ISSN 0412-1961
CN 21-1139/TG
Started in 1956

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Present Research Situation and Prospect of Multi-Scale Design in Novel Co-Based Superalloys: A Review
LIU Xingjun, CHEN Yuechao, LU Yong, HAN Jiajia, XU Weiwei, GUO Yihui, YU Jinxin, WEI Zhenbang, WANG Cuiping
Acta Metall Sin    2020, 56 (1): 1-20.   doi:10.11900/0412.1961.2019.00159
Accepted: 24 October 2019

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In recent years, the development of material genetic methods, together with multi-scale material design theory and calculation methods has provided new ideas for the alloy design of novel Co-based superalloys. Based on the published results of multi-scale design and the research work of our laboratory, this paper systematically summarizes the present research status of multi-scale design methods in the field of novel Co-based superalloys. A review of multi-scale calculation methods including first-principle calculation, CALPHAD, phase field simulation, and machine learning is presented in this paper. The development trend of multi-scale design in novel Co-based superalloys is prospected.

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Review on Research Progress of Steel and Iron Wear-Resistant Materials
WEI Shizhong, XU Liujie
Acta Metall Sin    2020, 56 (4): 523-538.   doi:10.11900/0412.1961.2019.00370
Accepted: 24 December 2019

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In this paper, the development history of iron and steel wear-resistant materials is introduced, and the composition, microstructure, wear property, antiwear mechanism and modification technology of three typical wear resistant materials, namely high manganese steel, high chromium cast iron and high vanadium high-speed steel, are mainly reviewed. The wear-resistant steel represented by high manganese steel relies on the matrix with high strength and toughness to resist wear, while the wear-resistant alloy represented by high chromium cast iron and high vanadium high-speed steel mainly relies on the wear-resistant phase with high hardness to resist wear. High vanadium high speed steel has better wear resistance than high chromium cast iron, which is related to VC characteristics with high hardness and good shape. It is proposed that high performance wear-resistant materials should have three elements: high strength and toughness matrix, multi-scale synergistic action of high quality wear-resistant phase with high hardness and good morphology, as well as good bonding interface between wear-resistant phase and matrix.

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Progress in Research on the Alloying of Binary Immiscible Metals
HUANG Yuan, DU Jinlong, WANG Zumin
Acta Metall Sin    2020, 56 (6): 801-820.   doi:10.11900/0412.1961.2019.00451
Accepted: 16 April 2020

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Materials based on binary immiscible metal systems are widely used in aerospace, nuclear fusion engineering, electronic packaging, anti-armor weapons and other fields. However, due to the positive formation heat and the large differences in the properties of the component, the direct alloying of binary immiscible metals and the preparation of the corresponding materials are very difficult. Varieties of methods have been developed for direct alloying of binary immiscible metals at home and abroad, and the thermodynamic and diffusion mechanism of these methods have been studied. In this review, firstly the principle and thermodynamic mechanism of mechanical alloying, physical vapor deposition and ion beam mixing, as well as their applications in binary immiscible metal powder alloys and nano-multilayer films are reviewed. Then the irradiation damage alloying (IDA) and high-temperature structure induced alloying (HTSIA) methods that are proposed and developed by our group are introduced. Besides, the principle, interfacial microstructure, thermodynamic mechanism, diffusion mechanism and application of these two methods were described in detail. Finally, the development trend of the research on alloying of binary immiscible metals is proposed.

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Acta Metall Sin    doi:10.11900/0412.1961.2020.00199
Accepted: 28 August 2020

Effect of Ageing Treatment at 650 ℃ on Microstructure and Properties of 9Cr-ODS Steel
PENG Yanyan, YU Liming, LIU Yongchang, MA Zongqing, LIU Chenxi, LI Chong, LI Huijun
Acta Metall Sin    2020, 56 (8): 1075-1083.   doi:10.11900/0412.1961.2019.00445
Accepted: 01 June 2020

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Oxide dispersion strengthened (ODS) steel has excellent high-temperature performance and corrosion resistance. It has broad application prospect and development space in the key field of high temperature structural materials for nuclear power. 9Cr-ODS steel has become one of the most promising candidate materials in advanced nuclear reactors because of its excellent high temperature mechanical properties and radiation resistance. In this work, 9Cr-ODS steel was designed and prepared by powder metallurgy process. The as-hot isostatically pressed (HIPed) microstructure of the steel was studied and analyzed, including matrix grain distribution characteristics, micron-scale large size precipitated phase, and nanoscale oxide particles. In addition, the high temperature microstructure thermal stability of 9Cr-ODS steel aged at 650 ℃ for different time was researched by means of XRD, SEM, TEM and hardness test, and the microstructure change of matrix and hardness properties were analyzed. Based on the contrast analysis of the matrix microstructure and hardness properties, the hardness change of the austenitic ODS steel at high temperature was obtained. The results showed that the original as-HIPed microstructure of 9Cr-ODS steel is mainly composed of martensite lath and large amount of Y2O3. During ageing process, the lath martensite of 9Cr-ODS steel gradually coarsens and the number of dislocations decreases with ageing time increasing, and the Cr23C6 carbides begin to precipitate along the grain boundary and grow up. At the same time, the Laves phases with large size begin to precipitate in ageing and then grow with the increase of ageing time. Meanwhile, ageing treatment makes Y2O3 phase with larger size further grow, while Y2O3 phase with smaller size precipitate increase. This phenomenon can probably be associated with the dissolution of the fine particles induced from the particle coarsening, generally called the Ostwald-Ripening mechanism. The change of microhardness during ageing was related to the size of lath martensite and the number and density of the second phase precipitation, especially Cr23C6. The hardness test results show that the microhardness first decreases and then tends to be stable with the increase of ageing time.

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Recent Progress of Microstructure Evolution and Performance of Multiphase Ni3Al-Based Intermetallic Alloy with High Fe and Cr Contents
WU Jing,LIU Yongchang,LI Chong,WU Yuting,XIA Xingchuan,LI Huijun
Acta Metall Sin    2020, 56 (1): 21-35.   doi:10.11900/0412.1961.2019.00137
Accepted: 20 August 2019

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Owing to the high temperature resistance, excellent high temperature oxidation and corrosion resistance, low density and production cost, Ni3Al-based intermetallic alloys have broad applications and attract much attention. In order to widen the application field of the Ni3Al-based superalloy, it is urgently important to improve the high-temperature performance on the basis of good weldability. Under this background, in the composition design of Ni3Al alloy, the high Fe and Cr contents can effectively enhance the phase composition and weldability of Ni3Al-based intermetallic alloys. Based on this, the microstructural characterization and phase separation sequences during solidification of a newly designed multiphase Ni3Al-based intermetallic alloy modified with high Fe and Cr elements are analyzed. On account of the typical solidification structure of the multiphase Ni3Al-based intermetallic alloy comprising γ'+γ dendrite, interdendritic β and γ'-envelope, etc., the microstructural evolutions of the alloy under different solution cooling rates, high temperature annealing, and long-term ageing processes are summarized. The effects of its corresponding complex microstructural variables (size of primary γ' phase, morphology of β, phase evolution in the interior of β, widening of γ'-envelope) on the creep behaviors of the multiphase Ni3Al-based intermetallic alloy are systematically discussed. Recent advances in welding and joining of multiphase Ni3Al-based intermetallic alloy are summarized, and the development of multiphase Ni3Al-based intermetallic alloy is also prospected.

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Effect of Dilution Ratio of the First 309L Cladding Layer on the Microstructure and Mechanical Properties of Weld Joint of Connecting Pipe-Nozzle to Safe-End in Nuclear Power Plant
ZHANG Maolong, LU Yanhong, CHEN Shenghu, RONG Lijian, LU Hao
Acta Metall Sin    2020, 56 (8): 1057-1066.   doi:10.11900/0412.1961.2019.00449
Accepted: 25 May 2020

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The transition joint between austenitic stainless steel pipe and low alloy steel nozzle of the pressure vessel has attracted much attention due to the occurrence of failure during application. Usually, the low alloy steel vessel nozzle should be firstly buttered with several layers of austenitic stainless steel and then welded to the austenitic stainless steel pipe. Cracking phenomenon in the austenitic cladding layer sometimes occurs during fabrication of the transition joint, and the cracking mechanism is not very clear. It is worth noting that microstructure in the first buttering layer is largely dependent on the welding condition, because the variation of the buttering welding parameters would lead to different dilution ratios in the cladding layer. Therefore, it is essential to investigate the effect of dilution ratio of the cladding layer on the mechanical properties of the weld joint. In this work, microstructure of the 309L cladding layer under two kinds of buttering welding parameters was analyzed using OM, SEM, XRD, EPMA and EBSD, and its effects on the mechanical properties of the weld joints were further studied. The results show that duplex microstructure (austenite+martensite) are present in the 309L cladding layers under two kinds of buttering welding parameters, but the dilution ratio could determine the morphology and amount of martensite phase. Microstructure consisting of austenite and lath martensite is found in the 309L cladding layer with a lower dilution ratio. A higher dilution ratio could increase the amount of lath martensite. The formation of needle-like martensite occurs when the dilution ratio exceeds a critical value. The dilution ratio in the 309L cladding layers directly affects the mechanical properties of weld joint. For the weld joint with a lower dilution ratio, no cracking phenomonen is observed during three-point bending test, and the specimens fracture at the weld fusion zone after tensile test. For the weld joint with a higher dilution ratio, cracking phenomenon initiated at the 309L cladding layer is present during three-point bending test, and a significat reduction in the tensile strength and elongation is observed. During deformation, the strain incompatibility between needle-like martensite and austenite is produced, leading to the formation of microcracks at the interfaces. The preferential cracking at the 309L cladding layer with a higher dilution ratio leads to the degradation of mechanical properties of the weld joint.

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Effects of Hot Isostatic Pressure on Microdefects and Stress Rupture Life of Second-Generation Nickel-Based Single Crystal Superalloy in As-Cast and As-Solid-Solution States
HE Siliang, ZHAO Yunsong, LU Fan, ZHANG Jian, LI Longfei, FENG Qiang
Acta Metall Sin    2020, 56 (9): 1195-1205.   doi:10.11900/0412.1961.2020.00020
Accepted: 21 May 2020

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Due to the excellent high temperature comprehensive performance and cost effective, the second-generation nickel-based single crystal superalloy has been widely used in the high-pressure turbine blades of advanced aero-engines. Microdefects such as micropores and interdendritic eutectic are seriously harmful to the high temperature mechanical properties of nickel-based single crystal superalloys. Hot isostatic pressure (HIP) technology, which has been widely used in powder and casting superalloys, can effectively reduce the micropores, interdendritic eutectic and other structural defects formed in the turbine blades during manufacturing, and improve the service reliability of turbine blades. However, the effect of HIP process on the high temperature stress rupture life of nickel-based single crystal superalloys is still controversial, especially with regard to the initial microstructure state of the nickel-based single crystal superalloys, i.e. the as-cast microstructure state or the as-solid-solution state. In this work, a kind of second-generation nickel-based single crystal superalloy with as-cast state or as-solid-solution state was selected as the research object. Through two-stage heat/booster type heat treatment process, in combination with microdefects quantitative analysis, quantitative characterization of alloying element segregation and high temperature stress rupture tests at 980 ℃ and 250 MPa, the effects of HIP process on the microdefects and high temperature stress rupture life of the used superalloy with different initial microstructures were studied. The results indicated that the solid-solution treatment can significantly promote the diffusion of alloying elements, such as Re, W, Al, and Ta, reduce the area fraction of interdendritic eutectic, but significantly increase the average area fraction and size of micropores in the used alloy with as-cast state. While, HIP process can effectively reduce the average area fraction and size of microspores in the used alloy with as-cast state or as-solid-solution state, but cannot eliminate the interdendritic eutectic as remarkable as the solid-solution treatment. By HIP process of the used alloy with as-solid-solution state, the area fraction of micropores is reduced to 0.005%, the eutectic structure is basically eliminated, and the dendrite segregation of Re, W, Al, Ta and other elements is significantly alleviated, resulting in the higher stress ruputure life of the used alloy, about 40% over that of the used alloy with the standard heat treatment state. Performing HIP process on nickel-based single crystal superalloy alloy with as-solid-solution state is of benefit to the high temperature stress rupture life due to the reduction of microdefects and the homogenization of alloying elements, in comparison with performing HIP process directly on the alloy with as-cast sate.

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Tensile Properties of Selective Laser Melted 316L Stainless Steel
YU Chenfan, ZHAO Congcong, ZHANG Zhefeng, LIU Wei
Acta Metall Sin    2020, 56 (5): 683-692.   doi:10.11900/0412.1961.2019.00278
Accepted: 25 September 2019

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Selective laser melting (SLM), as the most common additive manufacturing (AM) method, is capable of manufacturing metallic components with complex shape layer by layer. Compared with conventional manufacturing technologies such as casting or forging, the SLM technology has the advantages of high degree accuracy, high material utilization rate and environmentally friendly, and has attracted great attention in the fields of aerospace, nuclear power and medicine. The 316L austenitic stainless steel is widely used in the industrial field because of the excellent corrosion resistance and plasticity. It is also one of the commonly used material systems for SLM. In this work, the tensile properties and fracture mechanism of 316L stainless steel fabricated via SLM technology were investigated. The microstructure of the SLMed 316L specimens after tensile fracture was characterized and analyzed. The results show that the SLMed 316L stainless steel has a relatively desirable combination of strength and ductility, and its tensile performance is obviously better than that of 316L stainless steel prepared by traditional methods. The nanometer-scale cell structure inside the grain contributes to the improvement of strength. Deformation twins were observed in the SLMed 316L stainless steel after tensile test. The appearance of twins is oriented-dependent, and it is easy to occur in the grain with the direction near <110>-<111>.

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Progress and Perspective of Ultra-High Strength Steels Having High Toughness
LUO Haiwen,SHEN Guohui
Acta Metall Sin    2020, 56 (4): 494-512.   doi:10.11900/0412.1961.2019.00328
Accepted: 15 November 2019

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Ultra-high strength steels have been widely used in the critical engineering structures in both military and civilian applications due to the combination of ultra-high strength and excellent toughness. In this paper, firstly, the typical ultra-high strength steel grades that have been employed were introduced, and their compositions, mechanical properties, application and histories of development were summarized with the emphasis on their microstructures and strengthening/toughening mechanism; secondly, the latest progress on the emerging ultra-high strength steel grades was reviewed, including their compositions, microstructures, strengthening mechanism and mechanical properties; thirdly, the newly emerging demands on replacing the currently employed ultra-high strength steels in China were defined, including steels for low-density but ultra-strong armors, the large ball grinding mill, cutters of tunnel boring machine and high pressure fracturing pump; finally, recent research results on ultra-high strength and high-toughness medium Mn steel were presented, which overcame the trade-off of strength and toughness to a greater extent; on this basis, some suggestions were put forward for the future development of these steel grades to meet the urgent national demands.

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A Review of Research Status of Hydrogen Embrittlement for Automotive Advanced High-Strength Steels
LI Jinxu,WANG Wei,ZHOU Yao,LIU Shenguang,FU Hao,WANG Zheng,KAN Bo
Acta Metall Sin    2020, 56 (4): 444-458.   doi:10.11900/0412.1961.2019.00427
Accepted: 15 January 2020

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This paper overviewed the current research status and important results of the hydrogen embrittlement (HE) of the representative steel types from 1st to 3rd generation advanced high-strength steel (AHSS): transformation induced plasticity (TRIP) steel, twinning-induced plasticity (TWIP) steel, quenching & partitioning (QP) steel and medium manganese steel. The main conclusions are as follows: the HE sensitivity of TRIP steel is mainly reflected in the reduction of plasticity and the small loss of strength. The HE sensitivity of TWIP steel depends heavily on the strain rate, i.e., the HE susceptibility is significantly increased as the strain rate decreases. Deformation twin boundaries and ε/γ phase interfaces are generally prone to hydrogen-induced cracking, while Σ3 annealing twin boundaries are not. However, the ε/γ phase interfaces with Nishiyama-Wassermann orientation relationship, which is similar to the Σ3 twin boundaries, could hinder the propagation of hydrogen-induced cracks. HE sensitivity of QP steel is similar to that of TRIP steel. For medium manganese steel containing a large volume fraction of austenite phase, which result in a strong TRIP effect during deformation, the HE susceptibility represented by plasticity loss and strength loss is very high. For TRIP steel, QP steel and medium manganese steel with austenite structure, the main strategy to improve their hydrogen embrittlement is to control the morphology and distribution of austenite structure; for TWIP Steel, the measures to improve hydrogen embrittlement can be taken by controlling the prestrain rate and Al Alloying.

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A Review of Current State and Prospect of the Manufacturing and Application of Advanced Hot Stamping Automobile Steels
JIN Xuejun,GONG Yu,HAN Xianhong,DU Hao,DING Wei,ZHU Bin,ZHANG Yisheng,FENG Yi,MA Mingtu,LIANG Bin,ZHAO Yan,LI Yong,ZHENG Jinghua,SHI Zhusheng
Acta Metall Sin    2020, 56 (4): 411-428.   doi:10.11900/0412.1961.2019.00381
Accepted: 09 March 2020

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Ultrahigh strength steels are highly competitive materials for vehicles to concurrently meet the increasing demand of the weight reduction and passenger safety. Hot stamping is the key forming technology to manufacture automobile components with high strength. Hot stamping steel and its manufacturing technology experienced a fast development in the past decade. This paper reviewed the state of the art of the manufacturing and applications of hot stamping steels/components in the following aspects: (1) hot stamping steels (from traditional MnB steels to recently newly developed hot stamping steels); (2) forming technologies (from traditional hot stamping process to industry 4.0 intelligent production); (3) novel hot stamping + quenching & partitioning (Q&P) process and fundamentals of deformation assisted heat treatments; (4) simulation techniques for hot stamping process (modeling of the temperature-stress field, microstructure field and simulation of the manufacturing process); (5) the assessments of in-service performance of hot stamped components. Finally, the trends of the development of hot stamping steels and related forming technologies in the future will be discussed.

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Motion Characteristics of <c+a> Edge Dislocation on the Second-Order Pyramidal Plane in Magnesium Simulated by Molecular Dynamics
LI Meilin, LI Saiyi
Acta Metall Sin    2020, 56 (5): 795-800.   doi:10.11900/0412.1961.2019.00305
Accepted: 23 March 2020

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Magnesium has a hcp lattice structure, in which insufficient independent slip systems are available to accommodate applied plastic deformation at room temperature. The ductility of Mg is intimately related to the fundamental behaviors of pyramidal <c+a> dislocations, which are the major contributor to c-axis strain. In this study, the motion of <c+a> edge dislocation on the second-order pyramidal plane in Mg under external shear stress of different magnitudes and directions are simulated by molecular dynamics at 300 K, and the motion and structural evolution of dislocations are studied. The results show that the effective shear stress causing dislocation motion is lower than the external applied one and the dislocation velocity increases linearly with increasing applied shear stress. Under the same level of external shear stress, the dislocation velocity in shearing leading to c-axis tension deformation is higher than that for shearing leading to c-axis compression, and in both cases the corresponding viscous drag coefficients are significantly higher than those for basal and prismatic edge dislocations at the same temperature. The tension-compression asymmetry of dislocation motion is essentially related to the effect of applied shear stress on the extended dislocation width.

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Low-Cycle Fatigue Behavior of 1100 MPa Grade High-Strength Steel
ZHOU Hongwei, BAI Fengmei, YANG Lei, CHEN Yan, FANG Junfei, ZHANG Liqiang, YI Hailong, HE Yizhu
Acta Metall Sin    2020, 56 (7): 937-948.   doi:10.11900/0412.1961.2019.00291
Accepted: 13 March 2020

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The low-cycle fatigue (LCF) behavior of 1100 MPa grade tempered high strength steel under symmetrical strain control conditions was studied at the strain amplitude ranges of 0.4%~1.2% in this work. The LCF properties of quenching and tempering high strength steel were examined by means of OM, SEM and TEM. The microstructure changes, fracture morphology, crack propagation characteristics and inclusion morphology were studied in detail. The results show that the cyclic hardening and cyclic softening depend on strain amplitude. At the low strain amplitude of 0.4%, rapid cyclic hardening occurs in initial 10 cyc, and then the stress remains almost unchanged until the sample breaks. While at the strain amplitude ranges of 0.5%~1.2%, the cyclic hardening reaches a peak at the first few cycles, followed by the remarkable cyclic softening until the sample fails. The cyclic softening is mainly due to the recovery of some martensite lath under low-cycle fatigue loading and the decrease of dislocation density in the laths. 1100 MPa grade high-strength steel is found to obey LCF Manson-Coffin relationship. The high-strength steel has excellent LCF performance for two main reasons, which is related with the shape and size of inclusions. One is that the shape of the inclusion is nearly circular, and the diameter is 2~5 μm, which is lower than the critical dimension of the inclusions causing fatigue crack initiation. The crack is initiated on the surface of the sample. This increases fatigue crack initiation life. The other one is that the original austenite grain boundary, the martensite packet/block boundary and the inclusions or cavities can induce the crack deflection, reducing the crack propagation rate and increasing fatigue crack propagation life.

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Diffusion Bonding of Copper and 304 Stainless Steel with an Interlayer of CoCrFeMnNi High-Entropy Alloy
DING Wen, WANG Xiaojing, LIU Ning, QIN Liang
Acta Metall Sin    2020, 56 (8): 1084-1090.   doi:10.11900/0412.1961.2019.00404
Accepted: 07 May 2020

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During the dissimilar materials bonding of copper and 304 stainless steel, micro-voids and micro-cracks can propagate into the bond region because of Kirkendall effect, and have a strong impact on the mechanical and physical properties of conjunct. Copper and 304 stainless steel was bonded by utilizing vacuum solid-state diffusion method with an interlayer of CoCrFeMnNi high-entropy alloy, and the influence of temperature on diffusion reaction mechanism and properties was investigated by using SEM, EDS and microhardness test. The second Fick's law was adopted to calculate the diffusion coefficient of Cu/Fe in CoCrFeMnNi high-entropy alloy. The phase components of the diffusion interface were detected by XRD, and the famous phase-selection-criteria was also used to discuss the phase formation. The results showed that the diffusion interface was well bonded and all the elements diffused mutually at the temperature range of 800~900 ℃, the diffusion rate of Cu/Fe in CoCrFeMnNi high-entropy alloy was increased with the increasing temperature, and no intermetallic compounds were detected at the diffusion interface, and the microhardness increased continuously near the diffusion interface. It was investigated that CoCrFeMnNi high-entropy alloy can be used as an effective diffusion barriers for dissimilar materials bonding of Cu/304 stainless steel.

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Research Progress of Laser Additive Manufacturing of Maraging Steels
TAN Chaolin,ZHOU Kesong,MA Wenyou,ZENG Dechang
Acta Metall Sin    2020, 56 (1): 36-52.   doi:10.11900/0412.1961.2019.00129
Accepted: 21 June 2019

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Additive manufacture is recognized as a world-altering technology which triggered a world-wide intensive research interest. Here the research progress and application of the laser additive manufacturing maraging steel (MS) are systematically outlined. The advantages of selective laser melting (SLM) additive manufacture of MS is emphasized. The processing parameter and properties optimizations, build orientation based anisotropies, age hardening mechanism, gradient materials, and applications in die and moulds of SLM-processed MS are reviewed in detail. Achieving relative density of >99% in SLM-processed MS is effortless, owing to the wide SLM process window of MS. Mechanical properties of MS produced with optimized SLM processing parameters and post heat treatments are comparable to traditionally wrought parts. The build orientation hardly affects the property anisotropies of MS. The age hardening behaviour in MS follows Orowan bowing mechanism. MS-based gradient multi-materials (such as MS-Cu, MS-H13, etc.) with high bonding strength are fabricated by SLM, which provides a new approach to produce high-performance functionally gradient multi-materials components. Lastly, the application in conformal cooling moulds of SLM-processed MS is elucidated, and future research interests related to MS are also proposed.

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Microstructure and Corrosion Behavior of Fe-15Mn-5Si-14Cr-0.2C Amorphous Steel
ZHAO Yanchun, MAO Xuejing, LI Wensheng, SUN Hao, LI Chunling, ZHAO Pengbiao, KOU Shengzhong, Liaw Peter K.
Acta Metall Sin    2020, 56 (5): 715-722.   doi:10.11900/0412.1961.2019.00275
Accepted: 23 December 2019

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Amorphous steels exhibit ultra-high strength but room-temperature brittleness and strain-softening behavior as loading, which restricted the application of amorphous steels as high-performance structural material. Developing in situ crystals is an effective way to toughen the amorphous alloys. However, the crystals may sacrifice the corrosion resistance of amorphous steels. In this work, austenite and ferrite duel phases were introduced to the amorphous phase, via transformation induced plasticity (TRIP) of the austenite as loading, to enhance the ductility and improve the work-hardening behavior; and via the synergy of ferrite and amorphous phase to ensure the corrosion resistance. A novel amorphous steel Fe-15Mn-5Si-14Cr-0.2C was fabricated by magnetic suspension melting in a water-cooled copper crucible, and negative pressure suction casting into a copper mold. The microstructure and mechanical properties of the amorphous steel were characterized by XRD, EBSD and the electronic universal testing machine. The corrosion behavior in artificial seawater was studied on an electrochemical work station with a three-electrode system, and the corrosion morphology and corrosion products were characterized by SEM with EDS analysis. The results showed that the as-cast amorphous steel consisted of the amorphous matrix, CFe15.1 super-cooled austenite and Fe-Cr ferrite phases. From surface to inner, amorphous phases mainly exist in the margin, while crystalline phases are abundantly distributed in the center. The amorphous steel exhibited excellent comprehensive mechanical properties at room temperature, and its yield strength, fracture strength and plastic strain were up to 978 MPa, 2645 MPa and 35.8%, respectively. In artificial seawater, compared with 304 stainless steel, the amorphous steel showed high self-corrosion potential, low self-corrosion current density and high polarization resistance, large resistance arc radius, only one high frequency resistance arc and low corrosion kinetic rate. Moreover, the stable and dense passivation film was observed on the corrosion surface. Their excellent corrosion resistance and mechanical properties endow the amorphous steel with the potential to become a novel corrosion-resistant structural material for marine engineering.

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Microstructure Evolution of K4169 Alloy During Cyclic Loading
WU Yun, LIU Yahui, KANG Maodong, GAO Haiyan, WANG Jun, SUN Baode
Acta Metall Sin    2020, 56 (9): 1185-1194.   doi:10.11900/0412.1961.2020.00026
Accepted: 05 June 2020

Abstract216)   HTML0)    PDF (4614KB)(182)      

K4169 nickel-based superalloy has been widely used to fabricate high-strength components in aircraft engine. When in service, especially affected by vibration and start-stop process, this alloy is inevitably affected by the external cyclic stress. Therefore, it is of great significance for researchers to understand the microstructure evolution in K4169 while cyclic loading. In the present study, the microstructure evolution of K4169 during cyclic loading has been examined and discussed in detail by using investment casting, cyclic loading and microstructure characterization methods. The cyclic loading test with stress amplitude of 380 MPa was carried out on a pull-push type fatigue machine at room temperature. The dependence of cycle times or fatigue life of specimens with different casting conditions on microporosity content has been discussed. Special emphases have been put on investigating the deformation and fracture characteristics of Laves and δ-Ni3Nb phases under the influence of microporosity. The results show that the cyclic life was mainly dominated by the content of microporosity. The crack initiation occurred mainly near the microporosity of the specimen surface. The specimen with high microporosity content exhibits the characteristic of complete brittle fracture, while the specimen with low microporosity content exhibits obvious transgranular fracture characteristics. In addition, the fracture of Laves phase was not apparently affected by cycle number. At the beginning of cyclic loading, the long-striped Laves phase near the microporosity was easy to crack, which became the sensitive area of crack growth, and extending in the manner of parallel secondary cracks. The δ-Ni3Nb plates near microporosity exhibited two obvious cyclic deformation and fracture characteristics depending on their arrangement (or growth orientation) relative to external loading axis: cracking along length direction (or denoted as branch cracking); and exhibiting slip lines and cracks on the surface of δ-Ni3Nb plates. At the initial stage of cyclic loading, δ-Ni3Nb plates were prone to crack along the length direction, while the surfaces of the δ-Ni3Nb plates far from microporosity appear the characteristics of slipping, bending and fracture in turn with the decrease of microporosity content or increase of cyclic cycles. Edge dislocations have been found within δ-Ni3Nb plates, indicating the transition from screw dislocations to edge dislocations under cyclic loading. Additionally, the twinning deformation of γ-Ni matrix during cyclic loading has been scrutinized through TEM and TKD analyses. The results have been linked to the evolutions of Laves and δ-Ni3Nb phases, i.e., the evolutions were influenced by the increase of strain localization around Laves and δ-Ni3Nb phases.

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Effect of Electromagnetic Stirring on Microstructure and Mechanical Properties of TiB2 Particle-Reinforced Steel
ZHANG Lin, GUO Xiao, GAO Jianwen, DENG Anyuan, WANG Engang
Acta Metall Sin    2020, 56 (9): 1239-1246.   doi:10.11900/0412.1961.2020.00019
Accepted: 30 April 2020

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TiB2 is a promising strengthening phase in steels applying in lightweight transportation systems due to its high Young's modulus and low density. However, the density difference between TiB2 particles and matrix leads to segregation during solidification. TiB2 particle-reinforced steels were solidified with a vertical linear-type electromagnetic stirring device. The effects of electromagnetic stirring on the morphology and size distribution of TiB2 particles were studied. Vickers hardness, mechanical properties in the tensile test were also discussed. The results show that electromagnetic stirring effectively refined the primary TiB2 particles in the steel, and the average particle size decreased with the increase of exciting current. The particles distributed dispersively and the structure was more homogenous under a higher exciting current. Besides, the defects of crackle around particles were eliminated under high current. Electromagnetic stirring reduced the macrosegregation of TiB2 particle-reinforced steels, which decreased the hardness discrepancy in the ingot at various heights. A higher exciting current attributed to higher average hardness, and the steel reached a hardness of 275 HV under 350 A exciting current. The ultimate tensile strength and the strain at break were both enhanced by electromagnetic stirring, and reached 520.2 MPa and 8.5% respectively under an exciting current of 350 A. The refinement of particles was caused by the smashing process under a strong convection driven by the moving magnetic field, and the effect of electromagnetic force acting on the particles. The influence factors of electromagnetic force were analyzed, which show the force increases with increasing magnetic intensity, decreases with increasing temperature of melt, and increases with increasing particle size.

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Online Electromagnetic Measurement of Molten Zinc Surface Velocity in Hot Galvanized Process
ZHENG Jincan, LIU Runcong, WANG Xiaodong
Acta Metall Sin    2020, 56 (7): 929-936.   doi:10.11900/0412.1961.2020.00024
Accepted: 11 May 2020

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The behavior of zinc flow in the zinc bath plays an important role in hot galvanizing process, which has an important influence on the temperature distribution, the composition of zinc coat, the control of air knife, and so on, thus affecting the surface quality of zinc products (surface oxidation, rake slag). However, due to the high temperature, strong activity, opacification of the zinc bath and harsh, complex industrial environment, it is difficult to directly measure the flow behavior of zinc in the zinc bath through conventional methods. In this work, based on the principle of electromagnetic induction, Lorentz force velocimetry (LFV) method was used to measure and analyze the velocity of zinc flow in the bath during the galvanizing process for the first time. The LFV has the characteristics and advantages of non-contact, online and continuous measurement, and can realize the real-time quantitative measurement of molten metal flow by reasonable design and ingenious implementation. The key parameters of LFV, such as the gap between device and molten zinc, penetration depth and geometry of the applied model, were discussed through numerical analysis, the LFV device suitable for the characteristics of zinc plating process was designed, and the in-plant measurement was carried out. The results show that the fluctuation range of zinc flow velocity in the zinc bath is almost 0.13~0.20 m/s, which is within typical range referenced in previous studies. In addition, the flow behavior and flow field characteristics of zinc liquid were analyzed, and these discussions reflect the capacity of zinc slag or ash in the zinc flow at the monitoring position. The work promoted in this study revealed that this LFV method can measure the surface velocity of zinc liquid in real time, on-line and quantitatively, which provides a new way for the velocity monitoring of high temperature liquid metal in metallurgical industry.

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Microstructure Evolution and Deformation Mechanisms by Direct Hot-Pack Rolling for As-Cast Ti-46Al-8Nb Alloys
LI Tianrui, LIU Guohuai, YU Shaoxia, WANG Wenjuan, ZHANG Fengyi, PENG Quanyi, WANG Zhaodong
Acta Metall Sin    2020, 56 (8): 1091-1102.   doi:10.11900/0412.1961.2019.00379
Accepted: 07 May 2020

Abstract199)   HTML0)    PDF (5376KB)(108)      

TiAl alloys are considered attractive structural materials because of their low density, excellent high-temperature strength, and oxidation resistance. However, their intrinsic characteristics, including low-temperature brittleness, poor workability, and narrow processing window, restrict their wide use in industrial applications. Various hot-work processes are conducted to enhance the inherent ductility of TiAl alloys, especially hot-pack rolling. In this work, the hot processing maps at different strains were developed based on isothermal compression tests and dynamic material model (DMM). The optimum hot-working parameters were selected and a crack free Ti-46Al-8Nb (atomic fraction, %) sheet was directly fabricated by hot pack rolling from ingot. Moreover, microstructure evolution and hot deformation behavior of the as-rolled alloys were investigated. The processing maps showed two typical dynamic recrystallization (DRX) domains which would facilitate the hot-work process, of which the temperature was at 1200 ℃, strain rates was 1 s-1 with a peak efficiency of power dissipation of 0.38 and temperature of 1150~1200 ℃, strain rate of 0.01 s-1 with a peak efficiency of power dissipation of 0.45. The instable-area temperature was 1100~1200 ℃ and strain rate was 0.06~1 s-1 at low strain, which was expanded to low strain rate with the increasing strain. As the strain increased to 0.4, the region with the temperature of 1250 ℃ and strain rate of 0.006 s-1 always became instable. The Ti-46Al-8Nb alloy sheet with thickness of 0.85 mm was produced within processing windows of 1150~1200 ℃, 0.01~0.03 s-1 with engineering strain 18% per pass. The produced sheet showed uniform microstructure as a whole, though the local flow softening and deformation bands were inevitable. Furthermore, the main softening mechanism of Ti-46Al-8Nb alloy was DRX which began with the pile-up of dislocations, the formation of sub-boundaries and mechanical twins. Then the substructures would rearrange to inducing the formation of DRXed grains with the cumulative reduction increasing. The phase transitions of Lamellae (α/γ)→γ+α+B2/β and αγ during hot-pack rolling combining with the growth of DRXed grains were simultaneously a main softening mechanism. The formations of plentiful mechanical twinning and twin lamellae also contributed to the uniformity of as-rolled microstructure.

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Numerical Simulation of Stress Evolution of Thin-Wall Titanium Parts Fabricated by Selective Laser Melting
KE Linda,YIN Jie,ZHU Haihong,PENG Gangyong,SUN Jingli,CHEN Changpeng,WANG Guoqing,LI Zhongquan,ZENG Xiaoyan
Acta Metall Sin    2020, 56 (3): 374-384.   doi:10.11900/0412.1961.2019.00198
Accepted: 18 November 2019

Abstract196)   HTML2)    PDF (9051KB)(367)      

Selective laser melting (SLM) is a very promising additive manufacturing (AM) technology for fabrication of thin-walled parts due to its high forming accuracy with complex shape. The higher temperature gradient in rapid heating and cooling process is prone to produce larger thermal stress, which will induce warpage deformation of SLMed parts. However, most of the current SLM stress studies focus on the residual stress, and only a few reports on the transient stress in the thermal cycle during SLM. In this work, a thermal-mechanical coupled transient dynamic finite element model was established to study the effects of laser scan rate and layer thickness on stress evolution during SLM processing. The results show that under the action of thermal cycle, the internal stress evolution in SLM of titanium alloy thin-walled parts presents a thermal stress cycle. Under the relief annealing of the thermal stress cycle, the peak thermal stress increases first and then decreases in the heating stage, and stabilizes and approaches the value of residual stress in the cooling stage. The residual stress of SLMed thin-walled parts is less than the transient peak stress during heating. After several thermal cycles with stress relief annealing effect, the peak thermal stress of SLM thin-walled parts can be reduced by more than 30%.

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Strength, Ductility and Fracture Strain ofPress-Hardening Steels
YI Hongliang,CHANG Zhiyuan,CAI Helong,DU Pengju,YANG Dapeng
Acta Metall Sin    2020, 56 (4): 429-443.   doi:10.11900/0412.1961.2020.00003
Accepted: 11 March 2020

Abstract194)   HTML3)    PDF (21659KB)(343)      

Press-hardening steels (PHS) are increasingly used for vehicle body structure components because of their lightening potential owning to superiorly high strength, adequate ductility and fracture resistance. New PHS grades with higher strength and enhanced fracture resistance are being widely studied now for achieving further vehicle weight reduction, and the recent development in this field is reviewed in this article. Combining quenching and partitioning (Q&P) with the hot stamping process has been explored by some researchers, as well as tempering after the hot stamping using the medium-Mn steels. A certain amount of austenite could remain by the above processes and the resulted tensile strength can exceed 1500 MPa while tensile ductility of 10%~16% can be achieved utilizing the transformation-induced plasticity (TRIP) effect. A V micro-alloyed steel (34MnB5V) for hot stamping has been designed, utilizing both grain refinement and precipitation strengthening of VC. The tensile strength of the newly developed 34MnB5V exceeds 2000 MPa which is much higher than that of the most commonly used PHS 22MnB5 (1500 MPa). Meanwhile, the ductility and bending properties of the above two steels are comparable. Al-Si coated PHS is usually adopted to avoid oxidation during heating and improve its corrosion resistance after stamping. However, its bendability after forming is lower than that of the bare grade when surface decarburization is absent. The thickness of the brittle Fe2Al5 phase was reduced and the carbon enrichment at the interface of α-Fe and martensite matrix was weakened after hot stamping by thinning of the Al-Si coating. Thus, the bending property was improved. The applicability of the new designed processes for the existing production lines should be considered in future studies. The bending test should be adopted for the deformability evaluation rather than the uniaxial tensile test simply. The welding property and the mechanism of hydrogen embrittlement should also be studied for industrial application of the new developed steels.

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A Method to Calculate the Dislocation Density of a TWIP Steel Based on Neutron Diffraction and Synchrotron X-Ray Diffraction
LI Yizhuang,HUANG Mingxin
Acta Metall Sin    2020, 56 (4): 487-493.   doi:10.11900/0412.1961.2020.00016
Accepted: 16 March 2020

Abstract193)   HTML2)    PDF (2167KB)(460)      

The modified Williamson-Hall method, which has been widely used to calculate dislocation densities of high-strength steels and other structural alloys, is re-examined in this work, and is further applied to calculate the dislocation density of a deformed twinning-induced plasticity (TWIP) steel by using its neutron diffraction patterns and synchrotron X-ray diffraction patterns. This paper aims not only to promote the proper use of the method but also to shed light on its underlying pre-requisites and assumptions, and is thus expected to help avoid any errors during its usage.

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Mechanical Properties of AlSiMg Alloy Specifically Designed for Selective Laser Melting
GENG Yaoxiang, FAN Shimin, JIAN Jianglin, XU Shu, ZHANG Zhijie, JU Hongbo, YU Lihua, XU Junhua
Acta Metall Sin    2020, 56 (6): 821-830.   doi:10.11900/0412.1961.2019.00306
Accepted: 03 January 2020

Abstract190)   HTML2)    PDF (4779KB)(356)      

Using complex shapes and precise structural parts is becoming a strong trend in modern advanced manufacturing. However, traditional manufacturing technology hardly achieves the complex geometric parts directly. Selective laser melting (SLM) is an advanced manufacturing technology for metallic materials, enables production parts with complex geometry combined with the enhancement of design flexibility. The cooling rate of molten pool can reach 103~106 K/s during the SLM process. In this case, the solid solubility of the alloying elements in the matrix can be greatly enhanced. Aluminum alloy has been widely used in industry. At present, the strength of SLM-formed aluminum alloys is far lower than that of high-strength aluminum alloys obtained from a traditional process. It is necessary to develop high-strength aluminum alloy composition based on SLM technical characteristics. The present study is devoted to design high-strength AlSiMg1.5 aluminum alloy specifically for SLM using the local structure model based on the liquid-solid structural compatibility of the alloy and the technical characteristics of the liquid quenching in SLM. The effect of the ageing treatment on the microstructure, the hardness, and the compressive properties of the SLM-formed AlSiMg1.5 alloy was systematically studied. Almost completely dense samples were obtained by adjusting the parameters of SLM process. When the ageing temperature was 300 ℃, the super-solid solution Si precipitated and grew in the island-like Al-rich structure, and the reticular Si-rich structure decomposed and spheroidized gradually with the increases of ageing time of SLM-formed AlSiMg1.5 samples. In this case, the hardness and the strength of the samples decreased, but the elongation increased significantly. The microstructures of the SLM-formed AlSiMg1.5 samples did not change obviously when the ageing temperature was 150 ℃. But the hardness and yield strength of the samples significantly increased first and then decreased slightly. The maximum microhardness and compressive yield strength of SLM-formed AlSiMg1.5 samples aged at 150 ℃ were (169±1) HV and (453±4) MPa, respectively, and the elongation of samples exceeds 25%. In this study, a special Al91.0Si7.5Mg1.5 (mass fraction, %) aluminum alloy specifically for SLM with excellent formability and mechanical properties was designed.

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M3 Microstructure Control Theory and Technology of the Third-Generation Automotive Steels with HighStrength and High Ductility
WANG Cunyu,CHANG Ying,ZHOU Fengluan,CAO Wenquan,DONG Han,WENG Yuqing
Acta Metall Sin    2020, 56 (4): 400-410.   doi:10.11900/0412.1961.2019.00371
Accepted: 07 January 2020

Abstract190)   HTML2)    PDF (6927KB)(400)      

An important topic is the achievement of high strength and high plasticity for the development of automotive steels. Present article reviews the M3 (multiphase, metastable and multiscale) microstructure and property control theory and technology of high-strength and high-ductility third-generation automotive steels, as well as new challenges. M3 microstructure and property-microstructure control theory provide theoretical support for the development of steels with high strength and high plasticity. Transformation induced plasticity (TRIP) effect of metastable austenite has a significant influence on properties and microstructure of steels. On the one hand, it can enhance the work-hardening rate and thereby improve strength and plasticity of steels. On the other hand, it causes some new problems, such as the increase of the shear edge crack sensitivity, the decrease of hydrogen induced delayed fracture properties, and more complex transformation behavior of metastable austenite under cyclic loading. At present, the quality consistency and basic research on application are insufficient for the high-strength and high-plasticity steels with metastable austenite. As a widely-applied product, the automotive steels need be evaluated in microstructure evolution and properties from the whole chain including composition design, microstructure control, cutting process, forming process, joining process and service performance. The evaluation results will provide the basis for the improvement of microstructure control theory and technology. Full consideration will be given in the technical applicability and cost of products.

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Effect of Cold Deformation and Solid Solution Temperature on σ-phase Precipitation Behavior in HR3C Heat Resistant Steel
CAO Tieshan, ZHAO Jinyi, CHENG Congqian, MENG Xianming, ZHAO Jie
Acta Metall Sin    2020, 56 (5): 673-682.   doi:10.11900/0412.1961.2019.00267
Accepted: 24 October 2019

Abstract187)   HTML1)    PDF (3791KB)(268)      

HR3C steel, widely applied in ultra-supercritical power plant, suffers an intergranular embrittlement problem during long-term high-temperature ageing or service, which will be enhanced by the precipitation of σ phase. Research has showed that the precipitation behaviors of σ phase are different significantly as the difference of manufacturers, which relates to the preparation process of cold-deformation & solid-solution treatment. In this work, the effects of cold deformation and solution treatment on the precipitation kinetics of σ phase and related mechanical properties for HR3C steel during the ageing process were studied. The results show that cold-deformation and solid solution temperature both have a significant influence on the precipitation of σ phase in the steel. The increase of cold-deformation will promote the precipitation of σ phase, and rising solution temperature helps to inhibit the growth of σ phase but increase the grain size. The precipitation kinetics study of σ phase in HR3C steel with different pre-treatment shows that σ phase growths slowly at first, and then gets into a rapid precipitation period, and finally reaches a steady-state with a value of about 5.7% (volume fraction). The impact toughness analysis shows that the increase of cold-deformation would lower down the impact toughness of HR3C steel during the ageing procedure, while the rise of the solid-solution temperature increases the impact toughness before ageing and reduces it during ageing.

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As-Cast Microstructure Characteristic and Homogenization of a Newly Developed Hard-Deformed Ni-Based Superalloy GH4975
XIANG Xuemei, JIANG He, DONG Jianxin, YAO Zhihao
Acta Metall Sin    2020, 56 (7): 988-996.   doi:10.11900/0412.1961.2019.00429
Accepted: 13 April 2020

Abstract186)   HTML2)    PDF (3295KB)(200)      

Alloy GH4975 is a newly developed hard-deformed Ni-based superalloy which can keep high performance at elevated temperatures. And it is expected to be applied above 850 ℃. The as-cast microstructure, hot deformation of as-cast alloy, and the microstructural evolution during homogenization of alloy GH4975 were investigated utilizing a combination of FESEM, EBSD and extractive phase analysis. The results show that the γ′ phase, primary MC carbide and eutectic phase are the main precipitates in the as-cast alloy. Alloying elements Ti, Nb and W exhibit severe microsegregation during solidification. Cracking phenomenon can be observed in the hot-deformed samples of as-cast alloy due to the incoordination deformation between matrix and the MC carbide, primary coarse γ′ phase and eutectic phase. Microsegregation of alloying elements is eliminated after heat treated at 1180 ℃ for 50 h. Furthermore, besides of the redissolution of eutectic phase, the morphologies and size of MC carbide also evolved during homogenization process. Thermoplasticity and deformability can be improved obviously after homogenization due to improvement of the coordinated deformation capacity of MC carbide and strengthening phase.

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Relationship of Inclusions and Rolling Contact Fatigue Life for Ultra-Clean Bearing Steel
SUN Feilong, GENG Ke, YU Feng, LUO Haiwen
Acta Metall Sin    2020, 56 (5): 693-703.   doi:10.11900/0412.1961.2019.00337
Accepted: 20 November 2019

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The cleanliness of bearing steels produced in China has been greatly improved due to the significant progress in the steelmaking technologies in the past decade, leading to their total oxygen (T.O.) contents lowered to no more than 6×10-6. Under such a high cleanliness, it is then expected that the influence of non-metallic inclusions on fatigue property should be different from the previous knowledge, because both the size and quantity of inclusions are reduced greatly. Therefore, both inclusions and fatigue properties for three ultra-clean GCr15 (100Cr6) bearing steels containing T.O. around 6×10-6, which were manufactured via different industrial production processes, were studied for this purpose. First, inclusions were characterized by ASPEX SEM and then statically analyzed by the statistics of extreme values (SEV) and the generalized Pareto distribution (GPD). Next, their rolling contact fatigue lives (RCF) L10 and L50 were measured by flat washer tests. Only the largest inclusion in each sample is required for predicting the characteristic sizes of maximum inclusion (CSMI) for the three steels using the SEV method. The calculated CSMIs, however, are not consistent with the variation of either L10 or L50, indicating they are not relevant. In contrast, the types of inclusions above threshold (u) size can be classified and their number density of inclusions quantified when the GPD method is employed. In particularly, the CSMIs of different types of inclusions can be determined. In this case, it has been found that the CSMI of TiN inclusion, which is the most dangerous for initiating cracking, is in a good agreement with the low probability rolling fatigue life (L10), suggesting that they are very correlated. This, however, cannot explain the variation of high-probability fatigue life (L50). Instead, the density of total inclusions also played an important role on the L50 of ultra-clean bearing steels in addition to the CSMI of TiN inclusions. This is reasonable because cracking shall be initiated at not only the most dangerous TiN inclusion during the early failure but also some other highly dense inclusions particularly during the late failure. Therefore, it is then concluded that the L10 is much more related to the CSMI of most dangerous TiN inclusion; whilst the L50 is strongly affected by the number density of total inclusions.

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