20 February 2021, Volume 45 Issue 2
    

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  • Whether Nanograins Formed in Surface Mechanical Attrition Treatment
    HE Jiawen
    Materials For Mechanical Engineering. 2021, 45(2): 1-6. https://doi.org/10.11973/jxgccl202102001
    Abstract ( ) Download PDF ( )   Knowledge map   Save
    Shot peening or surface rolling holds the same nature of wear in terms of microstructure refining. In the 1970s and 1980s, systematic studies of surface deformed microstructure were carried out from the perspective of tribology, and the conclusions obtained have become important achievement addressed in the textbook of materials science. However, following the fashion of nano tide at the beginning of the 21st century, the surface mechanical attrition treatment (SMAT) group neglected the contributions of the former generation and claimed that the deform-refined structure was nanograins. The early year study shows that the size of the deformed structure has little influence on the performance and the key factor is misorientation angles. But the SMAT group insists the finer nanosize the better. The tribological study indicates that to meet the characteristics of nanograin, a critical surface rotation to a certain value is required; but this kind of rotation will destroy the continuity and cause the surface material to peel off. As the SMAT group recognized the misorientation angle dominating the mechanical behavior and manipulated excessive rolling to realize the high angle nanograins, the surface was covered with cracks. The nature and characteristics of the deformed structure are clarified. It is pointed out that the dynamic recrystallization of nanograins initiated at the cell walls is specious, and the difference between cells and grains associated with Hall-Petch relation is described by the different resistant stress of cells and grains to dislocations.
  • Research Progress on Failure Mechanisms of TP347H Boiler Steel Tube
    ZHANG Jun, ZHENG Zhunbei, YANG Zhanjun, SUN Xingxin, LI Mengyang, ZHANG Jianwei
    Materials For Mechanical Engineering. 2021, 45(2): 7-14. https://doi.org/10.11973/jxgccl202102002
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    The austenitic heat-resistant steel TP347H is widely used in heating tubes of thermal power unit boilers. Combining the service performance of austenitic heat-resistant steel tubes and the failure cases of TP347H steel tube bursts, the six common failure mechanisms, including intergranular corrosion, stress corrosion cracking, microstructure aging, oxidative corrosion, original defects and martensite transformation, of TP347H steel heating tubes during boiler operation are reviewed. The failure characteristics, analysis methods and preventive measures are described.
  • Stress Corrosion Behavior of AISI4340 High Strength Steel in High Temperature Water Containing Oxygen and/or Cl-
    MA Tao, ZHANG Xiaofei, HUA Xiaochun, ZHAO Li, RAO Sixian
    Materials For Mechanical Engineering. 2021, 45(2): 15-19,60. https://doi.org/10.11973/jxgccl202102003
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    Slow tensile stress corrosion tests and stress corrosion crack growth tests were used to study the stress corrosion behavior of AISI4340 steel in water at 100 ℃ containing saturated oxygen and/or 0.1 mol·L-1 Cl-. The results show that the presence of oxygen or Cl- in water at 100 ℃ could increase the stress corrosion tendency of AISI4340 steel. The stress corrosion tendency in deoxygenated 100 ℃ water containing Cl- was not significant, and the slow tensile fracture retained some ductile fracture characteristics; complete brittle fracture of AISI4340 steel occurred in the high temperature water containing saturated oxygen, and the stress corrosion tendency was significant. Oxygen or Cl- could increase the stress corrosion crack growth rate of AISI4340 steel in water at 100 ℃. There was an interaction between oxygen and Cl-, so their co-existence significantly increased the stress corrosion tendency and caused rapid crack growth after cracking.
  • Effect of Slurry Impact Angle on Erosion Behavior of Cr30AHigh Chromium Cast Iron
    HE Jianjun, ZHOU Chengwei, YU Renqiang, LI Wei, REN Yanjie, BAO Jiangyong
    Materials For Mechanical Engineering. 2021, 45(2): 20-24. https://doi.org/10.11973/jxgccl202102004
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    In simulated wet desulfurization slurry flow environment, the effect of impact angle of NaCl solution+quartz sand acid slurry with sample impact surface on the erosion behavior of Cr30A high chromium cast iron was studied. The results show that the mass loss of the cast iron was relatively large under the interaction of corrosion and abrasion after the slurry impacted the cast iron at angles of 60° and 30°. The mass loss of the cast iron was relatively small after the slurry impacted the cast iron vertically, and the main cause of the mass loss was impact abrasion. The mass loss of the cast iron was the smallest after the slurry impacted cast iron parallelly, and the main cause of the mass loss was pitting. When the cast iron was subjected to the paralled impact of the slurry, its erosion mechanism was mainly micro-cutting. When subjected to the 30° angle impact of the slurry, the erosion mechanism was mainly cutting and plowing, and extrusion edges, impact pits and shallow flaky fatigue peeling existed on the cast iron surface. When subjected to the 60° angle impact of the slurry, the erosion mechanism was mainly deep material layer peeling and severe corrosion abrasion. When subjected to the vertical impact of the slurry, the erosion mechanism was mainly the shedding of surface material, accompanied by slight corrosion abrasion.
  • Effect of Nb on Microstructure and Toughness of Heat Affected Zone of Shipbuilding Steel under High Heat Input Welding
    HAN Limei, LI Li, TIAN Meng, QU Jinbo
    Materials For Mechanical Engineering. 2021, 45(2): 25-30,101. https://doi.org/10.11973/jxgccl202102005
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    Two DH36 shipbuilding steels, containing 0.025wt% Nb and Nb-free, were produced by a controlled rolling and controlled cooling (TMCP) process, and then welded by 150 kJ·cm-1 high heat input gas electric vertical welding. The microstructure and toughness of heat-affected zones of welded joints were studied. The results show that the addition of Nb could delay the transformation of ferrite and pearlite, and promote the formation of granular bainite and bainite ferrite, leading to less grain boundary ferrite and more granular bainite and bainite ferrite in the coarse-grained zone, and the slow precipitation of ferrite and pearlite in the fine-grained zone of the heat-affected zone of the Nb-bearing steel. The microstructure and toughness in different locations of the welded joints of Nb-free steel were good, and every single value of impact energy at -20 ℃ was above 102 J, which was far higher than the requirements of classification society. The Nb-bearing steel welded joint had single values of impact energy at -20 ℃ lower than 24 J, which did not meet the requirements of classification society, but every single value of impact energy at -20 ℃ in other locations was above 143 J. The addition of Nb was detrimental to the toughness of the welded joints of DH36 shipbuilding steel with high heat inputs.
  • Continuous Cooling Transformation Behaviour of 20Cr1Mo1VTiB Steel
    LI Xingdong, LI Yan, AI Di, LI Yufeng, LIU Ruiliang
    Materials For Mechanical Engineering. 2021, 45(2): 31-36,89. https://doi.org/10.11973/jxgccl202102006
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    The bainite transformation point at different quenching temperatures (950-1 100 ℃) and the continuous cooling phase transformation point at the optimal quenching temperature of 20Cr1Mo1VTiB steel were measured on a thermal dilatometer. Then the continuous cooling transformation curves were drawn by combination of the phase transformation points, microstructure observation and hardness test. The relationship between the phase transformation point or transformation amount and the cooling rate was established with empirical equations, and the phase transition activation energy was calculated. The results show that the bainite transformation temperature of the test steel decreased with increasing quenching temperature; the preferred quenching temperature was 1 050 ℃. The supercooled austenite transformation products were proeutectoid ferrite, pearlite and bainite with cooling rates not higher than 0.5 ℃·s-1, and were single bainite with cooling rates higher than 0.5 ℃·s-1. The fitting curves of phase transformation point- and phase transformation amount-cooling rate were in good agreement with the test results. The activation energy of proeutectoid ferrite and bainite transition was 744.8, 274.9 kJ·mol-1, respectively.
  • Comparison of Sulfide Morphology and Mechanical Properties of As-cast andForged Titanium-Sulfur Containing Free-Cutting Steel
    WANG Yinghu, SONG Lingxi
    Materials For Mechanical Engineering. 2021, 45(2): 37-42,48. https://doi.org/10.11973/jxgccl202102007
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    Two titanium-sulfur containing free-cutting steels with titanium content (0.09wt% and 0.21wt%) were smelted and forged at 1 200 ℃. The morphology, size and quantity of sulfides in the structure and the mechanical properties of the as-cast and forged test steels were compared and studied. The results show that in the as-cast test steel, most of the manganese sulfides were short-rod-like and spherical, and distributed in chains or nets along the grain boundaries. After forging, the manganese sulfides extended along the forging direction, the statistically obtained length-width ratio increased, and the quantity per unit area decreased. The increase of titanicum content increased the quantity per unit area of manganese sulfides. The tensile fracture surface of the as-cast test steel showed cleavage terrace and river pattern, and the fracture mode was brittle fracture. The tensile fracture after forging showed cleavage and dimple mixed morphology, and the fracture mode was ductile fracture. The tensile properties and the impact toughness of the forged test steel were better than those of the as-cast steel, indicating forging was helpful to improve the mechanical properties of the titanium-sulfur containing free-cutting steel.
  • Damage Behavior of U71Mn Steel Rail Gas Pressure Welded Joint
    JIANG Wenjuan, WANG Wenjian, DING Haohao, GUO Jun, LIU Qiyue
    Materials For Mechanical Engineering. 2021, 45(2): 43-48. https://doi.org/10.11973/jxgccl202102008
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    The hardness, crack morphology and microstructure of U71Mn steel rail gas pressure welded joint after service were tested and observed by hardness tester, profilometer, optical microscope and scanning electron microscope. The damage behavior in different regions of the joint was analyzed. The results show that the depressions were located at 30-50 mm away from the joint center on both sides, and their positions were consistent with the softening zones in the joint. The hardness of the joint increased because of wheel-rail contact, but the hardness of the softening zone was always lower than that of the base metal. According to morphology of pearlite, the joint was divided into four regions from the joint center to heat-affected zone on both sides: lamellar pearlite area, partial pearlite spheroidization area I, pearlite spheroidization area and partial pearlite spheroidization area Ⅱ. The softening zone was located in pearlite spheroidization area. The depths of cracks and the expansion angles of crack tips in pearlite spheroidization and partial pearlite spheroidization areas were relatively large, because of the relative small resistance of granular pearlite to crack growth.
  • Electrochromic Properties of Reticular Tungsten Trioxide/PolythiopheneNanowire Hybrid Structure Film
    LI Guyue, ZHENG Tianxiang, JIANG Da, XU Jinfeng, SUN Mingjun, SHI Yingdi, ZHANG Yong
    Materials For Mechanical Engineering. 2021, 45(2): 49-54,95. https://doi.org/10.11973/jxgccl202102009
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    The hydrothermal parameters were optimized by orthogonal tests to prepare WO3 nanowires with sodium tungstate as tungsten source, hydrochloric acid as regulator, ammonium sulfate as auxiliary agent, and then the reticular WO3/poly 3,4-ethylenedioxythiophene (PEDOT) nanowire hybrid structure film was prepared by electrodeposition method. The structure and electrochromic properties of the film were studied. The results show that the electrochromic performance of WO3 was optimal under hydrothermal reaction temperature of 190 ℃, time of 4 h and pH of 2, and the contrast was 80.6%. The WO3/PEDOT film with hybrid structure of score-shell nanowire was successfully prepared by electrochemical deposition under optimal hydrothermal conditions, and the outer side of the WO3 monocrystalline nucleus was covered by an amorphous thin shell. Compared with WO3 nanowire and PEDOT, WO3/PEDOT film showed excellent electrochromic and optical modulation properties, fast response speed and good cycling stability. The contrast of the film was 89.1% wth the coloring time of 3.4 s and the fading time of 2.2 s, and after 1 000 cycles, the contrast was retained 98.8%.
  • Formation Mechanism of Irregular Carbides in Raceway ofGCr15 Steel Bearing Inner Ring
    HU Runchuan, LAI Chengban, MIN Yongan
    Materials For Mechanical Engineering. 2021, 45(2): 55-60. https://doi.org/10.11973/jxgccl202102010
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    The carbide morphology in the GCr15 steel bearing inner ring structure after forging, spheroidizing annealing, and quenching and tempering was observed. Combining with thermal expansion tests and thermodynamic analysis, the dissolution, precipitation and growth behavior of secondary carbides were studied. The forming reasons for large irregular carbides with sharp corners were analyzed. The results show that the secondary carbides precipitated in the alloy element segregation zone of the raceway surface of GCr15 steel bearing inner ring during rolling and forming at a lower temperature were broken, resulting in the appearance of large irregular carbides with sharp corners in the raceway surface structure. During the spheroidizing annealing process, the broken irregular granular carbides on the raceway surface of the bearing inner ring grew up and became round, and these large carbides partially dissolved into the austenite matrix during quenching heating, resulting in the reappearance of edges and corners. The conditions for the appearance of irregular carbides on the raceway surface of bearing inner ring included the presence of a serious alloy element segregation zone on the raceway surface, and the large secondary carbides precipitated from the segregation zone.
  • Effect of Service Time on Microstructure and Mechanical Properties of High Temperature 1Cr5Mo Steel Fastening Nut
    ZHANG Tengyu, HAN Tao, WU Shuquan, JIANG Feng, ZHANG Zhibo, JIANG Shikai
    Materials For Mechanical Engineering. 2021, 45(2): 61-65. https://doi.org/10.11973/jxgccl202102011
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    The hardness of the 1Cr5Mo steel fastening nut of a supercritical unit steam turbine in a thermal power plant was reduced significantly after service. Aiming at this phenomenon, the aging tests (equivalent to service) of the 1Cr5Mo steel nut were carried out at 566 ℃. The effect of aging time on the microstructure and mechanical properties was studied, and the microstructure and performance were compared with those of the nut served at 566 ℃ for 105 h. The results show that at temperatures not higher than 566 ℃, the microstructure of the 1Cr5Mo steel nut changed slowly; the microstructure was still martensite and carbide after aging for 2 880 h. But the microstructure changed to tempered sorbite after service for 105 h. With the extension of the aging/service time, the carbides migrated and concentrated from the inside grain to the grain boundary, and became obviously coarsened. The deterioration of the structure led to the reduction of the hardness, strength, and impact energy of the nut.
  • Corrosion Resistance of New BGNDMA Low-Alloy Steel
    ZHANG Qinglian, LIU Cheng, GUO Jinbao
    Materials For Mechanical Engineering. 2021, 45(2): 66-70,77. https://doi.org/10.11973/jxgccl202102012
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    The microstructure, tensile properties and corrosion resistance of a new BGNDMA low-alloy steel were studied and compared with the properties of 09CrCuSb steel and 2205 duplex stainless steel. The results show that the microstructure of the BGNDMA steel was ferrite and pearlite, and the tensile properties were better than those of 09CrCuSb steel. When immersed in 70 ℃, 50wt% H2SO4 solution for 24 h, the corrosion rates of most samples were not higher than 80 g·m-2·h-1, and the statistical average value was about 36.367 g·m-2·h-1, indicating the excellent resistance to sulfuric acid corrosion. In the dead green liquor and the industrial flue gas dew point (not higher than 80 ℃) corrosion environment, the BGNDMA steel underwent uniform corrosion, and 2205 duplex stainless steel underwent local corrosion such as pitting corrosion and crevice corrosion; BGNDMA steel was expected to replace 2205 duplex stainless steel in H2SO4-HCl mixed acid dew point corrosion environment. In an industrial flue gas dew point corrosion environment, the corrosion resistance of the hot rolled BGNDMA steel was better than that of the normalized steel.
  • Hot Deformation Behavior and Hot Processing Maps of 4Cr5MoSiV1 HotWorking Die Steel
    QIU Yu, YUAN Fei, ZENG Yuansong, MENG Qiang, LUO Rui, DONG Jihong, ZHAO Huaxia
    Materials For Mechanical Engineering. 2021, 45(2): 71-77. https://doi.org/10.11973/jxgccl202102013
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    Single-pass isothermal compression tests were carried out on 4Cr5MoSiV1 hot working die steel by Gleeble-3500 thermal simulator. The hot deformation behavior of the steel at deformation temperatures of 750-1 050 ℃ and strain rates of 0.001-0.1 s-1 was studied, and the microstructures after deformation were observed. On the basis of true stress-true strain curves obtained from the tests, the Arrhenius high-temperature constitutive model at true strain of 0.3 was established. The hot processing maps were drawn by the dynamic material model to obtain the reasonable hot processing window of the steel. The results show that the deformation resistance of 4Cr5MoSiV1 steel decreased significantly with increasing deformation temperature or decreasing strain rate. The hot deformation activation energy of 4Cr5MoSiV1 steel was 594.52 kJ·mol-1. Within the range of test parameters, the reasonable hot processing window of 4Cr5MoSiV1 steel was at a deformation temperature of 1 050 ℃ and strain rates of 0.001-0.01 s-1; at this time the carbide was fine and dispersed, and the strengthening effect of the second phase was significant.
  • Finite Element Simulation of Temperature Rise and Strain of AISI1015 SteelWorkpiece During Mini Mill Rolling
    XU Leqian, GAO Yubo, MENG Xiaoling
    Materials For Mechanical Engineering. 2021, 45(2): 78-84. https://doi.org/10.11973/jxgccl202102014
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    A mini mill rolling model was established with Deform software and then was used to simulate temperature rise and strains of the AISI1015 steel workpiece with different specifications (φ8 mm, φ10 mm, φ13 mm) and different initial temperatures (800, 850, 900, 950 ℃) during the rolling process. The reason for the difference in rolling temperature rise was analyzed. The results show that the relative errors were within 13% between the mini mill rolling model simulation and the test results of the surface temperature of the workpiece when exiting the rolling mill. During the rolling process, the temperature rise and accumulated equivalent strains in the core of the workpiece were the largest, and the smaller the rolling specification, the greater the temperature rise and strain in the core. The lower the initial temperature of the workpiece, the greater the temperature rise and the torque. The uneven temperature rise of the workpiece with different specifications was mainly caused by the uneven distribution of equivalent strains; the obvious difference of the temperature rise in the core at different initial temperatures was caused by different rolling loads.
  • Finite Element Simulation of Molding Residual Stress of T300/AG80 Composite U-shaped Structure Part
    YANG Kang, YAN Zhaowei, LIANG Yu, WANG Ji, DING Wenxi
    Materials For Mechanical Engineering. 2021, 45(2): 85-89. https://doi.org/10.11973/jxgccl202102015
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    A U-shaped thin-walled structure part model of T300 carbon fiber reinforced AG80 epoxy resin (T300/AG80) composite was established by ABAQUS finite element software. The stress and springback amount (synchronous demolding by simulation) changes during hot-press curing molding and the residual stress distribution after demolding were studied. The simulation of residual stress was verified by the small-hole test method. The results show that during the molding process, the stress and springback amount of the side wall and bottom surface of the U-shaped structure part increased with time. The stress and springback amount increased with the distance from the bottom surface or the distance from the center of symmetry. After demolding, the residual stress at the center of symmetry of the U-shaped structure part was the smallest, and the residual stress increased with the distance from the bottom surface or the distance from the center of symmetry. The greater the residual stress released before and after demolding, the greater the springback amount. The relative error between the residual stress measured by the small-hole method and the simulation was less than 10%, indicating the simulation was accurate.
  • Reasons for Cracking of Nozzle Socket of Hot Section Drain Pipe of a660 MW Ultra-supercritical Unit
    LIU Ming, LIU Qi, BAI Jia, CHEN Rui
    Materials For Mechanical Engineering. 2021, 45(2): 90-95. https://doi.org/10.11973/jxgccl202102016
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    After short operation of the 660 MW ultra-supercritical unit of a thermal power plant, the nozzle socket of the drain pipe cracked in the hot section, causing steam leakage. The chemical composition, hardness and microstructure of the nozzle socket were analyzed. A three-dimensional solid model was established according to the actual layout of the pipe, and the force of the structure was simulated and analyzed by the thermo-solid coupling finite element method. The results show that the chemical composition, hardness, and microstructure of the nozzle socket met the requirements. The maximum stress obtained by the finite element simulation was located at the lower edge of the connection weld between the nozzle socket and the main pipe, which was consistent with the actual cracking position. The cracking was due to the limited ability of the drain pipe to absorb deformation, and the higher tensile stresses in the nozzle socket area induced by the blocked thermal expansion of the pipe because of the unreasonable pipe layout. After adding a U-shaped expansion bend to enhance the flexibility of the pipe and reduce the structural stress, the nozzle socket of the drain pipe did not crack during operation for about 1 a.
  • Causes of Surface Imprints of Laser Welded DP780 High-Aluminum Dual-Phase Strip Steel after Cold Rolling
    LIU Ying, ZHOU Chaogang, WANG Yunhui, WANG Shuhuan, AI Liqun, WANG Hailong
    Materials For Mechanical Engineering. 2021, 45(2): 96-101. https://doi.org/10.11973/jxgccl202102017
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    Imprints appeared on the surface of DP780 high-aluminum (aluminum mass fraction of 0.55%-0.70%) dual-phase strip steel by hot rolling, head-to-tail laser welding, pickling and cold rolling. Causes of the imprints were studied by the microstructure, chemical composition and hardness analysis. The results show that during welding of DP780 dual-phase steel, the aluminum element in the weld metal was oxidized and alumina formed. The alumina was not effectively removed by pickling. During cold rolling, the hard alumina caused damage of the work roll, leading to the imprints on the surface of the strip steel. The surface imprint of the strip steel was completely eliminated by increasing the welding speed, the concentration of acid solution and the pickling bath temperature.
  • Reason for Leakage of a Heater Spiral Tube
    JIANG Qiaohong, BA Fahai
    Materials For Mechanical Engineering. 2021, 45(2): 102-105,110. https://doi.org/10.11973/jxgccl202102018
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    The spiral tube of a heater leaked in operation. The cause of the leakage was analyzed by macromorphology and microstructure observation, chemical composition analysis, scanning electron microscopy, energy disperse spectroscopy, and so on. The results show that creep cracking occurred in the spiral tube, and the cracking form was brittle intergranular cracking. Because of the low working temperature, flake carbides precipitated on grain boundaries of the welding heat-affected zone of the spiral tube, causing the formation of chromium-depleted zones nearby. Under the bending stress, vibration stress and welding residual stress, many cavitation defects formed around the carbides, and the cavitation expanded and grew, forming cracks and finally leading to leakage.
  • Cause of Surface Cracks of Water-Wall Tube in Soot Blower Cover of a Supercritical Boiler
    ZHANG Zhibo, FAN Zhidong, ZHANG Chunxiang, NIU Kun, MA Yichao, LIU Chengxin
    Materials For Mechanical Engineering. 2021, 45(2): 106-110. https://doi.org/10.11973/jxgccl202102019
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    Dense transverse cracks were found on the backfire surface of the water-wall tube in the soot blower cover after the supercritical boiler ran for 1.2×104 h. By data investigation, material composition inspection, and analysis of microstructure, fracture morphology and tensile properties, the causes of surface cracks of the water-wall tube were investigated. The results show that the material of the water-wall tube was normal, and the cracks on the tube outer wall of the backfire surface were thermal fatigue cracks. The outer diameter of the soot blower cover was too large, making the cover being topped on the water-wall tube around the soot blowing hole. Meanwhile, the temperature was low, making the soot blowing steam carrying water. The water dropped along the gap between the water-wall tube and the soot blowing sleeve to the backfire surface of the water-wall tube, causing the bearing of long-term heat and cold alternating stresses, and finally thermal fatigue cracking occurred.
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