Sintering, microstructure, and mechanical properties of PM manganese-molybdenum steels

The effects of 0.5 wt-%Mo addition on the processing, microstructure, and s trength of PM Fe-3.5Mn-0.7C steel are described. Water atomised and sponge irons, Astaloy 1.5Mo, milled ferromanganese, and graphite were the starting powders. During sintering in 75H(2)/25N(2) or pure hydrogen the dewpoint w as controlled and monitored; in particular the effects of improving it from -35 to -60 degreesC were investigated. Faster heating rates (greater than or equal to 20 K min(-1)), sufficient gas flowrates, milling the ferro allo y under nitrogen, a low dewpoint (<-60<degrees>C), and a getter powder can all contribute to the reduction or prevention of oxidation of the manganese , in particular formation of oxide networks in the sintered steels. For 600 MPa compaction pressure densities up to 7.1 g cm(-3) were obtained; these were not significantly affected by sintering at temperatures up to 1180 deg reesC. The sintered microstructures were sensitively dependent on the cooli ng rate. Irrespective of the presence of Mo, slow furnace cooling at simila r to4 K min(-1) resulted in mainly pearlitic structures with some ferrite a nd coarse bainite, whereas fast cooling at similar to 40 K min(-1) produced martensite and some retained austenite, very fine pearlite, bainite, and s ome ferrite. Young's modulus, determined by tensile and ultrasonic tests, w as in the range 110-155 GPa. Sintering with -60 degreesC dewpoint resulted in tensile and transverse rupture strengths of 420 and 860 MPa for the Mn s teel, rising to 530 and 1130 MPa as a result of the Mo addition. This contr asts with strength decreases observed when processing included use of high oxygen containing ferromanganese and sintering with -35 degreesC dewpoint. PM/0811.