PSI - Issue 5

Marek Smaga et al. / Procedia Structural Integrity 5 (2017) 989–996 Marek Smaga et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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induced  -martensite and very localized  ´-martensite formation which was recently observed in metastable austenitic stainless steel AISI 347 cyclically loaded at 300°C in both HCF regime Sorich et al (2014) and VHCF regime Smaga et al. (2017b). Furthermore, very recent detailed scanning electron microscopy investigations of surface after focused ion beam preparation have shown that during turning with appropriately chosen parameters nano-crystalline surface layers were formed. More details were published elsewhere Smaga et al (2017a). Conclusion Cryogenic turning with CO 2 snow cooling allows in metastable austenitic stainless steel AISI 347 the production of net-shape geometry combined with the formation of martensitic surface layers due to deformation induced transformation processes. The morphologies of cryogenically machined and conventionally turned specimen surfaces were characterized by the roughness-, phase-, micro hardness- and residual stress measurements. X-ray diffraction measurements showed in cryogenically turned samples the existence of three phases: f.c.c.  -austenite, b.c.c.  ’ - martensite and h.c.p.  -martensite in the surface layer of thickness up to 300 µm. In stress-controlled fatigue tests at ambient temperature and T = 300°C, the martensitic surface layer resulted in (i) a clear improvement of fatigue life compared to specimens with fully austenitic structure on the surface, even in case of higher surface roughness, and (ii) in a reduction of plastic strain amplitude. Acknowledgements The authors thank the German Research Foundation (DFG) for the financial support within the CRC 926 ‘‘Microscale Morphology of Component Surfaces’’ . References Altenberger, I., Scholtes, B., Martin, U., Oettel, H., 1999. Cyclic deformation behavior and near surface microstructure of shot peened or deep rolled austenitic stainless steel AISI 304. Mater. Sci. Eng. A 264, 1 – 16. Angel, T., 1954. 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