Characterization of deformation induced near-surface martensite and fatigue behaviour of cryogenic machined austenitic steel
Annika Boemke, Institute of Materials Science and Engineering/ University of Kaiserslautern, Kaiserslautern, GermanyRobert Skorupski, Institute of Materials Science and Engineering/ University of Kaiserslautern, Kaiserslautern, GermanyMarek Smaga, Institute of Materials Science and Engineering/ University of Kaiserslautern, Kaiserslautern, GermanyDietmar Eifler, Institute of Materials Science and Engineering/ University of Kaiserslautern, Kaiserslautern, GermanyTilmann Beck, Institute of Materials Science and Engineering/ University of Kaiserslautern, Kaiserslautern, Germany
Within the Collaborative Research Center 926 "Microscale Morphology of Component Surfaces" at the University of Kaiserslautern, it was shown that by variation of the cutting parameters during cryogenic turning, different surface morphologies up to 300 micrometer depth can be produced in the metastable austenitic stainless steel X6CrNiNb1810. Increasing feed rate especially causes higher martensite contents up to larger depths under the machined surface. In addition to fcc gamma-austenite and bcc α'-martensite hexagonal epsilon-martensite is observed as a further microstructural phase. The maximum concentration of α'-martensite shifts deeper under the surface if feed rate is increased from 0,15 mm/rev to 0,35 mm/rev. ε-martensite could only be detected directly underneath the surface. Decreasing micro-hardness from the surface to the volume can be measured for both surface variations.
The modified surface morphology significantly influences fatigue strength in the low-cycle (LCF) and high-cycle fatigue (HCF) regime. Even though cryogenic turning results in higher surface roughness, these samples show significantly higher fatigue strength compared to conventionally machined, purely austenitic samples.
Recent research focusses on the fatigue behaviour of metastable austenitic steel X6CrNiNb1810 and the stable austenitic steel X1NiCrMoCu25205 in the VHCF-regime. Therefore, an ultrasonic fatigue testing machine was developed and built up at the authors' institute. From the HCF-results outlined above and literature data on VHCF behaviour of conventionally machined metastable austenite, it is expected that cryogenic turning has the potential to increase fatigue strength in the VHCF-regime even though crack initiation at internal defects can occur. To separate the influence of transformation induced martensite formation from microstructure modification in the retained austenite, the stable austenitic steel will be turned with the same parameters as the metastable variant and fatigued performance and damage mechanisms will be compared from the LCF to the VHCF regime.
The authors thank the German Research Foundation (DFG) for funding the presented research within CRC 926 "Microscale Morphology of Component Surfaces (MICOS)".