The Effect of Sub-critical Quenching Temperature on Fatigue Properties of Press-fitted Axles

Abstract

In order to apply a sub-critical quenching method for the improvement of fatigue strength of press-fitted parts of car axles, the effect of sub-critical quenching temperature (500700°C) on the microstructure, hardness, residual stress distribution and fatigue strength was investigated using 12 mmφ notched specimens (0.35%C steel) and press-fitted specimens (0.4%C steel) of 50 mmφ.
(1) The maximum surface hardness, about 1.2 times higher than the normalized state, was obtained by 650°C sub-critical quenched specimens. The cause is attributable to the solution hardening effect of carbon into ferrite. And it can be considered that the hardness increase in the surface would have a beneficial effect to improve the fatigue limit for the crack initiation, σ_w1, of notched or press-fitted parts.
(2) The maximum value of the compressive residual stresses in the surface was generated by 650°C sub-critical quenching, but depths of the compressive residual stress zone were almost independent on the sub-critical quenching temperatures.
(3) The maximum value of the fatigue limit for failure, σ_w2, was obtained by 650°C sub-critical quenching. Moreover, a linear relationship between σ_w2 and the values of the compressive residual stress in the surface was clarified by the sub-critical quenched specimens and the normalized ones.
(4) It is concluded from the above results that the optimum sub-critical quenching temperature is about 650°C.