A Novel Method for the Prediction of Critical Inclusion Size Leading to Fatigue Failure
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INTRODUCTION
INCLUSIONS are known to be present in commercial steels of any type. The inclusions might be impurities, enclosed exogenous materials, or results of deoxidizer addition during manufacturing.[1,2] It is well known that inclusions play a significant role in the fatigue life of steels. Numerous investigations have shown that stress localization at the inclusion–matrix interface is the origin of fatigue cracks.[3–9] The size, quantity, and distribution of inclusions must be controlled in steelmaking process so as to improve the fatigue properties of steels. Although it is possible to manufacture steels with minor inclusions, the mass production of the same is not feasible because of economical concerns. Fortunately, it is possible to manufacture steels with desirable fatigue properties because a critical inclusion size has been determined below which the fatigue crack will not originate from the inclusions. Murakami et al.[10–12, pp. 16–17] proposed an inclusion effective projected area model in which the fatigue properties of steels are estimated by the square root of the inclusion projected area perpendicular to the applied stress axis. The authors conducted a comprehensive study on the aggregate effect of surface roughness and nonmetallic inclusions in 30MnVS6 steels via this model.[13] However, it is critical to find a reliable way to estimate the critical inclusion size in steels for quality control considerations. The aim of this study is to investigate the critical inclusion size in 30MnVS6 steels, which are used mostly in automotive components. In this regard, two types of the same steel with similar
strengths but different inclusion sizes were prepared, and their fatigue properties were investigated based on the rotating bending fatigue test and the Murakami’s model in this study. The different models for the estimation of critical inclusion size was reviewed precisely, and finally, a novel method was proposed to determine this value for steels in the specified strength and fatigue range.
II.
MATERIALS AND EXPERIMENTS
Two commercial 30MnVS6 steels with different inclusion sizes were used in this study. The chemical compositions of these steels are shown in Table I. The average sizes of the observed inclusions were approximately 19 lm for steel A and 35 lm for steel B (measured according to ASTM E 45). The specimens were austenitized at 1473 K (1200 C) for 30 minutes followed by air cooling. The two steels were observed to have a ferritic-pearlitic microstructure. Table II shows the mechanical properties of the steels obtained after heat treatment. The standard rotating bending specimens were prepared as shown in Figure 1. To eliminate surface roughness effects, the specimens were ground after machining with a series of sandpapers to a mesh number of 2000. Rotating bending fatigue tests were carried out at room temperature and at 5800 rpm with the stress ratio of R = –1. Furthermore, all fracture surfaces were investigated using a scanning electron microscope (SEM).
III. S. SABERIFAR, Graduate Student,
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