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

About the Journal

Most Read
Published in last 1 year |  In last 2 years |  In last 3 years |  All
Please wait a minute...
For Selected: Toggle Thumbnails
Progress in Materials Genome Engineering in China
SU Yanjing, FU Huadong, BAI Yang, JIANG Xue, XIE Jianxin
Acta Metall Sin    2020, 56 (10): 1313-1323.   doi:10.11900/0412.1961.2020.00199
Accepted: 28 August 2020

Abstract868)   HTML2)    PDF (1951KB)(945)      

Materials genome engineering (MGE) is a frontier technology in the field of material science and engineering, which is well capable to revolutionize the research and development (R&D) mode of new materials, greatly improve the R&D efficiency, shorten the R&D time, and reduce the cost. This paper reviews the progress of MGE in China from the aspects of the fundamental theory and methods, key technology and equipment, the R&D of new materials and related engineering application, talents training, formation and promotion of new concept of material genetic engineering. The paper also looks forward to the future development of MGE in China.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract766)   HTML10)    PDF (16289KB)(851)      

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.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract755)   HTML58)    PDF (24999KB)(1060)      

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.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract395)   HTML6)    PDF (3903KB)(444)      

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.

Table and Figures | Reference | Related Articles | Metrics
Microstructure and Mechanical Properties of HSLA Steel Containing 1.4%Cu
DU Yubin, HU Xiaofeng, ZHANG Shouqing, SONG Yuanyuan, JIANG Haichang, RONG Lijian
Acta Metall Sin    2020, 56 (10): 1343-1354.   doi:10.11900/0412.1961.2020.00012
Accepted: 26 May 2020

Abstract388)   HTML0)    PDF (4726KB)(409)      

The Cu bearing high strength low alloy (HSLA) steels exhibit high-strength, high toughness and good weldability, which have been widely used in shipbuilding, offshore structures etc. Due to the extremely poor impact energy when attained peak strength, the Cu bearing HSLA steels are usually used at overaged state, which have a good combination of impact energy and strength. In order to clarify the effect of Cu on mechanical properties especially on the impact energy for HSLA steels at peak ageing state, two HSLA steels without Cu (0Cu) and with 1.4%Cu (1.4Cu), were prepared by vacuum induction melting in this study. The influence of Cu on the microstructure of HSLA steel was investigated by OM, SEM and EBSD. Meanwhile, the Cu-riched clusters were characterized by APT and the mechanical properties were measured by tensile test and impact test. The results show that the Cu is completely solid-solutioned into the matrix after quenching, and there are a great number of Cu-riched clusters precipitated in the matrix and boundaries after tempering. Cu element has no obvious effect on the prior austenite grain size, microstructure and effective grain size of tempered HSLA steel, but has significant influence on the strength and impact energy for tempered HSLA steel. After tempered at 450 ℃, the 1.4Cu steel attained the maximum yield strength (1053 MPa), higher than that of 0Cu steel. It is worth noting that the impact energy of 1.4Cu steel tempered at 450 ℃ is only 24 J at room temperature and the impact fracture is a quasi-cleavage brittle fracture mode dominated by river patterns. However, 0Cu steel exhibits a completely ductile fracture mode dominated by dimples at room temperature and the impact energy is 127 J. The APT results show that both 0Cu and 1.4Cu tempered steels have the segregation of C, Cr, Ni, Mn elements at the lath boundary. Compared with 0Cu steel, there precipitate a great number of Cu-riched clusters at the lath boundary for 1.4Cu steel, which will result in the stress concentration and then promote the crack initiation at the lath boundary. In addition, the Cu-rich clusters precipitated at the lath boundary could prevent the Mo segregated at the lath boundary, which will decrease the bonding energy and then promote the crack propagation along the lath boundary. Besides, the negative effect of strengthening due to the Cu-riched clusters at matrix will also accelerate the crack propagation in the matrix, which will decrease the impact energy of 1.4Cu steel. Therefore, the impact energy of 1.4Cu steel is much lower than that of 0Cu steel at room temperature.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract375)   HTML1)    PDF (3597KB)(218)      

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.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract369)   HTML6)    PDF (26034KB)(402)      

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.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract354)   HTML4)    PDF (26345KB)(724)      

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.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract354)   HTML5)    PDF (10980KB)(866)      

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.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract352)   HTML6)    PDF (11030KB)(487)      

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.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract351)   HTML2)    PDF (2906KB)(295)      

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.

Table and Figures | Reference | Related Articles | Metrics
Effect of Al on Hardenability and Microstructure of 42CrMo Bolt Steel
LU Chaoran, XU Le, SHI Chao, LIU Jinde, JIANG Weibin, WANG Maoqiu
Acta Metall Sin    2020, 56 (10): 1324-1334.   doi:10.11900/0412.1961.2020.00045
Accepted: 26 May 2020

Abstract338)   HTML0)    PDF (5761KB)(418)      

42CrMo steel has a good combination of strength and toughness after quenching and tempering treatment, which make it an ideal candidate material for high strength bolt. Nevertheless, with the increase of bolt diameter in wind power field, the hardenability of 42CrMo steel is inadequate to manufacture the high strength bolt with diameter over 36 mm. Recent study indicates that Al addition is an economical and effective way to affect the phase transformation product during quenching process. In order to improve the hardenability of 42CrMo bolt steel, the effect of Al on the hardenability of 42CrMo was investigated by Jominy test and cross section hardness distribution test. OM and SEM were used to analyze the morphology of the grain size; chemical phase analysis test was used to detect the precipitation in Al addition steels; the isothermal transformation diagram (TTT curve) was measured to study the phase transformation of the steels; the three dimensional atom probe (3DAP) was used to analyze the Al distribution in matrix; the tensile and impact toughness properties of Al addition steels were also examined. It was found that the hardenability of 42CrMo bolt steel could be improved significantly by Al-Ti and Al-B addition, the hardness was increased by 6 HRC at the position of 25 mm from quenched end, the center hardness in diameter of 42, 48 and 56 mm was increased by 7, 10 and 14 HRC, respectively. The improvement of hardenability for Al-Ti addition steel can be attributed to the increasing dissolved Al content in the matrix because of the Ti addition, which suppresses the formation of bainite during the quenching process. The hardenability of Al-B addition steel is better than that of Al-Ti addition steel, which can be ascribed to the dissolved Al and B inhibiting the phase transformation of ferrite and pearlite. Moreover, Al can play an important role in increasing dissolved B content by means of AlN formation, in which the dissolved Al dispersive distribution in matrix is favorable to improve the hardenability of 42CrMo steel. Meanwhile, the tensile strength and Charpy V-notch impact energy at -40 ℃ of Al addition steels are adequate to manufacture grade 12.9 high strength bolt.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract336)   HTML1)    PDF (4614KB)(288)      

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.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract336)   HTML7)    PDF (2537KB)(437)      

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.

Table and Figures | Reference | Related Articles | Metrics
Effects of Tempering Temperature on Microstructure and Mechanical Properties of a Mn-Cr Type Bainitic Forging Steel
WANG Zhanhua, HUI Weijun, XIE Zhiqi, ZHANG Yongjian, ZHAO Xiaoli
Acta Metall Sin    2020, 56 (11): 1441-1451.   doi:10.11900/0412.1961.2020.00139
Accepted: 27 July 2020

Abstract329)   HTML0)    PDF (5869KB)(215)      

With continuous demands for cost reduction and environmental protection, bainitic forging steels, which have notably higher strength and toughness combination than ferritic-pearlitic forging steels, have been developed and gained an increasingly applications in a variety of critical automotive parts. In order to optimize the microstructure and properties of bainitic forging steel, the influences of tempering temperature ranging from 200 ℃ to 500 ℃ on the microstructure and mechanical properties of a Mn-Cr type bainitic forging steel were investigated based on microstructural observations and mechanical property tests. The results show that the microstructure in the as-forged condition of the tested steel is a mixture of lower lath-bainite and granular bainite. With the increase of tempering temperature (Ttemp), the microstructure began to recover and the large blocky martensite/austenite (M/A) constituents decomposed granularly with the precipitation of fine cementites. Further increasing Ttemp to 500 ℃, the blocky M/A constituents decomposed completely and the cementites were spheroidized. Consequently, the ultimate tensile strength (UTS) decreases gradually from 1418 MPa of the as-forged specimen to 1094 MPa of the specimen tempered at 500 ℃ with increasing Ttemp, while the yield strength (YS) increases gradually with increasing Ttemp at first, reaching a peak at 400 ℃, and then decreases with further increasing Ttemp. As a result, the yield strength ratio (YS/UTS) increases continuously from 0.73 in the as-forged state to 0.93 of the specimen tempered at 500 ℃. Unlike those of the strengths, the impact energy increases at Ttemp of 200 ℃ at first, then it decreases and reaches a valley at 400 ℃, and finally it increases notably again at Ttemp of 500 ℃, an increase of about 27% than that of the as-forged one. It is concluded that suitable tempering treatment after forging can obtain better strength and toughness balance of the tested bainitic forging steel, and thus help to expand its application range.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract328)   HTML3)    PDF (5339KB)(299)      

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.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract316)   HTML4)    PDF (3487KB)(448)      

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>.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract313)   HTML1)    PDF (1698KB)(195)      

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.

Table and Figures | Reference | Related Articles | Metrics
Microstructure and Wear Resistance of Ni-Based WC Coating by Ultra-High Speed Laser Cladding
ZHANG Yu, LOU Liyan, XU Qinglong, LI Yan, LI Changjiu, LI Chengxin
Acta Metall Sin    2020, 56 (11): 1530-1540.   doi:10.11900/0412.1961.2020.00033
Accepted: 14 July 2020

Abstract310)   HTML0)    PDF (2847KB)(169)      

Steel materials are highly sourced construction materials owing to their robust mechanical properties, and they are widely used in the construction industry for building bridges, tunnels, skyscrapers, towers, ship-metal parts, and other industrial metal applications. However, as steel has poor surface wear resistance, parts are susceptible to failure due to friction damage. To improve the surface wear resistance of steel materials, Ni-based WC coating was prepared by ultra-high-speed laser cladding. Using low-speed laser cladding as a reference, the surface morphology, microstructure, and wear resistance of ultra-high-speed laser cladding of Ni-based WC coatings were studied using SEM, EDS, and XRD, respectively. Experimental results revealed that the Ni-based WC coating prepared by ultra-high-speed laser cladding exhibited better surface quality compared with that prepared by low-speed laser cladding. Comparatively, ultra-high-speed laser cladding requires a smaller heat input and a faster cooling rate. However, the dilution rate of the coating is significantly reduced. In addition, ultra-high-speed laser cladding significantly reduces thermal damage in the WC coating; it inhibits the precipitation of carbides and formation of porosities and promotes the uniform distribution of the WC in the coating, thereby significantly reducing stress localization in the coating and also inhibits crack nucleation in the coating. Because of the reduction of porosities, cracks, and other surface defects in the coating and uniform distribution of WC particles, the Ni-based WC coating prepared by ultra-high-speed laser cladding possesses better wear resistance than that prepared by low-speed laser cladding, and the wear mechanism is abrasion.

Table and Figures | Reference | Related Articles | Metrics
Electrochemistry and In Situ Scratch Behavior of 690 Alloy in Simulated Nuclear Power High Temperature High Pressure Water
LI Xiaohui, WANG Jianqiu, HAN En-Hou, GUO Yanjun, ZHENG Hui, YANG Shuangliang
Acta Metall Sin    2020, 56 (11): 1474-1484.   doi:10.11900/0412.1961.2020.00091
Accepted: 08 June 2020

Abstract305)   HTML0)    PDF (3121KB)(129)      

The abnormal shutdown of the pressurized water reactor (PWR) nuclear power plants can be primarily attributed to the rupturing of the heat transfer tube of the steam generator. Regardless, stress corrosion cracking is the most important ageing mechanism associated with the primary water of the PWR. In this work, the damage behavior of alloy 690 was systematically investigated using high-temperature and high-pressure in situ scratching and electrochemical techniques to understand its corrosion behavior and failure mode and provide a reference for controlling the manufacturing, processing, and installation of the alloy 690 tubing. Further, the polarization behavior of alloy 690 at different temperatures was investigated using the self-built high-temperature and high-pressure water circulation circuit system and the high-temperature and high-pressure in situ scratching device. Subsequently, the single-pass scratch in air and in situ reciprocating scratch of alloy 690 obtained using high-temperature and high-pressure water for 11 and 100 h, respectively, were studied. The samples after scratching were observed and analyzed via SEM and EDS. The results revealed the occurrence of microcracks at the bottom of the scratch during the single-pass scratch of alloy 690. The TiN inclusions with large particles were prone to fragmentation, whereas those with smaller particles were susceptible to cracking at the joint of the matrix. During the reciprocating scratch process in high-temperature and high-pressure water, a portion of the metal substrate debris at the bottom of the scratch groove was peeled off along with oxide particles, microcracks, and chipped debris. Further, the TiN inclusions with large particles were fragmented, whereas those with smaller particles easily cracked at the bonding interface of the substrate. The electrochemical signals of alloy 690 during the reciprocating scratch processes were measured using the high-temperature and high-pressure in situ electrochemical technology. The instantaneous peak current density at the scratch during the scratch process is 149~326 times of that associated with the substrate.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract284)   HTML2)    PDF (3391KB)(360)      

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.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract284)   HTML4)    PDF (1661KB)(149)      

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.

Table and Figures | Reference | Related Articles | Metrics
Effect of Al Content on Microstructure and Mechanical Properties of Mg-Sn-Ca Alloy
WU Huajian, CHENG Renshan, LI Jingren, XIE Dongsheng, SONG Kai, PAN Hucheng, QIN Gaowu
Acta Metall Sin    2020, 56 (10): 1423-1432.   doi:10.11900/0412.1961.2020.00086
Accepted: 30 July 2020

Abstract278)   HTML0)    PDF (3084KB)(199)      

There is considerable demand for high-performance, low-cost, and rare-earth-free magnesium alloys in several industrial applications because of their energy conservation potential. However, the mechanical properties of the currently available rare-earth-free magnesium alloys cannot satisfy the industrial requirements. Therefore, a novel rare-earth-free magnesium alloy with high strength, excellent ductility, and good formability must be urgently developed. In this study, the microstructure and mechanical properties of the Mg-2.5Sn-2Ca-xAl (x=2, 4, and 9, mass fraction, %) alloys in the as-cast and extruded states when different amounts of Al content are added are systematically studied. As indicated by the results, the strength and elongation of the alloy decrease and increase, respectively, with the increasing Al content. The yield strengths of the Mg-2.5Sn-2Ca-2Al, Mg-2.5Sn-2Ca-4Al, and Mg-2.5Sn-2Ca-9Al alloys are approximately 370, 325, and 290 MPa, respectively, and their elongations are approximately 6.2%, 11.0%, and 12.0%, respectively. The type and content of the nanosecond phase of the Mg-Sn-Ca-based alloy changed because of the addition of the fourth type of Al element. High-density G.P. zones and a second phase of Mg17Al12 can be observed in the extruded Mg-2.5Sn-2Ca-2Al and Mg-2.5Sn-2Ca-9Al alloys, respectively; however, nanophase precipitation cannot be observed in case of the extruded Mg-2.5Sn-2Ca-4Al alloy. The high-density G.P. zones hinder the growth of the recrystallized grains more efficiently than the Mg17Al12 nanophase; thus, the recrystallized grains of the extruded Mg-2.5Sn-2Ca-2Al alloys are finer (approximately 0.5 μm) than the extruded Mg-2.5Sn-2Ca-9Al alloy. Based on TEM images, high-density dislocations can be observed inside the extruded Mg-2.5Sn-2Ca-2Al alloy grains and G.P. zones can be observed toward the side of the dislocations; thus, the high density subgrain lamella structure is retained in the alloy (lamella thickness: 0.2~1.0 μm). The movement of the newly generated dislocations is inhibited by the large number of G.P. zones and residual dislocations, increasing the yield strength and decreasing the plasticity of the Mg-2.5Sn-2Ca-2Al alloy. The Mg17Al12 nanophase that was formed in the Mg-2.5Sn-2Ca-9Al alloy because of the addition of high Al content exhibits a weak ability to hinder the movement of the dislocations, resulting in low-density residual dislocation. Therefore, the Mg-2.5Sn-2Ca-9Al alloy, exhibits a large grain size, low yield strength and high plasticity.

Table and Figures | Reference | Related Articles | Metrics
Peritectic Solidification Characteristics and Mechanism of 15CrMoG Steel
LI Yaqiang, LIU Jianhua, DENG Zhenqiang, QIU Shengtao, ZHANG Pei, ZHENG Guiyun
Acta Metall Sin    2020, 56 (10): 1335-1342.   doi:10.11900/0412.1961.2020.00002
Accepted: 23 June 2020

Abstract277)   HTML0)    PDF (2256KB)(224)      

Cast defects of hypo-peritectic steel such as uneven growth of strand shell, crack formation and oscillation marks formation were found to occur frequently during continuous casting of steels. In industry, measures such as high-basicity casting powder, hot-top mold and reduction of mold cooling strength were usually used in the investigations, but a reasonable explanation for these measures has been lacking. In this work, solidification of 15CrMoG steel at different cooling rates were observed with an ultra high temperature confocal scanning laser microscope. The precipitation of the δ-phase was in a cellular manner when the cooling rates were 5 and 15 ℃/min, whereas it was in a dendrite manner when the cooling rate was increased to 100 ℃/min. Thermodynamic analysis of the peritectic phase nucleation showed that a concentration gradient existed at the L/δ interface during the solidication of initial δ phase which led to an increase in the Gibbs free energy barrier for the nucleation of the peritectic γ phase. As the cooling rate increased, the concentration gradient across the L/δ interface became steeper, resulting in an increase in the nucleation undercooling of the peritectic γ phase. This, in turn, decreased the temperature and increased the peritectic reaction rate. In addition, an increase in the cooling rate led to a change in the mode of peritectic transformation (δγ). A diffusion-controlled δγ transformation occurred due to the progression of planar and cellular interfaces at cooling rates of 5 and 15 ℃/min, respectively. However, a large δγ transformation, which was controlled by the interface process, occurred when the cooling rate was increased to 100 ℃/min. The difference in volume shrinkage of the different modes of peritectic transformation (δγ) led to a discussion of the control mechanism of continuous casting of hypo-peritectic steel.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract276)   HTML3)    PDF (6927KB)(446)      

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.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract269)   HTML2)    PDF (1970KB)(229)      

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.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract264)   HTML4)    PDF (21659KB)(408)      

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.

Table and Figures | Reference | Related Articles | Metrics
Research and Application Progress in Ultra-HighStrength Stainless Steel
LIU Zhenbao,LIANG Jianxiong,SU Jie,WANG Xiaohui,SUN Yongqing,WANG Changjun,YANG Zhiyong
Acta Metall Sin    2020, 56 (4): 549-557.   doi:10.11900/0412.1961.2019.00453
Accepted: 10 March 2020

Abstract263)   HTML2)    PDF (1599KB)(474)      

In the present work, the development and research on ultra-high strength stainless steels (UHSSS) have been systematically reviewed. Specifically, the focus was primarily placed on the precipitation hardening and austenite phase toughening mechanisms. And, the hydrogen-induced stress corrosion cracking (SCC) and hydrogen embrittlement (HE) behaviors of high-strength stainless steels were also retrospected. It is suggested that the future development of UHSSS is on the basis of computer-aided alloy designing system, strengthening via multiple high-coherency precipitates and toughening by filmy high-stability austenite phase. Besides, verification of the SCC and HE underlying mechanisms is vital to further optimizing the performance of UHSSS.

Table and Figures | Reference | Related Articles | Metrics
Corrosion Behaviors and Mechanisms of ODS Steel Exposed to Static Pb-Bi Eutectic at 600 and 700 ℃
BAO Feiyang, LI Yanfen, WANG Guangquan, ZHANG Jiarong, YAN Wei, SHI Quanqiang, SHAN Yiyin, YANG Ke, XU Bin, SONG Danrong, YAN Mingyu, WEI Xuedong
Acta Metall Sin    2020, 56 (10): 1366-1376.   doi:10.11900/0412.1961.2020.00035
Accepted: 03 June 2020

Abstract262)   HTML1)    PDF (3522KB)(164)      

With good neutron properties, anti-irradiation performances, heat transfer properties and inherent safety characteristics, liquid lead or Pb-Bi eutectic (LBE) has been a primary candidate coolant for accelerator driven system and advanced nuclear reactors. However, corrosion of structural materials is a critical challenge in the use of liquid lead and LBE in high temperature nuclear reactors. Therefore, research on corrosion compatibility of structural materials with LBE at elevated temperatures is of great significance. In this work, the long-term corrosion experiments in static LBE for a oxide dispersion strengthened (ODS) steel were carried out at 600 and 700 ℃. The temperature effects on different corrosion behaviors were studied by the analyses of XRD, SEM and EDS, and the underlying mechanisms were clarified. After exposing to LBE at 600 ℃ for up to 2000 h, a typical double-layer oxide scale with the thickness of about 10 μm was formed on the surface of ODS steel, which was composed of outer layers of Pb-Fe-O and Fe3O4 and inner layer of Fe-Cr-Al spinal. In addition, a thin Al-rich layer was also formed under the inner layer. Due to the protective effect of the relatively dense inner layer and the Al-rich layer, ODS steel showed excellent resistance to LBE corrosion at 600 ℃ with a significantly lower corrosion rate. On the contrary, when exposed to LBE at 700 ℃ , the structure and thickness of the oxide scale formed on the surface of the ODS steel were obviously different. After exposure for 100 h, a dense protective Al2O3 oxide layer with a thickness of about 500 nm was formed, greatly reducing the corrosion rate. With the corrosion time prolonging to 500 h at 700 ℃, most of Al2O3 layer was still remained. However, a few of nodular-like oxides were formed originated from local weak areas, which broken off the continuity of protective Al2O3 and led to deeper corrosion by LBE.

Table and Figures | Reference | Related Articles | Metrics
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

Abstract261)   HTML3)    PDF (2167KB)(572)      

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.

Table and Figures | Reference | Related Articles | Metrics