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Microstructural characterisation of nickel rich areas and their influence on endurance limit of sintered steel Bernier, F.; Plamondon, P.; Baïlon, J.-P.; L'Espérance, G. Nickel is an often used alloying element in powder metal steel to achieve high hardenability. However, when nickel is added, the slow diffusion rate between iron and nickel leads to the formation of nickel rich areas (NRAs). Two steel alloys were studied: a Fe-6?4Ni-0?7Mo-0?7C with standard sized nickel powder additions and a Fe-2?4Ni-0?7Mo-0?7C with a finer sized nickel powder. Microstructural characterisation of the parts revealed that sufficient hardenability was achieved for both materials, but that NRAs were observed when standard sized nickel is used. Xray energy dispersive spectrometry and electron diffraction show that the NRAs are composed of martensite and austenite under rapid cooling conditions. Three-point bending fatigue tests were carried out on both alloys to evaluate the effect of these soft austenitic areas on the fatigue properties of powder metal steel parts. The analysis of the endurance limit results shows that NRAs are not a governing factor.

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Fatigue life prediction of welded joints needs an accurate and exhaustive theoretical Fracture Mechanics characterization of weld toe crack propagation. The method proposed by Albrecht et al. leads rapidly to accurate solutions of the LEFM AK-parameter. However, non-LEFM short crack behavior within the notch (weld toe) plastic zone must be taken into account. Available information on notch fatigue is surveyed, and practical cases where short crack growth is likely to occur are identified. Based on an elastoplastic finite element analysis, the LEFM validity limits and errors resulting from the misuse of LEFM in fatigue life prediction are quantified. NOMENCLATUREcrack length preexisting defect length, final crack length extent of the notch plastic zone plate thickness, stiffener thickness major semi-diameter of a semi-elliptical crack Paris law coefficients crack growth rate size of the crack tip cyclic (monotonic) plastic zone Young's modulus plastic constraint factor plastic modulus stress intensity factor, range of K, effective range of K value of K at crack closure weld leg length correction factors in K-expression fatigue life S~,,iSrn,, nominal stress, range of S value of S at crack closure relative fraction of the nominal stress range for which the crack tip is open stabilized value of U,, outside the notch plastic zone singularity of a V-notch stress field gamma function conventional AK threshold effective AK threshold weld toe angle Poisson-ratio local stress * cyclic yield stress, monotonic yield stress correction factor in K-expression 180"-0 (V-notch angle)

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