MECHANICAL PROPERTIES AND MICROSTRUCTURE OF PM Mn-Cr-Mo STEELS WITH LOW CARBON CONCENTRATION

The effect of chemical composition of the sintering atmosphere on the microstructure and mechanical properties of PM structural low-carbon steels is presented. The base powders were Astaloy CrL, Astaloy CrM, low carbon ferromanganese and graphite C-UF. From the base powders two mixtures with compositions of Fe-3%Mn-(1.5/3%)Cr-(0.2/0.5)%Mo-0.2%C were prepared. Following pressing in a steel rigid die, compacts were sintered at 1250 degrees C for 60 min in a semi-closed container. 5%H2-95%N-2 mixture and air were the sintering atmospheres. For sintering in air, lumps of ferromanganese were placed with the compacts in the container. After sintering, half of the samples were tempered at 200 degrees C for 60 minutes in air. Mechanical tests (tensile, bend, toughness, hardness) and microstructural investigations were performed. The microstructures of the steels were inhomogeneous, mainly ferritic-bainic. Tempering of steel based on Astaloy CrM sintered in an atmosphere of 5% H2-95% N-2 slightly reduced tensile strength and toughness: from 748 to 734 MPa and from 7.15 to 6.83 J/cm(2), respectively. Chemical composition had a greater effect; steels based on Astaloy CrL and Astaloy CrM had tensile strengths 526-665 and 672-748 MPa, hardness 280-325 and 388-421 HV, respectively. The best properties were obtained after sintering in air of Fe-3%Mn-3%Cr-0.5%Mo-0.2%C without heat treatment: tensile strength 672 MPa, toughness 6.93 J/cm(2), hardness 421.1 HV, 0.2 % offset yield strength 395 MPa.