Effect of Cooling Rate on A3 Transformation Temperatures of Iron and Iron-Nickel Binary Alloys

Effects of cooling rate on A3 transformation temperatures of a few pure irons, iron-low carbon alloys and iron-nickel alloys have been measured at cooling rates between 100°C/sec and 60000°C/sec by using a gas quench apparatus. A3 transformation temperature 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 A3), but in the iron containing more than 0.006 wt%C that temperature falls discontinuously to the Ms temperature at a cooling rate of about 20000°C/sec. Maximum supercooled A3 temperatures of these irons are affected seriously by a very small amount of carbon below 0.005 wt%C and descend remarkably from about 720°C at 0.001 wt%C to about 650°C at 0.003 wt%C with increasing carbon content. A3 transformation temperatures of iron-nickel alloys descend with increasing cooling rate and fall discontinuously by about 100°C at a certain cooling rate which depends on the nickel concentration. The drop of the transformation temperatures is independent of further cooling rate. The critical cooling rate decreases from about 25000°C/sec at 1%Ni to about 2000°C/sec at 20%Ni. The Ms temperatures 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 concentrations.