Microscopic and nanoscopic observations of metallurgical structures around inclusions at interior crack initiation site for a bearing steel in very high‐cycle fatigue

Abstract

A B S T R A C T In some high-strength steels, a fatigue crack tends to occur at the interior inclusion after a long-term sequence of the cyclic loadings at low stress levels, although the crack takes place at the surface in the usual life region at high stress levels. Thus, we have the duplex S-N curves consisting of the respective S-N curves for usual life region and very highcycle regime. It is well known that a significant fracture surface having the fine granular morphology is formed around the interior inclusion at the crack initiation site. This surface area is sometimes called as 'fine granular area'. In this work, metallurgical structures around the interior inclusion at the fatigue crack initiation site were carefully observed by combining several special techniques such as focused ion beam technique and highresolution scanning electronic microscopes. Based on the current observation results, it was found that the microstructure around the interior inclusion was changed into the penny-shape fine granular layer from the usual martensitic structure during long-term cyclic loadings. Then, debondings along with the boundaries of the matrix and the fine granular layer have produced the small cracks inside the metallic material, and these interior cracks caused the final fatigue fracture after definite loading cycles of the crack propagation.Keywords bearing steel; fine acicular area (FAA); fine granular area (FGA); interior crack initiation; rotating bending; very high-cycle fatigue. N O M E N C L A T U R Eα = stress concentration factor D = distance from definite section ΔK th = threshold value of stress intensity factor range in crack propagation N = number of cycles N f = number of cycles to failure R = stress ratio (minimum stress/maximum stress) σ = stress σ a = stress amplitude σ B = tensile strength σ w = fatigue limit I N T R O D U C T I O NAs a common understanding in the Metal Fatigue, it is well known that ferrous metals such as structural steels reveal the clear fatigue limit at the number of stress cycles less than 10 7 cycles. 1 However, non-ferrous metals such as aluminium alloys have no fatigue limit such that the S-N curve tends to decrease continuously in the very long life region, longer than 10 7 cycles. Usually, the fatigue limit for the structural steels is proportional to the tensile strength, and we have σ w = σ B /2, approximately, in the wide variety of the strength levels. 2,3 In recent years, practical structures such as railway wheels and axles, offshore structures, energy conversion and transportation systems have been used in a long term, sometimes, beyond their original design life

Very-high-cycle fatigue behavior of a structural steel with and without induced surface defects

“…Thus, the initiated microcracks connected to each other and ran through the fine granular layer region. A similar explanation of ''local grain refinement at the crack tip" [29] was subsequently proposed, which together with the previous one by Sakai [27,28] considered that the fine grains were produced before the related crack formation.…”

“…The crack propagation was independent of the spherical carbide decohesion after FGA reached a critical size. Sakai [27,28] stated that a fine granular layer was first generated in the vicinity of an inclusion during a large number of cyclic loadings. The microcracks formed by the debonding between the fine granular layer and the matrix.…”

The formation mechanism of characteristic region at crack initiation for very-high-cycle fatigue of high-strength steels

“…To reveal the formation mechanism of this crack initiation characteristic region is extremely essential for understanding the unique behavior of VHCF and predicting the fatigue life of VHCF for high-strength steels. Several mechanisms have been proposed for this aspect, including ''hydrogen assisted crack growth" [11,19], ''decohesion of spherical carbide" [20], and ''formation and debonding of fine granular layer" [2,21].…”

“…For the mechanism of ''formation and debonding of fine granular layer" [2,21], it was regarded that FGA was caused by the debonding of the fine granular layer and the matrix to form the rough morphology, and the fine granular layer was generated (before crack initiation) by the intensive polygonization in the vicinity of an inclusion during the long sequence of cyclic loading. It was supposed that when a penny-shape crack of FGA was formed http://dx.doi.org/10.1016/j.ijfatigue.2015.11.029 0142-1123/Ó 2015 Elsevier Ltd. All rights reserved.…”

Failure mechanisms and fatigue strength reduction factor of a Cr-Ni-Mo-V steel welded joint up to ultra-long life regime

“…It is believed that the interior microdefects being the crack initiation site is responsible for the disappearance of traditional fatigue limit. An enlarged observation around the inclusion illustrates the occurrence of fine granular area (FGA), according to Sakai et al [19], with the characteristic of rough but fine surface. Here, the width of FGA at the radical direction is measured to be about 35 mm.…”