This study has been made to improve fatigue strength of press-fitted parts of car axles by sub-critical quenching; water quenching from the temperature just below Ac
1.
The effect of sub-critical quenching on behaviours of fatigue crack propagation, the relation between σw
2 (fatigue limit for failure) and sub-critical quenching temperature as well as the relation between hardness of ferrite structure and residual compressive stress at surface were investigated using notched specimens of 15mm in diameter (0.35%C steel) and are press-fitted specimens of 50 mm in diameter (0.4%C steel).
Results obtained are as follows.
(1) In sub-critical quenched specimens, both notched and press-fitted, fatigue cracks initiated and propagated to some extent and stopped, when the stress was applied above σ
w1 (fatigue limit for crack initiation) and below σ
w2. The crack length depended probably on the applied stress.
(2) Maximum value of σ
w2 was obtained in the specimens, both notched and press-fitted, sub-critical quenched from about 650-750°C.
(3) In the surface of sub-critical quenched specimens, hardness of ferrite structure was considerably increased. It was presumed that about 60% of total hardness increase was provided by age hardening after quenching, and about 40% of it by residual compressive stress generated on the surface by the quenching.
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