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The effect of surface compressive residual stresses (RS) induced by surface grinding and polishing on the gigacycle fatigue behavior of medium-carbon high-chromium alloy cold work tool steel was evaluated. Two test series were performed: Specimens of series I revealed high compressive RS of about -800 MPa at the surface, resulting from grinding with fine emery paper, which treatment had definitely a beneficial influence on the fatigue endurance strength. The existence of surface RS was also revealed to be responsible for the location of the fatigue crack initiation. High compressive RS favored internal crack origins. In this case crack nucleation sites were primary carbide clusters in the interior of the specimen, forming so-called fish-eyes at the fracture surface. In contrast, specimens of test series II had only very low RS, which enabled crack initiation near/at the surface at primary carbides/clusters. Furthermore, it has been shown that the high initial RS are prone to partial relaxation through cyclic loading for which the mechanisms are currently unknown. In this case near-surface induced failure was obtained. It was possible to confirm the experimentally obtained data by the use of the concept of local fatigue strength as function of effective RS. The relaxation of high initial RS was experimentally confirmed through RS measurements at runout specimens (10 10 cycles without failure

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In this work, the gigacycle fatigue response of several tool steel grades, produced by ingot metallurgy and powder metallurgy, has been studied using an ultrasonic frequency resonance testing device. In all cases the S–N curves exhibited a consistent drop up to 10 billion cycles, i. e. there is no fatigue “limit” with these materials. Surprisingly, there was virtually no effect of the composition and hardness of the materials, both for powder metallurgy and ingot metallurgy grades cold work tool steels and high speed steels exhibiting virtually the same S–N curves, however the powder metallurgy steels revealed significantly higher endurance strength levels than ingot metallurgy grades. In the ingot metallurgy tool steel grades, crack initiation started at large primary carbides or carbide clusters, while in the powder metallurgy grades, markedly smaller non-metallic inclusions turned out being the critical defects. Furthermore, it has been shown that it is very important to avoid introducing residual stresses into the specimen surfaces during preparation, since these stresses significantly affect the endurance strength levels and the location of crack initiating sites.

Alkyd resins are polyesters containing unsaturated fatty acids that are used as binding agents in paints and coatings. Chemical drying of these polyesters is based on heavy metal catalyzed cross-linking of the unsaturated fatty acid moieties. Among the heavy-metal catalysts, cobalt complexes are the most effective, yet they have been proven to be carcinogenic. Therefore, strategies to replace the cobalt-based catalyst by environmentally friendlier and less toxic alternatives are under development. Here, we demonstrate for the first time that a laccase-mediator system can effectively replace the heavy-metal catalyst and cross-link alkyd resins. Interestingly, the biocatalytic reaction does not only work in aqueous media, but also in a solid film, where enzyme diffusion is limited. Within the catalytic cycle, the mediator oxidizes the alkyd resin and is regenerated by the laccase, which is uniformly distributed within the drying film as evidenced by confocal laser scanning microscopy. During gradual build-up of molecular weight, there is a concomitant decrease of the oxygen content in the film. A new optical sensor to follow oxygen consumption during the cross-linking reaction was developed and validated with state of the art techniques. A remarkable feature is the low sample amount required, which allows faster screening of new catalysts.