\n ABSTRACT\n Owing to difficult economical conditions, machines and structures often have to be used beyond the design lifetime. In this study, fatigue properties of a bearing steel in the long‐life region were experimentally examined under cyclic axial loading. The complicated S–N behaviour was well explained as a combination of S–N curves for surface‐induced fracture and interior inclusion‐induced fracture. Fish‐eye marks were always observed on the fracture surfaces of specimens, which failed in the latter fracture mode, and an inclusion was found at the center of the fish‐eye. Finally, it was found that the fatigue fracture of this steel in the long‐life region occurred through the following three processes: (i) formation of the characteristic area as a fine granular area (FGA), (ii) crack propagation to form the fish‐eye and (iii) rapid crack propagation to cause the catastrophic fracture.\n

\n Effect of shot‐peening on fatigue behaviour in the gigacycle regime was investigated in order to clarify the duplex\n S–N\n curve characteristics. Cantilever‐type rotary bending fatigue tests were performed in laboratory air at room temperature by using hourglass‐shaped specimens of high‐carbon–chromium bearing steel, JIS SUJ2. Fatigue crack initiation site changed from the surface of untreated specimen to the subsurface of the specimen because of hardening and compressive residual stress with shot‐peening in the region of high‐stress amplitude. On the other hand, no difference in fatigue life controlled by the subsurface crack initiation between untreated specimen and shot‐peening one was observed in high‐cycle region. It was suggested that the\n S–N\n curve corresponding to the internal fracture mode is inherent in the material, as compared with the\n S–N\n curve of surface fracture mode, which is affected by surface conditions, environmental conditions and so on. Subsurface crack initiation and propagation behaviour were discussed under the detailed measurement of crack initiation area and shape of the fish‐eye fracture surface.\n

Advanced bainitic steels with the multiphase structure of bainitic ferrite, retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue (HCF/VHCF) regimes. The subsurface microstructural fatigue crack initiation, referred to as “non-inclusion induced crack initiation, NIICI”, is a leading mode of failure of bainitic steels within the HCF/VHCF regimes. In this regard, there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF. To address this aspect, we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure. This was accomplished by uniquely combining electron microscopy, three-dimensional confocal microscopy, focused ion beam, and transmission Kikuchi diffraction. Interestingly, the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels, including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI, which resulted in the different distribution of fine grains in the crack initiation area. The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries. The formation of fine grains assisted the advancement of small cracks. Another important aspect discussed is the role of retained austenite (RA) during cyclic loading, on crack initiation and propagation in terms of the morphology, distribution and stability of RA, which determined the development of localized cyclic plastic deformation in multiphase structure.

CrCoNi-based high-entropy alloys have demonstrated outstanding mechanical properties, particularly at cryogenic temperatures. Here we investigate the fatigue-crack propagation properties of the equiatomic, single-phase, face -centered cubic, medium-entropy alloy (MEA), CrCoNi, that displays exceptional strength, ductility and toughness, all of which are enhanced at cryogenic temperatures. Fatigue-crack growth is examined, at a load ratio of 0.1 over a wide range of growth rates, from similar to 10(-11) to >10(-7) m/cycle, at room (293 K) and cryogenic (198 K, 77 K) temperatures for two grain sizes (similar to 7 and 68 mu m), with emphasis on near-threshold behavior. We find that the Delta K-th fatigue thresholds are increased with decreasing temperature and increasing grain size: from 5.7 MPa root m at 293 K to 8 MPa root m at 77 K in the fine-grained alloy, and from 9.4 MPa root m at 293 K to 13.7 MPa root m at 77 K in the coarse grained alloy. Mechanistically, transgranular cracking at 293 K transitions to a mixture of intergranular and transgranular at cryogenic temperatures, where the increased propensity of nano-twins appears to inhibit growth rates by deflecting the crack path. However, the main factor affecting near-threshold behavior is roughness-induced crack closure from interference between the crack flanks, which is enhanced by the rougher fracture surfaces at low temperatures, particularly in the coarser-grained microstructure. Fatigue-crack propagation behavior in CrCoNi is comparable to nickel-based superalloys but is superior to that of the high-entropy CrMnFeCoNi (Cantor) alloy and many high-strength steels, making the CrCoNi alloy an excellent candidate material for safety-critical applications, particularly involving low temperatures. Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Wheel–rail contact creates high stresses in both rails and wheels, which can lead to different damage, such as plastic deformation, wear and rolling contact fatigue (RCF). It is important to use high-quality steels that are resistant to these damages. Mechanical properties and failure of steels are determined by various microstructural features, such as grain size, phase fraction, as well as spatial distribution and morphology of these phases in the microstructure. To quantify the mechanical behavior of bainitic rail steels, uniaxial tensile experiments and hardness measurements were performed. In order to characterize the influence of microstructure on the mechanical behavior, various microscopy techniques, such as light optical microscopy (LOM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), were used. Three bainitic grades industrially known as B360, B1400 plus and Cr-Bainitic together with commonly used R350HT pearlitic grade were studied. Influence of isothermal bainitic heat treatment on the microstructure and mechanical properties of the bainitic grades was investigated and compared with B360, B1400 plus, Cr-Bainitic and R350HT in as-received (AR) condition from the industry. The results show that the carbide-free bainitic steel (B360) after an isothermal heat treatment offers the best mechanical performance among these steels due to a very fine, carbide-free bainitic microstructure consisting of bainitic ferrite and retained austenite laths.

Recently, the steel plates used in the ship, pipeline and bridge generally required not only high strength but also excellent low temperature toughness. As a competitive candidate, the ultra-low carbon high strength low alloyed (HSLA) steel has been developed widely. The low temperature toughness depends on the microstructure of the steels. Therefore, the relationship of low temperature toughness and microstructure should be studied in detail. In the present work, the steel plates with 25 mm thickness after hot rolling were immediately water quenched to 550, 450 and 350 ℃(finish cooling temperature), respectively, and subsequently air cooled to room temperature. The effect of finish cooling temperature on the microstructure and low temperature toughness of Mn-series ultra-low carbon HSLA steel was investigated by SEM, TEM and crystallographic analysis. The results show that the granular bainite, lath bainite and martensite were obtained with finish cooling temperatures decreasing. There are three blocks with different orientations in a single packet for lath bainite microstructure in the sample with finish cooling temperature of 450 ℃, leading to the refinement of effective grain size and large amount of high-angle grain boundaries. Electron backscattered diffraction analyses of the cleavage crack path show that the bainite block boundaries can strongly hinder fracture propagation, and thus the refinement of bainite blocks can improve the low temperature toughness of Mn-series ultra-low carbon HSLA steel. Finally, the yield strength of 775 MPa and ductile-brittle transition temperature of -55 ℃can be achieved when the finish cooling temperature is 450 ℃.

研究了终冷温度(550, 450和350 ℃)对Mn系超低碳高强度低合金钢组织及低温韧性的影响. 力学性能的测试结果表明, 在终冷温度为450 ℃时, 实验钢获得良好的强韧性配合, 屈服强度为775 MPa, 韧脆转变温度为-55 ℃. 组织观察及晶体学表征结果表明, 随着终冷温度的降低, 组织逐渐由粒状贝氏体向板条贝氏体和板条马氏体转变; 终冷温度为450 ℃时, 组织以板条贝氏体为主, 多数的板条束包含三组不同的板条块, 有效晶粒尺寸最小, 大角晶界比例达到最大. 解理裂纹扩展路径的观察结果表明, 具有大角晶界的贝氏体板条块对解理裂纹扩展具有显著的阻碍作用, 因此板条块尺寸细化、大角晶界比例增加是低温韧性改善的主要原因.

About 90% of all mechanical service failures are caused by fatigue. Avoiding fatigue failure requires addressing the wide knowledge gap regarding the micromechanical processes governing damage under cyclic loading, which may be fundamentally different from that under static loading. This is particularly true for deformation-induced martensitic transformation (DIMT), one of the most common strengthening mechanisms for alloys. Here, we identify two antagonistic mechanisms mediated by martensitic transformation during the fatigue process through in situ observations and demonstrate the dual role of DIMT in fatigue crack growth and its strong crack-size dependence. Our findings open up avenues for designing fatigue-resistant alloys through optimal use of DIMT. They also enable the development of physically based lifetime prediction models with higher fidelity.