The effect of work hardening on the fatigue behaviour of the austenitic high interstitial steel Cr18Mn19C0.35N0.61

Reza Abbassi, Universität Duisburg-Essen, ITM, Lehrstuhl für Werkstofftechnik, Duisburg, Germany
Sedat Gueler, Universität Duisburg-Essen, ITM, Lehrstuhl für Werkstofftechnik, Duisburg, Germany
Alfons Fischer, Universität Duisburg-Essen, ITM, Lehrstuhl für Werkstofftechnik, Duisburg, Germany

The beneficial combination of strength, ductility, and corrosion resistance renders applications in biomedical, automotive, mechanical and process engineering for austenitic stainless steels. Further improvement of the chemical and mechanical properties was achieved by alloying nitrogen. In order to avoid melting under pressure and decrease production costs, a part of N can also be substituted by C. This leads to austenitic high interstitial steels (AHIS).

The aim of this work is a microstructural study and analysis on the deformation mechanisms of work-hardened Cr18Mn19C0.35N0.61 (C+N=0.96) under monotonic and cyclic loading using electron backscatter diffraction (EBSD) technique. Thus, strain-controlled uniaxial fatigue tests were carried out. The comparison was conducted using total strain controlled axial LCF and HCF fatigue tests at room temperature on 35% cold worked samples. The tests were carried out up to the failure of the specimens or stopped with the achievement of the ultimate number of 2x106 load cycles. Subsequent microstructural examination was performed by optical microscopy, hardness measurement, fractography and the EBSD-analyses. Cold working enhanced the strength of CrMn-based high interstitial steel CN0.96 by 40% without a decrease of the elongation to fracture; the endurance limit did not follow this trend. Microstructural study after cold working indicates twinnings in grains of <111> direction and dislocation in all other directions. By analyzing the stress-strain-hysteresis, it was noticed that cyclic strengthening takes place within the very first cycles followed by softening.

Microstructural investigations show that no martensitic transformation did occur after cold working and fatigue test; deformation is performed through dislocation movements and twinnings. After the fatigue tests the kernel average missorientation increases. There is a lot of twinning in microstructures; however, the majority of deformation mechanisms refer to dislocation generation under loading and their movement. Regarding the same tests, which had been done on solution-annealed state of that material, it can be concluded that for Cr18Mn19C0.35N0.61, there is no direct relation between the amount of strength increase and endurance limit improvement. These results do not resemble previous research.

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