112 D.V. Nelson and Z. Long 107 106 105 1 2 3 4 5 6 7 8 9 Number of cycles to failure (Nf) ΔKinc ΔKODA (MPa ) ΔK m 0 0.2 0.4 0.8 1 2 3 4 5 6 7 8 Depth from the surface (mm) ΔKinc ΔKODA 0.6 (MPa ) ΔK m a b Fig. 14.12 Stress intensity factor range at inclusions and ODAs for steel B vs. (a) cycles to failure and (b) depth of inclusions region of an ODA may differ from the “macro” residual stresses, but little appears to be known about such differences. For example, computational modeling [38] suggests that the residual stresses near inclusions the develop from shot peening of hardened steel can differ significantly from stresses measured in typical volumes probed by X-ray diffraction with conventional beam diameters. The distributions of residual stresses that develop locally around inclusions from martensitic transformations, as in case hardening, are difficult to predict with confidence owing to the complexities involved in material behavior at the crystalline size scale. A better undemanding of residual stresses around inclusions may improve prediction of cracking within ODAs. Different mechanisms have been proposed for development of cracking within ODAs. Murakami et al. [15] suggested that hydrogen played an important role in cracking, perhaps by embrittlement. Shiozawa & Lu [39] proposed that ODAs are formed by carbide decohesion, growth of microcracks along boundaries between carbides and matrix and microcrack coalescence. Larger precipitated carbide particles (approx. 1 m in size) of Mo, V and tungsten were observed on fracture surfaces within ODAs. Finer particles (e.g., a fraction of a micron) of chromium carbide and iron carbide were also found. Fractography revealed that larger particles ruptured and smaller ones peeled off. Hydrogen is known to have the potential to reduce cohesive strength at carbide-to-matrix interfaces [40]. It seems possible that hydrogen may promote the mechanism suggested by Shiozawa et al. [9], and future study of this potential influence may enhance the understanding of how ODAs develop. Sakai [3] suggested a different mechanism for formation of ODAs in which very fine sub-grains with different crystallographic orientations are formed by micro-scale polygonization and that debonding occurred along their boundaries, along with coalescence of the micro-debondings. Grad et al. [41] have found that cyclic loading at the stress concentration created by inclusions causes significant grain refinement within ODAs. As Li [4] has noted, hydrogen assisted local plasticity may encourage dislocation activity and multi-slip around inclusions, encouraging mechanisms such as polygonization. It may of interest in future studies of fish eye fatigue to investigate how hydrogen content varies with depth within carburized cases using a measurement approach like that applied by Murakami & Matsunaga [42] to hydrogen charged specimens. 14.5 Conclusions 1. Previous studies on fish eye fatigue in carburized specimens in which depths of crack origins were reported revealed that origins were at depths where compressive residual were minimal and hardness values were close to those in the core. By contrast, all fish eye fatigue cracks started at sub-surface inclusions within the case of carburized specimens in this study, at depths where compressive residual stresses were significant and where hardness values were much higher than in the core. 2. Murakami’s parea parameter model was found to be effective in predicting fatigue strength when taking into account hardness and compressive residual stresses at the locations of inclusions from which cracks originated. 3. Values of 4Kinc at the border of inclusions, computed to account for residual stresses by superposition with applied stresses, were similar for both steels tested and in the range of 2.5–3.6, MPapm comparable to values in the literature for specimens that had minimal residual stresses at locations where fish eye cracks originated. Values of 4KODA at the outer
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