The effect of quenching heat treatments on the microstructure of cast and worked commercial steel AISI M2 and hot isostatically pressed as well as IPEN vacuum sintered AISI T15 have been studied. The quenching treatments were carried out from 1160, 1185, 1210 and 1235°C. The average grain size, volume fraction and average size of the primary carbides M6C and MC were determined by scanning electron microscopy and by using the digital image analysis method "Quantikov". An increase in average austenite grain size with increase in quenching temperature was observed for the AISI M2 commercial steel and vacuum sintered specimens. In the case of the AISI T15 steel specimens, increase in quenching temperature did not result in an increase in average grain size, both for the commercial and the vacuum sintered steels. The average size and volume fraction of M6C and MC carbides remained unaltered with increase in quenching temperature.