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The contributions of different strengthening mechanisms to yield strength of bainite/martensite multiphase rail steel with different finish cooling temperatures in the controlled cooling process were quantitatively investigated. Dislocation density and substructure size of the rail steel were measured by scanning electron microscopy, electron backscatter diffraction and X-ray diffraction. The results show that the dislocation density increases with the decrease in block width in rail steel. Based on the correlation among dislocation density, block width and yield strength, a physical model was proposed to predict the yield strength of rail steel. The variation of block width and dislocation density in different positions of rail head microstructure was integrated with temperature field simulation. Dislocation density and block width reveal significant correlations with the finish cooling temperature.

Rolling contact fatigue (RCF) failure is an important concern for wheel and rail system of railway transportation. In this work, the effect of non-uniform microstructure of the pearlite- ferrite wheel steel on RCF was investigated. Some bainite regions dispersedly distributed in the matrix of ferrite and pearlite. The hardness and chemical composition of different regions of wheel steel were determined by Vickers hardness tester and electron probe microanalysis (EPMA) respectively. As the hardness was different in these two regions, the initiation and propagation of cracks occurred at the boundary of abnormal microstructure mainly. EPMA results showed that the contents of Mn and Si were higher in positive segregation region than those of the matrix. In addition, the thermodynamic model was used to calculate the evolution of time-temperature-transformation (TTT) diagram of steel with element segregation. Result showed that the increase of Mn and Si in positive segregation region changed the shape of the TTT curves and caused the formation of abnormal microstructure.

Multiphase microstructure with carbide-free bainite, martensite and a small amount of film-like retained austenite was obtained in a new Mn-Si-Cr-C steel, designed to promote bainite transformation. The microstructural constituents were studied by a combination of electron microscopy, x-ray diffraction, dilatometry and high temperature laser confocal microscopy. A critical analysis of experimental data and theoretical analysis underscored that the austenite stability is enhanced by bainitic transformation, during quenching and partitioning process, besides carbon partitioning. The nucleation of bainite divided the untransformed austenite and refined the grain size of retained austenite, benefiting the mechanical properties, especially low temperature toughness.

Quenching and partitioning (Q&P) steels have been attracted much attention due to excellent combination of strength and ductility. However, the actual Q&P microstructures may contain the carbon-depleted martensite (i.e., initial martensite, M1), bainitic ferrite (BF), secondary martensite (M2) and retained austenite (RA). Hence, it is challenging to build a quantitative model of the yield strength of Q&P microstructures, since it requires prior knowledge of the yield strength of each constituent phase that is not always in a clear state in Q&P microstructures. In the present work, based on the analysis with respect of microstructural heterogeneities inevitably existing during martensitic transformation, both as-quenched and Q&P microstructures are simplified and regarded as a mixture of α′ phase (containing M1, M2 or BF) and γ phase (RA). The carbon content, dislocation density and substructure size of the α′ phase and γ phase were measured by SEM, EBSD and XRD. Then we build two generalized physical models to predict the yield strength of as-quenched and Q&P steels with the carbon content in range of 0.06–0.42 wt%. The physical models show the yield strength of α′ phase varies as the reciprocal of the square root of the block width (Hall-Petch relation), but varies as the inverse of the lath thickness (Langford-Cohen relation). The deviation analysis indicates that the assumption and simplification with regard of microstructure do not greatly affect the accuracy of the prediction of yield strength for both as-quenched and Q&P steels, with uncertainty less than 10%. The effect of carbon content and microstructural features on the yield strength of as-quenched and Q&P steels was discussed in terms of the developed physical models. It reveals that the residual carbon atom in α’ phase is still the dominant factor controlling the yield strength of Q&P steels, despite of possible highly uneven carbon distribution.

A novel isothermal bainitic transformation combined with quenching and partitioning (i.e. ‘disturbed’ bainitic austempering) process was employed in an Mn–Si–Cr–C low-alloyed steel to reduce the fr...

In this paper, we attempt to review our recent works on the ultrahigh strength Mn–Si–Cr–C steels treated by BQ&P and BQ–P–T processes. The core of these novel processes is to incorporate the format...

A low-carbon Mn-Si-Cr alloyed steel was treated by two different heat treatment routes, quenching- partitioning- tempering (Q-P-T) and quenching- partitioning- cryogenic- tempering (Q-P-C-T). The strength and toughness were enhanced concurrently after Q-P-C-T treatment (i.e., ultimate tensile strength: 1236 MPa and the impact energy at 20 °C: ~ 113 J cm−2, respectively). Compared to the toughness of Q-P-T, the toughness of Q-P-C-T was improved by 23%. Microstructural characterization was carried out by conducting scanning electron microscopy, X-ray diffraction, electron backscatter diffraction and transmission electron microscopy. The enhanced toughness was attributed to the increasing high-angle boundary and newly formed martensite, which was caused by cryogenic treatment of Q-P-C-T. At last, a rational model was built to explain the toughening mechanism by microstructural evolution during Q-P-C-T. The work provides a meaningful idea that cryogenic treatment can cooperate with advanced heat treatment scientifically to acquire optimized performance of conventional steels.

Transition metal carbonitrides play an important role in high-temperature structural ceramics, high-temperature coatings and alloy reinforcements. In this paper, we have searched for more than a dozen compounds of the type MxCyNz, with the special focus on M2CN compounds. The electronic structure and mechanical properties of transition-metal carbonitrides M2CN (M = V, Ti, Ta, Nb, Hf and Zr) are investigated by the first-principle calculations based on the density functional theory. The stability of ternary compounds M2CN is evaluated by calculating cohesive energy, formation energy and phonon dispersion spectrum. Electronic structures and chemical bonding characteristics are analyzed by the density of states and Mulliken population analysis. The mechanical properties, such as bulk, shear, Young's modulus, hardness and Poisson’s ratio, are obtained, the modulus of M2CN is generally higher than that of other carbonitrides. The anisotropic mechanical properties of M2CN are interpreted, anisotropic index and three-dimensional surface contours show that Ti2CN has the weakest elastic anisotropy. The simple mixed values HM, BM, GM and EM are calculated and are compared with the earlier calculated and experimental values.

An inverted multi-step bainitic austempering process is proposed, where a low austempering temperature step is followed by a high temperature counterpart: after austenitization, the specimens were cooled to the first austempering step at 260 °C for 600 s, then were raised to the second step at 360 °C and held for 1800 s. As comparison, the conventional multi-step bainitic austempering processes were carried out: the first step is at 360 °C for 1800 s, while the second step is at 260 °C for 7200 s. Through inverted multi-step bainitic austempering process, the efficacy of reducing blocky austenite is also achieved and is comparable to that obtained by conventional multi-step bainitic austempering process. But the total time used for inverted multi-step bainitic austempering treatment is reduced markedly due to accelerating bainitic ferrite formation. Furthermore, the novel heat treatment can enhance the strength of a Mn-Si-Cr-C low-alloyed steel without sacrificing ductility, which is attributed to refining austenite and bainitic ferrite blocks.

Twin boundary of a nano-sized {1 0 −1 2} extension twin far away from its tip has been experimentally characterized to be about 3.5° from the fully coherent twin boundary (CTB), which is the twinning plane of (1 0 −1 2)t//(1 0 −1 2)m. Subscripts ‘t’ and ‘m’ refer respectively to twin and its matrix. In atomic scale, the twin boundary consists of PtBm-CTB step, PtBm-BtPm serration, and BtPm-CTB-PtBm serration, where PtBm and BtPm refer respectively to (1 0 −1 0)t//(0 0 0 1)m and (0 0 0 1)t//(1 0 −1 0)m. The relationship between the deviation angle of a twin boundary from CTB and the structure of the twin boundary is deduced on the basis of an explicit geometric model. When the deviation angle is 3.5° as measured, the ratio of the average length of CTB terraces to the average height of PtBm-CTB steps is calculated to be 15.28, representing a twin boundary dominated by CTB with relatively small strain.

Borides are very important in cast TiAl alloys. Among various morphologies and structures, the long curve ribbon borides are harmful to mechanical properties and need to be avoided or refined. The knowledge on curve ribbon is relatively less than fine boride particles. In the present work, a long ribbon with blocky borides at both ends is studied by SEM, HRTEM， and STEM-EDS method. The ribbon consists of nanoflakes of 4 boride structures and a B2 layer. The blocky boride is Ti3B4 with a D7b structure. The ribbon contains flakes with the structures of D7b, B27, Bf, C32, and B2. The block is adhesive to the ribbon by a B2 layer with a width of several nanometers. Mn is found to segregate to B2/borides interfaces inside the ribbon. Nb is rich in borides but poor in the B2 phase.

Microstructure difference between the center and the surface caused by temperature gradient during quenching process may lead to inhomogeneous properties of large-size products. A new finite element model (FEM) is proposed in the present work to precisely simulate temperature distribution of a bainite/martensite multiphase material during water-air alternating surface quenching process. Parameters like nonlinear variation of thermo-physical parameters, nonlinear variation of heat transfer coefficient, phase transformation latent heat, and transformation kinetics were considered when developing the FEM. In particular, the experimental investigation of bainite and martensite transformation kinetics provides the basis for the accurate calculation of microstructure. The model helps identify the temperature distribution, cooling rate profile, microstructure and hardness characteristic of Mn-Si-Cr bainitic/martensitic large-size products. A quarter of axle block experiments were conducted to validate the proposed FEM, which shows high precision. The error of simulated temperature was lower than 5 % and that of hardness was within 2 %. Then, the model is used to simulate the discrepancy between surface and center location under as-employed quenching. Both experiment and simulation results confirmed that the different cooling rate leads to the temperature gradient. Different temperature distribution resulted in inhomogeneous properties of large-size product. Effects of the intensity of water spraying on large-size product are compared thoroughly in the simulation analysis. It is reflected that the improvement of microstructure and corresponding diminution of non-uniformity in hardness can be realized through optimizations of key processing parameters. Therefore, it is helpful for quenching process design of large-size products.

A medium carbon Mn-Si-Cr alloyed steel was treated by a novel bainite-based quenching and partitioning (BQP uniform elongation and total elongation: 26.2% and 31.8%; the reduction of area: 47.9%). Besides the transformation-induced plasticity effect of the retained austenite and the composite effect of the multiphase after BQ&P treatment, the formation of carbide free bainite also plays a significant role on the enhanced mechanical properties. The carbide-free bainite could improve the damage resistance of the multiphase due to the additional strain-hardening capacity within the local plasticity deformation zone near the tip of micro-cracks. In this case, the fraction and distribution of CFB should be controlled properly and the macrosegregation should be avoided.

The bainite/martensite (B/M) multiphase microstructure was studied in 0.22C–2.0Mn–1.0Si–0.8Cr–0.8(Mo+Ni) (wt%) bainitic steel subjected to deep cryogenic treatment (DCT) to elucidate the positive effect of DCT on structure and mechanical properties. The study indicates that DCT can improve mechanical properties and wear resistance. It reduces the content of blocky martensite/austenite (M/A) constituents by eliminating unstable retained austenite (RA). At the same time, RA is relatively more enriched in carbon after DCT, compared to the tempering process. During DCT, the brittle martensite is also avoided, since the enhanced recovery reduces the carbon concentration during tempering. Meanwhile, the contraction of unit cell at low temperature promotes the precipitation of fine dispersed carbides and contributes to wear resistance.

A bainite/martensite multiphase rail is treated by the controlled-cooling process with different finish-cooling temperatures. The simulated temperature&ndash, time curves of the position of 5 mm and 15 mm below the rail tread (P5 and P15) express different trends. P5 has greater impact toughness and lower tensile strength than P15. Microstructural characterization was carried out by conducting scanning electron microscopy, X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. The greater tensile strength is due to the dispersed &epsilon, carbides hindering the movement of dislocations. The greater impact toughness is attributed to the filmy retained austenite and the smaller effective grain with high-angle boundary. Finite element modeling (FEM) and microstructural characterization reasonably explain the changes of mechanical properties. The present work provides experimental and theoretical guidance for the development of rail with excellent mechanical properties.

The effect of microstructure on the spalling damage in a heavy-haul 20Mn2SiCrMo bainitic wing rail was investigated. The results show that spalling damage is strongly correlated to the detailed microstructure factors which contribute to the crack propagation resistance at different levels. It is demonstrated that comparing with blocky martensite/austenite (M/A) constituents, film-like M/A constituents have better performance to retard crack propagation. Moreover, the type and proportion of M/A constituents are affected by the isothermal holding temperature during heat treatment, where typical blocky M/A constituents are found in the temperature range from 360 °C to 400 °C.

The bainite/martensite multiphase microstructure was studied in 0.22C-2.0Mn-1.0Si-0.8Cr-0.8(Mo+Ni) (wt%) bainitic steel subjected to austempering between 325 °C and 400 °C to elucidate the toughening mechanism and the determining role of martensite/austenite (M/A) constituents obtained through various heat treatment. Steel austempered at higher temperature had lower strength and poor toughness, because of the wide bainitic ferrite lath and blocky M/A constituents. Bainite with similar crystallographic orientation to lath martensite was formed and exhibited a typical K–S relationship. The samples austempered below martensite-start (Ms) temperature exhibited an excellent impact toughness at room temperature, due to the superfine sub-plates in lower bainite blocks. Meanwhile, air-cooled toughness at low temperature was superior to the steel austempered at 325 °C, because of its finer effective grain size. The relationship between microstructure and mechanical properties was studied using a combination of SEM, TEM and crystallographic analysis.

We elucidate here the mechanistic contribution of the application of quenching and partitioning (Q&P) concept to a high carbon Mn-Si-Cr steel in obtaining a multiphase microstructure comprising of martensite/austenite and nanostructured bainite (bainitic ferrite and nanometer-sized film-like retained austenite) that exhibited tensile strength of 1923 MPa and total elongation of 18.3%. The excellent mechanical properties are attributed to the enhanced refinement of blocky austenite islands obtained by the Q&P process. The austenite was stabilized by both carbon partitioning from martensite and bainite transformation. Compared with conventional heat treatment to produce nanostructured bainite, the total time is significantly reduced without degradation of mechanical properties.

We elucidate the effect of microstructure on the rolling contact fatigue (RCF) performance of bainitic rail steel via microstructural characterization and simulated field tests. The RCF performance of bainitic rail steel is enhanced through microstructural design and the thin film-like retained austenite plays a significant role on RCF crack propagation.

The microstructure and the mechanical properties of the low carbon high strength (LCHS) Mn–Si–Cr steel treated by the novel quenching and dynamic-partitioning (QDP) process are investigated in detail. All samples exhibit a better performance of tensile strength and impact toughness compared with conventional LCHS steels. The higher impact toughness is closely related to the stability of retained austenite (RA), which is determined by the morphology and the carbon content. The filmy RA between martensite laths as well as the nano-sized RA inside martensite laths can effectively improve toughness. The crystallographic features of the RA are studied in depth and the crystal plane indices of facets are accurately characterized. The near-coincidence-sites (NCS) model is applied to further analyze the interfacial structures of the nano-sized RA in the given orientation relationship. The observed facets tend to contain the high-planar density of NCS clusters. The study provides some references for the research on the phase transformation of high-strength steels.

The morphology and crystallographic features of the nano-sized retained austenite (RA) in low carbon Mn-Si-Cr steel were explored in the present work. The nano-sized RA between the sub-laths inside the martensite laths is perpendicular to the lath boundary and shows particular long-axis direction, i.e. (3 5 1)γ. The stacking faults connected with the dislocation outcrops on the interfaces are observed in the nano-sized RA. The phase transformation stress accompanying the martensitic transformation can induce plastic deformation by the slip of both the stacking faults in the nano-sized RA and the Shockley partial dislocations in the interface.

A “disturbed” bainitic austempering process (isothermal bainitic transformation in conjunction with quenching and partitioning) was employed in a Mn–Si–Cr–C low-alloyed steel to reduce the fraction and size of the blocky martensite/austenite (M/A) islands. The M/A islands formed during the conventional bainitic austempering process are coarse and undesirable for property optimization. Owing to the novel heat treatment, the strain hardening capacity of the steel is improved remarkably, leading to an excellent combination of strength and ductility (e.g. ultimate strength: 1515 MPa; total elongation: 31.2%). The improvement of mechanical properties is attributed to both the transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) effects provided by the refined filmy austenite constituent.

We obtained a good combination of strength and ductility in a 0.4C–2.0Mn–1.7Si–0.4Cr (wt%) steel, namely, ∼1.7 GPa of ultimate tensile strength and ∼26% of elongation, by conducting a Q–P–T (quenching–partitioning–tempering) process incorporating the formation of carbide-free bainite. The tempering behavior of this steel was discussed by using experimental finding (scanning electron microscopy, X-ray diffraction (XRD), transmission electron microscopy and dilatometer) and CCE (constrained carbon equilibrium) modeling. The XRD results combined with CCE calculation prove that carbon partitioning from martensite to austenite occurs during tempering. Consequently, the thermodynamic stability of retained austenite is enhanced. This idea can be utilized to design novel Q–P–T processes in future.

Three heat-treatment routes incorporating bainite formation, namely bainite-based quenching plus tempering, bainite austempering and bainite-based quenching plus partitioning (BQP total elongation: 25.2%; U-notch impact toughness at � 40 C: 48 J cm � 2 ). The enhanced mechanical properties were attributed to an increased amount of refined filmy retained austenite (22 vol.%, nanometer width range

Quenching-partitioning-tempering (Q-P-T) process were applied to a quenched carbide-free bainite/martensite (CFB/M) multiphase steel 40Mn2Si2Cr and its effect on the mechanical properties was studied. The results showed that the partitioning time of Q-P-T treatment can be increased greatly to 90 min or even longer. The tensile strength, elongation and product of strength and elongation (PSE) of the experimental steel can achieve 1650 MPa, about 20% and over 33 GPa · %, respectively. The significant increase of partitioning time and excellent mechanical properties could be related to the CFB microstructure introduced in the quenching process during the Q-P-T treatment.

The surface of bainitic steel was remelted by fiber laser, and the microstructure and mechanical properties of the melted layer were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), a nanoindentation instrument, and wear equipment. The study of changing the laser scanning speed showed that the depth of the melted layer increases with decreases of the laser scanning speed. The wear-resistance property increased by 55% compared with the matrix and decreased with the reduction of laser scanning speed within a certain range. In the study of changing the laser-scanning space, the thermal effect of laser melting in the back channel on the front channel was further validated. At the same time, it was found that the solidified layer surface of hardness alternating with softness can be obtained by appropriately expanding the scanning space, which is conducive to improving the wear-resistant properties of the steel surface, and properly improving the production efficiency of the laser remelting treatment.

A novel quenching–partitioning–tempering (Q–P–T) process was employed in a Mn–Si–Cr alloyed steel to obtain a triplex microstructure comprising carbide-free bainite, martensite and retained austenite. The carbide-free bainite formed during the quenching step (rather than partitioning step) of Q–P–T process can enhance the mechanical properties (e.g. the product of strength and elongation, ∼31.4 GPa%) by extending the partitioning time up to 30 min. A good combination of strength and elongation (Ultimate tensile strength: ∼1655 MPa; Elongation: ∼19%) has been realized for the Mn–Si–Cr alloyed high strength steel without expensive alloying, e.g. Ni, Mo, Nb in previous Q&P/Q–P–T processes. The enhanced mechanical properties could be attributed to the various types of retained austenite in the Q–P–T treated carbide-free bainite/martensite/austenite triplex microstructure.