Optimisation of Processing Parameters for Direct Vacuum Sintering of a T15 High Speed Steel

Abstract

An attempt was made to achieve, by direct vacuum sintering of water atomised T15 powder, the microstructure and optimum transverse rupture strength (TRS), ∼3200 MN m⁻², characteristic of the containerlessly hipped undersintered counterpart. The processing variables were: compacting pressure 400–800 MPa, sintering temperature 1260–1290°C, and sintering time 5–60 min. Ground, ground and polished, and hipped and unhipped specimens were tested, and the failure process investigated by metallographic and fractographic techniques. The highest strengths were associated with the absence of porosity and grain boundary films, which are responsible for cracking of oversintered specimens, and uniform distribution of fine (<10 μm) carbides. There are indications that cracks through these carbides, generated by stresses, are subcritical in the fracture mechanics sense, and crack growth needs to take place before catastrophic failure can occur. A directly sintered (30 min at 1265°C) batch with the highest mean TRS of 2800 MN m⁻² and a Weibull modulus of 8 is to be compared to a TRS of 3200 MN m⁻² and Weibull modulus of 6 for the hipped, undersintered (1260°C) batch. An additional feature of this investigation was a critical comparison of TRS values for ground specimens with those for ground and polished specimens, which showed the importance of polishing for fundamental studies of fracture mechanisms originating from natural or naturally produced flaws, but indicated the general acceptability of grinding for producing industrial quality control specimens. PM/0543