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Using Martensite Formation during Tube-Forming to Optimize Fatigue Strength



Metastable austenitic stainless steels can undergo a deformation-induced phase transformation from austenite (fcc) into ϵ-martensite (hcp) and the harder α′-martensite (bcc). This class of materials is widely used in sheet metal forming processes because of a beneficial combination of strength, ductility, and corrosion resistance. In such manufacturing processes phase transformation can be exploited in order to optimize the monotonic and cyclic strength of the components locally. In this paper it is shown that the amount of transformation can be controlled by a variation of process parameters such as deformation rate, temperature, and amount of deformation. An increase in initial temperature and strain rate reduces the rate of martensite formation. The effect of martensite volume fraction on the fatigue behavior is systematically investigated and the damage mechanisms in the VHCF regime are analyzed. For a very high number of cycles (beyond 107) a high martensite volume fraction (of more than 50%) leads to internal crack initiation from oxidic inclusions. Form and dimension of the inclusion are found to determine the number of cycles up to failure. An optimum volume fraction of the martensite phase is proposed for both the HCF as well as for the VHCF regime.

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