Radial crushing strength and microstructure of molybdenum alloyed sintered steels

Prealloyed iron-based powders with different Mo–Cu–Ni–C contents were uniaxially compacted at 700 MPa and sintered at 1120°C in a nitrogen–hydrogen atmosphere. After sintering, all the materials were thermochemically treated by carburizing (endogas atmosphere) and carbonitriding. Treatment conditions for the carburizing process were heating at 875°C during 100 min with two different potentials of carbon: 0.88 and 0.55% for the two cycles. The carbonitriding process took place at 820°C with two cycles: carbonitriding-40 during 90 min and carbonitriding-20 during 45 min. Materials were evaluated through the study of radial crushing strength and the complete microstructural study was carried out using optical microscopy and SEM. Finally, the fracture surface was analysed through SEM. Other properties like friction coefficient, density and microhardness were evaluated. Crushing strength test results show that, in all cases, treated materials present higher strength than sintered base materials. Carburizing is also more positive than carbonitriding, reaching the maximum properties always for the first cycle (low carbon content) of carburizing. A maximum crushing strength of 1290 MPa was obtained when using Fe–1.5% Mo as base material.