Fatigue properties of high-manganese steel: effects of microstructure, internal hydrogen and SFE

Abstract

High-manganese (high-Mn) steel is a newly-developed material with high strength and ductility for structural components. Due to such excellent properties, high-Mn steel is being evaluated as an alternative material in various industries. However, studies of the fatigue characteristics of high-Mn steels are insufficient. Therefore, in this study, fatigue properties of high-Mn steels were studied. The improvement of fatigue resistance by modification of microstructure and the change of fatigue characteristics due to internal hydrogen were studied. The first research is an investigation of the effect of microstructures on tensile and fatigue properties of a high-Mn austenitic steel. Changing the amount of pre-strain, and the pre-straining temperature yielded various microstructural components, such as deformation-induced twin boundaries and martensite. Tensile tests and fully-reversed strain-controlled fatigue tests were conducted on plate-type samples. The presence of a small amount of ε-martensite increased yield stress and fatigue resistance by fostering deflected-and-branched propagation of fatigue cracks, whereas a high fraction of ε-martensite degraded the mechanical properties. In contrast, the presence of mechanical twin boundaries always improved both tensile and fatigue properties under the conditions used in this work. The sizes of dislocation cells decreased and spacing of fatigue striations narrowed as the fraction of mechanical twin boundaries increased. Results suggest that a microstructure composed of ~20% ε-martensite and numerous mechanical twin boundaries can obtain superior tensile and fatigue properties. The second topic is the effect of high-cycle fatigue properties of cold-drawn high-Mn steel, a favored candidate for replacing fully pearlitic (FP) steels in wire applications. For comparison, the high-cycle fatigue tests were conducted on cold-drawn TWIP and FP steels that had comparable ultimate tensile strength. Fatigue strength of both TWIP and FP steels increased with the tensile strength, but in TWIP steel that had been cold-drawn to a tensile strength of 1.5 GPa, the ratio of fatigue strength to tensile strength was very low, and deviated far from the predicted linear relationship. Fracture surface analysis showed that crack initiation mainly occurred at the ferrite matrix in FP steels, but either at grain or a twin boundaries in TWIP steels where a large density of dislocations piled up during cold drawing. In TWIP steels, notch sensitivity increased with tensile strength, so the presence of inclusions at grain boundaries led to high local stress concentration and caused early intergranular fatigue cracking. Subsequent annealing after cold-drawing increased fatigue strength of TWIP steels. I suggest that TWIP steel that has both high tensile strength and excellent high cycle fatigue strength could be a promising alternative to conventional FP steels. The third study was performed to clarify the effect of Al addition on low-cycle fatigue (LCF) properties of high-Mn twinning-induced plasticity (TWIP) steels, which were electrochemically charged with hydrogen. For this purpose, two alloys, Fe-17Mn-0.8C (0Al), and Fe-17Mn-0.8C-2Al (2Al) were used. Internal hydrogen clearly degraded the resistance to fatigue fracture in both steels. At high strain amplitude, Al addition improved the resistance to hydrogen embrittlement (HE) by preventing intergranular cracking. In contrast, at low strain amplitude, LCF life was reduced more in 2Al than in 0Al, because large-size inclusions in 2Al promoted initiation of fatigue cracks. These results demonstrate that Al addition does not always provide a beneficial effect on LCF behavior of TWIP steels in a hydrogen environment, but that the behavior depends strongly on strain mode or amplitude. The final study was an investigation of the effect of carbon content on tensile and low-cycle fatigue (LCF) properties of high-Mn steels, which were electrochemically charged with hydrogen. For this purpose, Fe-17Mn-xC (x = 0.5, 0.7 or 0.9 weight%) steels were used. Monotonic tensile test and fully-reversed strain-controlled fatigue test were conducted using plate-type samples. Increase in carbon content clearly led to increase in tensile properties (e.g. yield and tensile strength, total elongation), but decrease in LCF life in hydrogen-uncharged condition. In contrast, addition of carbon shows exactly opposite effect in resistance to hydrogen embrittlement (HE). The resistance to HE decreased with increasing carbon content in tensile, but increased with increasing carbon content in LCF test. These results are interpreted as the effect of the operated mechanisms according to loading mode; twinning is operated during monotonic loading, but only the slip-induced deformation is dominantly operated during cyclic loading. As carbon content increase in monotonic loading, thinner and more numerous twin boundaries are formed which increase stress and hydrogen concentrations at grain boundaries. On the other hand, the increase of stacking fault energy (SFE) due to the increase of carbon content served as an important factor in cyclic loading; the cross slip was frequently operated due to the increased SFE and the hydrogen concentration was lowered at grain boundaries.