.22B- x C steels hardly showed evidences of intergranular liquid phase whereas all experimental vacuum-sintered steels showed intergranular boride. Deboronization is believed to contribute to the intergranular boride absence in the hydrogen-sintered steels. However, when the hardening effect was taken into account, the strengthening by intergranular liquid phase in the sintered steels was less important than precipitation strengthening. Advantage of ductility was only obtained in the vacuum-sintered steels with C contents £ 0.1 wt.%, whose microstructures contained discontinuous boride networks along polygonal ferrite grain boundaries. In contrast, the presence of continuous and thick boride networks caused embrittlement to the sintered steels.