The effect of double austenitization on the microstructure and toughness of AISI M2 high speed steel

In this study, the effect of double austenitization on microstructure and toughness of AISI M2 high speed steel was investigated. For double austenitization treatment, the specimens, which are hardened initially at 1220 degrees C and quenched in air, were hardened for a second time in the temperature range 1150 - 1050 degrees C. For comparison purposes, another set of specimens is austenitized singly in the temperature range 1150 - 1050 degrees C. Tempering process was carried out between 500 - 640 degrees C. A double austenitization causes a fine carbide precipitation in the matrix, having sizes in the range of 0.10 +/- 0.05 mu m and volume fractions of between 1 and 6%. It is shown that a double austenitization treatment causes a decrease in fracture toughness (K-Ic), when compared with single austenitized ones. The reason for the lower K-Ic Values of double austenitized specimens are attributed to these fine carbide precipitates: It is suggested that they limit the plastic deformation capability of the matrix and yield lower fracture toughness values