On the fatigue properties of high-strength steels

Summary

The paper gives the results of an investigation of the optimal tensile strength level suitable for obtaining the maximum fatigue limit of a Mn-Cr-Ni-Mo steel produced by current technology with constant amount of alloying elements, but three different carbon contents, 0·21–0·41%, after conventional (CT) and thermomechanical treatment (TMT). The study was carried out in an effort to explain the considerable difference in the fatigue limit data of high strength steels published by different authors [1, 2, 3]. For a tensile strength of about 260 kp/mm² the fatigue limits were found from 38 up to 116 kp/mm².

In addition to confirming the influence of the TMT on the tensile strength and ductility already known, it was found that the maximum of the fatigue properties for medium carbon content can be obtained in the tensile strength area of cca 220 kp/mm². When the tensile strength is raised beyond this limit the fatigue limit falls. The values of the fatigue limit after CT as well as after TMT, but in comparison to the same tensile strength range, are very similar. The fatigue limits of about 90 kp/mm² lie within the great range of other published data.

Suggested causes for the decreased fatigue limit at high strength such as the method of testing, the shape of the test bar, the effect of ductility, the grain size and the level of internal stresses, are discussed and the idea is advanced that the main cause of the great differences is the increasing influence of the size of non-metallic inclusions as the strength increases.

As a result, a preliminary hypothesis was made to explain the relation of the fatigue limit of steels to the size of non-metallic inclusions, referred to different limits of strength, based on the results of Stulen and co-workers [4]. By means of extrapolation the fatigue limit of a steel containing an inclusion of 0 μm diameter was also determined as a function of the tensile strength. This can be considered as the theoretical fatigue limit of the metal matrix itself, where the mechanism of the fatigue fracture is controlled by the fatigue phenomena in the matrix itself. Below this limit, on the contrary, the mechanism of the fatigue fracture is controlled by the presence of nonmetallic inclusions.