Abstract
In high-strength martensitic steels, the inclusions significantly affect the material performance especially in terms of fatigue properties. In this study, a numerical procedure to investigate the effect of the inclusions types and shapes on the residual stresses during the cooling process of the martensitic steels is applied systematically based on the scanning electronic microscopy (SEM) and energy dispersive spectrometer (EDS) results of different types of inclusions. The results show that the maximum residual stress around the interface between Mg-Al-O inclusion and the matrix is the largest, followed by TiN, Al-Ca-O-S, and MnS when the inclusions are assumed as perfect spheres for simplicity. However, these results are proved to be 28.0 to 48.0% inaccurate compared to the results considering actual shapes of inclusions. Furthermore, the convex shape of inclusion will lead to stress concentration in the matrix while the concave shape of inclusion will lead to stress concentration in the inclusion. The residual stress increases with the increase of inclusion edge angle. The increase rate is the largest for TiN inclusions on the concave angle, which leads to extreme stress concentration inside TiN inclusion.
Highlights
Driven by the increasing demand on the higher mechanical performance of engineering structures, high-strength steels are steadily developed and widely applied in recent decades in multiple areas, e.g., automotive, high-speed trains, and aerospace
For the recently developed high-strength steels, it is found that even when stress amplitude is lower than fatigue limit, the fatigue crack still will occur when fatigue life reaches a certain level [9,10,11,12]
Nonmetallic inclusions were observed with scanning electronic microscopy (SEM) and the compositions of inclusions were analyzed with energy dispersive spectrometer (EDS)
Summary
Driven by the increasing demand on the higher mechanical performance of engineering structures, high-strength steels are steadily developed and widely applied in recent decades in multiple areas, e.g., automotive, high-speed trains, and aerospace. Murakami et al [9] reported that the fatigue limit increases with the increase of steel hardness and decreases with the increase of the defect area. Increasing the strength of materials is leading to a higher hardness and eventually a higher fatigue strength limit assuming the same defect size. For the recently developed high-strength steels, it is found that even when stress amplitude is lower than fatigue limit, the fatigue crack still will occur when fatigue life reaches a certain level [9,10,11,12]. For higher stress and lower fatigue life, fatigue crack usually occurs from the component surface; for lower stress and longer fatigue life, fatigue crack usually occurs from inner microstructure [13]
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