Effect of Modified Ausforming on Fatigue Strength of Low Alloy Steel

Abstract

The effect of modified ausforming on fatigue properties of_Ni-Cr and Ni-Cr-Mo steels has been studied. These steels were rolled to reduce the thickness tO 30, 54 and 72 % in the temperature range between 880°C and 700°C, and cooled to form martensite. Their fatigue properties under alternate bending stress were then compared with those of the conventionally treated specimen after tempering at 3OO°C or 6O0°C for 1 hour. Fatigue limit increased approximately linearly with the reduction up to 54 % and the increase of fatigue limit amounted to about 0.17 - 0.22 kg/mm² per 1 % reduction. The maximum improvement of fatigue limit was 39 % and fatigue life was prolonged by the process. Endurance ratio was then improved by modified ausfoming. Moreover, modified ausforming restrained the intergranular fracture of these steels after low temperature tempering. Matallographic observations revealed that the modified-ausformed specimen kept refined subgrains even after higher temperature tempering. It was found that the refinement of subgrains was an important factor in the improvement of fatigue strength of these steels.