Steels for bearings

Fatigue crack propagation behaviour of an ultra-high strength, silicon-modified AISI 4340 alloy steel (300-M) has been investigated in moist air over an extremely wide range of growth rates from 10−8 to 10−1 mm/cycle. Particular emphasis has been devoted to the influence of microstructure on fatigue-fracture behaviour near the threshold stress intensity, ∆K 0 below which crack growth cannot be detected. By varying microstructure through quench and tempering and isothermal transformations, the threshold stress intensity and near-threshold crack-propagation rates are observed to be influenced by mean stress (load ratio), material strength, grain size, and impurity segregation. The threshold ∆K 0 for crack propagation is found to be inversely related to the strength of the steel, and a relationship between ∆K 0 and cyclic yield stress is observed. It is shown how near-threshold crack -growth resistance can be improved by (i) cyclic softening, (ii) coarsening the prior austenite grain size, and (iii) ...

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Influence of microstructure on near-threshold fatigue-crack propagation in ultra-high strength steel

Correlation of cavitation erosion behavior with mechanical properties of metals

Fatigue crack propagation behavior of an ultra-high strength steel (300-M) has been investigated in humid air over a very wide spectrum of growth rates from 10/sup -8/ to 10/sup -1/ mm/cycle. Particular emphasis has been devoted to the influence of mean stress (or load ratio R = K/sub min//K/sub max/) and microstructure on fatigue crack growth near the threshold stress intensity for crack propagation, ..delta..K/sub 0/. Increasing the load ratio from R = 0.05 to 0.70 was found to lead to increased near-threshold growth rates, and a decrease in the threshold stress intensity. Similarly, increasing material strength, by varying the microstructure through quench and tempering and isothermal transformation, resulted in higher near-threshold growth rates, and a marked reduction of ..delta..K/sub 0/. These effects are contrasted with behavior at higher growth rates. The infuence of strength on ..delta..K/sub 0/ is rationalized in terms of the cyclic hardening or softening response of the material, and hence it is shown that cyclic softening can be beneficial to fatigue crack propagation resistance at very low growth rates. The results are discussed in the light of crack closure and environmental contributions to fatigue crack growth at low stress intensities.

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Near-threshold fatigue crack propagation in ultra-high strength steel: influence of load ratio and cyclic strength

Some of the recent developments in fatigue-crack initiation and growth are reviewed. With respect to fatigue-crack initiation, persistent slip bands can be importantly involved, and the understanding of PSB formation should in turn lead to a better understanding of the crack -initiation process. In the crack growth stage the topics of crack closure and overload effects are of interest. In the near-threshold region the observed high closure levels may be associated with crack growth in Mode II. It is shown that the overload retardation effect is a surface-related phenomenon, and the dependence of closure and overload effects on specimen geometry are discussed. Additional consideration is given to environmental effects, minimum flaw size limitations in the fracture approach, and the modelling of the crack -growth process.

Current aspects of fatigue

Cyclic stress strain relations and strain-controlled fatigue of 4140 steel

The fatigue crack propagation rates, da/dN, of 4140 steel were measured in dry argonvs tempering temperature. In specimens 3.2 mm thick at a given ΔK between 15 and 30 MN/ m3/2, da/dN decreases with increasing tempering temperature, reaches a shallow minimum for tempering at 400°C. The rate for as-quenched specimens increases withR ratio; this is not the case for the 400, 550 and 650°C tempers. Reducing the specimen thickness to 1.3 mm has little effect on the 650°C temper but causes a large decrease in da/dN for the asquenched condition and 200°C temper. Edge notch specimens tempered at 550 and 650°C are subject to crack arrest from cycling prior to crack initiation. The results are discussed in terms of the metallurgical structures and various fatigue crack propagation equations which have been proposed. The results cannot be explained on the basis of da/dN being determined only by Young’s modulus andK
 c.

Paul N. Thielen

Papers

Cyclic stress strain relations and strain-controlled fatigue of 4140 steel

Fatigue crack propagation in 4140 steel

Cyclic strength differential effect in 4140 steel