HEAT-TREATMENT OF INVESTMENT CAST PH 13-8 MO STAINLESS-STEEL .1. MECHANICAL-PROPERTIES AND MICROSTRUCTURE

The microstructure of investment cast PH 13-8 Mo stainless steel heat- treated to various conditions was studied using light and electron mic roscopy, electron probe microanalysis, and Mossbauer spectroscopy. The mechanical properties were investigated by using uniaxial tensile tes ting, hardness testing, and Charpy impact testing. The beta-NiAl stren gthening precipitates, though detectable by electron diffraction, were difficult to resolve by transmission electron microscopy (TEM) in spe cimens aged at low temperatures (566-degrees-C and below). A high disl ocation density was observed in the lath martensitic structure. The hi gher strength and lower ductility observed at low aging temperatures w as attributed to both the high dislocation density and the precipitati on of beta-NiAl. When samples were aged at high temperatures (>566-deg rees-C), a lower dislocation density and a reverted austenite fraction on the order of 15 pct were observed. Spherical beta-NiAl precipitate s were observed in the overaged condition. The decrease in strength an d corresponding increase in ductility observed in samples aged at temp eratures above 566-degrees-C were attributed to the reverted austenite and recovery. Mechanical properties were improved when the homogenizi ng temperature and time were increased. Electron probe microanalysis q uantified the increased homogeneity realized by increasing homogenizin g temperature and time. Elimination of the refrigeration step, which n ormally follows the solution treatment, did not degrade the mechanical properties. Mossbauer spectroscopy showed only minor decreases in the fraction of retained austenite when refrigeration followed the soluti on treatment.