Development of life prediction model and improvement of a die steel for hot forging die

A life prediction model was established for a hot forging die for crankshafts. Stress analysis under thermal and mechanical loadings was conducted by the finite element method. Fatigue crack initiation and propagation lives were quantitatively predicted based on the results of analysis, and on the creep-fatigue properties and fatigue crack propagation properties of die steels. Die lives predicted for a conventional SKD61 steel and improved die steels corresponded to those in endurance tests with an actual press forging machine. The lives of improved die steels were about 1.5 times that of the conventional steel. It was found that improvement of high temperature strength of a die steel is effective for extension of the crack initiation life, and that improvement of fracture toughness is effective for extension of the crack propagation life. Because the life prediction model has potential to predict relationships between material properties and die lives, it makes it possible to select a suitable die steel, with the longest die life according to the configuration and loading conditions of the die.