The effect of quenching and defects size on the HCF behaviour of Boron steel
Abstract This work investigates the effect of natural and artificial surface defects and quenching on the fatigue strength of a Boron steel (22MnB5). A vast experimental campaign has been undertaken to study the high cycle fatigue behaviour and more specifically the fatigue damage mechanisms observed in quenched and untreated materials, under different loading conditions and with differents artificial defects sizes (from 25 μm to 370 μm radius). In order to test the sheet metal in shear an original test apparatus is used. The critical defect size is determined to be 100 ± 50 μm. This critical size does not appear to depend on the loading type or the microstructure of the material (i.e. ferritic–perlitic or martensitic). However, for large defects, the quenched material is more sensitive to the defect size than the untreated material. For a defect size range of 100–300 μm the slope of the Kitagawa–Takahashi diagram is approximately −1/3 and −1/6 for the quenched and untreated materials respectively. A probabilistic approach that leads naturally to a probabilistic Kitagawa type diagram is developed. This methodology can be used to explain the relationship between the influence of the heat treatment and the defect size on the fatigue behaviour of this steel.
Transition of fatigue failure mode of Ni–Cr–Mo low-alloy steel in very high cycle regime
Abstract Transition of the fatigue failure mode in very high cycle regime was investigated based on the experimental results obtained under a fully reversed tension–compression test in air at room temperature using a hourglass shaped specimen of a Ni–Cr–Mo low-alloy steel, JIS SNCM439. A clear duplex S–N curve existed in an emery-polished specimen as a result of the change from surface-induced failure to internal-induced failure with GBF area. On the other hand, a smooth and continuous S–N curve appeared in a specimen treated by a shot-peening, even though the failure mode changed depending on applied stress amplitude level. An internal-induced failure without GBF area was observed on the fracture surface of the shot-peened specimen in addition to the above two failure modes. Stress amplitude for the transition of the failure mode was explained by considering an effect of compressive residual stress in the surface layer, and competing subsurface crack growth rate with surface one. A map for transition of failure mode was proposed through the experimental and theoretical investigation.
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