The effect of quench rate on the as-quenched hardness and carbide or carbon segregate particle size was studied for two alloys steels with M//s temperatures of 310 degree C and 300 degree C. The quench rate was varied form 681 to 93,000 degree C per second. The as-quenched hardness and particle size were both found to decrease as the quench rate increased. Analysis of the data ruled out autotempering of the martensite as a likely basis for the observed quench rate effects. The effects of aging at room temperature (20 degree C) on the hardness and particle size for these alloys were also studied.