Microstructural characterization of cyclic deformation behavior of metastable austenitic stainless steel AISI 347 with d
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In the present work, specimens of the metastable austenitic stainless steel AISI 347 with different surface morphologies were investigated in stress-controlled fatigue tests in the high cycle fatigue (HCF) regime at ambient temperature. Specific surface morphologies were generated by cryogenic turning with CO2 snow cooling. As a result of the metastable austenite microstructure, phase changes from paramagnetic austenite to ferromagnetic martensite take place in the near-surface regime during cryogenic turning as well as in the whole specimen volume during monotonic and/ or cyclic elastic–plastic deformation. The metastability of AISI 347 was characterized according to the MS-temperature determined from the chemical composition and by X-ray diffraction measurements with in situ cooling. Microhardness and strength of both phases were measured. Near-surface microstructure was analyzed by optical and scanning electron microscopy after focused ion beam preparation. Besides a partially martensitic surface layer, a thin nanocrystalline layer, both induced by cryogenic turning, was observed. In case of cyclic loading, the martensitic surface layer leads to a reduction of plastic strain amplitude as well as a retardation of crack initiation and consequently to an increase in fatigue life.
I. INTRODUCTION
The most widely used austenitic stainless steels are those of the AISI 300 series, i.e., Fe–C–Cr–Ni alloys containing approximately 18 wt% Cr and 8 wt% Ni with a very low carbon content ,0.08 wt%.1,2 The chemical composition not only influences the mechanical properties, formability, passivity but also significantly affects the stability of the face centered cubic austenitic microstructure. Hence, the paramagnetic austenite can transform due to plastic deformation to a more stable microstructure, i.e., paramagnetic e-martenisite and/or ferromagnetic a9-martensite.3–9 Consequently, in the AISI 300 series, several austenitic steels are metastable, e.g. AISI 304, 321, 348, or 316L5–9 and intensive investigations of monotonic-induced and cyclic-induced phase transformations as well as their influences on monotonic and cyclic strength were performed. According to their excellent mechanical and technological properties as well as corrosion resistance, austenitic Cr–Ni steels are widely used for components in nuclear power and chemical plants as well as in a big variety of industrial, architectonic and biological applications.1,2,10 Therefore, improvement of fatigue properties of these
Contributing Editor: Mathias Göken a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.318
steels is required. Since fatigue damage generally initiates at the surface,11,12 improved surface treatments and finishing processes are continuously developed. In this context, investigations on e.g. cryogenic laser shot peening,13 surface mechanical attrition treatment14 and cryogenic deep rolling15 were performed. Using a lowtemperature turning process with carbon dioxide snow as cooling medium in the cutting zone, a
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