P/M Processing of Rare Earth Modified High Strength Steels

摘要

Material processing studies were continued directed toward developing powder metallurgy methods for producing rare earth modified high strength 4340 steel with improved resistance to hydrogen embrittlement. Three different methods of making rare earth modified 4340 steel powders were investigated: hydrogen gas atomization, mechanical alloying, and rapid solidification atomization. Hot extrusion consolidation of hydrogen gas atomized 4340 steel powders at 2150 F (1450 K) resulted in much better mechanical properties after heat treatment than hot isostatic pressing (HIP) consolidation at 2100 F (1420 K), but the ductility and impact resistance properties of the extruded powders were generally lower than those of similarly heat treated, rare earth modified, wrought 4340 steels evaluated previously, at approximately the same rare earth (cerium) content. Hot extrusion consolidation of 4340 steel powder which was mechanically alloyed with either 75% Ce-25% Ni or LaNi5 alloy powders by attriting in a special high-intensity ball mill resulted in higher strength but lower ductility and impact resistance after heat treatment than the previously evaluated wrought 4340 steels at approximately the same rare earth content. Rapid solidification atomization of 4340 steel prealloyed with cerium by adding a 75% Ce-25% Ni alloy to the melt, using both the rapid solidification rate (RSR) and commercial rapid solidification (CRS) processes, resulted in very low cerium recoveries due to vaporization of the cerium in the RSR process and oxidation of the cerium in the CRS process. Because these difficulties could not be resolved within the scope of the program, no further work was performed on rapidly solidified powders. (Author)