Owing to difficult economical conditions, machines and structures often have to be used beyond the design lifetime. In this study, fatigue properties of a bearing steel in the long-life region were experimentally examined under cyclic axial loading. The complicated S-N behaviour was well explained as a combination of S-N curves for surface-induced fracture and interior inclusion-induced fracture. Fish-eye marks were always observed on the fracture surfaces of specimens, which failed in the latter fracture mode, and an inclusion was found at the center of the fish-eye. Finally, it was found that the fatigue fracture of this steel in the long-life region occurred through the following three processes: (i) formation of the characteristic area as a fine granular area (FGA), (ii) crack propagation to form the fish-eye and (iii) rapid crack propagation to cause the catastrophic fracture.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1460-2695.2002.00574.x

Characteristic S-N properties of high-carbon-chromium-bearing steel under axial loading in long-life fatigue

In recent years, mechanical structures such as railway wheels, rails, offshore structures, bridges, engine components, load bearing parts of automobiles, etc. have to endure for a long term up to 108-1010 loading cycles in order to save resources and to reduce the cost together with the environmental load to the globe. Thus, the fatigue behavior of structural materials in the very high cycle regime of 108-1010 cycles has become an important subject of the research. In this paper, a review of the current studies in this area performed by many researchers is described in order to provide a certain milestone in the history of the research on fatigue behavior of the metallic materials in the very high cycle regime.

Review and Prospects for Current Studies on Very High Cycle Fatigue of Metallic Materials for Machine Structural Use

The fracture surfaces of specimens of a heat-treated hard steel, namely Cr-Mo steel SCM435, which failed in the regime of N= 10 5 to 5 x 10 8 cycles, were investigated by optical microscopy and scanning electron microscopy (SEM). Specimens having a longer fatigue life had a particular morphology beside the inclusion at the fracture origin. The particular morphology looked optically dark when observed by an optical microscope and it was named the optically dark area (ODA). The ODA looks a rough area when observed by SEM and atomic force microscope (AFM). The relative size of the ODA to the size of the inclusion at the fracture origin increases with increase in fatigue life. Thus, the ODA is considered to have a crucial role in the mechanism of superlong fatigue failure. It has been assumed that the ODA is made by the cyclic fatigue stress and the synergetic effect of the hydrogen which is trapped by the inclusion at the fracture origin. To verify this hypothesis, in addition to conventionally heat-treated specimens (specimen QT, i.e. quenched and tempered), specimens annealed at 300 °C in a vacuum (specimen VA) and the specimens quenched in a vacuum (specimen VQ) were prepared to remove the hydrogen trapped by inclusions. The specimens VA and VQ, had a much smaller ODA than the specimen QT. Some other evidence of the influence of hydrogen on superlong fatigue failure are also presented. Thus, it is concluded that the hydrogen trapped by inclusions is a crucial factor which causes the superlong fatigue failure of high strength steels.

On the mechanism of fatigue failure in the superlong life regime (N>107 cycles). Part 1: influence of hydrogen trapped by inclusions

The fatigue fracture surfaces of specimens of heat treated hard steels which failed in the regime of N =10 5 to 5 x 10 8 cycles , were investigated by optical microscopy and SEM. Specimens having a longer fatigue life had a particular morphology beside the inclusion at the fracture origin. The particular morphology looked optically dark and in the previous paper it was named the Optically Dark Area, ODA. The roughness inside ODA is larger than outside ODA. The relative size of the ODA to the size of the inclusion at the fracture origin increases with increase in fatigue life. Thus, the ODA is considered to have a crucial role in the mechanism of ultra long life fatigue failure. Direct evidences of existence of hydrogen at the inclusion at fracture origin are presented. It is presumed that the ODA is made by the cyclic stress coupled with the hydrogen which is trapped by the inclusion at the fracture origin. To verify the influence of hydrogen, specimens containing different levels of hydrogen were prepared by different heat treatments. The results obtained by fatigue tests of these specimens suggest that the hydrogen trapped by inclusions is a crucial factor which causes the ultra long fatigue failure of high strength steels. Aspects of the double S-N curve are also discussed in terms of experimental methods, specimen size and statistical distribution of inclusions sizes.

Mechanism of fatigue failure in ultralong life regime

The effect of inclusion size on fatigue behaviour of high strength steels in the very high cycle fatigue (VHCF) regime (>10(7) - 10(9) cycles) is reviewed. Internal fatigue fractures of high strength steels in the VHCF regime initiate mostly at non-metallic inclusions. The critical inclusion size below which it is hard to initiate fatigue cracking of high strength steels in the VHCF regime is found to be about half the critical value characteristic of the high cycle fatigue (HCF) regime (about 10(5) - 10(7) cycles). A stepwise or duplex S-N curve is observed in the VHCF regime. The shape and form of the S-N curves are affected by inclusion size and other factors including surface condition, residual stress, environment and loading modes. Fatigue strength and fatigue life for high strength steels have been found to obey inverse power laws with respect to inclusion size D of the form sigma(w)proportional to D-n1 and N-f proportional to D-n2 respectively. For fatigue strength, the exponent n(1) has been reported to be similar to 0.33 in the literature for the HCF regime and, more recently, to fall in the range 0.17-0.19 for the VHCF regime. For fatigue life, the exponent n(2) is reported to be similar to 3 in the HCF regime, and in the range 4.29-8.42 in the VHCF regime. A special area was often observed inside a 'fish eye' mark in the vicinity of a non-metallic inclusion acting as the fracture origin for specimens having a long fatigue life. The major mechanisms of formation for this special area are discussed. To estimate the fatigue strength and fatigue life, it is necessary to know the size of the maximum inclusion in a tested specimen, and to be able to infer this value using data from a small volume of steel. The statistics of extreme value (SEV) method and the generalised Pareto distribution (GPD) method are introduced and compared. Finally, unresolved problems and future work required in studying the VHCF of high strength steels are briefly presented.

Effects of inclusions on very high cycle fatigue properties of high strength steels

The characteristic fatigue behavior of carburized and quenched steel with internal oxides and non-martensitic microstructure near the surface was presented through rotating bending fatigue test. The S-N diagram revealed two knees and the specimens continued to fail over 107 stress cycles. The fatigue limit could not be obtained even at 108 stress cycles. The test results were compared to those for the specimens without surface structure anomalies to show the effect of structure anomalies on the fatigue behavior of carburized steel.

Fatigue behavior of carburized steel with internal oxides and nonmartensitic microstructure near the surface

A method for carburizing low alloy steel having carbon content of no more than 0.5% employing two-step carburizing process. The method comprising pre-carburizing the steel in a carburizing atmosphere having a carbon potential of less than Acm to produce a carburized case having hypereutectoid composition, cooling the steel to transform the case into a bainite, pearlite or martensite structure, and carburizing the steel by heating thereof to a carburizing temperature for a sufficiently long time to produce carbides in a spheroidal or spherical form, the carburizing being carried out in such a manner that the heating speed of the steel is being controlled under 20° C. per minute in a region between Ac1 temperature and 750° C.-950° C. and the carbon potential in the carburizing atmosphere is being maintained over Acm when the temperature is over Ac1.

Carburization of ferrous alloys

Un nouveau procede de cementation suivi de carbonitruration ameliore la resistance aux piqures de frottement et a la fatigue par flexion des dents d'engrenages en aciers allies au Cr-Mo-Ni, au Cr-Mo, au Cr et au B-Mn. Relation avec la proportion d'austenite residuelle

Takeshi Naito

Papers

Fatigue behavior of carburized steel with internal oxides and nonmartensitic microstructure near the surface

Carburization of ferrous alloys

Improvement on the Fatigue Strength of Case Hardened Gears by a New Heat Treatment Process

Fatigue behavior of carburized steel with internal oxides near the surface

On the Heat-Treatment Characteristics of Containing Silicon-Nickel-Chromium-Molybdenum Carburizing Steel