抄録
An investigation has been made on the influence of ausforming of stable austenite at a comparatively low temperature on the fatigue crack growth behaviour of Mn-Cr-B spring steel under alternate bending stress. The following results have been obtained.
(1) The fatigue strength of the notched specimen is improved by this ausforming, and about 20-35% improvement in the endurance limit has been realized within 56% reduction in thickness. It seems that ausforming has no influence on the fatigue notch sensitivity.
(2) Ausforming is an effective treatment for lowering the crack growth rate in the range of stable propagation from the early stage to the middle stage. In the range of unstable propagation, however, the crack growth rate of the ausformed specimen is higher than that of the conventionally heat-treated one, and in this case, the power m in the Paris' formula increases with the reduction ratio in ausforming process, while the coefficient c drops gradually.
(3) The fractography reveals that the fracture surface of the conventionally treated sample consists of both the intergranular and the transgranular fracture, while the ausformed specimen gives only the transgranular fracture. The lowering of the crack growth rate by ausforming is related to the disappearance of the intergranular fracture, and this improvement of the fracture mode may be attributed to the precipitation tendency of carbide.
