A Fundamental Study of Tool Steels Processed from Rapidly Solidified Powders.

A detailed microstructural examination has been made of T15 and Rex 25 high speed tool steels processed from rapidly solidified powders. Gas atomized powders were screened into selected size fractions, hot isostatically pressed to full density and heat-treated. Two HIPing temperatures, two austenitizing temperatures and three tempering temperatures were included in the study. Atomized and consolidated materials were characterized by means of optical and electron microscopy, x-ray diffraction and hardness. Both cellular and dendritic structures are present in the atomized powders depending on particle size, with MC carbides present primarily at cell boundaries. The partitioning of alloying elements in the cell structure decreases with decreasing particle size. 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 M6C carbides is dependent on prior particle size fraction it is broader for the coarser size fractions.