Effects of Austempering Temperature on Fatigue Crack Rate Propagation in a Series of Modified (Cu, Ni, and/or Mo) Nodular Irons

Studies on austempered nodular cast irons were carried out to establish the optimum isothermic heat treatment at a given chemical composition that rendered the highest fatigue crack propagation resistance. Seven nodular iron chemical compositions with different concentrations of copper, nickel, and or molybdenum were tested at three austempering temperatures achieving ausferritic microstructures. Three-point bend tests for crack growth rates were performed at room temperature in a close loop servo hydraulic machine. Crack opening displacement measurements were performed using a controlled displacement telescope. A simple linear statistical analysis indicated that the lower the austempering isothermal temperature, the higher the fatigue strength of the alloys. Cu and Mo additions along with a good spheroidicity of graphite nodules in the iron favored this effect.