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

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The effect of aluminum on the solidification microstructure of M2 high speed steel, particularly the morphology and microstructure of eutectic carbides, has been investigated by OM, TEM, SEM, EBSD and XRD. The results show that the as-cast microstructure consists of dislocation martensite and M2C eu. . .
Acta Metall, 2014 Vol. 50 (7): 769-776    DOI: 10.3724/SP.J.1037.2013.00621
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Cryogenic Processing High-Strength 7050 Aluminum Alloy and the Controlling of the Microstructures and Mechanical Properties PDF (2321KB)
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      10 June 2017, Volume 53 Issue 6 Previous Issue   
    Orginal Article
    Influence of Cooling Rate on Microstructural Formation of Melt-Spun Fe-Al-Nb Ternary Alloy
    Qianqian GU,Ying RUAN,Haizhe ZHU,Na YAN
    Acta Metall. 2017, 53 (6): 641-647.   DOI: 10.11900/0412.1961.2016.00415
    Abstract   HTML   PDF (4838KB)

    Fe-Al-Nb ternary alloys as a sort of high-temperature structure materials are paid more attention in recent years. The pseudobinary eutectic composed of Nb(Fe, Al)2 and α-Fe phases in Fe-Al-Nb alloy transformed from lamellar shape to fiber with the increase of growth rate in directional solidification. Heat treatment techniques were applied to investigate the strengthening mechanism related to microstructural formation. However, influence of cooling rate on microstructure especially pseudobinary eutectic is not clear yet. In this work, rapid solidification and the microstructural formation of Fe67.5Al22.8Nb9.7 ternary alloy were investigated by melt spinning technique to reveal the rapid solidification mechanism of the alloy. As the wheel rate increases from 10 m/s to 40 m/s, the thickness of alloy ribbon decrease by one order of magnitude, i.e. from 67.70 μm to 4.69 μm, the cooling rate increases by seven times, i.e. from 1.24×106 K/s to 9.53×106 K/s. Consequently, the sample shape transforms from regular ribbon to regular ribbon, fishbone-like ribbon and droplets. The microstructure consists of Nb(Fe, Al)2 and α-Fe phases. The rise of wheel rate leaded to the microstructural transition and refinement, as well as the refinement in terms of eutectic interlamellar spacing and grain size (i.e. grain diameter) measured using Image-Pro Plus software. On condition that the wheel rate is less than 40 m/s, the ribbon microstructural characteristics are divided into two regions, i.e. primary α-Fe phase plus lamellar pseudobinary eutectic near free surface region and anomalous pseudobinary eutectic near roller surface region. As the wheel rate increases from 10 m/s to 30 m/s, lamellar eutectic becomes fragmented and the amount of anomalous pseudobinary eutectic enlarges. Once the wheel rate is up to 40 m/s, anomalous pseudobinary eutectic is the only microstructure of the fishbone-like ribbon. Meanwhile, the alloy droplets with the diameter size ranging from 90 μm to 1500 μm were achieved at the wheel rate of 40 m/s. Owing to the relative low cooling rate, the microstructure of the alloy droplet consist of primary α-Fe phase and lamellar pseudobinary eutectic. As the droplet diameter decreases, the primary α-Fe phase transforms from dendrite to equiaxed grain and the pseudobinary lamellar eutectic is refined.

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    Effect of Nb on the Continuous Cooling Transformation Rule and Microstructure, Mechanical Properties of Ti-Mo Bearing Microalloyed Steel
    Xianling HE,Gengwei YANG,Xinping MAO,Chibin YU,Chuanli DA,Xiaolong GAN
    Acta Metall. 2017, 53 (6): 648-656.   DOI: 10.11900/0412.1961.2016.00437
    Abstract   HTML   PDF (10892KB)

    In recent years, with the fast development of automotive industry, more and more attention has been focused on developing high strength automobile steels with excellent formability. The microalloying elements, such as Nb, Ti, Mo, which can facilitate grain refinement and precipitation hardening, were added into steels to achieve high strength and good formability. The Ti-Mo and Ti-Mo-Nb microalloyed high strength ferritic steel were developed. In this work, the continuous cooling transformation curves (CCT) of Ti-Mo and Ti-Mo-Nb steels were obtained. And the effect of Nb on the microstructure and mechanical properties of Ti-Mo low carbon microalloyed steel was investigated by means of SEM, HRTEM and EDS. The results showed that Nb could raise the Ac1 and Ac3 temperatures, and restrain the ferrite-pearlite and bainite transformation. Moreover, Nb could also refine the microstructure and harden the matrix of steel which attributed to the strain-induced precipitation of nano-sized (Ti, Nb, Mo)C particles identified by HRTEM and EDS. It was also found that the strain-induced precipitation of (Ti, Mo)C was existed in the Ti-Mo steel. And both of (Ti, Mo)C and (Ti, Nb, Mo)C particles were NaCl type structure. The lattice constants/the average particle sizes of (Ti, Mo)C and (Ti, Nb, Mo)C were 0.432 nm and 0.436 nm / 12.11 nm and 8.69 nm, respectively.

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    Effect of Mn, Ni, Mo Contents on Microstructure Transition and Low Temperature Toughness of Weld Metal for K65 Hot Bending Pipe
    Liming DONG,Li YANG,Jun DAI,Yu ZHANG,Xuelin WANG,Chengjia SHANG
    Acta Metall. 2017, 53 (6): 657-668.   DOI: 10.11900/0412.1961.2016.00403
    Abstract   HTML   PDF (15758KB)

    To increase transport efficiency and to lower the costs of pipeline construction, longitudinally submerged arc welded (LSAW) pipes with larger diameters and thicker walls have been increasingly used by the pipeline industry. For example, in Russia, the LSAW pipeline in the Bovanenkovo-Ukhta project was recently constructed with K65 steel (the highest grade of the Russian natural gas pipeline), which is similar in specifications and yield strength requirement (550 MPa grade) to API X80 but has a stricter low temperature toughness value of 60 J at -40 ℃ (compared to -20 ℃ for API X80 grade) due to the extreme Arctic environment. Although weld metal with acicular ferrite (AF) has been developed to meet the requirement of low temperature toughness, the main objective of the present work was to clarify the microstructural evolution and the resulting changes in mechanical properties after the bending process. Hot bending pipes are necessary links in the construction of pipeline lying, which make more strin gent standards for the strength and low temperature toughness. That puts forward a challenge especially to the weld bead because of the deterioration of toughness during the hot bending process. In this work, submerged arc welding wire with high strength and toughness was developed for K65 hot bending pipes, and the alloying elements of Mn, Ni, Mo were considered to estimate the microstructure evolution and the effect of low temperature toughness for the weld metal. The results showed the low temperature toughness at -40 ℃ reached 90~185 J and 65~124 J for weld metal of straight seam pipe and hot bending pipe respectively, which reflect the excellent role of alloying elements of Mn, Ni, Mo. Microstructure characterization revealed that the weld metal, which originally consisted mainly of AF in the as-deposited condition, became predominantly composed of bainitic ferrite (BF) after hot bending. In addition, the large size cementite along the grain boundary was also the reason for the deterioration of toughness. It is found that reaustenisation caused a small austenite grain-sized matrix, which brought about a very high volume fraction of bainite. However, the low temperature toughness for hot bending pipe was improved to 124 J for the weld metal with 0.2%Mo, in which about 67.1% of high angle grain boundary were found. It is clear that the process of reaustenitisation during the bending process plays an important role in successful microstructural design for the steel weld metals.

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    Effects of Tempering Temperature on Microstructure and Mechanical Properties of Drill Pipe Steel 26CrMo
    Zhiqiang SHU,Pengbin YUAN,Zhiying OUYANG,Danmei GONG,Xueming BAI
    Acta Metall. 2017, 53 (6): 669-676.   DOI: 10.11900/0412.1961.2016.00406
    Abstract   HTML   PDF (6909KB)

    The effects of tempering temperature on microstructure and mechanical properties of steel 26CrMo were studied based on mechanical property tests and microstructure observation. The results show that a phase matrix gradually occurs recovery and recrystallization with increasing temperature during 540~690 ℃ temper process, martensite morphology fades away gradually, flake or rocklike carbides separate out along the martensite boundaries, and then change into granulated dispersed distribution, at 690 ℃ tempering carbides happen aggregation and growth on grain boundaries. With tempering temperature increasing, the strength of 26CrMo steel is gradually reducing, plasticity and toughness are gradually increasing. The tensile property and impact energy can meet all different grade drill pipe requirements in API 5DP standard with different tempering conditions. The total impact energy, crack initiation energy and crack propagation energy of 26CrMo steel are gradually increasing with the tempering temperature rising, the crack propagation energy is three times of crack initiation energy which shows great anti-crack propagation capability, but their ratio has no obvious change. The change of impact pro-perty is closely related to the strength and plasticity change, impact toughness stand or fall depends on high or low plasticity.

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    Influence of Cr Content and pH Value on the Semi-Passivation Behavior of Low Cr Pipeline Steels
    Lining XU,Jinyang ZHU,Bei WANG
    Acta Metall. 2017, 53 (6): 677-683.   DOI: 10.11900/0412.1961.2016.00412
    Abstract   HTML   PDF (1282KB)

    More than 60% corrosion failure of oil/gas pipeline happened in carbon steel pipeline transporting fluid containing CO2. Adding appropriate amount of Cr to carbon steel can greatly suppress localized corrosion. So, anti-CO2 low Cr steel and its corrosion mechanism become research hotspot. This work found that active-passive transition region occurred when low Cr steel was anodic polarized. But stable broad passive region like typical passivation was not found. This phenomenon was called semi-passivation. The anodic polarization behavior of low Cr steels with Cr content of 1%~5% and the effect of Cr content on semi-passivation in CO2 corrosion environment had been studied and discussed respectively. The anode potentiodynamic polarization curves of five kinds of low Cr steels in solutions with different pH values had been tested, subsequently the critical pH value which made the occurrence of the semi-passivation of low Cr steels with different Cr contents had been explored. The components of the corrosion product films on the low Cr steel samples polarized in solutions with various pH values had been compared through the Raman spectroscopy. The results showed that with the increase of Cr content, the semi-passivation characteristics was more obvious. The pH value of solutions in the CO2 environment contributes to the precipitation of Cr(OH)3. The Cr3+ ions dissolved from the substrate generate the corrosion films of Cr(OH)3 on the surface, resulting in the semi-passivation characteristics in the tests of anode potentiodynamic polarization.

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    Incipient Melting Behavior and Its Influences on the Mechanical Properties of a Directionally Solidified Ni-Based Superalloy with High Boron Content
    Hongwei ZHANG,Xuezhi QIN,Xiaowu LI,Lanzhang ZHOU
    Acta Metall. 2017, 53 (6): 684-694.   DOI: 10.11900/0412.1961.2016.00495
    Abstract   HTML   PDF (19817KB)

    A new directionally solidified Ni-based superalloy is developed for industrial gas turbine applications, which has high strength and excellent hot corrosion resistance at high temperatures. The high strength of the alloy is primarily derived from precipitation hardening by ordered L12 γ′ phase. To achieve a uniform distribution of precipitated γ′ particles for optimized mechanical properties, the suitable heat treatments are used. However, the heat treatment temperature in Ni-based superalloys is limited by the problem of incipient melting. Incipient melting microstructrue evolution during heat treatment has been hardly reported. Therefore, the behaviors of incipient melting and its effect on mechanical properties in the new directionally solidified superalloy DZ444 with high boron have been investigated in this work. The results show that some interdendritic regions of the as-cast DZ444 sample exhibit many of γ′/γ eutectic, MC carbides and multi-phase eutectic-like constituent which are composed of boride, Ni5Hf and η phases. During solution treatments, incipient melting does not occur in boride or Ni5Hf phase with low melting point firstly, but appears in γ matrix around multi-phase eutectic-like constituent which is affected significantly by borides. Compared to DZ444 alloy with the normal boron content, incipient melting occurs at the lower temperature in the range between 1160 ℃ and 1170 ℃. Incipient melting can occur significantly with the increase of the solid solution temperature or time. Incipient melting consists of typical γ dentrites and a lot of tiny precipitation particles after the water quenching (WQ) method following solution treatment. However, incipient melting forms multi-phase eutectic-like constituent, γ matrix and γ′/γ eutectic successively during air cooling (AC) following solution treatment, and the morphology of multi-phase eutectic-like constituent is similar to that of as-cast alloy. Firstly, a so-called incipiently melted circle (IMC) forms around multi-phase eutectic-like constituent; with the increase of the solid solution temperature or time, IMC extends inwards which makes γ matrix and multi-phase eutectic-like constituent in this circle melt successively. Finally, a incipiently melted pool forms gradually. Incipient melting is limited to the IMC below 1200 ℃ and the area of incipient melting changes with temperature or time correspondingly. However, incipiently melted region (IMR) expands outwards continuously which makes γ matrix outside the incipiently melted circle melt when the temperature is higher than 1210 ℃. Especially, IMR swallows up plenty of γ matrix, and many matrix islands, regions unmelted, exist in IMR above 1250 ℃ which destroys the continuity of the matrix significantly. The incipient melting has a minor effect on the tensile properties, nevertheless, decreases the creep-rupture properties remarkably. The degradation of mechanical properties mainly results from the increasing of the incipient melting area fraction and size.

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    Precipitation Behavior of δ Phase of Deformation Induced GH3625 Superalloy Hot-Extruded Tube
    Yutian DING,Yubi GAO,Zhengyi DOU,Xin GAO,Dexue LIU,Zhi JIA
    Acta Metall. 2017, 53 (6): 695-702.   DOI: 10.11900/0412.1961.2016.00508
    Abstract   HTML   PDF (6704KB)

    GH3625 alloy is a wrought nickel-based superalloy mainly used in aeronautical, aerospace, chemical, nuclear, petrochemical, and marine applications industry due to its good mechanical properties, processability, weldability and resistance to high-temperature corrosion on prolonged exposure to aggressive environments. However, in medium and high temperature environment during long-term service, the γ'' is a metastable phase, easily transformed into stable δ phase, or δ phase directly formed in the γ matrix so that alloy performance was deteriorated, leading to the result of alloy failure. At the present work, mass fraction of δ phase in GH3625 superalloy hot-extruded tube cold deformed to different reductions and then aged at 800 ℃ for different times, were measured by XRD. The effect of cold deformation on the law and kinetics of δ phase precipitation was investigated by SEM, EDS and Image-Pro Plus metallographic analysis. The results show that δ phase first precipitates at the deformation twin and grain boundaries as well as deformation bands, and then precipitates in the grains. The amount of δ phase at the deformation bands increases with the increase of cold deformation. The morphologies of δ phase change gradually from needles to spheroids or rodlike with increasing cold deformation. With the extend of ageing time, the average size of δ phase increases which grows according to LSW theory. At 800 ℃, the relationship between the precipitation content of δ phase and ageing time follows Avrami equation. As cold deformation increases, the content of δ phase increases, the time index n decreases, whereas the δ phase precipitation rate increases. Cold deformation promotes the precipitation of δ phase. The solute drags of Nb in soild solution and pinning of δ phase inhibits the grain growth during ageing process of cold deformed GH3625 superalloy hot-extruded tube. The hardness of the alloy increases with the extension of the holding time at ε =35% but no obvious change at ε ≥50%.

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    Effects of Re-Melting Processes on the Tensile Properties of K452 Alloy at High Temperature
    Jinxia YANG,Futao XU,Donglin ZHOU,Yuan SUN,Xingyu HOU,Chuanyong CUI
    Acta Metall. 2017, 53 (6): 703-708.   DOI: 10.11900/0412.1961.2016.00286
    Abstract   HTML   PDF (3849KB)

    K452 alloy is a nickel-based cast superalloy having the good tensile properties at high temperature and excellent corrosion resistance. It has been applied as a blade material of engines when environmental temperature is not above 950 ℃. It is found that the tensile properties of the alloy have become more scattered and unstable although its chemical compositions are not changed. Hence, the tensile properties of the alloy were studied in order to increase its stability at high temperature and improve its applied properties. Tensile specimens were prepared using the different re-melting processes. Tensile tests were done at 900 ℃. When the pouring temperature was 1430 ℃, tensile properties were not only lower than expected, but also had great degree of dispersion, i.e., the vales of ultimate strengths changed in the range of 410 MPa and 510 MPa, and the elongations changed in the range of 3.5% and 22.0%, the average contents of O and N were the highest among three tested conditions. The highest N content was 0.0028%. And the shrinkage area was higher than those in other two re-melting processes. When the pouring temperature was 1500 ℃, the tensile properties were improved, and their changing scopes became small, the average contents of O and N decreased, the shrinkage area decreased. When the refining temperature was 1590 ℃ and the holding time was 5 min, both average contents of O and N were decreased greatly, the shrinkage was not seen in the fracture surfaces. And the tensile properties were improved. Furthermore, their changing scopes were very small.

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    Effect of N Doping on Microstructure, Mechanical and Tribological Properties of V-Al-C Coatings
    Xin WANG,Zhenyu WANG,Zaixin FENG,Peiling KE,Aiying WANG
    Acta Metall. 2017, 53 (6): 709-718.   DOI: 10.11900/0412.1961.2016.00523
    Abstract   HTML   PDF (8662KB)

    The crises of resource shortage have prompted ocean exploitation to spring up all over the world. Some crucial frictional components of marine equipment have to be directly faced with the conjoint action of wear and corrosion. Transition metal nitrides or carbides hard coatings have been widely used to improve tribological performance in various applications. However, the poor toughness, wear and corrosion resistance of coatings cannot meet the harsher marine environment, which needs to obtain multi-functional hard coatings providing complex properties. The nanocomposite structure coatings containing nanocrystalline phase embedded in an amorphous matrix allow tailoring their properties to desired value by designing chemical composition and nanostructure. In this work, V-Al-C and V-Al-C-N coatings were deposited on silicon and high speed steel (HSS) substrates by magnetron sputtering. The crystal microstructure, chemical composition, surface morphology, cross-sectional structure, mechanical property and friction behavior of the coatings under different contact conditions (air, distilled water and artificial seawater) were studied by XRD, XPS, SEM, nano-indentation and ball-on-disc tribometer. The results showed that the V-Al-C coating displayed columnar structure with coarse grain. When the nitrogen was incorporated, the coating structure evolved into nanocomposite structure composed of nanocrystallite and amorphous carbon. The hardness increased from (14±0.48) GPa to (24.5±0.8) GPa, and the toughness was significantly improved (H/E>0.1). In air condition, the friction coefficient decreased from 0.70 to 0.42, owing to the synergy interaction between V2O5 and amorphous carbon during sliding. The friction coefficients of the both coatings in distilled water and artificial seawater were lower than those in air owing to the boundary lubrication forming lubricative film by absorbed water. The friction coefficient in seawater was lower than those in distilled water, resulting from the formation of Mg(OH)2 and CaCO3 during sliding. However, the wear rates of the both coatings in artificial seawater were larger than that in distilled water, which demonstrated a synergism between corrosion and wear in artificial water. The V-Al-C coating was all worn out under different contact conditions owing to severe abrasive wear, while the V-Al-C-N coating showed better wear resistance, with a wear rate of 3.0×10-16 m3/(Nm) in air and 1.4×10-15 m3/(Nm) in artificial water, respectively.

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    Microstructure and Property of Cu/Al Joint Brazed with Al-Si-Ge Filler Metal
    Zhiwei NIU,Zheng YE,Kaikai LIU,Jihua HUANG,Shuhai CHEN,Xingke ZHAO
    Acta Metall. 2017, 53 (6): 719-725.   DOI: 10.11900/0412.1961.2016.00342
    Abstract   HTML   PDF (5395KB)

    Cu/Al brazing has good prospect for applications in the air conditioning and refrigeration industry. A suitable filler metal is the key of Cu/Al brazing. The chemical and physical properties of the filler metal have great influence on the brazing process and parameters. And the strength of the brazing joint is closely related to the properties of the filler metal and the brazing process. While the previous studies have not developed a kind of Cu/Al brazing filler metal which can achieve a tough joint at a low brazing temperature. In this work, the Al-5.6Si-25.2Ge filler metal was first used to braze Cu/Al dissimilar metals, and the melting characteristics of the filler metal, spreading wettability, Cu interfacial structure and strength of brazed joint were investigated systematically. Additionally, the common Zn-22Al filler metal was also used for comparison. The results show that the Al-5.6Si-25.2Ge filler metal possesses low melting temperature (about 541 ℃) and excellent spreading wettability on Cu and Al base metals. The interfacial structure of Al-5.6Si-25.2Ge/Cu was CuAl2/CuAl/Cu3Al2. The thickness of planar CuAl and Cu3Al2 phases was only 1~2 μm, and the thickness of cellular CuAl2 phase was about 3 μm. The interfacial structure of Zn-22Al/Cu was CuAl2/CuAl/Cu9Al4, but the average thickness of the CuAl2 layer was up to 15 μm. The test results of the shearing strength show that the shearing strength of the Cu/Al joint brazed with Zn-22Al filler metal was only 42.7 MPa, but the shearing strength brazed with Al-5.6Si-25.2Ge filler metal was higher (53.4 MPa).

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    Effect of Fe on Microstructure and Coercivity of SmCo-Based Magnets
    Shaoting GONG, Chengbao JIANG, Tianli ZHANG
    Acta Metall. 2017, 53 (6): 726-732.   DOI: 10.11900/0412.1961.2016.00402
    Abstract   HTML   PDF (1989KB)

    High-temperature permanent magnets have an important application in the aerospace and other high-tech fields, among which 2:17-type SmCo magnets have become the first choice for high-temperature permanent magnets due to the strong magnetic anisotropy and high Curie temperature. Although there are studies on the effect of Fe on the remanence and coercivity, the role that Fe plays on coercivity mechanism of SmCo magnets is still unclear. In this work, Sm(CobalFexCu0.08~0.10Zr0.03~0.033)z (x=0.10~0.16, z=6.90 and 7.40) magnets are prepared and the magnetic properties under different temperatures are investigated. The magnets with an intrinsic coercivity of 603.99 kA/m and a maximum energy product of 87.30 kJ/m3 at 500 ℃ are obtained. It is revealed that at room temperature the coercivity of the magnets increases with increasing Fe content, however, at 500 ℃ the coercivity shows an opposite dependency on Fe content. Moreover, the effect of Fe on coercivity is more obvious at low z value. The phase structure and composition analyses were characterized by XRD and TEM. The results show that with the increase of Fe content, the size of the 2:17R cell phase increases, the volume ratio of cell boundary 1:5H phase decreases, and furthermore, both Fe content in the 2:17R phase and Cu content in the 1:5H phase increase. The variations of Fe and Cu contents in both phases lead to the change of the domain wall energy difference. With the increase of Cu content of 1:5H phase, the domain wall energy of 1:5H phase (γ1:5) drops faster at room temperature, the coercivity is determined by γ2:17-γ1:5, so the coercivity increases with increasing Fe content. While at 500 ℃, due to γ1:5 at its Curie temperature, the coercivity is mainly determined by the domain wall energy of 2:17R phase (γ2:17), which decreases with increasing Fe content. The increase of Fe content at the low z value results in a smaller growth of cell size, which leads to a more significant change in coercivity.

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    Finite Element Simulation and Experimental Verification of Quenching Stress in Fully Through-Hardened Cylinders
    Yu LIU, Shengwei QIN, Xunwei ZUO, Nailu CHEN, Yonghua RONG
    Acta Metall. 2017, 53 (6): 733-742.   DOI: 10.11900/0412.1961.2016.00509
    Abstract   HTML   PDF (1871KB)

    Quenching is one of the most important heat-treatment processes for improving the mechanical properties of steel components in manufacture industry. The quenching stress is a source of cracking, which is frequently detrimental to steel properties. Therefore, the investigation of quenching stress is very important for the control of distortion, cracking and residual stress distributions of components. In the study of quenching stress, the measurement of stress distribution is necessary to the stress analysis and design of quenching process. However, in most cases, the cracking of a quenched component is caused by transient stress during quenching, while experiment can only measures the final internal stress (residual stress), rather than transient stress. As a result, the measurement of residual stress associated with finite element simulation (FES) has been a mainstream direction in the investigation of quenching stress. In this work, a full through-hardened 40CrNiMo cylinder with 60 mm diameter was water-quenched, and cooling curves at three positions along the radius of cylinder were measured. Then, an optimized heat transfer coefficient as a function of surface temperature was obtained by fitting with the measured cooling curves using the trial and error method. Based on an exponent-modified (Ex-Modified) normalized function describing transformation plasticity kinetics proposed, the thermo-elasto-plastic constitutive equations were deduced. The commercial finite element software, Abaqus/Standard, was used to solve the coupled temperature field, microstructure field and stress (strain) field. The results indicate that the quenching stress and its distribution predicted by FES is well consistent with those measured by XRD, which verified that the models employed in coupling of thermal field, phase transformation field and stress field including transformation plasticity function proposed are correct. Meanwhile, the features of residual stress distribution were revealed that compressive stress exists in the core and surface of cylinder and the maximum tensile stress exists at subsurface. The separated calculation of thermal stress and phase transformation stress by FES reveals the origin of residual stress distribution feature in quenched cylinders, that is, the relative higher phase transformation compressive stress and lower tensile thermal stress at the core of cylinder make the residual stress to be compressive, while at the surface of cylinder the compressive stress is predominantly from thermal stress, because it is much larger than the tensile stress caused by phase transformation stress. The tangential residual stress distributions in cylinders with several diameters from 3 mm to 100 mm were predicted by FES, and the results indicate that when diameter is less than 5 mm, the tensile stress at the surface increases with increasing diameter until to 5 mm, then decreases with increasing diameter to 20 mm, finally the tensile stress becomes compressive stress. Besides, with the increase of diameter, maximum tensile stress shifts from the surface to the location of 0.6 radius. The effects of different quenching media on quenching stress were also investigated by FES. The results demonstrated that although there is compressive stress at surface of cylinder quenched in water or salt solution, the maximum stress locates the subsurface, meaning that cracking easily occurs at the subsurface, which is consistent with cracking in practical components. This work is helpful for the analysis of cracking from quenching stress in components with different sizes and under different quenching media.

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    Experimental Studies and Numerical Simulation on the Nitriding Process of Grain-Oriented Silicon Steel
    Guimin ZENG,Haiwen LUO,Jun LI,Jian GONG,Xianhao LI,Xianhui WANG
    Acta Metall. 2017, 53 (6): 743-750.   DOI: 10.11900/0412.1961.2016.00463
    Abstract   HTML   PDF (1318KB)

    Grain-oriented silicon steel (GOSS) is an important functional material used as lamination cores in various transformers. Its magnetic properties are strongly dependent on the sharpness of Goss texture, which is developed during the secondary recrystallization annealing of product. In order to save energy and reduce cut-down operation costs, Nippon steel first lowered the slab-reheating temperature from 1350~1400 ℃ to 1150 ℃ and adopted the nitriding process to form nitride inhibitors before recrysta-llization annealing in 1970s. In this new process, nitriding is the critical process because it controls the size, distribution and volume fraction of nitride precipitates, which then determines the subsequent deve-lopment of Goss texture. Although it is of great importance for good quality control of industrial GOSS product, unfortunately, a quantitative mathematic modeling on nitriding kinetics is still in lack. In this work, nitriding kinetics were both measured experimentally and simulated by modeling. The N contents after various nitriding periods and N concentration gradient across thickness were both measured. It has been found that the N content increases slowly at the beginning of 60 s and then much more rapidly during nitriding. There exists a sharp N concentration gradient within the depth of 0.03 mm to the steel sheet surface, which diminishes after about 0.04 mm depth. With the different assumptions on N-transfer coefficient from gas to the steel matrix, the first mathematic modeling on nitriding kinetics of GOSS has been successfully established and solved numerically. The simulation results suggest that only when the N-transfer coefficient, f, changes with time following the Avrami function, f=A(1-exp(-ktn)), the calculated nitriding kinetics are consistent with the measurements. Such an Avrami-type dependence results from the reduction kinetics of oxide layer on the surface of silicon steel sheet during nitriding, in which both plate-like and spherical oxides were observed at the beginning but most of them became spherical after nitriding.

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    Point Defect Concentrations and Interactions in D019-Ti3Al from First-Principles Calculations
    Huijin TAO,Shan ZHOU,Yu LIU,Jian YIN,Hao XU
    Acta Metall. 2017, 53 (6): 751-759.   DOI: 10.11900/0412.1961.2016.00464
    Abstract   HTML   PDF (2257KB)

    The intermetallics D019-Ti3Al has low specific density and high thermal resistance for both bulk and coating applications in engineering area. The point defects such as thermal vacancy, compostion vacancy and antisite defect have great influence on the properties of D019-Ti3Al, but are usally neglected. According to available research data from both theory and experiment, it is commonly considered that the thermal vacancies in D019-Ti3Al provide paths for atomic migration and diffusion, the antisite defects play an important role in the disordering of D019-Ti3Al, and the interaction between composition vacancy and antisite defect may have important influence on atomic diffusion and dislocation movement. So it is necessary to explore the mechanism of interaction between composition vacancy and antisite defect for more accurate understanding of the atomic diffusion, dislocation movement and plastic deform in D019-Ti3Al. In this work, the formation enthalpy, equilibrium concentration, and binding energy of composition vacancy and antisite defect in D019-Ti3Al intermetallics were calculated by using both the Wagner-Schottky model of point defect thermodynamics and the plane wave pseudopotential method in first-principles. Results suggest that, in the whole composition range of interest, the point defect concentrations increase with the increase of temperature. In particular, the concentrations of antisite defects are higher than those of vacancies, and the vacancy concentration of Ti is higher than that of Al. At the stoichiometric composition, the concentrations of antisite defects of Ti and Al are very close. At the Ti-rich side of component, the antisite defect of Ti dominates in concentration, while at the Al-rich side, that of Al dominates in concentration. For the calculated results of 3 types of point defect pairs, AlTi-TiAl, TiAl-TiAl and VAl-AlTi, they may have the strong trend to aggregate, while others may show the tend to diffuse into the matrix.

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    Phase-Field Modeling of Austenite-to-Ferrite Transformation in Fe-C-Mn Ternary Alloys
    Jun ZHANG,Wenxiong CHEN,Chengwu ZHENG,Dianzhong LI
    Acta Metall. 2017, 53 (6): 760-768.   DOI: 10.11900/0412.1961.2016.00468
    Abstract   HTML   PDF (2800KB)

    The effect of Mn on the austenite-to-ferrite transformation has been widely studied by both physical models and experiments due to its technological importance for alloy design in steel industries. In recent years, an increasing interest of this issue is moved onto the effect of alloying element on the migrating interface during the austenite-to-ferrite transformation. For ternary Fe-C-Mn alloys, the interfacial condition is more complicated than that of binary Fe-C alloys in view of the large difference in the diffusivity between the interstitial and substitutional alloying elements. Generally speaking, there are two main concepts, i.e. the paraequilibrium model and the local-equilibrium model, which have been proposed to describe the phase transformation kinetics in ternary Fe-C-Mn alloys based on different assumptions about the diffusion of the substitutional elements. And many modeling attempts have been made to study the effect of Mn on the migration kinetics by using these theories. In this work, a multi-phase-field (MPF) model coupling with a Gibbs-energy dissipation model was developed to simulate the isothermal austenite-to-ferrite transformation in ternary Fe-C-Mn alloys. This model has considered the Mn diffusion inside the migrating interface in a physical manner and takes its effect on the transformation kinetics into account. Comparison simulations were made to analyze the difference in the transformation kinetics and ferrite morphologies with and without considering the energy dissipation at the moving interface. It shows that the incomplete transformation phenomenon does occur due to the Mn diffusion inside interface. The modified MPF model was then used to study the effect of Mn contents on the microstructures and kinetics of the phase transformations. It is found that the ferrite growth along the austenite/austenite boundaries is faster than that in the perpendicular direction. This difference is intensified with increasing the Mn concentration, which hence leads to the ferrite morphology changed from elliptical to flat alike. It also produces a slower transformation kinetics and a larger degree of the incomplete transformation when increasing the Mn concentration.

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