FGA formation mechanism for X10CrNiMoV12‐2‐2 and 34CrNiMo6 for constant and variable amplitude tests under the influence of applied mean loads

Abstract

The aim of the present work is to clarify the fine granular area (FGA) formation mechanism in two steels (tempered 34CrNiMo6 and X10CrNiMoV12‐2‐2) causing grain refinement in the early state of fatigue for internal crack initiation and propagation in the very high cycle fatigue regime at pure tension‐compression loading (R = −1) and for applied mean stresses (R ≠ −1). Fatigue tests were performed with constant and variable amplitude at several R values using ultrasonic fatigue testing setups. Failed specimens were investigated using high‐resolution scanning electron microscopy and focused ion beam technique with special attention paid to the crack origin and the surrounding microstructure. To prove models for FGA formation proposed in literature, a numerical model to evaluate effective R values and contact stresses between the fracture surfaces depending on the crack length has been realised. The aim of these investigations is to estimate the influence of crack closure effects on FGA formation. FGA formation due to repeating contact of the fracture surfaces according to the model postulated by Hong et al correlates well with the findings for numerical simulations.

Structure characterization within the vicinity of the fine granular area by synchrotron radiation nano‐CT

“…A series of slices was observed carefully, especially near the surface of the FGA. It is worthy to note that nanograin layers with a dimension of hundreds nanometres in depth reported by previous references 24, 30 are not observed in Figure 6A. Only a series of subgrains can be observed.…”

“…The field of view for nano‐CT at the Beijing Synchrotron Radiation Facility is 15 μm, and the corresponding spatial resolution is better than 30 nm 29 . Considering the sizes of the FGA and the small granular area, 21,24,30 the field of view is near the size of the FGA and the spatial resolution is smaller than the size of the small granular area. Thus, it is worth trying to get the spatial structure within the vicinity of the FGA using nano‐CT at the Beijing Synchrotron Radiation Facility.…”

Long-Life Fatigue of Carburized 12Cr2Ni Alloy Steel: Evaluation of Failure Characteristic and Prediction of Fatigue Strength

“…Based on Equation (20) and test data of carburized 12Cr2Ni alloy steel, the distribution of σ w versus V is shown in Figure 14. As can be seen, the variation trend of predicted sizes of σ w increased with the decrease in V. Compared with Figures 13 and 14, the maximum defect size increased with the increase in V, while the predicted value of σ w decreased with the increase in V; the variation trend of the two predicted values was just the opposite.…”

“…The three stages of the FGA formation process are as follows [15]: (1) grain refinement, (2) coalescence of microcracks, and (3) FGA formation. Over the years, researchers have attempted to explain the FGA formation process in different ways, such as depressive decohesion of spherical carbide [16], hydrogen embrittlement-assisted cracking [17], cyclic compression between crack faces [18], numerous cyclic pressing [19,20], grain refinement and local stress threshold decreasing [21], and nanograin refinement at negative stress effect [2]. The varied theories on the formation of the FGA means the topic needs further study and discussion.…”

Interior induced fatigue of surface‐strengthened steel under constant and variable loading: Failure mechanism and damage modeling

“…However, the current VHCF studies focus primarily on the condition of constant amplitude loading. The investigation of VHCF properties of structural materials under VA loading is relatively rare . Furthermore, although a lot of cumulative damage models such as damage curve models, continuum damage mechanics models, and energy‐based damage models, etc.…”