MICROSTRUCTURAL CHARACTERIZATION OF RAPIDLY SOLIDIFIED ULTRAHIGH BORON TOOL STEELS

Argon atomized powders of two modified baron-containing-tool steels in the rapid solidification condition as well as the consolidated materials after hot isostatic pressing at different temperatures were investigated. The microstructure of the powders of both tool steels consists of primary dendrites of solidification and a network of the M(23)(B, C)(6) borocarbide forming a continuous phase. The metastable M(3)B boride is present because of the high solidification rates involved in the atomization process. This microstructure is gradually lost when the temperature of consolidation by hot isostatic pressing is increased. A fine and uniform microstructure is developed after consolidation at 1100 degrees C. An increase of hardness with an increase of consolidation temperature is observed which is associated to the presence of martensite since both alloys have a composition characteristic of air-hardening steels. After two consecutive tempering treatments at 650 degrees C for 2 h the martensite is decomposed and a decrease of hardness with increasing consolidation temperature is observed.