| A detailed microstructural examination has been made of T15 high speed tool steel processed from gas atomized powder and associated strength, toughness and hardness at elevated temperatures evaluated. Powders were screened into selected size fractions to include the rapid solidification regime, hot isostatically pressed and heat-treated. Some of the HIPed material was not-forged prior to heat-treatment. Two HIPing temperatures, two austenitizing temperatures and three tempering temperatures were included in the study. Atomized and consolidated materials were characterized using x-ray diffraction, optical and scanning electron microscopy. A segregated network structure is present in the atomized powder independent of the particle size, with MC and M(,2)C carbides present primarily at cell boundaries. The weight fraction and the overall carbide compsition are insensitive to particle size; the proportion of MC carbides increases with decreasing particle size. After HIPing, MC, M(,6)C and M(,23)C(,6) carbides are present in the form of individual particles in a ferritic matrix. Independent of prior particle size fraction, the carbide size distribution after HIPing is skewed to larger carbide sizes with increasing HIPing temperature, but with no significant change in volume fraction of carbides. At a given HIPing temperature the carbide size distribution for MC and M(,6)C carbides is dependent marginally on prior particle size fraction, being broader for the coarser size fractions. M(,23)C(,6) goes into solution during the austenitizing if the temperature is high enough and does not reprecipitate on tempering. Prior austenite grain size is influenced by the HIPing and austenitizing temperatures as well as the intermediate hot-forging step. MC and M(,6)C carbide size distributions in the fully heat-treated material, are influenced primarily by the HIPing temperature and to a lesser extent by the intermediate hot-forging step. Strength and toughness are enhanced by hot-forging. Property levels are similar to those of the corresponding commercial P/M tool steel. Hardness retention at elevated temperature is superior to that of the commercial material. The dependence of property levels on powder characteristics and processing parameters is understood in terms of the associated microstructures and densification mechanisms. |
