Powder metallurgy T15 tool steel: Part I. Characterization of powder and hot isostatically pressed material

The microstructure and constitution of T15 tool steel processed from gas-atomized powder have been characterized. From the atomized powder, four particle size ranges (≤840, 250 to 840, 44 to 100, and ≤44 Μm) were consolidated to full density by hot isostatic pressing (“hipping”) at 1130 ‡C or 1195 ‡C. Both atomized powder and consolidated material were examined by means of optical and electron microscopy, X-ray diffraction, chemical analysis, and micro-hardness. A segregated structure exists in the gas-atomized powder, independent of particle size; MC and M₂C carbides are present, primarily at cell boundaries. The matrix of the powders is a mix of martensite and retained austenite. Weight fraction and overall composition of the carbides are insensitive to particle size, but the proportion of MC carbides increases with decreasing particle size. After consolidation, MC, M₆C, and M₂₃C₆ carbides are present in a ferrite matrix. The carbide size distribution is skewed to larger carbide sizes at the higher consolidation temperature, independent of the prior particle size fraction, but there is no significant change in carbide volume fraction. For a given consolidation temperature, the size distribution of the MC and M₆C carbides is broader for the coarser particle size fractions.