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ISSN 1005-0302
CN 21-1315/TG
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      20 December 2017, Volume 33 Issue 12 Previous Issue    Next Issue
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    Orginal Article
    Hydrogen transport in metals: Integration of permeation, thermal desorption and degassing
    Galindo-Nava E.I., Basha B.I.Y., Rivera-Díaz-del-Castillo P.E.J.
    J. Mater. Sci. Technol., 2017, 33 (12): 1433-1447.  DOI: 10.1016/j.jmst.2017.09.011
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    A modelling suite for hydrogen transport during electrochemical permeation, degassing and thermal desorption spectroscopy is presented. The approach is based on Fick's diffusion laws, where the initial concentration and diffusion coefficients depend on microstructure and charging conditions. The evolution equations are shown to reduce to classical models for hydrogen diffusion and thermal desorption spectroscopy. The number density of trapping sites is found to be proportional to the mean spacing of each microstructural feature, including dislocations, grain boundaries and various precipitates. The model is validated with several steel grades and polycrystalline nickel for a wide range of processing conditions and microstructures. A systematic study of the factors affecting hydrogen mobility in martensitic steels showed that dislocations control the effective diffusion coefficient of hydrogen. However, they also release hydrogen into the lattice more rapidly than other kind of traps. It is suggested that these effects contribute to the increased susceptibility to hydrogen embrittlement in martensitic and other high-strength steels. These results show that the methodology can be employed as a tool for alloy and process design, and that dislocation kinematics play a crucial role in such design.

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    Precipitation and hot deformation behavior of austenitic heat-resistant steels: A review
    Zhou Yinghui, Liu Yongchang, Zhou Xiaosheng, Liu Chenxi, Yu Jianxin, Huang Yuan, Li Huijun, Li Wenya
    J. Mater. Sci. Technol., 2017, 33 (12): 1448-1456.  DOI: 10.1016/j.jmst.2017.01.025
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    The austenitic heat resistant-steels have been considered as important candidate materials for advanced supercritical boilers, nuclear reactors, super heaters and chemical reactors, due to their favorable combination of high strength, corrosion resistance, perfect mechanical properties, workability and low cost. Since the precipitation behavior of the steels during long-term service at elevated temperature would lead to the deterioration of mechanical properties, it is essential to clarify the evolution of secondary phases in the microstructure of the steels. Here, a summary of recent progress in the precipitation behavior and the coarsening mechanism of various precipitates during aging in austenitic steels is made. Various secondary phases are formed under service conditions, like MX carbonitrides, M23C6 carbides, Z phase, sigma phase and Laves phase. It is found that the coarsening rate of M23C6 carbides is much higher than that of MX carbonitrides. In order to understand the thermal deformation mechanism, a constitutive equation can be established, and thus obtained processing maps are beneficial to optimizing thermal processing parameters, leading to improved thermal processing properties of steels.

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    Recent progress in medium-Mn steels made with new designing strategies, a review
    Hu Bin, Luo Haiwen, Yang Feng, Dong Han
    J. Mater. Sci. Technol., 2017, 33 (12): 1457-1464.  DOI: 10.1016/j.jmst.2017.06.017
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    After summarizing the relevant researches on the medium Mn steels in references, two new targets on the tensile properties have been defined. One is that both transformation-induced (TRIP) and twinning-induced plasticity (TWIP) could be realized for the steel with a relatively low Mn content, which exhibits the similar tensile properties to the classical TWIP steels with higher Mn content. The other is to achieve ultrahigh ultimate tensile strength (>1.5 GPa) without sacrificing formability. To achieve these goals, new designing strategies was put forward for compositions and the processing route. In particular, warm rolling was employed instead of the usual hot/cold rolling process because the former can produce a mixture of retained austenite grains with different morphologies and sizes via the partial recrystallization. Consequently, the retained austenite grains have a wide range of mechanic stability so that they can transform to martensite gradually during deformation, leading to enhanced TRIP effect and then improved mechanic properties. Finally, it is succeeded in manufacturing these targeted medium Mn steels in laboratory, some of them even exhibit better tensile properties than our expectation.

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    Effect of pre-annealing prior to cold rolling on the precipitation, microstructure and magnetic properties of strip-cast non-oriented electrical steels
    Xu Yunbo, Jiao Haitao, Zhang Yuanxiang, FengFang, Lu Xiang, Wang Yang, Cao Guangming, Li Chenggang, Misra R.D.K.
    J. Mater. Sci. Technol., 2017, 33 (12): 1465-1474.  DOI: 10.1016/j.jmst.2017.08.002
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    A novel processing route involving strip casting, pre-annealing treatment, cold rolling and recrystallization annealing was applied to a Fe-2.6%Si steel to improve the magnetic properties. The impact of as-cast strip pre-annealing on the microstructure, texture, precipitation and magnetic properties were investigated by electron probe micro-analysis, transmission electron microscopy, and X-ray diffraction analysis, etc. It was found that the precipitation of second-phase particles during strip casting was restrained by rapid solidification. The absence of pre-annealing led to the occurrence of a large amount of 20-50 nm MnS precipitates in the final annealed sheets, which is responsible for fine grains and high core loss (4.01 W/kg) due to grain boundary pinning effect. Although the microstructure and texture of 900-1000 °C pre-annealed samples were similar to those of as-cast strip, significant grain coarsening together with the strengthening of λ-fiber texture was observed in the 1100 °C pre-annealed strips. In comparison with the case of as-cast strip, a higher amount of large-sized precipitates consisting of manganese sulfide and/or aluminum nitride occurred in matrix after pre-annealing. Correspondingly, in the final annealed sheets, the number density of precipitates with sizes smaller than 100 nm was substantially reduced, and 100-200 nm and 200-500 nm sized particles became more dominant in samples subjected to 30-min and 120-min pre-annealing treatments respectively. In addition, the average grain size of final annealed sheets increased with the pre-annealing temperature and time because of the weakened pining effect of coarsen precipitates. Ultimately, the magnetic induction of samples subjected to pre-annealing was slightly increased and ranged from 1.73 T to 1.75 T owing to the enhancement of {100} recrystallization texture, and simultaneously the core loss significantly decreased until a minimum of 3.26 W/kg was reached. Nevertheless, large number of 200-500 nm particles presented during pre-annealing for 120 min could weaken the improvement in core loss which is likely associated with the pinning effect on magnetic domain wall.

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    Effect of substitution of Si by Al on the microstructure and mechanical properties of bainitic transformation-induced plasticity steels
    Zhu Kangying, Mager Coralie, Huang Mingxin
    J. Mater. Sci. Technol., 2017, 33 (12): 1475-1486.  DOI: 10.1016/j.jmst.2017.09.002
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    The effect of partial or full substitution of Si by Al on the microstructure and mechanical properties has been extensively studied in multi-phase transformation-induced plasticity (TRIP) steels with polygonal ferrite matrix, but rarely studied in bainitic TRIP steels. The aim of the present study is to properly investigate the effect of Al and Si on bainite transformation, microstructure and mechanical properties in bainitic steels in order to provide guidelines for the alloying design as a function of process parameters for the 3rd generation advanced high strength steels (AHSS). It is shown from the dilatometry study, microstructural investigations and tensile properties measurements that the Al addition results in an acceleration whereas Si addition leads to a retardation in bainite transformation kinetics. The addition of Al retards the decomposition of austenite into pearlite and carbides at holding temperatures higher than 450 °C whereas Si retards the decomposition of austenite into carbides at temperatures lower than 450 °C. Consequently, the Al-added bainitic steel has a better strength-elongation combination at bainitic holding temperatures higher than 450 °C while Si-added steel has a better strength-elongation combination at temperatures lower than 450 °C. The higher yield strength of Al-added steel is mainly attributed to its finer bainitic lath. The higher tensile strength of Si-added steel is not only related to the stronger contribution of Si on work hardening during deformation, but also due to the higher volume fraction of martensite or martensite/austenite (MA) blocks in all heat treatment conditions, as well as the lower mechanical stability of retained austenite in this steel.

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    Effect of rhenium on the microstructure and mechanical behavior of Fe-2.25Cr-1.6W-0.25V-0.1C bainitic steels
    Antonio Jiménez José, Carsí Manuel, Antonio Ruano Oscar
    J. Mater. Sci. Technol., 2017, 33 (12): 1487-1493.  DOI: 10.1016/j.jmst.2017.08.001
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    A new ferritic creep resistant steel has been developed by eliminating Nb and adding 1.5 mass % Re to a ferritic steel grade T/P23 with the aim of enhancing its mechanical properties at high temperature. Cast ingots of both steels, new grade and ASTM T/P 23, were hot rolled at 900 °C and then submitted to a thermal treatment consisting of solubilization at 1050 °C and tempering at 700 °C. Tempered bainitic microstructures obtained contain second phases reinforcing carbide particles, mainly M6C and M23C6 at the boundaries of both, prior austenite grains and bainitic ferrite laths, as well as MC within the grains. Mechanical properties at temperatures ranging from 540 to 600 °C were studied by strain-rate-change tests in compression at strain rates between 10-7 and 10-4 s-1. These tests showed high stress exponents (n ≥ 20) and activation energies (Q ≈ 400 kJ/mol) for both alloys, which were associated with a dislocation movement mechanism with a strong interaction between dislocations and precipitates. On the other hand, a creep exponent of 5 was derived for the stress dependence of minimum creep rate from conventional-type creep tests at 600 °C. Although this stress exponent is usually related to a dislocation climb controlled creep mechanism, remarkable microstructural degradation observed with increasing creep time makes difficult to elucidate the true deformation mechanism controlling creep.

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    Effect of boron on bainitic transformation kinetics after ausforming in low carbon steels
    He Binbin, Xu Wei, Huang Mingxin
    J. Mater. Sci. Technol., 2017, 33 (12): 1494-1503.  DOI: 10.1016/j.jmst.2017.05.006
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    The addition of boron (B) is frequently adopted to increase the hardenability of bainitic steels. Although it is well known that B can retard the bainitic transformation kinetics, it is still not clear how the B affects the bainitic transformation kinetics after ausforming. By systematic high-resolution dilatometry tests, the present work reveals that the bainitic transformation kinetics is accelerated in a low C steel with B addition after ausforming from all aspects including incubation time, transformation velocity and transformed volume fraction. In contrast, for the same steel without B addition, both transformation velocity and transformed volume fraction are retarded after ausforming. It is proposed that ausforming can reduce B segregation at prior austenite grain boundaries as some boron can interact with dislocations and therefore enhance bainite nucleation rate. Furthermore, auforming can refine the average volume of bainitic sheaf. Based on the competing mechanisms between increase of nucleation rate and refinement of bainitic sheaf, the effects of B and ausforming on the bainitic transformation kinetics are discussed.

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    Relationship between microstructure and hydrogen induced cracking behavior in a low alloy pipeline steel
    Li Jing, Gao Xiuhua, Du Linxiu, Liu Zhenguang
    J. Mater. Sci. Technol., 2017, 33 (12): 1504-1512.  DOI: 10.1016/j.jmst.2017.09.013
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    Hydrogen induced cracking (HIC) behaviors of a high strength pipeline steel with three different microstructures, granular bainite & lath bainite (GB + LB), granular bainite & acicular ferrite (GB + AF), and quasi-polygonal ferrite (QF), were studied by using corrosion experiment based on standard NACE TM 0284. The HIC experiment was conducted in hydrogen sulfide (H2S)-saturated solution. The experimental results show that the steel with GB + AF and QF microstructure present excellent corrosion resistance to HIC, whereas the phases of bainite lath and martensite/austenite in LB + GB microstructure are responsible for poor corrosion resistance. Compared with ferrite phase, the bainite microstructure exhibits higher strength and crack susceptibility of HIC. The AF + GB microstructure is believed to have the best combination of mechanical properties and resistance to HIC among the designed steels.

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    Predicting the Transition between Upper and Lower Bainite via a Gibbs Energy Balance Approach
    Yang Zenan, Xu Wei, Yang Zhigang, Zhang Chi, Chen Hao, van der Zwaag Sybrand
    J. Mater. Sci. Technol., 2017, 33 (12): 1513-1521.  DOI: 10.1016/j.jmst.2016.11.028
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    The transition temperature between upper bainite and lower bainite is calculated with an extended Gibbs energy balance model, which is able to quantitatively describe the evolution of carbon supersaturation within bainitic ferrite sheaves during the entire thickening process. The nucleation rate of intra-lath cementite precipitation on a dislocation is calculated based on of the degree of carbon supersaturation. Upper bainite and lower bainite are thus distinguished by the effective nucleation density and therefore a numerical criterion can be set to define the transition. The model is applied to Fe-xC-1Mn/2Mn/1Mo ternary alloys. Results show that the transition temperature increases with bulk carbon content at lower carbon concentration but decreases in the higher carbon region. This prediction agrees very well with the experimental observations in Mn and Mo alloyed systems. Moreover, the highest transition temperature and the carbon content at which it occurs in the Fe-xC-2Mn system are in good agreement with reported experimental data. The inverse “V” shaped character of the carbon concentration-transition temperature curve indicates two opposite physical mechanisms operating at the same time. An analysis is carried out to provide an explanation.

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    Opposite Relationship between Orientation Selection and Texture Memory in the Deformed Electrical Steel Sheets during α→γ→α Transformation
    Zhang Louwen, Yang Ping, Mao Weimin
    J. Mater. Sci. Technol., 2017, 33 (12): 1522-1530.  DOI: 10.1016/j.jmst.2016.11.007
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    The undesired {111} texture component for the magnetic properties mainly exists in the sheets of electrical steels by the conventional process, whereas the sheets with the non-{111} texture can be obtained by α→γ→α transformation. In this paper, we mainly investigate the opposite relationship between orientation selection and texture memory in the deformed ultra-low carbon steel sheet during α→γ→α transformation annealing. A 0.5 mm thick hot-rolled sheet is directly subjected to transformation. The result shows that the specific transformation textures are not possible to generate in the sheets without deformation. Besides, transformation annealing is conducted on the recrystallized sheets in hydrogen and vacuum, respectively. The near {100} and {110} grains have the growth advantage at the atmosphere/metal interface, and the initial ferrite textures are retained in vacuum. Cold-rolled sheets with different thicknesses are annealed for transformation in vacuum, hydrogen and nitrogen, respectively. The near {100} and {110} textures are still the preferential orientations at the atmosphere/metal interface. When the surface grains have sufficiently large growth advantage, the {111} grains developed by texture memory effect will be annexed. Otherwise, the {111} grains at the center layer of the sheets are hard to be replaced, and they are retained after α→γ→α transformation cycle. The results of deformed sheets annealed with different heating rates in hydrogen show that the growth of initial recrystallization grains has a great effect on variant selection.

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    Rietveld refinement, microstructure and high-temperature oxidation characteristics of low-density high manganese steels
    Huang Zhenyi, Jiang Yueshang, Hou Along, Wang Ping, Shi Qi, Hou Qingyu, Liu Xianghua
    J. Mater. Sci. Technol., 2017, 33 (12): 1531-1539.  DOI: 10.1016/j.jmst.2017.09.012
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    Three forged low-density high manganese steels Mn28Al10, Mn28Al8 and Mn20Al10 were used as experimental materials in this study. The forged microstructure and external oxidation characteristics at 1323 K and 1373 K for 5-25 h in air were investigated by microstructural observation and X-ray diffraction (XRD) technique. The phase compositions and abundance in the forged and oxidized samples were quantitatively obtained by Rietveld method on the basis of XRD pattern analysis. The results showed that an austenitic microstructure formed in steels Mn28Al10 and Mn28Al8, and 18.02 wt% ferrite could be found in Mn20Al10. The relative amount of ~5.28 wt% κ-carbide (Fe3AlC0.5) in Mn28Al10 was far greater than that in Mn28Al8 and Mn20Al10. The oxidation kinetics of forged steels oxidized at 1323 K for 5-25 h had two-stage parabolic rate laws; and the oxidation rate of the first stage was lower than that of the second stage. When they were oxidized at 1373 K for 5-25 h, the oxidation kinetics followed only a parabolic law and the oxidation rates were respectively greater than those at 1323 K for 5-25 h. When they were oxidized at 1323 K for 25 h, detached external scales contained Fe2MnO4 and α-Fe2O3 oxides. α-Al2O3 and (Fe, Mn)2O3 oxides could only be indexed in steels Mn28Al8 and Mn28Al10, respectively. When they were oxidized at 1373 K for 25 h, Fe2MnO4, Fe3O4, α-Fe2O3 and α-Al2O3 oxides could all be indexed in the external detached scales. The main phase of detached external scales was Fe2MnO4; and the relative amount of α-Al2O3 in steel Mn28Al8 was higher than that in steels Mn28Al10 and Mn20Al. The external oxidation layers of these three forged steels oxidized at 1323 K and 1373 K for 25 h were essentially followed the sequence of α-Al2O3, Fe2MnO4, Fe3O4, FeMnO3, and Fe2O3 from the substrate to the outside surface. The forged Mn28Al10 steel with austenitic microstructure and a certain amount of κ-carbide (~5.28 wt% in the present work) possessed a better combination of strength, ductility, specific strength, and oxidation rate when compared to that of the forged Mn28Al8 and Mn20Al10 steels.

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    Experimental and simulated characterization of creep behavior of P92 steel with prior cyclic loading damage
    Zhang Wei, Wang Xiaowei, Gong Jianming, Jiang Yong, Huang Xin
    J. Mater. Sci. Technol., 2017, 33 (12): 1540-1548.  DOI: 10.1016/j.jmst.2017.09.006
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    The effect of prior cyclic loading on creep behavior of P92 steel was investigated. Creep tests on prior cyclic loading exposure specimens were performed at 650 °C and 130 MPa. In order to clarify the influence of prior cyclic loading on creep behavior, optical microscope, scanning electron microscope and transmission electron microscope were used. Experimental results indicate that the prior cyclic loading degrades the creep strength significantly. However, the degradation tends to be saturated with further increase in prior cyclic loading. From the view of microstructural evolution, the recovery of martensite laths takes place during prior cyclic loading exposure. This facilitates the dislocation movement during the following creep process. Therefore, premature rupture of creep test occurs. Additionally, saturated behavior of degradation can be attributed to the near completed recovery of martensite laths. Based on the effect of prior cyclic loading, a newly modified Hayhurst creep damage model was proposed to consider the prior cyclic loading damage. The main advantage of the proposed model lies in its ability to directly predict creep behavior with different levels of prior cyclic loading damage. Comparison of the predicted and experimental results shows that the proposed model can give a reasonable prediction for creep behavior of P92 steel with different level of prior cyclic loading damage.

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    Microstructure and abrasive wear resistance of an alloyed ductile iron subjected to deep cryogenic and austempering treatments
    Cui Junjun, Chen Liqing
    J. Mater. Sci. Technol., 2017, 33 (12): 1549-1554.  DOI: 10.1016/j.jmst.2017.08.003
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    To further improve the mechanical performance of a new alloyed austempered ductile iron (ADI), deep cryogenic treatment (DCT) has been adopted to investigate the effect of DCT time on the microstructure and mechanical behaviors of the alloyed ADI Fe-3.55C-1.97Si-3.79Ni-0.71Cu-0.92Mo-0.64Cr-0.36Mn-0.30 V (in wt.%). With increasing the DCT time, more austenite transformed to martensite and very fine carbides precipitated in martensite in the extended period of DCT. The amount of austenite decreased in alloyed ductile irons, while that of martensite and carbide precipitation increased. The alloyed ADI after DCT for 6 h had the highest hardness and compressive strength, which can be attributed to the formation of more plate-like martensite and the finely precipitated carbides. There was a gradual decrease in hardness and compressive strength with increasing the DCT time to 12 h because of the dissolution of M3C carbide. After tempering, there was a decrease in mechanical properties compared to the direct DCT sample, which was caused by the occurrence of Ostwald ripening of precipitated carbides. The optimum wear resistance was achieved for the alloyed ADI after DCT for 6 h. The wear mechanism of the alloyed ADI in associating with DCT is mainly consisted of micro-cutting wear and some plastic deformation wear.

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    Effect of Cr on mechanical properties and corrosion behaviors of Fe-Mn-C-Al-Cr-N TWIP steels
    Yuan Xiaoyun, Zhao Yang, Li Xing, Chen Liqing
    J. Mater. Sci. Technol., 2017, 33 (12): 1555-1560.  DOI: 10.1016/j.jmst.2017.08.004
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    By using scanning electron microscopy (SEM) equipped with electron back-scattered diffraction (EBSD) system, transmission electron microscopy (TEM) and CorrTest4 electrochemical workstation, effects of chromium content (1.35wt% - 3.95 wt%) on the mechanical properties and anti-corrosion behaviours of high manganese Fe-Mn-C-Al-Cr-N twinning-induced plasticity (TWIP) steels were studied. The results show that Cr content has an obvious influence on the mechanical properties and fracture behaviors of the high manganese TWIP steels. The yield and ultimate tensile strengths of the steel sheets were improved with increasing Cr content while the elongation was reduced. In addition, with the increase of Cr content, the fracture mode changed from ductile fracture pattern with coarse dimples and tear ridges (Cr content ≤ 2.35%) to intergranular fracture (when Cr content is 3.95%). Furthermore, Cr content has a tremendous effect on anti-corrosion behaviors of the high manganese TWIP steels. The increase of Cr content enhanced the corrosion resistance of the annealed steel sheets by improving the proportion of low-angle boundary.

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    Microstructure and properties in dissimilar/similar weld joints between DP780 and DP980 steels processed by fiber laser welding
    Di Hongshuang, Sun Qian, Wang Xiaonan, Li Jianping
    J. Mater. Sci. Technol., 2017, 33 (12): 1561-1571.  DOI: 10.1016/j.jmst.2017.09.001
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    The microstructure and mechanical properties (strength, fatigue and formability) of dissimilar/similar weld joints between DP780 and DP980 steels were studied. The microstructure in fusion zone (FZ) was lath martensite (LM), and alloying elements in the FZ were uniformly distributed. The hardness in the FZ of dissimilar weld joint was similar to the average value (375 HV) of the two similar weld joints. The microstructural evolution in heat affected zone (HAZ) of dissimilar/similar weld joints was as follows: LM (coarse-grained HAZ) →finer LM (fine-grained HAZ) →M-A constituent and ferrite (intercritically HAZ) →tempered martensite (TM) and ferrite (sub-critical HAZ). Lower hardness in intercritically HAZ and sub-critical HAZ (softening zones) was observed compared to base metal (BM) in dissimilar/similar weld joints. The size of softening zone was 0.2-0.3 mm and reduction in hardness was ~7.6%-12.7% of BM in all the weld joints, which did not influence the tensile properties of weld joints such that fracture location was in BM. Formability of dissimilar weld joints was inferior compared to similar weld joints because of the softening zone, non-uniform microstructure and hardness on the two sides of FZ. The effect of microstructure on fatigue life was not influenced due to the presence of welding concavity.

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    Grain refinement of non-magnetic austenitic steels during asymmetrical hot rolling process
    Li Changsheng, Ma Biao, Song Yanlei, Zheng Jianjun, Wang Jikai
    J. Mater. Sci. Technol., 2017, 33 (12): 1572-1576.  DOI: 10.1016/j.jmst.2017.06.002
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    Asymmetrical hot rolling (ASHR) was proposed to acquire productive grain refinement for Fe-20Mn-4Al-0.3C and Fe-18Cr-18Mn-0.5N non-magnetic austenitic steels. The intensive additional shear deformation caused by ASHR promotes the nucleation of recrystallization and grain refining of steel plates. With the speed ratio of 1.2, the austenitic grains were refined to ~5 μm on the surface, the recrystallization fraction was enhanced to ~34.7%, and the thickness of fine-grained surface layer increases to ~450 μm for Fe-20Mn-4Al-0.3C steel. The Fe-18Cr-18Mn-0.5N steel also exhibited an effective surface grain refinement with an average size of ~3 μm, and the recrystallization fraction reached ~76.9% at the speed ratio of 1.15.

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    Charting the ‘composition-strength’ space for novel austenitic, martensitic and ferritic creep resistant steels
    Lu Qi, der Zwaag Sybrandvan, Xu Wei
    J. Mater. Sci. Technol., 2017, 33 (12): 1577-1581.  DOI: 10.1016/j.jmst.2017.05.004
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    We report results of a large computational 'alloy by design' study, in which the ‘chemical composition-mechanical strength’ space is explored for austenitic, ferritic and martensitic creep resistant steels. The approach used allows simultaneously optimization of alloy composition and processing parameters based on the integration of thermodynamic, thermo-kinetics and a genetic algorithm optimization route. The nature of the optimisation depends on both the intended matrix (ferritic, martensitic or austenitic) and the desired precipitation family. The models are validated by analysing reported strengths of existing steels. All newly designed alloys are predicted to outperform existing high end reference grades.

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    Fracture mode identification of low alloy steels and cast irons by electron back-scattered diffraction misorientation analysis
    Rui Shao-Shi, Shang Yi-Bo, Qiu Wenhui, Niu Li-Sha, Shi Hui-Ji, Matsumoto Shunsaku, Chuman Yasuharu
    J. Mater. Sci. Technol., 2017, 33 (12): 1582-1595.  DOI: 10.1016/j.jmst.2017.03.020
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    The fracture modes of low alloy steels and cast irons under tensile and fatigue conditions were identified by electron back-scattered diffraction (EBSD) misorientation analysis in this research. The curves of grain reference orientation deviation (GROD) distribution perpendicular to the fracture surface were obtained by EBSD observation, and the characteristics of each fracture mode were identified. The GROD value of the specimen fractured in tension decreases to a constant related to the elongation of corresponding specimen in the far field (farther than 5 mm away from the fracture surface). The peak exhibits in GROD curves of two smooth specimens and a notched specimen near the fracture surface (within 5 mm away from the fracture surface), and the formation mechanisms were discussed in detail based on the influences of specimen geometries (smooth or notched) and material toughness. The GROD value of fatigue fractured specimen is close to that at undeformed condition in the whole field, except the small area near the crack path. The loading conditions (constant stress amplitude loading or constant stress intensity factor range △K loading) and the EBSD striation formation during fatigue crack propagation were also studied by EBSD observation parallel to the crack path.

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    Microbiologically influenced corrosion behavior of S32654 super austenitic stainless steel in the presence of marine Pseudomonas aeruginosa biofilm
    Li Huabing, Yang Chuntian, Zhou Enze, Yang Chunguang, Feng Hao, Jiang Zhouhua, Xu Dake, Gu Tingyue, Yang Ke
    J. Mater. Sci. Technol., 2017, 33 (12): 1596-1603.  DOI: 10.1016/j.jmst.2017.03.002
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    S32654 super austenitic stainless steel (SASS) is widely used in highly corrosive environments. However, its microbiologically influenced corrosion (MIC) behavior has not been reported yet. In this study, the corrosion behavior of S32654 SASS caused by a corrosive marine bacterium Pseudomonas aeruginosa was investigated using electrochemical measurements and surface analysis techniques. It was found that P. aeruginosa biofilm accelerated the corrosion rate of S325654 SASS, which was demonstrated by a negative shift of the open circuit potential (EOCP), a decrease of polarization resistance and an increase of corrosion current density in the culture medium. The largest pit depth of the coupons exposed in the P. aeruginosa broth for 14 days was 2.83 μm, much deeper than that of the control (1.33 μm) in the abiotic culture medium. It was likely that the P. aeruginosa biofilm catalyzed the formation of CrO3, which was detrimental to the passive film, resulting in MIC pitting corrosion.

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    In vitro study on infectious ureteral encrustation resistance of Cu-bearing stainless steel
    Zhao Jing, Ren Ling, Zhang Bingchun, Cao Zhiqiang, Yang Ke
    J. Mater. Sci. Technol., 2017, 33 (12): 1604-1609.  DOI: 10.1016/j.jmst.2017.03.025
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    Cu-bearing stainless steel has been found to have obvious inhibition performance against encrustation in vitro. This study was aiming to further investigate the inhibitory effect of a Cu-bearing stainless steel (316L-Cu SS) on the infectious encrustation based on its antimicrobial activity. The encrustation in presence of bacteria, antibacterial performance, urease production and Ca and Mg precipitation were examined by scanning electron microscopy, antibacterial assay, enzyme-linked immunosorbent assay and inductively coupled plasma-mass spectrometry, respectively. It was found that 316L-Cu SS could inhibit the formation of bacterial biofilm due to the release of Cu2+ ions and then decrease the urease amount splitting by bacteria, which produced a neutral environment with pH around 7. However, more encrustations coupled with bacterial biofilms on the surface of comparison stainless steel (316L SS) with an alkaline environment were recorded. It can thus be seen that the 316L-Cu SS highlights prominent superiority against encrustation in the presence of microorganisms.

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    Microstructure characterization and HCF fracture mode transition for modified 9Cr-1Mo dissimilarly welded joint at different elevated temperatures
    Shao Chendong, Lu Fenggui, Wang Xiongfei, Ding Yuming, Li Zhuguo
    J. Mater. Sci. Technol., 2017, 33 (12): 1610-1620.  DOI: 10.1016/j.jmst.2016.12.001
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    The high cycle fatigue (HCF) tests of modified 9Cr-1Mo dissimilarly welded joint were carried out at different elevated temperatures and the fracture mechanism was systematically revealed. The fatigue strength at 108 cycles based on S-N curve can be estimated as a half of weld joint’s yield strength for all conducted temperatures, which can be a reliable criterion in predicting the fatigue life. The results show that the inter-critical heat affected zones (IC-HAZs) of both sides are the weak zones due to their low hardness and inferior fatigue resistance property. HAZ of COST-FB2 (BM2) is the weakest zone at room temperature due to the existence of numerously distributed defects and the initiation of cracks, either in the surface or interior zone, impacting a crucial effect on the fatigue life of the joint. While at elevated temperatures, fatigue life was controlled mostly by the intrusion-extrusion mechanism at the specimen surface under high stress level and subsurface non-defect fatigue crack origin (SNDFCO) from the interior material under low stress amplitude. With increasing temperature, more and more fatigue failures began to occur at the HAZ of COST-E (BM1) due to its higher susceptibility of temperature. Besides, it is found that the δ-ferrite in the BM1 has no harm to the HCF behavior of the joint at the conducted temperatures.

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    Dependence of corrosion resistance on grain boundary characteristics in a high nitrogen CrMn austenitic stainless steel
    Qi Jianjun, Huang Boyuan, Wang Zhenhua, Ding Hui, Xi Junliang, Fu Wantang
    J. Mater. Sci. Technol., 2017, 33 (12): 1621-1628.  DOI: 10.1016/j.jmst.2017.09.016
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    Processing schedules for grain boundary engineering involving different types of cold deformation (tension, compression, and rolling) and annealing were designed and carried out for 18Mn18Cr0.6N high nitrogen austenitic stainless steel. The grain boundary characteristic distribution was obtained and characterized by electron backscatter diffraction (EBSD) analysis. The corrosion resistance of the specimens with different grain boundary characteristic distribution was examined by using potentiodynamic polarization test. The corrosion behavior of different types of boundaries after sensitization was also studied. The fraction of low-Σ boundaries decreased with increasing strain, and it was insensitive to the type of cold deformation when the engineering strain was lower than 20%. At the strain of 30%, the largest and smallest fractions of low-Σ boundaries were achieved in cold-tensioned and rolled specimens, respectively. The fraction of low-Σ boundaries increased exponentially with the increase of grain size. The proportion of low-angle grain boundaries increased with decreasing grain size. Increasing the fraction of low-Σ boundaries could improve the pitting corrosion resistance for the steels with the same grain size. After sensitization, the relative corrosion resistances of low-angle grain boundaries, Σ3 boundaries, and Σ9 boundaries were 100%, 95%, and 25%, respectively, while Σ27 boundaries, other low-Σ boundaries and random high-angle grain boundaries had no resistance to corrosion.

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ISSN: 1005-0302
CN: 21-1315/TG
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