THE RELATION OF HARDNESS TO TOUGHNESS AND RETAINED AUSTENITE CONTENT IN N2-H2-CH4 SINTERED-T6, SINTERED-T15 AND SINTERED-T42 HIGH-SPEED STEELS

The increase of toughness, 10 to 35 MPam1/2, with decreasing hardness, VH 950 to 500, is reported for sintered T6, T15 and T42 high-speed steels. This range of properties resulted from combinations of sintering temperatures (in N2-H2-CH4), HIPping and tempering treatments of steels processed from water-atomized powders. Toughness is related to the properties of the matrix, but more specifically to the retained austenite content, varying between 5% (HV10 ≃950, Klc ≃ 10 MPam1/2) and 70% (HV10 ≃ 500, Klc ≃ 35 MPam1/2). Austenite retention is caused by the stabilizing affect of nitrogen (0.3 to 0.7%), picked up from the sintering atmosphere. Fractographic examination revealed that, although failure proceeded by complex modes that could be described mainly as quasi-cleavage, microplasticity was observed in the high austenite materials, as opposed to the flatter fracture surfaces, with much less plasticity apparent, for the lower retained austenite-containing materials. © 1990 Chapman and Hall Ltd.