Initiation and propagation behaviour of fatigue cracks in hard‐shot peened Type 316L steel in high cycle fatigue

Abstract

A B S T R A C T Observations of fatigue crack growth behaviour were made during rotating-bend testing of hard-shot peened Type 316L steel. From the results of these observations, the crack that developed in the axial direction was observed and the mechanism of the fatigue crack properties was clarified as follows: (1) Small circumferential surface fatigue cracks were detected at 60% of the fatigue lifetime. These cracks propagated very slowly in both the circumferential and radial directions.(2) When a radial crack reached a depth of between 150 and 350 µm, axial fatigue cracks were formed.(3) In the next stage, either the radial or the axial fatigue cracks continued propagating, or an inwards growing radial crack formed from the axial crack. (4) In the final stage, the circumferential surface crack began to grow rapidly and resulted in fracture. (5) The fracture type of hard-shot peened Type 316L is a particular type of surface fracture.Keywords fatigue crack behaviour; fracture surface; hard-shot peening; high cycle fatigue; Type 316L stainless steel. N O M E N C L A T U R E d= distance of axial crack from surfaceHv = Vickers hardness Hv 0 = Vickers hardness of material N = number of cycles N f = number of cycles to failure σ a = stress amplitude σ r = residual stress I N T R O D U C T I O NSurface-hardening treatment is useful for improving high cycle fatigue strength, and fatigue properties of many surface-hardened materials have been investigated in detail for a long time. According to recent research papers, it has become clear that a surface-hardened material exhibits two types of fatigue fracture: a surface fracture and an internal fracture. 1 In the case of surface fracture, the fatigue crack initiates at an inclusion or slip plane of the surface. Moreover, the fatigue crack initiation and propagation behaviours can be investigated easily because it is possible to observe the fatigue crack directly using an optical microscope or a scanning electron microscope (SEM). If these direct observation methods are impossible, indirect observation methods, such as the replication method,are useful for fatigue crack investigation. In the case of internal fracture, the fatigue crack initiates at a subsurface defect, such as an inclusion or pore. 2 For internal fractures, it is difficult to investigate the fatigue crack propagation behaviour because the fatigue crack cannot be directly observed with a microscope during fatigue tests. In this case, the fatigue crack propagation behaviour can be inferred to some degree by fracture surface observation using the SEM. 3 However, it is difficult to know the exact internal crack behaviour, so it is necessary to have additional information that supports the inference. The authors have determined the rotating bending high cycle fatigue properties of hard-shot-peened Type 316L austenitic stainless steel (HSP-treated Type 316L) in a series of studies. 4-12 Although the fatigue crack propagation behaviour was investigated through SEM fracture surface observations, there are stil...

Influence of surface residual stresses on gigacycle fatigue response of high chromium cold work tool steel

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