In this investigation, the near surface morphology of metastable austenitic steel AISI 347 after defined machining processes was characterized in detail. Optical micrographs of the near surface microstructure and micro hardness measurements revealed that a modified surface layer with a thickness of about 300 µm develops during cryogenic turning. Quantitative X-ray analysis of the cryogenic turned surface layers showed that the feed rate has a great influence on the resulting phase distribution. In addition to paramagnetic austenite and ferromagnetic aplha-martensite a third phase, the paramagnetic epsilon-martensite, is formed. All considered turning parameters result in tensile residual stresses at the surface. At a distance of about 40 µm from the surface the residual stress state changes compressive stresses, converging to a stress-free state with increasing distance from the surface. For detailed visualization of the near surface microstructure focused ion beam milled cross sections were observed by SEM / EBSD. The sub-surface layer consists of a 4 - 8 µm thick nanocrystalline layer localized directly on the surface and an underlying austenitic/martensitic microstructure. The surface morphology influences significantly the fatigue strength in the low and high cycle fatigue regime. It should be noted that even though cryogenic turning results in a higher surface roughness, fatigue strength outperforms purely austenitic samples with polished surface.
The authors thank the German Research Foundation (DFG) for the support of the research within the CRC 926 „Microscale Morphology of Component Surfaces (MICOS)“.