The influence of retained austenite on fatigue of the martensitic high nitrogen (MHNS) cold-work tool steel: X30CrMoN15-1
Benedikt Schmitz, Universität Duisburg-Essen, Institut für Technologien der Metalle, Lehrstuhl der Werkstofftechnik, Lotharstr. 1, Duisburg, GermanySedat Güler, Universität Duisburg-Essen, Institut für Technologien der Metalle, Lehrstuhl der Werkstofftechnik, Lotharstr. 1, Duisburg, GermanyAlfons Fischer, Universität Duisburg-Essen, Institut für Technologien der Metalle, Lehrstuhl der Werkstofftechnik, Lotharstr. 1, Duisburg, Germany
The advantages of nitrogen martensitic steels, mainly its strength paired with the resistance to corrosion, are interesting for a lot of technical applications. In combination with the high amount of chromium, the sufficient amount of carbon, and nitrogen allows also for precipitates, which gain this unique combination of properties. Further research shows that nitrogen in solution provides excellent corrosion properties while in precipitates leads to good temper and creep resistance in such martensitic steels. Thus MHNS are attractive for bearings in aerospace as well as general mechanical engineering of parts and tools.
The following study deals with the microstructural analysis after total strain controlled uniaxial fatigue tests at different tempering conditions. Previous studies already showed the path to control the retained austenite by specific tempering conditions. Based on this information, the purpose of the present study was to compare the fatigue behaviour in relation to the content of retained austenite (RA). The latter was localized and quantified by scanning electron microscope (SEM) and electron back scatter detection (EBSD). The heat treatment measurements were: 1h/1100°C/Oil+2h/-196°C/Air. Furthermore, the temper has drawn at two different temperatures. Fatigue tests were conducted in laboratory air at room temperature and depending on its strain amplitude at 0.5 Hz to 2.5 Hz. The stress-strain hysteresis was analyzed as to the elastic and plastic strains by Manson-Coffin and Ramberg-Osgood's methods. The fatigue strength with RA was measured to be at 400 MPa for samples with RA while without RA, 540 MPa could be measured.
It was possible to show that under fatigue, cracks preferentially initiate at the RA, which appears as a banded structure as a consequence of segregation of N. Such RA-bands tend to remain at former austenite grain boundaries and are prone to bare tensile residual stressesafter heat treatment.