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High manganese steel has always been subjected to pre-hardening treatment before use owing to its low initial strength. Therefore, cast high manganese steel was pre-hardened in this work by surface impacting treatment under different parameters to evaluate its state. The microstructures, hardening characteristics, and wear resistances of the pre-hardened high manganese steel samples were all investigated. The results indicated that increasing the sample temperature, reducing the impact pin size, and increasing the impact times on a single-point can enhance the surface hardening effect to different degrees. Dislocations and deformation twins were produced on the top surface and within a certain depth of test steel under different impacting conditions. The optimal hardening effect of test steel was achieved at a temperature of 300 °C, a pin size of 10 × 10 mm2, and 9 impacts on a single point. The surface hardness of the treated sample reached 542 HV. For friction and wear testing, the best wear resistance was achieved at a temperature of 300 °C, a pin size of 10 × 10 mm2, and 3 impacts on a single point with a surface hardness of 446 HV. Further enhancement in impact hardening effect resulted in microcracking along the surface of test steel. In turn, the induced embrittlement led to the poor wear resistance. [ABSTRACT FROM AUTHOR]

In this paper, a new low-carbon alloy gear steel is designed via Si/Al alloying. The carburizing and austempering, at a temperature slightly higher than the martensitic transformation point (Ms) of the surface and much lower than the Ms of the core, for different times, were carried out on the newly designed gear steel. After heat treatment, a series of different microstructures (superfine bainitic ferrite + retained austenite, superfine bainitic ferrite + martensite + retained austenite, and martensite + retained austenite) were obtained on the surface, whilst the low-carbon lath martensitic microstructure was obtained in the core. The microstructure of the surface was examined using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The phase composition was analyzed using X-ray diffraction (XRD). The hardness and wear resistance of the surface as well as the hardness distribution of carburizing layer of the samples with different microstructures were studied. The results show that the Si/Al-rich gear steel, after carburizing and austempering at 200 °C for 8 h, not only has excellent mechanical properties but also has high wear resistance, which meets the technical requirements of heavy-duty gear steel. The research work in this paper can provide a data reference for the application of carburized steel with mixed microstructures of bainitic ferrite and martensite in the design of heavy-duty gear. [ABSTRACT FROM AUTHOR]

The hot deformation behavior of a medium-carbon bainitic steel was studied in a temperature range of 900-1100 °C and a strain rate range of 0.01-10 s. With increasing strain, the flow stress displays three tendencies: a continuous increase under most conditions and a peak stress with and without a steady-state region. Accurate constitutive modeling was proposed and exhibits a correlation coefficient of 0.984 and an average absolute relative error of 0.063 between the experimental and predicted stress values. The activation energy of the steel increased from 393 to 447 kJ/mol, when the strain increased from 0.1 to 0.4, followed by a slight fluctuation at higher strain. Finally, processing maps under different strains were constructed and exhibit a varied instability region with increasing strain. Microstructural observations show that a mischcrystal structure formed in the specimens that worked on the instability regions, which resulted from the occurrence of flow localization. Some deformation twins were also observed in certain specimens and were responsible for negative m-values. The optimum hot working processing parameters for the studied steel were 989-1012 °C, 0.01-0.02 s and 1034-1066 °C, 0.07-0.22 s, and a full dynamic recrystallization structure with fine homogeneous grains could be obtained. [ABSTRACT FROM AUTHOR]

A multiphase microstructure composed of nano-scaled bainite, martensite, undissolved spherical carbides and retained austenite was obtained on the top surface of carburizing 23Cr2Ni2Si1Mo steel after low-temperature austempered at 200 °C, with a dual-phase microstructure of fine martensite lath and carbon-enriched retained austenite in the center of steel. As compared with the conventional oil quenching and tempering (OQ) state, the low-temperature austempering specimens showed a higher surface residual compressive stress and a thicker residual compressive stress layer, and the wear resistance of the top surface of specimens austempered for 4 h, 8 h and 24 h was increased 40%, 58% and 12%, respectively. Moreover, the impact toughness of the center of these austempered specimens is also improved 20.8–33.3% as compared with that of the OQ specimen. Then, it can be deduced that applying the low-temperature austempering treatment for manufacturing carburizing-steel part is beneficial to improving its service performance for the better stress condition, higher wear resistance and higher interior toughness. [ABSTRACT FROM AUTHOR]

In this paper, we developed a strategy to circumvent the strength - ductility trade - off in high-strength carbide - free nanobainite steel. Two successive cycles of deformation were designed in the corresponding stacking fault energy ranges of supercooled austenite and retained austenite, which are respectively dominated by dislocation slip and twinning mechanism. The results indicate that the ausrolling and twinning process yielded a high density of dislocations and nanotwins in steel. A high density of dislocations in bainite ferrite trapped more carbon atoms, reducing the carbon content in retained austenite and the corresponding stability. Therefore, martensitic transformation of retained austenite was accelerated, during the tensile process, resulting in an excellent work hardening capacity and consequently ultrahigh strength in the resultant material. Meanwhile, multistage γ (face - centered cubic) → ε (hexagonal close - packed) → α' (body - centered cubic) martensitic transformation and interaction of nanotwins with dislocations provide superior ductility by alleviating shear strain accumulation and retarding crack initiation. The excellent strength (2.4 GPa) and ductility (15.7%) of the ausrolling and twinning steel indicates that the proposed strategy has potential applications in the development of high - strength, high - ductility materials. [ABSTRACT FROM AUTHOR]

Abstract Bainitic steel is applied in many cyclic loading projects due to its excellent comprehensive mechanical properties. In this study, the cyclic hardening/softening behavior and microstructure evolution of high-carbon nano-bainitic steel were investigated to analyze the cyclic deformation mechanism. Results showed that the optimal tempering temperature was determined to be at low temperature. During cyclic deformation, the retained austenite is transformed into martensite according to the Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) orientation relationships, and deformation twins are generated in the retained austenite. The cyclic deformation of high-carbon steels at a total strain amplitude of 0.6% manifests no obvious cyclic softening behavior after low-temperature tempering, mainly due to the high density of dislocation tangle around martensite and deformation twins induced by cyclic deformation. [ABSTRACT FROM AUTHOR]

In this study, the mechanism of increased strength and ductility of a martensite-austenite composite steel with different thickness ratios was analysed. Results revealed that with increasing the thickness ratio between the martensite and austenite layers, the strength and uniform ductility of the composite steel initially increased and then decreased gradually. The best combination was obtained in a 2:1 specimen, with the strength and the uniform ductility being increased by 9% and 190% respectively, as compared with the monolithic martensitic steel. The slight diffusion of carbon atoms from the austenite layer into the martensite layer strengthened the steel, which offset the weakening effect of introducing the austenite layer. Moreover, the diffusion of heavy elements between the neighbouring layers, such as manganese and silicon, formed a transition layer with varying concentration which effectively coordinated the deformation behaviour of the neighbouring martensite and austenite layers. [ABSTRACT FROM AUTHOR]

Abstract Given the transformation stasis after the incomplete transformation of carbide-free bainite, this paper aimed to study the effects of austempering time within transformation stasis on the bainite microstructure, monotonic deformation behavior, and especially cyclic deformation behavior using an ultrahigh silicon (2.59 wt%) steel. With increasing austempering time, the dislocation density of bainitic ferrite decreased, carbon content of retained austenite slight increased, and carbon distribution in retained austenite blocks gradually homogenized. The best combination of strength and ductility was obtained through a longer austempering time within transformation stasis. But the longer austempering time, indeed, did not result in longer fatigue life. The opposite trend could be explained by the fact that the primary factors affecting these mechanical properties were different. The lower density of mobile dislocation pre-existed in the starting microstructure of the samples austempered for a longer time was primarily responsible for its lower cyclic hardenability. Moreover, one retained austenite block with completely homogeneous carbon distribution was only transformed to one martensite grain, which increased the cyclic softening and degrade the fatigue life. By contrast, the deformation-induced martensite transformation from the retained austenite with higher mechanical stability in the samples austempered for a longer time enhanced strain hardenability at higher monotonic tensile strains and well delayed the necking, thus improving the combination of strength and ductility. [ABSTRACT FROM AUTHOR]

• Competing and synergistic effects of inclusion and microstructure on fatigue crack initiation are discussed. • Both inclusion-induced and microstructural small cracks are related to slip in the microstructure. • Coarse and soft microstructure has a small critical inclusion size and high sensitivity to inclusion. • Bainite/martensite microstructure with small-sized lath thickness and block length has superior VHCF performance. We emphasized the competing and synergistic effects of microstructure and non-metallic inclusion on subsurface fatigue crack initiation behaviors of bainitic/martensitic rail steels through multi-scale characterizations on the microstructure and inclusion features in the crack initiation area. Results show that the emergence of two failure modes, namely non-inclusion- and inclusion-induced crack initiation, is an outcome of the interaction between microstructure and inclusion. The refined bainite/martensite microstructure with small-sized lath thickness and block length can enhance the very high cycle fatigue performance and reliability via increasing the critical inclusion size and reducing the sensitivity to microstructure-induced crack initiation. [ABSTRACT FROM AUTHOR]

Making composite steel is an effective method to improve the strength and ductility of steel, but it often follows the mixture rule. Further increasing the strength and ductility simultaneously of composite steel is challenging. In this study, warm-rolling was introduced into a multilayered martensite/austenite composite steel after the compositing process. Results revealed that some C atoms diffused from the martensite layer into the neighbouring austenite layer, resulting in a reduced hardness in the martensite layer of common composite steel. Introducing warm-rolling deformation not only hardened each layer effectively, but also clearly improved the ductility of the austenite layer. This facilitated the coordination deformability of the austenite layer and resulted in simultaneous improvements in strength and ductility for warm-rolled composite samples. Better mechanical properties (tensile strength of 1546 MPa and a uniform elongation of 14.9%) were obtained for a sample with a lower degree of reduction (10%). The mechanism leading to improved mechanical properties is analysed in detail in this paper. [ABSTRACT FROM AUTHOR]