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

About the Journal

  Current Issue
    , Volume 46 Issue 2 Previous Issue    Next Issue
    For Selected: View Abstracts
    论文
    THEORETICAL ANALYSIS ON EFFECT OF TRANSFERENCE VELOCITY ON STRUCTURE OF POROUS METALS FABRICATED BY CONTINUOUS CASTING GASAR PROCESS
    LIU Yuan LI Yanxiang LIU Runfa ZHOU Rong JIANG Yehua LI Zhenhua
    Acta Metall Sin, 2010, 46 (2): 129-134.  DOI: 10.3724/SP.J.1037.2009.00527
    Abstract   PDF (1629KB) ( 1240 )

    Transference velocity is an important processing parameter to influence the final lotus–type porous structure fabricaed by continuus casting Gasar process. Via solving the solute diffusion equation, a theoretical model was developed to predict the effect of the transference velocity on the porosity, the pore diameter and the inter–pore spacing. The predicted values at different transference velocities have a good agreement with the corresponding experimental results obtained by Japanese esearcher. The average pore diameter and the inter–pore spacing L both have a prominent decrease with the increase of transference velocity v, but the porosity only has a slight decrease. The theoretical relationships between the pore radius rg, the inter–pore spacing and the transference velocity can be described by the two simple equations as r1.72g ·v = A and L1.72 ·v = B, where A and B are all constants depending on the used gas presures and the melt temperature.

    References | Related Articles | Metrics
    NUMERICAL MICROSTRUCTURE SIMULATION OF LASER RAPID FORMING 316L STAINLESS STEEL
    JIA Wenpeng TONG Huiping HE Weiwei LIN Xin HUANG Weidong
    Acta Metall Sin, 2010, 46 (2): 135-140.  DOI: 10.3724/SP.J.1037.2009.00192
    Abstract   PDF (1257KB) ( 1531 )

    The laser rapid forming (LRF) as an advanced solid freedom fabrication technology, has been developed rapidly in recent decade. By rapid prototyping with laser cladding, LRF realizes the direct net shaping of the components with irregular shapes and fine inner structures, and gives a short–route, low–cost and high–flexibility fabrication of aero components, aero–engine parts and biomedical implants. In the LRF, melting and solidification are happened in a dynamic non–equilibrium, high temperature gradient and rapid solidification manner, so that the microstructure of the laser rapid formed part is finer than that of ordinary cast or forge part and presents a characteristic of typical epitaxial growth. Therefore, to achieve the predict and control of the microstructure evolution is a key problem. Much efforts have been devoted to narrating the solidification and crystallization in melting pool, but little attention has been paid to stuy the microstructure of LRF part. In this paper, the evolution otemperture field and solidification of LRF part were concerned, the relationships between as–deposited microstructure and the local solidification conditions such as solidification velocity and temperaturgradient of moving melting pool were also investigeted. A coupled 2D transient finite element LRF epitaxial growth model was developed. The morphology evolution and first order dendrite arm space λ1 distribution in 2.8 mm high LRF 316L stainless steel wall were simulated. The results show that the microstructure of LRF 316L stainless steel wall is mainly columnar austenitic dendrites, and the λ1 gradually becomes larger from the bottom about 6.5 μm to the top about 17 μm which is in good agrement with te experimntal. Further more, on the basis of the validated model, morphology volution and λ1 distribution in 40 mm high LRF 316L stainless steel wall are also predicted.

    References | Related Articles | Metrics
    EVOLUTIONS OF MICROSTRUCTURE AND FERRITIC MICRO–ORIENTATION AND TEXTURE IN A PEARLITIC STEEL WIRE DURING COLD DRAWING
    ZHANG Xiaodan A. Godfrey LIU Wei LIU Qing
    Acta Metall Sin, 2010, 46 (2): 141-146.  DOI: 10.3724/SP.J.1037.2009.00247
    Abstract   PDF (4857KB) ( 1973 )

    Cold drawn high–carbon pearlitic steel wires have the highest strength of all mass–produced steel materials and are widely used in industry for a variety of applications, including cables for suspension bridges, steel cords for automobile tires and springs. At present the maximum tensile strength of high–carbon steel wires has already reached a value of 5.7 GPa. The properties of steel wires, including strength, fatigue properties and torsinal properties, are deeply affected by microstructure and ferritic micro–orientation and texture in the deformed pearlite. In this study, the evolutions of microstructure and ferritic micro–orientation and texture were investigated in a pearlitic steel wire during cld drawing using electron channel contrast (ECC) and electron backscatter diffraction (EBSD) techniques. The results show that there exist shear–bands (S–bands) in the deformed pearlite microstructure. Their appearance is related to the angle between cementite plates and the drawing axis: the larger the angle is, the more S–bands appear in the structure. The pearlite structure turns to the drawing direction and the angle between S–band and drawing axis decreases with the increase of strain.  The S–bands in the deformed pearlite colony induce the rapid change of local orientation of ferrite, and make the pearlite colony subdivide into several areas by high angle boundaries of ferrite. The strog h110i fibre texture f ferrite parallel to the drawing direction forms with the increase of strain, but the intensity of h110i fibre texture of ferrite is inhomogeneous from the cetre to surface in longitudinal section with the strongest in the centre and the weakest near the urface.

    References | Related Articles | Metrics
    ANALYSIS OF THE STATIC RECRYSTALLIZATION AT TENSION TWINS IN AZ31 MAGNESIUM ALLOY
    LI Xiao YANG Ping MENG Li CUI Feng’e
    Acta Metall Sin, 2010, 46 (2): 147-154.  DOI: 10.3724/SP.J.1037.2009.00533
    Abstract   PDF (5367KB) ( 3057 )

    Due to the poor plasticity of magnesium alloys at room temperature (about 15%), twinning plays an important role in the deformation of magnesium alloys, and twins will be the dominant recrystallization nucleation sites. There are at least two types of twinning in magnesium: the {1012}–type tension twinning and the {1011}–type compression twinning. Tension twinning proceeds much more easily than compression twinning since its volume fraction is much higher than that of compression twins, which may have a promotion effect on the recrystallization to a certain degree. Based on the previous research on the static recrystallization at compression twins, the evolution of microstructure and texture in AZ31 magnesium alloy during its static recystallization at tension twins was futher investigated; and the orientational characteristics of new grains formed at tension twins in the early stage of static recrystallization were analyzed by EBSD technique. The results showed that tension twins played only a subordinate role in recrystallization nucleation and suppressed recrystallization rate, thus failed to rfine grain size effectively. The strong basal texture waretained and weakened wih no new texture component being detectd dring annealing. New grains were observed to nucleae preferentially at the intersections of tension twin variants or the intersections between tension twins and compression twins. Their orientations were relative random and are strongly scattered from those of original tension twins or compression twins. A comparison of the recrystallization at tension twins and compression twins was made.

    References | Related Articles | Metrics
    MICROSTRUCTURE CONTROL OF HOT ROLLED TRIP STEEL BASED ON DYNAMIC TRANSFORMATION OF UNDERCOOLED AUSTENITE
    I. Prior Austenite Grain Size
    YIN Yunyang YANG Wangyue LI Longfei SUN Zuqing WANG Xitao
    Acta Metall Sin, 2010, 46 (2): 155-160.  DOI: 10.3724/SP.J.1037.2009.00431
    Abstract   PDF (2169KB) ( 1441 )

    Low–alloy multiphase transformation–induced plasticity (TRIP) steels offer excellent mechanical properties combining high–strength levels with a well ductility. This results from the complex synergy between different phases, i.e., ferrite, baintie and retained austenite in them. An in–depth understanding of the novel hot rolled TRIP processing based on dynamic transformation of undercooled austenite (DTUA) is required to study the microstructure evolution under different treatment conditions. The parameters determining the stability of the metastable austenite and mechanical property of TRIP steel were revived and investigated experimentally, with a special attention paid to the effect of prior austenite grain size and cooling rate after DTUA on its microstructure and mechanical property. The results show that the kinetics of ferrite during DTUA is accelerated when the prior austenite grain size is small. In addition, reducing the grain size of austenite, the distribution of ferrite, bainite and retained austenite becomes more uniform. Moreover, the size of bainite packets and bainitic ferrite becomes smaller, the volume fraction and carbon content of retained austenite higher, and the finer granular retained austenite vast and distributed uniformly, which result in the investigated steel having a higher strength and ductility.

    References | Related Articles | Metrics
    MICROSTRUCTURE CONTROL OF HOT ROLLED TRIP STEEL BASED ON DYNAMIC TRANSFORMATION OF UNDERCOOLED AUSTENITE
    II. Cooling Rate After Dynamic Transformation of Undercooled Austenite
    YIN Yunyang YANG Wangyue LI Longfei SUN Zuqing WANG Xitao
    Acta Metall Sin, 2010, 46 (2): 161-166.  DOI: 10.3724/SP.J.1037.2009.00432
    Abstract   PDF (1850KB) ( 1644 )

    TRIP–aided steels are ideal for lightweight automotive applications due to their high strength and ductility. Thermomechanical processing simulations were performed by hot compression on a Gleeble–1500 machine, in order to develop a comprehensive understanding of the effect of cooling rate after dynamic transformation of undercooled austenite (DTUA) on the microstructure evolution and mechanical properties of 0.2C–1.5Mn–0.5Si–1.0Al (mass faction, %) transformation–induced plasticity (TRIP) steel. The results show that the metastable austenite would be transformed to ferrite mainly by means of nucleation during cooling after DTUA. Decreasing the cooling rate after DTUA, the vlume fraction of ferrite increase, the static recovery of dislocations in ferrite produced during dynamic transformation could fully take place, thus the density of dislocations in ferrite decreased through by rearranging and merging, but the grain size of ferrite changes little. At the cooling rate of 30℃/s, the investigated steel has a moderate volume fraction of ferrite and dislocation density in ferrite and a higher volume fraction of retained austenite, resulting in the steel having a higher strength and plasticity.

    References | Related Articles | Metrics
    SUB–CRITICAL CRACK GROWTH IN AIR AND DOMAIN SWITCHING FOR BaTiO3 SINGLE CRYSTAL UNDER CONSTANT DEFLECTION
    ZHAO Xianwu CHU Wuyang QIAO Lijie
    Acta Metall Sin, 2010, 46 (2): 167-171.  DOI: 10.3724/SP.J.1037.2009.00281
    Abstract   PDF (2207KB) ( 965 )

    BaTiO3 single crystal with superior dielectric, pyroelectric, piezoelectric and electro–optic properties has triggered much attention due to numerous potential applications in microelectronic devices, sensors and micro/nanoelectromechanical systems. However, one of the main disadvantages for BaTiO3 single crystal is that after long–term working under mechanical field or electric field, cracks may be nucleated and propagated. The crack nucleation and propagation in ferroelectric materials are closely related to the domain configurations, and their relationship with crack propagation, nucleation and domain switching, is not known clearly, so it is imperative to take more effort to research this relationship. Here, the relationship of domain switching with crack propagating quickly as well as sub–critical crack growth in ambient air for constant deflection sample of BaTiO3 single crystal has been in situ tudied using a polarized light microscopy. The results indicate that domain switching occurs first and he volume fraction of swiched bnds increases dring loading. When the load reaches to a critical value, crack will be initiaed nd propagated qickly. Keeping the constant deflection, the domain switch could still occur but very slowly, i.e., adsorption of water in air is able to promote domain switchng. As a result of adsorption–enhanced domain switching, cracks are propagated lowly n air, that s say, sub–critical crack growth or stress corrosion cracking in air could occur for BaTiO3 single crystal under constant deflection.

    References | Related Articles | Metrics
    MOLECULAR DYNAMICS SIMULATION OF PHASE TRANSFORMATION of  γ-Fe→δ-Fe→LIQUID–Fe IN CONTINUOUS TEMPERATURE–RISE PROCESS
    LIU Yihu WU Yongquan SHEN Tong WANG Zhaoke JIANG Guochang
    Acta Metall Sin, 2010, 46 (2): 172-178.  DOI: 10.3724/SP.J.1037.2009.00327
    Abstract   PDF (3036KB) ( 1577 )

    Understanding high–temperature phase transformations of pure Fe is fundamental for quality control and product design of steels. Various theoretical methods have been used to determine dynamically the mechanism of phase transformations in pure Fe including  γ–Fe to δ–Fe and δ–Fe to liquid–Fe. Among these methods, molecular dynamics (MD) simulation has become a prospective method, in which atomic interactions play a key role in phase transformations. However, most attention ws focused on the MD simulation of temperature–drop phase transformtions rather than temprture–rise phase transformations befor. In the present study the isothermal–isobaric MD simulation at a wide temperature range of  γ-Fe→δ-Fe→liquid–Fe transformations in pure Fe was carried out by giving a set of long–rnge Finnis–Sinclair potential parameters. The results show that a better agreemenbetween simultion nd experimental results for the microstructures (including radial distribution functions and coordination numbers) and densities of transformed phases validate that the set of potential parameters for the MD simulation are reasonable. The larger difference between the calculated and experimental trnsfrmation temperatureiattributed to the effect of superheat degree induced by ultrafast heating speed in the MD simulation. Evolvement of microsructures exibits lattice–distorting and sldng induced by  γ–Fe to δ-Fe phase ransformation and melting of δ–Fe islands from δ–Fe to liquid–Fe. Finall, in the MD simulation stronger and stronger fluctuations of instantaneous energy ad density just before transformations, especially melting, show an apparent pregnant process in phase transfrmations.

    References | Related Articles | Metrics
    ZrO2∶Eu 3+ NANOCRYSTAL FABRICATION AND ITS LUMINESCENCE PROPERTIES BY ION EXCHANGE METHOD
    SHA Ren WANG Xigui LI Xia
    Acta Metall Sin, 2010, 46 (2): 179-183.  DOI: 10.3724/SP.J.1037.2009.00118
    Abstract   PDF (1609KB) ( 1136 )

    Eu3+ doped luminescent nano–materials have become a research focus due to their outstanding physical and chemical properties in light–emitting, magnetism, thermology, catalysis and chemical activity etc.. Furthermore, the relationship between crystal structure and energy levels transition of these nano–materials can be easily obtained by measuring the spectra of doped Eu3+. Among luminescence nano–materials, ZrO2 nano–crystal as a potential one has been attracted great attention in its higher refractive, good optical transparency and relatively low phonon energy. By now certain materials doped RE ion in the matrix ZrO2 for ZrO2∶Er3+–Y3+, ZrO2∶Pr3+ and ZrO2∶(Pr3+, Sm3+) etc., and mesoporous ZrO2 nano–crystals doped Eu3+ by hydrothermal way have been reported. In present study, the ZrO2∶Eu3+ nano–crystal was prepared with high purity and uniform composition by ion exchange method using strong OH as a precipitant. Its composition, morphology and structure were characterized by XRD, TEM, HRTEM and EDS. The experimental results show that it has a tetragonal crystal structure and its average grain size is 5—20 nm after calcined at 800 ℃ . It is found that the microstructure of ZrO2∶Eu3+ changes slightly with the increase of calcining temperature till a small amount of monoclinic phase forms after calcined at higher than 900  ℃. The luminescent properteof Eu3+ in the ZrO2 nano–crystal were measured by 3D emission and excitation spectra. The characteristic emissin bands of 590 nm (5D0 →7F1) and 606 nm (5D0 →7F2) of Eu3+ were observed at an excitation spectrum of λex=394 nm. The luminescent properties of ZrO2∶Eu 3+ are very sensitive to its microstructure change since slight changes in the ZrO2∶Eu 3+ microstructure cause the changes in the shape and intensity of its emission spectra.

    References | Related Articles | Metrics
    EFFECT OF MICROSTRUCTURES ON STRAIN HARDENING EXPONENT PREDICTION OF CAST ALUMINUM ALLOY
    MO Defeng HE Guoqiu HU Zhengfei LIU Xiaoshan ZHANG Weihua
    Acta Metall Sin, 2010, 46 (2): 184-188.  DOI: 10.3724/SP.J.1037.2009.00474
    Abstract   PDF (1323KB) ( 1977 )

    Strain hardening exponent (n) of a material is an important parameter reflecting its hardening property whose determination is of great importance. It has a widely application in material scientific research and engineering fields such as fatigue life prediction, stress–concentration–factor calculation, etc.. The value of strain hardening exponent varies with their microstructures in cast aluminum alloys, but a few theoretical and experimental investigations have been reported to understand the effects of the microstructural parameters on the strain hardening exponent in these alloys till now. In the present study, the influence of secondary dendrite arm spacing (SADS), aspect ratio and volume fraction of particles on internal stress in aluminum alloys were discussed, and a quantitative prediction of strain hardening exponnt was established on the basis of Hollomon and internal stress equations. It shows tat the strain hardening exponent represents their hardening ability in relatively large plastic deformation (larger than the upper limit for the no plastic relaxation regime). A new microstructural hardening parameter relatively to strain hardening exponent was defined. Besides, a group of A319 cast aluminum alloys with microstructural heterogeneities were tested. The calculated strain hardening exponents are in agreement with the experimental ones in A319 alloy as well as in some commonly used cast aluminum alloys. For the same grade of alloys, the microstructural hardening parameter and strain hardening exponent are quite sensitive to SDAS and particle aspect ratio while the influence of volume fraction of particles is reltivey little. As the values of SDAS and aspect ratio of particles increse, the vlue of te strain harening exponent decreases. A lier reltionship between microstructural hardeig parameter and strain hardening exponent was proposd. For A319 and A356/57 alloys, the optimum correctin coefficients are 0.17 and 0.11, respectively, and the mean prediction error of n is only bout 10%.

    References | Related Articles | Metrics
    ESTABLISHMENT AND APPLICATION OF FIXTURE CONSTRAINT MODELS IN FINITE ELEMENT ANALYSIS OF WELDING PROCESS
    ZHANG Zenglei SHI Qingyu YAN Dongyang CAI Zhipeng LI Decheng
    Acta Metall Sin, 2010, 46 (2): 189-194.  DOI: 10.3724/SP.J.1037.2009.00198
    Abstract   PDF (1819KB) ( 1150 )

    The mechanical interaction between welded panel and fixture has a remarkable influence on the distribution of weld–induced residual stress and distortion. Unfortunately, the influence of fixture is not generally included in conventional numerical simulations of welding process. In the present study, it is proposed that the numerical models in which the mechanical interaction between welded panel and fixture is considered as contact pair in the finite element analysis (FEA) of welding process. This could make the FEA of welding process more scientific and accurate. The fixture constraint plays an important role in accurate modeling of welding process. In the simulations, various mechanical constraint boundary conditions in welding process were thought about to establish the relationships of the normal and tangential mechanical interactions between welded panel and fixture. In Free model, the fixture restraint was not considered. In Mech model, the displacements of nodes where welded panel contacted with fixture were restricted in x, y, z directions. Hard model and Tabular model were contact models which included the fixture. In the normal direction, once welded panel and fixture were in contact, the transmitted contact pressure could be infinite in Hard model. While data pairs of contact pressure vs over–closure or clearance were specified to define a piecewise–linear relationship in the normal direction in Tabular model. A penalty friction coefficient of 0.3 was applied in tangential direction in both Hard and Tabular models. Fixture and welded panel were separated and the contact relationships were removed after the welding process in both Hard and Tabular models. The simulation results of contact models including fixture were more accurate, compared to models containing only welded panel with rigid retrictions. The distortiomode of welded panels under different fixture constrait conditions corresponded well with xperimental results. The differences between the distributions of transverse sresses on top and bottom surfaces f welded panel at the beginning of welding process had a crucial influence on the distortion mode f welded panel.

    References | Related Articles | Metrics
    NUMERICAL SIMULATION OF WELDING RESIDUAL STRESSES IN A MULTI–PASS BUTT–WELDED JOINT OF AUSTENITIC STAINLESS STEEL USING VARIABLE LENGTH HEAT SOURCE
    DENG De’an KIYOSHIMA Shoichi
    Acta Metall Sin, 2010, 46 (2): 195-200.  DOI: 10.3724/SP.J.1037.2009.00521
    Abstract   PDF (1165KB) ( 1512 )

    Recent discoveries of stress corrosion cracking at welded joints in pressurized water reactors and boiling water reactors have raised wide concerns about the safety and integrity of plant components. It has been recognized that residual stress and applied stress on their surfaces largely increase the expanding risk of initial stress corrosion cracking. Therefore, it is very important to investigate the welding residual stress in welded joints. It is very expensive and time–consuming to measure the residual stress, and sometime is impossible. As an alternative approach a computational procedure on the basis of finite element method is effective in solving non–linear problems such as thermal and mechanical nonlinearity in a welding process. Accurately simulating welding residual stress not only needs generally a long computational time, but also strongly depends on the analyst’s experience and know–how which is a main hindrane for the welding process simulation. Therefore, it is an urgent task to develop a time–effective numerical simulation procedure to calculate welding temperature field and residual stress distribution. In this study, a new method on the basis of the vaiable length heat souce was developed to simulate the welding residual stress in a multi–pass butt–welded joint of austenitic stainless steel. Meanwhile, the experiment was carried out to obtain the welding residual stress in the utt–welded joint. Comparing the simulated with experimental results, it was found that this method could not only save a large amount of computational time but also provide a highly accurate numerical result for the residual stress in multi–pass butt–welded joints.

    References | Related Articles | Metrics
    ELECTRICAL PROPERTIES OF PHOSPHORUS INCORPORATED TETRAHEDRAL AMORPHOUS CARBON FILMS
    LIU Aiping ZHU Jiaqi TANG Weihua LI Chaorong
    Acta Metall Sin, 2010, 46 (2): 201-205.  DOI: 10.3724/SP.J.1037.2009.00538
    Abstract   PDF (522KB) ( 936 )

    Great attention has been given to diamond–like carbon (DLC) or tetrahedral amorphous carbon (ta–C) films due to their potential applications in electronic devices as semiconductor materials. The controlled variation of electrical conductivity through doping is of primary importance. Active phosphorus element could be introduced into ta–C films to achieve n–type doping and the resulted films show potential applications as photovoltaic solar cells, semiconductor field emitters or biomedical coatings. However, an inconsistency of doping effect and graphitization of the bonding still exists in the role of phosphorus atoms in carbon films. A detailed study on the conduction mechanism related to the structural changes should be attempted to further understand the conductive behavior of the films. Based on this purpose, phosphorus incorporated tetrahedral amorphous carbon (ta–C∶P) films were deposited using filtered cathodic vacuum arc technology with PH3 as a dopant surce under negative substrate biases of 0—200 V. The structural characteristics of ta–C∶P films were investigated by X–ray photoelectron spectroscopy (XPS) and Raman spectroscopy, and the electrical behaviors of the films ere examined by measuring electrical conductivity at the temperature range of
    293—573 K and current–voltage curves. Results indicate that phosphorus implantation enhances the contents of sp2 sites in ta–C films and the numbers of localized electronic π and π* states as hopping sites, and improves the conductive ability of the films. ta–C∶P film obtained at −80 V shows the best conductive property. The carriers of ta–C∶P films represent the hopping conduction in localized band tail states and the thermally activated conduction in extended states in the temperament range of 293—573 K. The results of current–voltage curves indicate that ta–C∶P films are n–typsmiconuctor mateials.

    References | Related Articles | Metrics
    EFFECT OF DIFFERENT Nb–V CONTENTS ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF CR8–TYPE COLD WORK DIE STEEL
    CHI Hongxiao MA Dangshen LIU Jianhua CHEN Zaizhi YONG Qilong
    Acta Metall Sin, 2010, 46 (2): 206-212.  DOI: 10.3724/SP.J.1037.2009.00450
    Abstract   PDF (3248KB) ( 1588 )

    Cr8–type cold work die steels have been widely used in recent years. These steels have obvious secondary hardening effect due to high V content. Now it has been successfully used in high speed steel that Nb is substituted for V. However, it has been not widely applied in cold work die steel yet. In order to investigate the effect of different Nb–V contents on the microstructure and mechanical properties of this kind steel and the role of Nb in tool and die steels, in this paper the microstructure and morphology of three Cr8–type cold work die steels with different Nb–V contents were investigated by OM, SEM and EDS analysis, the mechanical properties including the hardness and impact toughness were measured at room temperature. Experimental results indicate that the precipitation temperature of MC–eutectic carbides increase with the increase of Nb content, and the improvement of their properties is closely related with Nb content. Moreover, the increase of Nb content causes the increase of metatectic and eutectic temperature and the disequilibrium of C distribution in primary γ phase. The granular carbides appear near the grain boundary of primary  phase when Nb content reaches to 1.32%; the variation in Nb/V ratio with the increase of Nb content will change the type of MC–eutectic carbides, the type of MC–eutectic carbides from mainly VC change to VC and a small amount of (Nb, V)C and then to mainly NbC and (Nb, V)C. So the shape of ledeburite is more straight. If Nb content reaches to 1.32%, the substitution of Nb for V increases quenching hardness, the peak of quenching hardness moves to low temperature region; the substitution of Nb for V is beneficial for secondary hardening in tested steels, and higher hardness and temper softening resistance can be obtained in them. In addition, their impact toughness decreases with the increase of Nb content.

    References | Related Articles | Metrics
    EVOLUTIONS OF MICROSTRUCTURES AND MECHANICAL PROPERTIES OF TWO CAST Ni–BASED SUPERALLOYS DURING LONG–TERM THERMAL EXPOSURE
    QIN Xuezhi GUO Jianting YUAN Chao HOU Jieshan ZHOU Lanzhang YE Hengqiang
    Acta Metall Sin, 2010, 46 (2): 213-220.  DOI: 10.3724/SP.J.1037.2009.00449
    Abstract   PDF (4382KB) ( 1614 )

    K452 and K446 alloys are two newly developed Ni–based cast superalloys, designed for microstructural component applications of gas turbines in marine and industrial fields. The two alloys perform well under laboratory conditions with good fatigue resistance, hot–corrosion resistance,
    and tensile– and stress–rupture properties, in addition to being completely oxidation resistant up to 900 ℃. However, due to the high contents of Cr, W and Mo, both K452 and K446 alloys tend to experience a severe microstructural degeneration when exposed at elevated temperatures. In this
    paper, the microstructural stabilities and their influences on the mechanical properties of the two alloys were comparatively examined during thermal exposure at 800—900 ℃ for 1×103—1×104 h. It is found that the  γ' phases in the two alloys keep coarsening and are both spherical during the whole exposure. The coarsening rate of the γ′ phase in K446 is in general faster than that in K452, the probable reason of which is that elements diffuse faster in K446 than in K452 due to the different heat treatment regimes they suffered. The thermal stabilities of primary MC carbides are closely related to the chemical compositions of both the carbides and the alloys. The carbide in K452 is instable and tends to completely decompose via three different reactions, whereas the carbide in K446 is stablead degenerates only ta tiny degree mainly via one reaction. Primary MC degeneration in K452 releases a significant amount of C into the supersaturated  γ matrix, facilitating the formation of M23C6 throughout the alloy, whereas the MC degeneration in K446 hardly releases C into the γ matrix so that the μ phase precipitates everywhere due to the lack of C. It seems that the kind of the phase precipitated from the supersaturated matrix during thermal exposure or service is dependent on the stability of primary MC. In K452 the blocky, closely spaced grain boundary M23C6 particles engulfed in  γ′ is the optimal structure of grain boundarywhere a stress–rupture life peak occurs, whereas in K446 the precipitation of a significant amount of μ phase degrades sharply the stress–rupture life.

    References | Related Articles | Metrics
    EFFECTS OF WATER QUENCHING PROCESS ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TWIP STEEL
    LI Jiguang DING Yajie PENG Xingdong LIU Jinwei
    Acta Metall Sin, 2010, 46 (2): 221-226.  DOI: 10.3724/SP.J.1037.2009.00180
    Abstract   PDF (2725KB) ( 1438 )

    In order to reduce greenhouse gas emissions, improve fuel economy and enhance safety of automobiles, a new high–strength and high–plasticity twinning induced plasticity (TWIP) steel containing medium carbon and high manganese has been developed. The effects of water quenching
    process on the microstructures of such TWIP steels and deformed ones were observed by OM, SEM and TEM, and effects on the mechanical properties were investigated by unidirectional tensile. The experimental results show that the volume fraction of annealing twins and the average size of grains, the plasticity and the strain hardening capability of TWIP steel increase with the increase of water quenching temperature, but the strength and the yield ratio decrease with it. Therefore, the samples could be obtained with a better comprehensive property, that is, the tensile strenth is 960 MPa, the elongation percentage is 60.5% and the strength–plasticity product achieves the maximum value of 6.096 ×104 MPa·%. It is also found that the austenite with a loof annealing twins can be transformed into deformation twins with the increase of the deformation degree, so that the strength and plasticity of TWIP steel are improved.

    References | Related Articles | Metrics
    HIGH VELOCITY COMPACTION AND CHARACTERISTICS OF Ti POWDER
    YAN Zhiqiao CHEN Feng CAI Yixiang CUI Liang
    Acta Metall Sin, 2010, 46 (2): 227-232.  DOI: 10.3724/SP.J.1037.2009.00383
    Abstract   PDF (1364KB) ( 1125 )

    High velocity compaction (HVC) is a recently developed technology for forming high density metallic parts with high efficiency, whose densification is realized through the strong impact wave generated by liquid–pressure controlled by heavy hamper. This technology is thought to have an excellent balance between properties and cost and has several advantages over other existing forming technologies, including high and homogeneous green density, low cost, low springback and high precision. It is extremely competitive to prepare powder metallurgy parts with high density, high strength, high precision and low cost. With a high hardening rate, Ti powder is difficult to be formed through traditional pressing methods, although advanced forming methods are effective for increasing its density, such as hot–pressing and isothermal–statistic pressing, they are very expensive. HVC would thus be highly attractive to solve these problems of low density and high cost in forming Ti powder. In this paper, the process of HVC forming Ti powder was studied. Two kinds of samples, the rings with 60 mm outer and 30 mm inner diameters and the cylindricals with 20 mm diameter were prepared through HVC, respectively. The influences of impact energy and filling weight on the green density were investigated. The results show that, for the ring samples, the maximum of green density is 4.00 g/cm3 and its relative density is 88.9% at an impact energy of 3.804 kJ. However, for the cylindrical samples, the maximum of green density is 4.38 g/cm3 and its relative density is 97.4% at an impact energy of 1.217 kJ. For the both kinds of samples, the green density increases with increasing impact energy while decreases with increasing filling weight. Impact energy per weight is helpful to characterize the green density obtained at different dimension, impact energy and filling weight.

    References | Related Articles | Metrics
    ANTI–FRICTION BEHAVIORS OF SEVERAL SYNTHESIZED PERRHENATES
    LIU Linlin LI Shu LIU Yang
    Acta Metall Sin, 2010, 46 (2): 233-238.  DOI: 10.3724/SP.J.1037.2009.00426
    Abstract   PDF (1442KB) ( 1260 )

    For exploring the possibility of a hybrid mode with smooth transition from fluid to boundary even to solid lubrication at the elevated temperature, i.e., searching a substance which should be the solid lubricant and oil additive with environmentally friendly characters, a series of perrhenates (the soft double oxide) were synthesized as the candidates because of their crystalline and decomposed products with a good anti–friction behavior, such as Re2O7 which might provide the lubrication during oil decomposition for its good sublimation at about 300 ℃. This investigation might be advantage of lubrication for rubbing parts of internal combustion or turbo engine such as piston ring and cylinder liner or exchanger, which were always operated in a cyclical variation of temperature (even to 800 ℃). In this paper, the properties of perrhenates of Fe, Co, Ni, Cu, Pb, Ca, Ba and La synthesized by aqua–solution method were described and their crystalline structures, compositions and morphologies were determined by XRD, SEM–EDS. For understanding their lubricating behaviors in both dry sliding and base oil pentaerythritol ester (PETE), their frictional behaviors between Si3N4 ball and superalloy disc were tested in pin-on-disc on universal micro–tribotester (UMT–2) from 20 to 750℃. The results show that most synthesized perrhenates are crystal powder except the synthesized gel–like ferric perrhenate, their crystalline structures are coincidence with powder diffraction file (PDF) cards, and compositions are very close to the stoichiometic values. Some of the synthesized perrhenates exhibit certain lubrication at 300—750 ℃, in both dry sliding and oil lubrication with addition of the synthesized perrhenate whch were verified in eciprocating sliding. The variation of friction coefficients with temperature probably results from the formation or removal of soft oxides at high temperature. The good anti–frictional behaviors of the synthesized perrhenates of Ca, Co and Cu in a wide temperature range seem to imply them to be developed as the candidates of additive in a hybrid lubricating mode for the engine operating in cyclical variation of temperature.

    References | Related Articles | Metrics
    EROSION CORROSION BEHAVIOUR OF J4 STAINLESS STEEL AND ELECTROLESS PLATING COATINGS OF Ni–P AND Ni–Cu–P IN LIQUID–SOLID TWO–PHASE FLOW
    FANG Xinxian BAI Yunqiang WANG Zhangzhong
    Acta Metall Sin, 2010, 46 (2): 239-244.  DOI: 10.3724/SP.J.1037.2009.00499
    Abstract   PDF (5454KB) ( 1550 )

    Components used really in industries such as mechanical, chemical, mining and petroleum industries, are often exposed to corrosive and erosive liquid–solid two–phase flow environments. These environments lead not only to high costs maintaining these components but also to shortening their service life. The behaviors of metal materials under corrosive and erosive conditions as well as the related mechanisms have been researched for several years. For reducing material failure caused by erosion–corrosion at surfaces of components, surface modification technology such as plating an Ni–P or Ni–Cu–P alloy coating with a high hardness and good corrosion resistance is an effective method. The behaviors of electroless Ni–P and Ni–Cu–P coatings, J4 and also 316L stainless steel as a contrast material were systematically investigated in liquid–solid two–phase flow (20% H2SO4+20 g/L Al2O3) by mass loss tst method at different temperatures (25 and 50 ℃) and different erosion rates (0.63—1.88 m/s). The sample surface morphologies and chemical compositions were also observed by SEM. Their erosion corrosion mechanisms were also explained. The results show that their erosion corrosion resistance performance under the test conditions is in the order of plated Ni–Cu–P coating, plated Ni–P coating, plated Ni–Cu–P coating after heat treatment, 316L stainless steel and J4 stainless steel. Their erosion corrosion resistance decreases with the increase of temperature of two–phase flow. Comparing to plated Ni–Cu–P coating, plated Ni–P coating and plated Ni–Cu–P coating after heat treatment, stainless steel 316L has over 8.5, 8 and 2.6 times higher erosion–corrosion rate at 25 ℃and over 392, 80 and 14.8 times higher erosion–corrosion rate at 50 ℃, respectively. The erosion–corrosion rate of J4 stainless steel is ver 28 times at 25 ℃and 13 times at 50 ℃higher than that of 316L stainless steel, respectively. The main corrosion characteristic of J4 stainless steel is selectie corrosion at 25 ℃and uniform corrosion at 50 ℃. However, it is slightly selective corrosion fr 316L stainless steel and uniform corrosion for Ni–P and Ni–Cu–P coatings. The results could be used as a useful reference for the application of electroless plating Ni–P and Ni–Cu–P coatings in erosion–corrosion environment.

    References | Related Articles | Metrics
    EFFECT OF NO2AND Cl ON THE CORROSION BEHAVIOR OF REINFORCING STEEL IN SIMULATED CONCRETE PORE SOLUTIONS
    QIAO Bing DU Ronggui CHEN Wen ZHU Yanfeng LIN Changjian
    Acta Metall Sin, 2010, 46 (2): 245-250.  DOI: 10.3724/SP.J.1037.2009.00376
    Abstract   PDF (784KB) ( 1206 )

    Under normal conditions, reinforcing steel in concrete maintains its passivity because of the high alkalinity of the concrete pore solution. However, steel passivity may be broken and the steel corrosion takes place due to concrete carbonation and/or chloride ingress. In fact, premature failure of reinforced concrete structures due to the steel corrosion has been a major problem in civil engineering. Therefore, the study of corrosion and protection of reinforcing steel is of indispensable importance to prolonging the service life of reinforced concrete structures. The application of inhibitors is an effective and economical method to prevent the corrosion of reinforcing steel. NaNO2 is a notable corrosion inhibitor which is effective in the steel corrosion control by reacting with ferrous ions to form a protective ferric oxide film on the steel surface, but unfortunately it can also accelerate the steel corrosion when the amount of nitrite is insufficient in a medium. In the present work, the linear polarization and electrochemical impedance spectroscopy were used to study the effect of nitrite and chloride ions on the corrosion behavior of reinforcing steel in simulated concrete pore solutions with different pH values. The results showed that the corrosion resistance of reinforcing steel is related with the pH value of the solution, nitrite ion concentrations and chloride in concentrations. The drop of pH value and the increase of chloride ion concentration would result in the decline of corrosion resstance of reinforcing steein the solution. The corrosion rate of reinforcing steel decreased with the NO2concentration increasing in the simulated concrete pore solutions with Cl, and the NO2 -ions had a good inhibiting effect on the steel when the concentration ratiof NO2 and Cl was 0.4 or higher for the solutions of pH=12.50 or pH=10.50.

    References | Related Articles | Metrics
    ANODIC ELECTROCHEMICAL BEHAVIOR OF X80 PIPELINE STEEL IN NaHCO3 SOLUTION
    ZHOU Jianlong LI Xiaogang DU Cuiwei LI Yunling LI Tao PAN Ying
    Acta Metall Sin, 2010, 46 (2): 251-256.  DOI: 10.3724/SP.J.1037.2009.00470
    Abstract   PDF (1328KB) ( 1326 )

    X80 steel has the potential to be widely used for building the gas transmission pipelines because of its high–intensity and high–toughness. Its corrosion performance is a key problem for the life expectancy, especially in soil environment. The anodic electrochemical behavior of X80 pipeline steel in NaHCO3 solution was studied using potentiodynamic polarization curve, dynamic electrochemical impedance spectroscopy (DEIS) as well as SEM observation. The results show that the shape of the polarization curves changes with HCO3- concentration. The corrosion rate of X80 pipeline steel first increases and then decreases with the increase of HCO3- concentration. 0.009 mol/L is the critical concentration of HCO3- for forming the'passive' film of the X80 pipeline steel. There is no anodic current peak in the solution when HCO3- concentration is below 0.009 mol/L, and one anodic current peak below 0.05 mol/L and two anodic current peaks above 0.1 mol/L. The results of the DEIS measurements are in complete agreement with the potentiodynamic polarization curve. The corrosion products chane with polarization potential and two equivalent circuits are used to explain the experimental spectra of X80 pipeline steel in the whole corrosion process. Therefore, the combination of dynamic polarization cure and DEIS methods can be well used to investigate the corrosion behavior of X80 pipeline steel in aHCO3 solution at various potentials. In addition, different corrosion products forming at various potentials and the corrosion mechanisms of X80 pipeline steel are briefly analyzed.

    References | Related Articles | Metrics