The Effect of Sub-critical Quenching on Fatigue Limits for Crack Initiation and for Failure of Press-fitted Part in Axle

Abstract

To improve the fatigue limits of press-fitted parts of axles by sub-critical quenching, that is, water quenching from the temperature just below A_c1 point, the effects of the heat treatment conditions on microstructure, hardness, residual stress distribution, rotating bending fatigue limits for crack initiation, σ_w1, and that for failure, σ_w2 were examined. Steels used for the experiment were normalized 0.35-0.45 %C steels for machine structual uses. For fatigue testing, press-fitted specimens of 50mm in diameter were mainly used.
(1) The microstructure of specimen was hardly changed by sub-critical quenching, but the surface hardness was slightly increased.
(2) High residual compressive stress, distributed from the surface to considerable depth of the core, was detected in sub-critical quenching specimens.
(3) σ_w1 and σ_w2 of press-fitted specimens were both improved considerably by sub-critical quenching. Moreover, the limit depth of non-propagating crack, measured after the fatigue test in which applied stress was just below σ_w2, was also remarkably increased.
(4) It was supposed that σ_w1 of the press-fitted specimen could be improved by the increase of surface hardness, and σ_w2 by the increase of residual compressive stress in surface zone, respectively.