Effects of cooling rate on A_3 points of a few pure irons, iron-low carbon alloys and iron-nickel alloys have been examined at cooling rates between 100℃/sec and 60, 000℃/sec by using a gas quench apparatus. A_3 point of iron containing less than 0.003 wt% C falls gradually with the increase in cooling rate and continuously approaches a fixed temperature (maximum supercooled A_3), but in the iron containing more than 0.006 wt% C it falls discontinuously to the Ms temperature at the cooling rate of about 20, 000℃/sec. Maximum supercooled A_3 points of these irons are affected seriously by a very small content of carbon below 0.005 wt% C, falling remarkably from about 720℃ at 0.001 wt% C to about 650℃ at 0.003 wt% C with increasing carbon content. In the case of iron-nickel alloys it falls with increasing cooling rate and discontinuously by about 100℃ at a certain cooling rate which depends on the nickel content. The drop of the transformation temperature is independent of a further cooling rate. The critical cooling rate decreases from about 25, 000℃/sec at 1% Ni to about 2, 000℃/sec at 20% Ni. The Ms points of these alloys are in agreement, at higher nickel concentrations, with those reported previously, but are displaced to a lower temperature side at lower nickel concentration
