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Residual δ-ferrite is ubiquitous in M50NiL bearing steel and seriously deteriorates impact toughness. In the present work, the formation mechanism of δ-ferrite in M50NiL steel and the influence of element segregation on its formation process were investigated. The residual δ-ferrite was proved to form directly from the final liquid phase at the end of solidification due to serious segregation of Mo and V elements. Calculation results indicate that the segregation of Mo and V elements expanded the “L + δ'' two-phase region and stabilized δ-ferrite. Due to the high concentration of Mo and V, large-sized V-rich MC and Mo-rich M2C carbides would precipitate in δ-ferrite at 900–1200 °C if the cooling rate is slow. When kept at 1200 °C for no more than 30 min, Mo and V elements derived from the decomposition of large-sized carbides are preferentially diffused into δ-ferrite, enhancing the stability of δ-ferrite. To achieve the effective elimination of δ-ferrite in M50NiL steel, it is necessary to keep the ingot at 1200 °C for at least 20 h.

The current trend of increasing the precision lifetime of high-performance bearings forces scientists and engineers to optimize the heat treatment and processing technology to endure higher fatigue frequencies from rolling/sliding motion. In this study, the influences of two crucial factors, hardness and spherical carbides, on the wear behavior of AISI 52100 bearing steel were investigated using elaborate heat treatments to control the variables. The results showed that avoiding excessive hardness and improving the content of spherical carbides could reduce the wear volume during sliding friction and enhance wear resistance. On the one hand, high hardness is correlated to high carbon concentration and high micro-plastic strain in the martensite matrix, inducing the propagation of microcracks and formation of large-size shallow micro-pitting, which accelerates wear loss. On the other hand, spherical carbides effectively inhibit severe plastic deformation of the martensite matrix, which inhibits the wear loss caused by micro-cutting and decreases the formation of wear debris. In this case, densely distributed carbides can improve the wear performance.

The effect of rare earth element Ce on creep damage behavior of 316LN austenitic stainless steel (316LN) at 700 °C was systematically investigated under stress ranging from 125 MPa to 200 MPa. The results show that the addition of 0.032 wt% Ce remarkably improves the rupture life of 316LN steel under various stress. Ce addition in 316LN steel decreases the nucleation stress, stabilizes stress concentrations at particles, and notably raises the cavity nucleation rate. Moreover, Ce addition suppresses cavity growth during creep, thus the cavity growth rate significantly decreases and the creep rupture ductility increases under high stress over 175 MPa. However, the anomalous growth rate was engendered in some cavities by Ce addition in 316LN steel under low stress below 150 MPa. Consequently, Ce addition in 316LN steel reduces the creep rupture ductility and changes the rupture mode from trans-granular dimple fracture under high stress to mixed fracture under low stress.

The hot compression bonding (HCB) in a Fe–9Cr-1.5W-0.3Ti alloy, under three conditions: (i) coarse-grained; (ii) fine-grained hot isostatic pressed (HIP); and (iii) fine-grained nano-oxide dispersion strengthened (ODS), was performed at 800 °C with strains of 10% and 30% and followed by soaking treatment at 1000 °C for 4 h. The effects of grain size and nano-oxide particles on the interfacial microstructure evolution and healing mechanism were systematically investigated. The results showed that grain boundary bulging and small recrystallized grains formed along the straight bonding interface of the coarse-grained alloy were related to discontinuous dynamic recrystallization behavior. In contrast, a layer of fine recrystallized grains was located at the bonding interface of the fine-grained HIP alloy, which was a typical rotation dynamic recrystallization behavior. Although the interfacial recrystallization behavior of the fine-grained ODS alloy was also rotation dynamic recrystallization, nano-oxide particles inhibited interfacial recrystallization and oxide transformation. Based on the interfacial bonding ratio considering different features of HCB, the ranking of interface bonding ability was coarse-grained alloy

In this work, a modified numerical algorithm that couples the quasi-static theory with the mixed thermal elastohydrodynamic lubrication (mixed-TEHL) model is proposed to examine the mechanical properties and lubrication performance of the spindle bearing that is used in a high-speed machine tool with spinning. The non-Newtonian fluid characteristics of the lubricant and the non-Gaussian surface roughness are also considered. Moreover, the mechanical properties and lubrication state of the bearing are examined in various service environments. The results indicate that the temperature reduces the lubrication efficiency, which in turn exerts a significant impact on the mechanical properties. The lubrication that either behaves in the manner of Newtonian or non-Newtonian fluid has a relatively negligible influence on the bearing working state, while the non-Gaussian surface roughness significantly alters the oil film thickness and temperature. Calculations with different operating conditions demonstrate that the operating parameters (i.e., axial load, rotation speed) will directly affect the performance of the bearings via the changes in the oil film thickness and the temperature.

This study aims to investigate the effect of rare earth (RE) elements on the microstructure and mechanical properties of bainite/martensite (B/M) bearing steel. For this purpose, GCr15 bearing steels with different RE contents were prepared and analyzed after experiencing B/M heat treatment. The results show that RE addition significantly delays the bainite transformation by reducing grain boundary energy and carbon diffusion rate, leading to a reduction of bainite volume fraction from 27.3% for bearing steel without RE addition to less than 14.5% for RE-treated steels, and the corresponding increase of martensite volume fraction from 39.1% to more than 50.2%. The volume fraction of retained austenite (RA) remains nearly unchanged. Meanwhile, the width of the bainite needles gets refined in RE-treated steels while martensite and RA blocks become bigger. Compared with bearing steel without RE addition, larger RA blocks in RE-treated steels unexpectedly have higher stability due to higher dislocation density and surrounding phases strength. Accompanied by the change of microstructure, the hardness in RE-Treated steels improves slightly, while the fracture toughness dramatically deteriorates which can be directly explained by two mechanical phenomena during toughness test. The one is that the RA in RE-treated steels is harder to transform into martensite and thus consumes less energy; The other one gives that cracks tend to propagate along grain boundaries or martensite interfaces with less energy consumption easily in RE-treated steels instead of breaking off bainite.

Hot-compression bonding with subsequent soaking treatment is employed to manufacture large high-speed homogeneous steels in the current study. The effects of the deformation temperature (1050, 1100, and 1150 °C), strain (10%, 30%, and 50%), and holding time (1 and 12 h) on the interfacial microstructure and tensile strength of joints were investigated. The results indicated that complete bonding occurred at 1050 °C/50%, 1100 °C/30%, 1100 °C/50%, 1150 °C/30%, and 1150 °C/50%, and the tensile strength of the obtained joints was comparable to that of the base material. In other cases, interfacial voids were formed along the straight interface, which deteriorated the bonding strength. These voids were eliminated by a post-holding treatment at 1150 °C for 12 h, promoting the transformation of a straight interface to bulged grain boundaries. This resulted in the complete healing of the interface, except for samples bonded at 1050 °C/10%. The bulging of the original bonding interface due to the plastic deformation and atomic diffusion was the main mechanism for interfacial healing. Stable interfacial oxide nanoparticles were formed under a high vacuum and did not affect the recovery of joint strength. To validate the practicality of this method, an integrated high-speed steel with a homogeneous tensile strength was manufactured by bonding four pieces of blanks.

Fe–10Cr–10Ni cryogenic maraging steel is one of the key candidate materials applied in harsh cryogenic condition due to its high strength and good cryogenic toughness. In this study, the effect of aging temperature on its microstructure and mechanical properties was systematically studied based on austenite reversion and nano-precipitation. It shows that a desirable combination of high strength (834 MPa, 25 °C) and excellent cryogenic impact toughness (164 J, −196 °C) can be obtained by aging treatment at 500 °C. Multi-scale characterizations were conducted to reveal the microstructure characteristics of the steel. It was found that obvious film-like reversed austenite nucleate and grow at the high-angle grain boundaries of martensite matrix in the steel aged at 500 °C, whereas the higher aging temperature resulted in a larger content of blocky reversed austenite in martensite blocks. Austenite reversion mechanism was proposed based on the double-spherical-cap model and diffusion kinetics of Ni element. Besides, it was found that the precipitation sites of Ti-rich particles are not only distributed in matrix but also located at the dislocations, and they were identified as the clusters of Ni3Ti precipitates. Finally, the origin of the above cryogenic toughness includes the transformation-induced plasticity (TRIP) effect from the soft film-like austenite, higher density of high angle grain boundaries of martensite and fine nanoscale precipitates. Moreover, the precipitation strengthening from the clusters of Ni3Ti precipitates contributes to the high strength of the steel aged at 500 °C.

Topological phononic (TP) materials are attracting wide attentions and it is more difficult to seek TP materials compared to electronic materials. Here, the authors present a high-throughput screening and data-driven approach to discover 5014 TP materials and further clarify the mechanism for the occurrence of various TPs.

The microstructures and segregations during solidification have been widely recognized in the past researches. The dendrite (termed as conventional dendrite in this paper) was recognized as the most basic entity for microstructures and microsegregation. Through careful analysis and observation in this paper, new nano-scale dendrites (with dendritic arm spacing about 500 nm) were revealed within the conventional dendritic arm (with dendritic arm spacing about 70 μm). The conventional dendrites should be recognized as macrosegregation patterns arisen in a subtle porous media. Accordingly, the research entity for microsegregation should change from the conventional dendritic arm to a more subtle entity. During solidification, temperature fluctuations with different directions, periods and intensities will superimpose to induce a dendritic temperature field on multi-scales. Because of differentiated solid fractions and liquid compositions, a dendritic macrosegregation field on multi-scales will be induced by the mass transfer. Verified by the in situ and real time observation, the subtle dendrites emerge at the later solidification stage much lower than the equilibrium eutectic temperature. Because nano-scale dendrites were recognized, new solutions were proposed for controlling macrosegregation within the conventional dendritic arm. In a more significant view, the nano-scale dendrites provide the basis for unifying the non-equilibrium solidification and the equilibrium solidification.

Macrosegregation is the key issue in the solidification field. Oxygen and its inclusions play the important role in driving the melt flow and the resulting macrosegregation in steel. Here, to reveal the inducing mechanism and quantitative model of oxygen content in real industrial steel ingots, we demonstrate for the first time that there exists the critical oxygen content in triggering the formation of channel-type segregation, the most undesirable macrosegregation type in steel. Our multiscale simulations from density functional theory calculations to multiphase/multicomponent macromodel, clarify the quantitative conditions initializing channel-type segregation and reveal two typical growth modes via oxide flotation. The oxygen content model and criterion to induce the channel onset is built accordingly, which are validated by the numerous full ingot dissections and experimental characterizations including the in situ electrolysis of inclusions, X-ray microtomography, scanning electron microscope, large-scale measurement system of inclusions and chemical analysis. With oxygen controlled below this critical value of 0.0008 wt%, channel disappears. This study quantitatively uncovers the novel role of oxygen in steel, changes the traditional sole concept of cleanliness, and highlights an innovative and controlling-effective route to fabricate homogenized steel.

By means of ab initio molecular dynamics (AIMD), we have evidenced the formation of the localized Nb clusters in the U-Nb liquid alloys even with the very low Nb concentrations. These localized Nb clusters are characterized, qualitatively and quantitatively, in terms of the pair correlation function, structure factor, coordination number and chemical short-range order parameters. The existence of the first highest peaks of the partial Nb-Nb pairs in both their pair correlation functions and structure factors reveals that the Nb atoms prefer to accumulate themselves together to form various localized clusters with three typical Nb networks of Nb2 dimers, opening chains (Nb3 or Nb4 chains) and closed triangles (Nb3 triangles) in the U-Nb liquid alloys. These Nb-Nb affinitive facts are further supported by the derived negative Nb-Nb chemical short-range order parameters. In contrast, both U-U and U-Nb interactions in these liquid alloys are further revealed to have the nearly zero, or positive, chemical short-range order parameters, corresponding to a relatively weak U-U, or repulsive U-Nb, interactions in the U-Nb liquid alloys.

The segregation produced during solidification usually deteriorates properties of materials. In this paper, the multi-scale segregation patterns of Al-5 wt% Cu alloy were investigated by various etching methods and measure tools. The multi-scale segregation patterns were simulated by the MATLAB software. Through investigation, the research entities for segregations were re-regulated. The entity for microsegregation is so tiny that it is usually concealed by back diffusion. The multi-scale macrosegregation patterns, including the typical dendritic pattern and triangle pattern, are reserved after solidification. After the propagation of the fine porous medium, ripples of solid fraction arise and superimpose in the fine porous medium. The superimposition of ripples with different directions, intensities and periods produces complicate multi-scale macrosegregation patterns. Based on the new mechanism, the multi-scale macrosegregation was controlled by a feasible quenching-holding-quenching route. This work re-regulates the research entities for microsegregation and macrosegregation, and establishes the superimposition principle of ripples. The ripple superimposition provides a promising vision to fabricate designed microstructures in the future. Keywords: Solidification, Multi-scale segregation, Dendrite, Ripple superimposition

Channel segregation (CS) is the most typical defect during solidification of NbTi alloy. Based on numerical simulation and experimental characterizations, we deeply elucidated its characteristics, formation mechanism, effecting factor and prediction criterion. According to acid etching, industrial X-ray transmission imaging, 3D X-ray microtomography and chemical analysis, it was found that in a casing ingot, by He cooling, finer grain size, weaker segregation and slighter CS can be obtained compared with air-cooled ingot. The simulation results of macrosegregation show that CS is caused by the strong natural convection in the mushy zone triggered by the thermo-solutal gradient. Its formation can be divided into two stages including channel initiation and growth. In addition, due to the stronger cooling effect of the He treatment, the interdendritic flow velocity becomes smaller, consequently lowering the positive segregation and CS and improving the global homogenization of the final ingot. Finally, to predict the formation of CS, the Rayleigh number model was proposed and its critical value was found to be 15 in NbTi alloy for the first time. When it is lower than the threshold, CS disappears. It provides an effective tool to evaluate and optimize the solidification parameters to fabricate the homogenized NbTi ingot in engineering practice.

The influence of hot top design on feeder channel segregates (F-CS) and centerline shrinkage porosities (C-SP) were investigated both experimentally and numerically. Two 100-ton 30Cr2Ni4MoV steel ingots with different insulating hot tops were longitudinally sectioned. The experimental results showed few channel segregates but severe shrinkage porosities appeared in the ingot with poorly insulated hot top, while it was the opposite case after the improved hot topping practice. By employing the finite element numerical simulation, the critical condition for the formation of F-CS in 30Cr2Ni4MoV steel was verified to be R2.1G ≤ 1.0 × 10−5 °C mm1.1 s−2.1. Through coupling with the published C-SP criterion (GR−0.5 ≤ 2.5 °C mm−1.5 s0.5), it was found out that the increase of hot-top height and preheating temperature would aggravate F-CS while alleviate C-SP contrarily. Hence, to comprehensively control those two defects, the optimum hot-top height and preheating temperature for 100-ton ingot were suggested to be 700 mm and 600 °C, respectively. Ultimately, the ratio of the solidification time for the whole ingot to the ingot body (tf/tb) was proposed as a novel criterion for hot top design. This practical criterion has been successfully utilized to optimize the hot top of a 5-ton steel ingot. Keywords: Hot top design, Feeder channel segregate, Shrinkage porosity, Steel ingot

The purity of rare earth metals is one of the most important factors to research and develop high technique materials. However, high purity rare earth metals are not easily achieved. This review summarizes the preparation and purification methods of rare earth metals. First, the preparation principle and process of molten salt electrolysis and metal thermal reduction are introduced. The main sources of metallic impurities and interstitial impurities in rare earth metals as well as the action mechanism of reducing the concentration of different impurities are analyzed and summarized. Then, the purification principle and process of vacuum distillation, arc melting, zone melting, and solid state electromigration are also discussed. Furthermore, the removal effect and function rule of metallic impurities and interstitial impurities in rare earth metals are outlined. Finally, the crucial issues in the development of high purity rare earth metals are put forward, and the development direction of high purity rare earth metals in future are pointed out on this basis.

A novel hot-work die steel, named 5Cr5Mo2, was designed to obtain superior thermal stability. The proposed alloy is evaluated in terms of its hardness, microstructure, and tempering kinetics. Compared with the commonly used H13 steel, the softening resistance of the designed steel is superior. Based on SEM and transmission electron microscopy (TEM) observations, a higher abundance of fine molybdenum carbides precipitate in 5Cr5Mo2 steel. Strikingly, the coarseness rate of the carbides is also relatively low during the tempering treatment. Moreover, owing to their pinning effect on dislocation slip, the dislocation density of the 5Cr5Mo2 steel decreases more slowly than that of the H13 steel. Furthermore, a mathematical softening model was successfully deduced and verified by analyzing the tempering kinetics. This model can be used to predict the hardness evolution of the die steels during the service period at high temperature.

Studies of H13 steel suggest that under appropriate conditions, additions of rare-earth metals (REM) can significantly enhance mechanical properties, such as impact toughness. This improvement is apparently due to the formation of finer and more dispersive RE inclusions and grain refinement after REM additions. In this present work, the microstructure evolution and mechanical properties of H13 steel with rare earth additions (0, 0.015, 0.025 and 0.1 wt.%) were investigated. The grain size, inclusions and fracture morphology were systematically studied by means of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that REM addition of 0.015 wt.% can result in good improvement of performance compared to the REM additions of 0.025 wt.% and 0.1 wt.%. It is found that the impact toughness is significantly enhanced with the addition of 0.015% REM, which can be improved 90% from 10 J to 19 J. Such an addition of REM can result in a huge volume fraction of finer and more dispersive inclusions which are extremely good to toughness.

The purpose of this study is to explore why type 2 diabetes mellitus (T2DM) patients are susceptible to pulmonary tuberculosis through detection of serum Toll-like receptor 4 (TLR4), an important immune-related receptor, especially in terms of content and TLR4 gene polymorphism. Patients with T2DM complicated by pulmonary tuberculosis (T2DMTB) were selected as the case group and T2DM patients without tuberculosis were selected as the control group. Forty patients in each group were randomly selected and their serum TLR4 levels were detected and compared. Determination of six sites of TLR4 gene polymorphism was carried out in 238 T2DMTB patients and 310 patients with T2DM, and results showed that the serum TLR4 content of the T2DMTB group was significantly lower than that of the T2DM group (p

The strength-toughness combination and hardness uniformity in large cross-section 718H pre-hardened mold steel from a 20 ton ingot were investigated with three different heat treatments for industrial applications. The different microstructures, including tempered martensite, lower bainite, and retained austenite, were obtained at equivalent hardness. The microstructures were characterized by using metallographic observations, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and electron back-scattered diffraction (EBSD). The mechanical properties were compared by tensile, Charpy U-notch impact and hardness uniformity tests at room temperature. The results showed that the test steels after normalizing-quenching-tempering (N-QT) possessed the best strength-toughness combination and hardness uniformity compared with the conventional quenched-tempered (QT) steel. In addition, the test steel after austempering-tempering (A-T) demonstrated the worse hardness uniformity and lower yield strength while possessing relatively higher elongation (17%) compared with the samples after N-QT (14.5%) treatments. The better ductility of A-T steel mainly depended on the amount and morphology of retained austenite and thermal/deformation-induced twined martensite. This work elucidates the mechanisms of microstructure evolution during heat treatments and will highly improve the strength-toughness-hardness trade-off in large cross-section steels.

A method called forging solidifying metal (FSM), which is applied for eliminating coarse dendrites and shrinkage porosity defects of ferrous alloys was proposed based on semi-solid forming technology (SSF). To verify its feasibility, the effects of liquid fraction (FL) on the microstructure of the deformed H13 steel were investigated experimentally. The coarse dendrites structure still existed and cracks appeared when the 0.1/s 50% FSM method was carried out at ~20% FL. What is significantly different from that is, the elimination of the coarse dendrites structure and shrinkage porosity defects became more significant, when this method was conducted at the end of solidification (FL < 10%). The microstructure of H13 steel was significantly refined and also became dense in such condition.

Studies of 4Cr5MoSiV1 die steel suggest that under appropriate conditions, additions of rare earth (RE) can enhance tensile property. This improvement is apparently due to the more uniform distribution of carbides and the enhancement of precipitation strengthening after RE additions. In this present work, the effect of the RE addition on the carbides evolution and tensile property of 4Cr5MoSiV1 steel with various RE contents (0, 0.018, 0.048 and 0.15 wt %) were systematically investigated. The two-dimensional detection techniques such as optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used to investigate the carbides evolution of as-cast, annealed and tempered with RE addition. The results indicated that the carbides in 4Cr5MoSiV1 steels were modified by adding the suitable amount of RE. The eutectic structure and coarse eutectic carbides were all refining and the morphology of the annealed carbides initiated change from strip shape to ellipsoidal shape compared with the unmodified test steel (0RE). In addition, the amount of the tempered M8C7 carbides increased initially and then decreased with the alteration of RE addition from 0.018 to 0.15 wt %. Notably, the tensile test indicated that the average value of ultimate tensile strength (UTS) and elongation rate of 0.048RE steel increased slightly to 1474 MPa and 15%, higher than the 1452 MPa and 12% for the unmodified test steel (0RE), respectively. Such an addition of RE (0.048 wt %) would have a significant effect on the carbides evolution of as-cast, annealed and tempered and resulting in the tensile property of 4Cr5MoSiV1 die steel.