Influence of inclusion size on fatigue behavior of high strength steels in the gigacycle fatigue regime

Abstract

The fatigue properties of four high strength steels with same strength class but containing different inclusion sizes were investigated using an ultrasonic fatigue testing machine in the gigacycle fatigue regime. The fracture surfaces were observed using field emission scanning electron microscopy (FESEM) and element distributions at the crack origins were measured by an electron probe microanalyzer (EPMA). The fatigue behavior can be divided into three categories:

• The S–N curve displays a continuous decline and the internal cracks initiated from the large oxide inclusions for commercial 50CrV4 steel in which the average inclusion size is about 29 μm.

• Step-wise S–N curves were observed for clean 54SiCrV6 and clean 50CrV4 steels in which the average inclusion sizes are about 3.0 and 2.4 μm respectively. Most fatigue failures originated from the VC inclusion clusters at the lower stress amplitudes.

• For clean 54SiCr6 steel in which the inclusion size is smaller than 1 μm, the fatigue cracks did not initiated from inclusions or inclusion clusters but from the region enriched with carbon. S–N curve shows that the fatigue failure hardly occurs from 10⁶ to 10⁹ cycles, in other words, the fatigue reliability can be substantially improved in the super long fatigue life regime.

Introduction

For low and medium strength steels, specimens do not fail after testing to 10⁷cycles, so the fatigue S–N curve is assumed to be a horizontal asymptote according to the conventional fatigue testing for steels. However, recent investigations have indicated that fatigue failure does occur in the regime of N > 10⁷ cycles when the applied stress amplitude is lower than conventional fatigue strength in high strength steels [1], [2], [3], [4], [5], [6], [7], [8]. Stickler et al. [8] and Weiss et al. [9] have made the numerous investigation of the fatigue behavior of PM steel and founded that crack initiation occurred mainly at singularities such as single larger, isolated pores or also at pore clusters. For low alloy high strength steels, the fatigue crack generally initiates from a larger non-metallic inclusions and the fracture surface displays a fish-eye pattern [1], [2], [3], [4], [5], [6], [7]. The stress localization at the interface between the inclusion and matrix is the origin of fatigue cracking. This arises from the differential thermal contraction coefficients of inclusions and matrix during cooling and the concentration of applied stresses due to the different elastic constants between the matrix and the inclusions. As a result of stress concentration due to the factors mentioned above, cracks can initiate easily at the interface between inclusions and matrix [10]. In clean steels the size and number of non-metallic inclusions decrease greatly and mechanical properties can be improved substantially. Kiedssling [11] supposed that crack would not initiate from inclusions when the size of non-metallic inclusion is less than 5 μm in clean steels from the point of view of fracture toughness. Similarly under fatigue conditions, there must be a critical inclusion size, below which the fatigue fracture origins will not initiate from the inclusions. In this paper, a commercial and three clean steels of same strength class containing different inclusion sizes were prepared and the fatigue properties were investigated using an ultrasonic fatigue testing machine in the gigacycle fatigue regime. The effects of inclusion sizes over or below the critical value on fatigue behaviors were discussed.