Abstract
Using a mild steel which is quench-aged, that is, overaged at 200, 100, 50 and 30°C, after the solution treatment for 30 minutes at 680°C and quenching into iced water, reversed bending fatigue tests of plain and notched specimens were carried out, and precipitated particles due to quench aging were observed by transmission electron microscope.
The results obtained are summarized as follows:
(1) The fatigue strength of mild steel is more increased by the quench aging at 30 or 50°C than by annealing or quenching.
(2) The increment of fatigue strength in various notched specimens due to thequench aging is more conspicuous than that in a plain specimen.
(3) Precipitated particles in ferrite matrix become fine and distributed densely, as the aging temperature is decreased. As the dislocation motion is disturbed by such particles, the stress concentration due to the dislocation pile-up decreases and then the fatigue limit of quench-aged steel is increased in proportion to the reciprocal of square root of mean free distance between particles.
(4) The quench aging is effective on decreasing the propagation rate of fatigue crack.
