Effect of Microstructure on Fatigue Strength and Resistance to Fatigue Crack Propagation in Quenched and Tempered Cr-Mo Alloy Steel

Abstract

The influence of microstructure on fatigue strength and resistance to fatigue crack propagation has been investigated by means of fracture mechanics and fractography in quenched and tempered Cr-Mo alloy steel, Then, the impact fatigue tests were carried out by using a rotating disk type impact fatigue testing machine, These results were compared with those for non-impact fatigue tests, The results obtained are summarized as follows.(1) For the non-impact fatigue, the fatigue strength and the resistance to fatigue crack propagation were markedly influenced by the microstructure. In other words, the 600°C tempered material had the most superior property of crack propagation resistance, while this material showed the most inferior resistance to crack initiation among the materials tested. The 300°C tempered material has a considerably inferior resistance to both the initiation and the propagation of fatigue crack because of low temperature temper brittleness. It was concluded from the present work that the temper treatment at 450°C was one method to improve the resistance to both the initiation and the propagation of fatigue crack.(2) For the impact fatigue, the resistance to crack propagation increased with decreasing tempering temperature. This result was not in agreement with the one described above for the non-impact fatigue. If the impact fatigue fracture behavior of component is estimated from the results of non-impact fatigue, it may involve a considerable risk.(3) The crack growth rate for impact fatigue was larger than that for non-impact fatigue. This difference increased with increasing tempering temperature. This behavior could be explained by the authors' concept of the equivalent stress ratio, in which the effect of impact loading is replaced with the stress ratio in non-impact fatigue.