No abstract

Due to the evolution of cleanness of bearing steels, the failures observed in service conditions move from deep initiated spalling on non-metallic inclusions to surface damage due to machining marks, lack or contaminated lubrication (dents). To take into account, by the most economic way, these request for more demanding applications, ASCOMETAL, SNR et VALTI have developed a new steel grade [1,2] based on the classical analysis of 100Cr6 (SAE 52100), with higher characteristics in terms of in-service properties (sustaining high loading, high temperature and contaminated lubrication) without deteriorated steelmaking or forming process by an optimised composition of the steel.

In this study, various experimental methods are employed to determine the anisotropic fatigue behavior of a 25MnCrSiVB6 forged steel (Metasco MC). This material has a bainitic microstructure and contains many elongated non-metallic inclusions in the rolled direction, which are grouped into clusters. Specimens with different orientations relative to the rolling direction have been extracted from a hot rolled bar and the ability of certain experimental techniques to capture the fatigue anisotropy has been tested. Results obtained from monotonic tensile tests and Charpy impact tests show that the material has isotropic fracture strength and anisotropic ductility. The influence of the inclusion clusters is clearly demonstrated via observation of the fracture surfaces. Concerning the fatigue behavior, results from a classical staircase experimental procedure are compared to results from self-heating fatigue tests. For specimens orientated at 0 • relative to the rolled direction, microcrack initiation is controlled by the material matrix and the prediction of the fatigue strength with the self-heating method has been observed to be correct. For specimens orientated at 45 • and 90 • , the elongated manganese sulfide inclusion clusters are the origin of crack initiation and the fatigue strength drops significantly. For this case, it appears that the self-heating method has difficulty predicting the fatigue behavior.

The objective of this paper is to determine the very long fatigue life of ferrous alloys up to 1 × 1010 cycles at an ultrasonic frequency of 20 kHz. A good agreement is found with the results from conventional tests at a frequency of 25 Hz by Renault between 105 and 107 cycles for a spheroidal graphite cast iron. The experimental results show that fatigue failure can occur over 107 cycles, and the fatigue endurance stress Smax continues to decrease with increasing number of cycles to failure between 106 and 109 cycles. The evolution of the temperature of the specimen caused by the absorption of ultrasonic energy is studied. The temperature increases rapidly with increasing stress amplitudes. There is a maximum temperature between 106 and 107 cycles which may be related to the crack nucleation phase. Observations of fracture surfaces were also made by scanning electron microscopy (SEM). Subsurface cracking has been established as the initiation mechanism in ultra‐high‐cycle fatigue (>107 cycles). A surface–subsurface transition in crack initiation location is described for the four low‐alloy high‐strength steels and a SG cast iron.