A study has been made on the fatigue limit of die forged crankshafts with 100 mm and 150 mm in journal diameter. It is found that the crankshaft shaped by die forging method has higher fatigue limit than those of the conventional forged ones, especially when the higher hardness layer than of their inner part is given on their outer layer by heat treatment. The required depth of this high hardness outer layer is estimated more than 0.2⋅R₄ to increase fifty per cent in fatigue limit, where R₄ is fillet radius.
In the last part, a proposal for the estimation method of the fatigue limit of solid type forged steel crankshafts has been made based on our whole fatigue test results obtained on the actual sized crankshaft specimens with various dimensions and shaped by various methods.