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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Nomenclature ASL p austenitic surface layer after mechanical and electrolytic polishing ASL t austenitic surface layer turned without CO 2 snow cooling MSL p martensitic surface layer after mechanical and electrolytic polishing turned with CO 2 snow cooling MSL t martensitic surface layer turned with CO 2 snow cooling f cutting feed [rev/min] α´ b.c.c. (body centered cubic)  ´-martensite  f.c.c. (face centered cubic)  -austenite  h.c.p.  hexagonal close packed)  -martensite   a,p plastic strain amplitude [-]  t total-strain [-]  a stress amplitude [MPa]  n nominal stress [MPa]  RS residual stresses [MPa]   fraction of magnetic phase, i.e. ferromagnetic  ´-martensite measured with Feritscope TM [FE-%] According to their excellent mechanical and technological properties as well as their corrosion resistivity, austenitic stainless steels are widely used for components in nuclear power and chemical plants as well as in a great variety of industrial, architectonic and biological applications – for a review see e.g. Lo and Shek (2009). After quenching from solution annealing temperature a large number of technically relevant chromium-nickel stainless steels exhibit austenite in a metastable state. Due to plastic deformation, local phase transformations from paramagnetic austenite into ferromagnetic martensite occur in these alloys Smaga et al. (2008) which can affect the mechanical properties of the material in a positive manner Marshall (1984). Besides typical surface hardening mechanisms like increase of dislocation density or introduction of compressive residual stresses, it is also possible to modify surface morphology of metastable austenitic structure by the deformation induced martensite formation – see Altenberger et al. (1999). In this context, it should be noted that the surface morphology of metastable austenite includes both topographic and microstructural features, i.e. volume fraction of phases, thickness of affected surface layer and distribution of martensite as well as residual stress state and surface roughness (Fig. 1a). (a) (b) 1. Introduction

Fig. 1. (a) Schematic representation of surface morphology; (b) fatigue testing specimen during cryogenic turning.

Since fatigue damage generally initiates at the surface Murghrabi (2009) a great effort is permanently exerted to the improvement of surface treatment and finishing processes which can considerably increase fatigue life of metals. Numerous diverse technologies of surface modification, e.g. cryogenic laser shot peening Ye et al. (2012) or abrasive ball blasting and cryogenic deep rolling Meyer (2012) are investigated at present. A possible variation in the morphology at the specimen surface can be realized also by a turning process utilizing carbon dioxide snow as cooling medium to achieve low temperatures in the cutting zone Aurich et al. (2014). In the case of metastable austenitic