A comparison of the fatigue and fracture behavior of high strength ultrafine grained medium carbon steel SAE 1045 with high strength bearing steel SAE 52100

Abstract

Ultrafine-grained metals feature very good mechanical properties like high hardness, high ultimate tensile strength, and a high fatigue limit, though all these properties are mostly reported for ultrafine-grained metals other than steels. Here we show fatigue results of ultrafine grained SAE 1045, which was produced by High Pressure Torsion (HPT), exhibiting significantly increased fatigue limits as compared to conventionally grained SAE 1045. Apart from applying the appropriate HPT-treatment it is also essential to use special carbide morphology in the initial state before HPT to reach high fatigue limit. Therefore we used SAE 1045 in normalized, spheroidized, tempered, and patented states. In our investigation the patented microstructure led to a degree of hardness and a fatigue limit equivalent to those of austempered SAE 52100, while the other carbide morphologies in SAE 1045 led to significantly lower fatigue limits. Morphology and crack initiation mechanisms were changed by severe plastic deformation. The fracture surfaces revealed mostly flat fatigue fracture surfaces with crack initiation at the surface or, more often, at non-metallic inclusions beneath the surface. All patented SAE 1045 as well as austempered SAE 52100 specimens failed from nonmetallic inclusions but with different features in the very long life time regime. While the bearing steel SAE 52100 showed fine granular areas (FGAs) around the crack initiation inclusions the ultrafine grained patented SAE 1045 did not produce these FGAs around the inclusions. This significant difference in failure mechanism in long fatigue life can be explained with the help of an analysis of the stress intensity factors of the crack initiating inclusions in both steels: the development of FGAs in SAE 52100 only acts around inclusions when the stress intensity factor at those inclusions is below the classical threshold value of long cracks. In this case a FGA is formed at the inclusions and reduces the threshold value, with the consequence that a fatigue crack starts from the inclusion with FGA and leads to failure. In contrast, the microstructure of the patented SAE 1045 is already ultrafine grained by HPT before fatigue loading so the threshold value for crack growth is lower than in austempered SAE 52100. Therefore in the ultrafine grained microstructure long cycle fatigue failure occurs without formation of FGAs around nonmetallic inclusions.