COMPARISON OF MECHANICAL PROPERTIES AND MICROSTRUCTURE OF Cr-Mn-Mo STEELS BASED ON ASTALOY CrL AND ASTALOY CrM PRE-ALLOYED POWDERS

摘要

Low carbon ferromanganese and graphite powders were admixed to Hoganas water atomised pre-alloyed Astaloy CrL and Astaloy CrM powders - to produce six variants of Fe-Cr-Mn-Mo-C steels. These were pressed into tensile ISO 2740 and bend 5x 10x55 mm specimens at 660 MPa, sintered in semi-closed containers for 1 hour in dry nitrogen or hydrogen at 1120°C or 1250°C and cooled at 65°C/min. Neither detectable densification nor swelling took place, whatever the powder and sintering conditions. The sintered specimens were subsequently tempered at 200°C for 60 and 240 minutes. Green and sintered densities were expectedly highest for Astaloy CrL base at ~7.1gcm~(-3), >0.1 gcm~(-3) larger than for the Astaloy CrM alloy. Sintered samples (as-sintered density was in the range 6.81-7.02 gcm~(-3)) were characterised for microstruc-turc, Vickers microhardness, three-point bending, tensile properties, as well as sintering behaviour in dilatometer. Recorded mechanical properties were as follows: tensile strength was up to 968 MPa, tensile elongation - up to 4.56%, and impact energy does not exceeded 15.18 Jcm~(-2). The TRS limit was up to 2034 MPa. Tensile strengths were higher for hydrogen sintering, generally by ~5%. Bend tensile strengths were somewhat higher for nitrogen sintering. The structure of investigated PM steels consisted of mainly bainite and martensite; also retained austenite and regularly arranged pores were observed during light microscopy examination. While properties were variable due to microstructure, results indicate that the material provides new possibilities. These findings, coupled with the superior hardenability of the alloy makes the material a candidate for a number of structural applications and warrants further and continued laboratory investigation.