Effect of composite addition of trace amounts of Mn, Zn and Ti on the microstructure and mechanical properties of Al-Cu-Li-Mg-Ag-Zr alloy was studied by using tensile testing machine, metallographic microscope, scanning electron microscope and transmission electron microscope. The results show that the composite addition of trace amounts of Mn, Zn and Ti elements can significantly refine the grain structure of the as-cast alloy, and at the same time, the insoluble second phase Al(CuMnFe) in the solution treated alloy is also refined. The composite addition of trace amounts of Mn, Zn and Ti elements can effectively promote the precipitation of the main strengthening phase T₁ in the T6 and T8 peak aged Al-Cu-Li-Mg-Ag-Zr alloy, improving the mechanical properties of the alloy. Compared with the T6 peak aged, the amount of T₁ phase precipitated in the T8 peak aged Al-Cu-Li-Mg-Ag-Zr alloy significantly increases, the size significantly decreases, the distribution is more uniform and dispersed, and the strength and plasticity are significantly improved.

Hot deformation behavior of 2Ni6Cr5Al2MoV bearing steel under deformation temperature of 900-1200 ℃ and strain rate of 0.01-10 s^-1 was studied by Gleeble-3800 simulation machine. The high temperature constitutive equation and recrystallization critical strain model were established and verified. Based on the theory of dynamic material model, the hot processing maps of the steel at true strain of 0.6 and 0.7 were drawn. The results show that the flow stress of the steel increases with the increase of strain rate and the decrease of deformation temperature. Increasing deformation temperature and decreasing strain rate are beneficial to the dynamic recrystallization. The maximum energy dissipation rate η of the steel is about 0.37, and the strain variable has little influence on the dissipation factor. The suitable hot working conditions for the steel are deformation temperature of 1100-1200 ℃ and strain rate of 0.01-0.1 s^-1.

Thermal and dynamic calculations were performed on 0.2C-5.0Mn-0.5Si-1.0Al medium-Mn steel, and combined with the analysis of the oxidation behavior of the tested steel annealed at 790 ℃ in a 5% H₂-N₂ atmosphere, the selective oxidation behavior of alloy elements Mn, Al and Si was studied. By thermodynamic calculations, the order of precipitation of the oxides in the tested steel is Al₂O₃>MnAl₂O₄>SiO₂>MnSiO₃, Mn₂SiO₄ or MnO, and the composition of the oxides at 790 ℃ and a dew point of -30 ℃(P_(O2)=9.83×10^-23 atm) is determined to be MnO+MnAl₂O₄+Mn₂SiO₄. The products of oxide layer of the tested steel obtained after annealing are in agreement with the thermodynamic calculations. The outer oxide layer primarily consists of a continuously distributed MnO layer, accompanied by Al and Si oxides which undergo selective oxidation first, distributed outside the MnO layer. The inner oxide layer contains Mn-Al-Si-O elements, which corresponds to the thermodynamic calculation results of MnO+MnAl₂O₄+Mn₂SiO₄. The Wagner model is employed to predict the internal oxidation behavior of Al and Si elements by utilizing the diffusion data in ferrite matrix, which provides theoretical substantiation for the control of selective oxidation on the surface of high-strength steel plates.

Microstructure, microhardness, friction coefficient, wear rate and worn surface morphology of solution treated Mg-xGd-1Zn-0.4Zr (x=3, 6, 9 wt%) alloys with different Gd contents and solution treated Mg-6Gd-yZn-0.4Zr (y=0.5, 1, 2 wt%) alloys with various Zn additions were studied by means of optical microscope, scanning electron microscope, energy spectrometer, microhardness tester and friction and wear testing machine. The results show that the grains refine, the microhardness increases, and the friction coefficient shows slight change with Gd or Zn content increasing. The wear rate of the alloys increases slightly with the increase of Gd content, whereas, it decreases with the increase of Zn content. Compared with the alloy containing 0.5%Zn, the wear rate of the alloy containing 2%Zn presents 24% reduction. The main wear mechanisms of the five alloys are abrasion, oxidation and plastic deformation, as well as adhesion for the alloy containing 9%Gd. The wear resistance of the alloys depends on the wear mechanisms and has no direct relation to the hardness.

Selecting Al_11.5Cr₁₆Fe₁₇Ni_51.5V₄ high-entropy alloy as the research object, the microstructure and tensile properties of the alloy with single-phase solid solution treated by solution treatment were studied after aging at 600 ℃ for different time. The results show that the microstructure of the alloy is single-phase FCC solid solution after recrystallization annealing/solution treatment at 1150 ℃ for 7 min. After aging at 600 ℃, nano-sized spherical Ni-rich and Al-rich γ′-Ni₃Al precipitates with L1₂ structure are formed. With the increase of aging time, the size and volume fraction of the precipitated γ′ phase increase monotonously, and the grain size increases slightly. The yield strength of the alloy increases from 306 MPa to 863 MPa, the tensile strength increases from 560 MPa to 1272 MPa, and the alloy maintains good toughness (uniform elongation of 34.1%-17.7%). During aging, the increase of strength is mainly attributed to the precipitation strengthening of the γ′ phase, where the mechanism is the dislocation cutting. The γ′ precipitation strengthening effect increases with the increase of size and volume fraction of the precipitated γ′ phase. The good toughness of the alloy depends on the coherent characteristics of the precipitated γ′ phase and the extremely small nano-size, which reduces the stress concentration at the precipitated phase/matrix interface.

Friction and wear performance of 75 kg/m pearlitic heavy-duty heat-treated rail steel was investigated by means of the ring-block butt-abrasion test at different test forces and cycle times, and compared with that of rail steel with RE, Nb and Ni elements adding. At the same time, the surface morphology, profile damage, microstructure evolution in the plastic deformation zone and elemental content changes of the specimens after wear were analyzed by using a combination of scanning electron microscope, transmission electron microscope and electron microprobe techniques. The results show that with the increase of test force, the wear loss increases, the wear form transitions from oxidizing wear to adhesive wear and fatigue wear, and the thickness of the white layer in the deformation zone increases. With the increase of friction wear cycle times, the grain size of the deformation zone is finer, and the thickness of the plastic deformation layer becomes larger. However, under the larger test force, the crack extension in the white layer intersects and causes the material to break, and the wear loss increases. The pearlitic lamellar spacing of the rail steel with adding RE, Nb and Ni elements is refined by 21.8% compared with that of the original state rail steel, and the smaller the pearlite lamellar spacing is, the smaller the thickness of the white layer is formed and is less prone to breaking, and thus it has better wear resistance.

Microstructure and precipitated phases of heat treated Q890 high strength steel were studied by using thermodynamic calculations and experimental equipment such as scanning electron microscope and transmission electron microscope. The results indicate that the microstructure of the quenched tested steel is lath martensite, a small amount of bainite, (Nb,Ti)C and Cu-rich nanoscale precipitated phase, the average size of precipitated phase is 42.09 nm, the volume fraction is 0.66%, and the precipitation strengthening component is 89.21 MPa. After low-temperature tempering at 200 ℃, the microstructure of the tested steel shows a little change, the precipitated phases grow slightly, and the precipitation strengthening component decreases. After high-temperature tempering at 600 ℃, the microstructure of the tested steel is tempered sorbite, a small amount of bainite and a large amount of Fe-rich M₂₃C₆ precipitate phases. The precipitated phase particle size significantly increases, with average size of 52.29 nm and volume fraction of 8.55%. The precipitation strengthening component is significantly increased to 271.08 MPa.

TC4 titanium alloy and atomic diffusion additive manufactured(ADAM) 17-4PH( stainless steel were diffusion bonded with different interlayers at different temperatures. The microstructure characteristics and properties of the joint interface were analyzed by means of optical microscope, scanning electron microscope, EDS analysis and microhardness testing. The results show that a well formed TC4 titanium alloy/ADAM 17-4PH stainless steel diffusion bonded joint is obtained by using a Cu foil+Ni foil composite interlayer at 960 ℃ and 920 ℃, with holding time of 60 min and welding pressure of 2 MPa. The interfacial area mainly includes four different zones, namely the diffusion affected zone on the TC4 titanium alloy side (DAZ 1), the interface reaction zone (IRZ 1 and IRZ 2) generated by the diffusion reaction of Cu foil+Ni foil and the diffusion affected zone on the ADAM 17-4PH stainless steel side (DAZ 2). When the diffusion bonding temperature is 960 ℃, CuTi+CuTi₂ eutectic phase and CuTi₂, Ti (Cu, Ni), α-Ti phases are generated in the IRZ 2, with the highest shearing strength of 163 MPa. With the diffusion bonding temperature increases from 920 ℃ to 960 ℃, the width of interface increases from 243.5 μm to 278.2 μm and the maximum microhardness of IRZ 2 decreases from 693 HV0.1 to 612 HV0.1 in the case of Cu foil+Ni foil as interlayers. When the diffusion bonding temperature remains constant (960 ℃), the peak microhardness of the diffusion bonded joint with Cu foil+Ni foil as interlayers is the highest, about 612 HV0.1, while that with Ni foil as interlayer is the lowest, approximately 495 HV0.1, in the IRZ.

Evolution of microstructure and properties of 2××× series aluminum alloy aluminum clad thin plate during solution treatment were studied by means of metallographic microscope, scanning electron microscope, energy dispersive spectroscopy, differential scanning calorimetry and room temperature tensile test. The results indicate that after solution treatment, there are still large-sized AlCuFeMn and Al₂CuMg phases in substrate of the cold-rolled aluminum clad thin plate. However, with the increase of solution treatment temperature and the extension of holding time, the content of retained second phases in the substrate gradually decreases. The diffusion degree of Mg and Cu elements is relatively high in the aluminum coating and the transition zone, but there is no failure problem of the aluminum coating caused by severe element diffusion. When the solution treatment process is 500 ℃×20 min, the retained second phase content in the alloy is the lowest of 1.101%. After natural aging for 96 h, the yield strength of the aluminum clad thin plate is 286.5 MPa, the tensile strength is 458.0 MPa, and the elongation after fracture is 23.1%, good combination of strength and plasticity is obtained.

Cooling curves of 28CrMoVNiRE steel oil well tube under different water mists, water pressures and nozzle height cooling conditions were measured by establishing a physical simulation platform of ultra-high pressure jet ultra-fast cooling heat transfer. The heat flux density and heat transfer coefficient of the specimen surface were calculated by inverse heat transfer method. The heat transfer mechanism of ultra-high pressure jet ultra-fast cooling of seamless steel tube was studied, and the accuracy of inverse heat transfer calculation was verified by finite element simulation. In order to explore the effect of ultra-high pressure jet ultra-fast cooling on the microstructure and properties of seamless steel tubes, the microstructure analysis and mechanical properties tests of the steel tube under original hot-rolling state and different ultra-fast cooling conditions were carried out respectively. The results show that when the nozzle height is 100 mm, the water pressure increases from 6 MPa to 7 MPa, the average heat transfer coefficient increases from 386.0 W/(m²·℃) to 859.1 W/(m²·℃), and the average heat flux density increases from 0.15 MW/m² to 0.35 MW/m². When the water pressure is 7 MPa and the nozzle height increases from 100 mm to 160 mm, the average cooling rate increases by 6.2%. The heat transfer coefficient with the decrease of temperature difference ΔT goes through two stages: high temperature, medium temperature near linear rapid growth stage and low temperature rapid decline stage. It is found that the martensite lath width of the tested steel can be refined to 0.35 μm on average under the cooling conditions of spray water pressure of 7 MPa and nozzle height of 160 mm. Compared with the original hot-rolled steel, the hardness and the impact absorbed energy increase, which verifies the feasibility of ultra-high pressure jet ultra-fast cooling technology.

As a steel used in construction machinery, the uniformity of microstructure and properties along the thickness of Q890D steel medium-thick plate has an important influence on the safety and life of the steel during service. The microstructure and properties of 50 mm thick Q890D steel quenched and tempered plate from a domestic steel mill were analyzed along its thickness direction. The metallographic microstructure, microhardness, orientation distribution and defects at different positions along the thickness direction were analyzed. The results show that the 50 mm thick Q890D steel quenched and tempered plate has a significant thickness cross-section effect, showing high hardness at the surface and low hardness at the center, respectively, which is the comprehensive result by the difference of microstructure and orientation. After quenching and tempering, the surface of the steel is dominated by tempered lath martensite with high defect density, while the center is dominated by tempered lath martensite and a small amount of bainite with low defect density. In addition, the surface of the steel has a clear [001] preferred orientation, while the center of the plate shows a less obvious [101] preferred orientation. The thickness cross-section effect is related to the temperature gradient during the quenching and tempering process. Large cooling rate on the surface is easy to form martensite, while slow cooling rate in the center is easy to form bainite. The transformation constraint effect of martensite on the surface is obviously smaller than that in the center, so a differentiated preferred orientation is shown along the thickness direction.

Microstructure of a high carbon silicon manganese steel quenched at 820 ℃ with oil cooling and then tempered at different temperatures was investigated, and the mechanical behaviour under high strain rates and various stress states was systematically examined by using the Hopkinson device. The results reveal that the quantity and size of carbides increase with the increase of tempering temperature. Specifically, as the tempering temperature rises from 580 ℃ to 620 ℃, the dynamic tensile (strain rate of 1400 s^-1) and compressive (strain rate of 3200 s^-1) strengths of the tested steel exhibit a decreasing trend, while an enhancement in dynamic tensile plasticity is observed. The Mises stress concentration in carbide and the shearing stress concentration at the micron carbide/matrix interface are crucial factors for microvoid nucleation during dynamic tensile tests of the tested steels. During dynamic compression, the plastic deformation of the steel is influenced by a coupling mechanism involving strain hardening and heat softening, thereby exhibiting significant strain rate sensitivity.

Microstructure transformation of a low yield ratio 890 MPa grade high strength steel during intercritical QT (quenching and tempering) treatment and its effect on mechanical properties were investigated. The heat-treated microstructure was observed by scanning electron microscope, and the effect of tempering temperature on microstructure and properties was analyzed via mechanical properties and XRD dislocation density calculation. The effect of quenching temperature on microstructure and properties was further investigated by Gleeble thermal simulation experiment. The results show that the intercritical QT treated microstructure is ferrite and tempered lath structure. With the increase of tempering temperature, the lath structure gradually decomposes, and the dislocation strengthening increment decreases, which lead to the decrease of tensile strength. The yield strength increases firstly under the coordination deformation effect of soft and hard phases, and then decreases with the decrease of overall dislocation density. The yield strength changes a little leading to the increase of yield ratio. With the increase of quenching temperature, the dislocation strengthening increment increases and the tensile strength increases, the dilation change during phase transition increases, the proportion of soft phase ferrite decreases, which leads to the increase of yield strength and yield ratio. The increasing proportion of hard phase tempered lath structure makes the low-temperature toughness decrease as the quenching temperature rises.

Effects of quenching from 840 ℃ to 1000 ℃ and tempering from 300 ℃ to 600 ℃ on microstructure and mechanical properties of the high carbon and high silicon SWRS96Si steel were studied. The results show that the undissolved carbides dissolve gradually, the acicular martensite coarsens and the amount of retained austenite increases gradually with the increase of quenching temperature. When the quenching temperature is greater than or equal to 920 ℃, the amount of retained austenite is 21%-23%. During tempering, the retained austenite decomposes from 300 ℃ to 400 ℃, the martensite decomposition and the nucleation and growth of cementite occur during tempering from 400 ℃ to 466 ℃. When the SWRS96Si steel is quenched from 840 ℃ to 960 ℃ and tempered at 480 ℃, with the increase of quenching temperature, the percentage reduction of area of the steel decreases from 32% to 24%, while the strength increases from 1569 MPa to 1968 MPa. When quenched at 920 ℃ and tempered from 300 ℃ to 600 ℃, with the increase of tempering temperature, the twin martensite in the microstructure gradually decomposes into cementite and ferrite, the hardness of the steel reduces from 61 HRC to 42 HRC gradually, while the impact absorbed energy at room temperature increases from 1.6 J to 9.5 J.

Taking CP780 steel as the research object, the treatments of pre-stretch deformation and stress relief annealing was conducted on it. The microstructures, hardness, tensile properties and tensile fracture morphologies of the CP780 steel in different states were systematically studied by means of the optical microscopy (OM), scanning electron microscope (SEM), Vickers hardness tester and tensile testing machine. The results show that the microstructure of the CP780 steel in different states are mainly the bainite and ferrite. After stress relief annealing at 400-550 ℃, the bainite banded structure at the center of the specimen wall thickness is the main reason for the delamination of the tensile fracture surface. With the increase of annealing temperature, the plasticity reduction of the CP780 steel due to pre-stretch deformation is improved, and the strength firstly decreases, then increases and then reduces. The pre-stretch deformed CP780 steel stress relief annealed at 600 ℃ obtains the highest strength-plasticity combination, with product of strength and elongation reaching 16.5 GPa·%.

Annealing temperature of Fe-2.0Mn-4.7Al-0.4C steel was determined to be 860 ℃ by using JMatPro simulation software. The effect of annealing time on the microstructure and mechanical properties of the hot-rolled Fe-2.0Mn-4.7Al-0.4C steel was studied by means of optical microscope, scanning electron microscope, and electronic universal material testing machine. The results show that after annealing treatment, the microstructure of the hot-rolled Fe-2.0Mn-4.7Al-0.4C steel transforms from δ ferrite, α ferrite, martensite, pearlite, and cementite to δ ferrite, pearlite, and retained austenite. When annealed for 120 min, the tensile fracture morphology of the specimen shows a transition from brittle fracture to ductile fracture. The tested steel has the best comprehensive mechanical properties with tensile strength of 515.6 MPa, yield strength of 361.0 MPa, maximum elongation of 35.4%, and strength-elongation product of 18.3 GPa·%. However, when the annealing time is too long to reach 180 min, the grain grows and aggregates, the dislocation movement disappears, resulting in a decrease in material toughness and an increase in material brittleness. The fracture surface is mainly composed of ductile dimples and relatively smooth cleavage surfaces.

To address the high costs of achieving lightweight automobiles while simultaneously obtaining high strength and high plasticity, a low-density medium manganese steel with the composition 0.36C-4.5Al-7.6Mn-0.31V-0.31Si-0.07Ti was developed. The effect of various reverse transformation annealing temperature on the microstructure, retained austenite (content, stability and dislocation density) and mechanical properties of the low-density medium manganese steel was explored by means of OM, SEM, EBSD, XRD and tensile testing machine. The results indicate that the reverse transformation annealed experimental steel comprises a mixed structure of elongated δ ferrite, lath martensite and blocky retained austenite along the rolling direction. As the reverse transformation annealing temperature increases from 650 ℃ to 780 ℃, the martensite content decreases, the retained austenite content rises from 50.14% to 58.97%, the average austenite grain size increases first and then decreases, while the austenite KAM value (proportional to dislocation density) shows the opposite trend. Optimal mechanical properties of the tested steel are achieved when reverse transformation annealed at 780 ℃ for 1 h, with yield strength of 739.1 MPa, tensile strength of 884.2 MPa, elongation of 37.80% and the product of strength and elongation of 33.42 GPa·%.

Effects of high temperature diffusion annealing and its sequence on the microstructure and properties of 4Cr5Mo2V hot working die steel were investigated by means of OM, SEM, TEM, XRD and in combination with impact tests. The results show that high temperature diffusion annealing can significantly improve dendritic segregation in the electroslag ingots of the 4Cr5Mo2V hot working die steel, so that the band segregation after hot rolling is lighter. The alloying elements such as Cr, Mo and V in the ingots are fully dissolved into the matrix, which ensures the diffusion and reallocation of the elements during the subsequent heat treatment, allowing for more uniformly distributed carbides in the spheroidizing annealed microstructure. It also increases the stability of the retained austenite to obtain a certain amount of thin-film retained austenite in the 4Cr5Mo2V steel, which can further enhance the impact property. Meanwhile, from the study of carbide evolution patterns, the carbides in the 4Cr5Mo2V steel prepared by the three processes after quenching and tempering are dominated by MC, M₃C, M₆C and M₇C₃ types. In addition, the high temperature diffusion annealing before hot rolling is the most cost-effective and efficient process to solve the band segregation and eutectic carbides in the electroslag ingots of the 4Cr5Mo2V steel and improve the impact property most obviously. The impact fractures of the specimens with high temperature diffusion annealing have more dimples and thus exhibit ductile fracture. The fractures of the specimens without high temperature diffusion annealing have more cleavage planes, and exhibit brittle fracture, meanwhile, the uneliminated eutectic carbides in the fracture accelerate cracking.

In order to reveal the precipitation behavior of B-containing carbides during tempering and its effect on the mechanical properties of heat-resistant steel, the morphology and distribution of B-containing carbides in a heat-resistant steel with boron content of 0.01% isothermally quenched at 950 ℃ and tempered at 700 ℃ for different time(10, 30, 60, 90, 120 and 150 min) were observed by means of optical microscope and scanning electron microscope. And the effect of precipitation behavior of B-containing carbides on Brinell hardness and tensile properties of the tested steel was studied by Brinell hardness test and room temperature tensile test. The results show that with the increase of tempering time, the matrix of the tested steel is lath tempered martensite, while the B-containing carbides grow in size gradually, with the shape changing from blocky to long blocky clustered. With the increase of tempering time, the hardness decreases first, then increases and finally decreases, while the tensile strength and yield strength increase first and then decrease. When the tempering time is ≥30 min, the tensile fracture of the tested steel is ductile fracture with dimples, and the strengths of the tested steel are the best when tempering at 700 ℃ for 60 min, with the tensile strength and yield strength of 931 MPa and 817 MPa, respectively.

Microstructure and precipitates of 15NiCuMoNb5 steel after isothermal treatment at different temperatures below the Ms point were analyzed by using scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that when isothermally treated at 370 ℃, M-A islands mainly exhibit a micro-sized long-striped morphology, and the precipitated phases are mainly M₃C and Cu-rich phases. After isothermal treatment at 310 ℃, the M-A island is dominated by traditional bulk morphology, and the precipitated phases are mainly spherical Nb-rich carbides. The difference of M-A island morphology and precipitated phase mainly depends on the difference of carbon diffusion rate and driving force of precipitated phase at different temperatures.

Effect of annealing temperature on microstructure, texture and magnetic properties of Fe-3.2%Si oriented silicon steel in rolling direction and transverse direction was studied. The results show that when the annealing temperature is 850-1025 ℃, the specimen does not undergo secondary recrystallization, and the main textures are α^* texture and γ texture. When the annealing temperature rises to 1025-1050 ℃, the secondary recrystallization takes place preferentially near the surface of the specimen in the rolling direction and transverse direction, and the grain grows abnormally rapidly with the increase of annealing temperature. At 1050-1100 ℃, the whole specimen is fully secondary recrystallized, forming a single Goss texture. When the annealing temperature is 850-1025 ℃, the corresponding magnetic inductances B₈ and P_15/50 have no obvious changes, but the magnetic properties are obviously improved when the temperature is raised to 1050 ℃, and when kept at 1050 ℃ for 1 h, the magnetic properties are the best, with B₈ of 1.92 T along the rolling direction, P_15/50 of 1.40 W/kg, relative permeability of 1.17×10⁴ and coercivity of 36.70 A/m. In the transverse direction, B₈ is 1.35 T, P_15/50 is 3.50 W/kg, and the relative permeability and coercivity are 531.75 and 77.51 A/m, respectively.

Effect of normalizing process on microstructure, texture and magnetic properties of non-oriented silicon steel for electric vehicle drive motors was investigated by taking industrial produced 2.8%Si+1.0%Al hot rolled plates as experimental materials. The results show that the hot rolled deformation microstructure is eliminated and the microstructure uniformity is improved by normalizing. Thereby the cold rolled and annealed microstructure is improved. The grain size of both normalized and annealed plates continues to increase with normalizing temperature increasing. Besides, the hot rolled texture is improved significantly by normalizing. {110} and {100} textures are strengthened, while {111} texture is weakened, with the increase of normalizing temperature. Due to the heritability of normalized texture, the {111} texture intensity in the annealed plate also weakens with the normalizing temperature rising. After normalizing at 900 ℃ for 4 min, cold rolling to a thickness of 0.27 mm and annealing at 1000 ℃ for 2 min, the grain size of the annealed finished plate is 117 μm, which is the optimal grain size. At this time, the magnetic properties of the annealed plate are the best, with the core loss P_1.5/50 of 2.044 W/kg, the core loss P_1.0/400 of 13.368 W/kg, and the magnetic induction intensity B₅₀ of 1.672 T.

To address the issue of uneven microstructure distribution in low alloy wear-resistant ZG30CrMnSiMoVTi steel after quenching and partitioning (Q-P), the water-air alternately timed quenching (ATQ) process was adopted. The cooling curves of the tested steel under different conditions were simulated and analyzed to determine the details of the ATQ process. The microstructure and mechanical properties of the tested steel after ATQ for different cycles were studied by means of optical microscope (OM), scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), Rockwell hardness tester, impact testing machine and tensile testing machine. The results indicate that a large amount of martensite and a small amount of bainite structure appear in the tested steel ATQ treated. Among all the tested steels, the one ATQ treated thrice has the most uniform microstructure distribution, the highest bainite content and the best mechanical properties, with the average Rockwell hardness of 46.45 HRC, impact absorbed energy of 29.1 J, tensile strength of 1551.69 MPa, yield strength of 1293.07 MPa and elongation after fracture of 12.6%.

50CrNiMoVNb steel belts after annealing were cold rolled by using cold rolling mill, and the cold rolled specimens were annealed in high temperature muffle furnace. The effects of cold rolling reduction rate, annealing temperature and annealing time on recrystallization were studied by means of metallographic microscope and microhardness tester. The results show that the growth rate of recrystallized grains is increased with the increase of cold rolling reduction rate, and the greater the reduction rate, the finer and more uniform recrystallized grains. The average grain size of the steel annealed at 800 ℃ for 1 h is reduced from 8.0 μm at reduction rate of 0% to 5.6 μm at reduction rate of 50%. At the same cold rolling reduction rate, when the steel is air cooled after annealing, the hardness decreases first and then increases with the increase of annealing temperature. The degree of recrystallization completion is improved with annealing temperature increasing and holding time prolonging. The completion time of recrystallization is 70 min for the steel with the reduction rate of 30% at the annealing temperature of 800 ℃. The relationships between the volume fraction x, the recrystallization time t_R, grain size d and the nucleation rate N, linear growth rate v, holding time t are x(t)≈Nv³t⁴,$t_{\mathrm{R}}=\sqrt[4]{\frac{3}{\pi N v^{3}}}$,$d=2 \sqrt[4]{\frac{3 v}{\pi N}}$, respectively.

Effect of isothermal bainite temperature on microstructure and mechanical properties of ship steel after intercritical rolling was investigated by means of scanning electron microscope (SEM), X-ray diffractometer (XRD) and mechanical properties tests. The results show that the microstructure is M/A island and a small amount of ferrite after hot rolling. When the isothermal temperature is 400 ℃ and 450 ℃ followed by air cooling, the microstructure is composed of the lath bainite, granular bainite and ferrite. When the isothermal temperature is 500 ℃, the microstructure is composed of granular bainite and ferrite. The yield strength of the tested steel ranges from 981 MPa to 1116 MPa, and the tensile strength ranges from 997 MPa to 1124 MPa after bainite isothermal treatment. Compared to the other two isothermal temperatures, the retained austenite volume fraction is the highest when isothermal treated at 500 ℃, resulting in the elongation and impact absorbed energy reaching the maximum values of 11.7% and 104.1 J, respectively. Through comprehensive comparison, the mechanical properties of the steel reaches the optimum when isothermal treated at 450 ℃, with the tensile strength of 1124 MPa, yield strength of 1116 MPa, elongation of 11.3%, and impact absorbed energy of 71.4 J.

Microstructure and hardness of 30CrMnSi steel after austempering treatment were studied by using metallurgical microscope and hardness tester. The results show that the hot-rolled tube of the 30CrMnSi steel is suitable for cutting processing after normalizing pretreating at 880 ℃ for 30 min instead of traditional annealing treatment. The optimal austempering process for the 30CrMnSi steel is optimized to be austenitizing at 890 ℃ for 30 min, followed by isothermal treating at 420-440 ℃ for 1-1.5 h. After treating by the optimal process, lower bainite can be obtained, with hardness ranging from 37 HRC to 39 HRC, meeting the design requirements of the product.

Sanicro25/G115 hetero-steel welded joint was prepared by using gas tungsten arc welding (GTAW), then the microstructure and mechanical properties of the joints as-welded, tempered at 760 ℃ and tempered at 780 ℃ were investigated, respectively, by means of optical microscope, scanning electron microscope and tensile testing machine. The results show that the microstructure of base metal and heat-affected zone (HAZ) at the Sanicro25 steel side is austenite with precipitated phase. The welding seam has a cellular dendritic morphology, and the HAZ and base metal at the G115 steel side are tempered martensite. After post-weld heat treatment (PWHT), the number of precipitated phases in CGHAZ, FGHAZ and base metal of the G115 steel side increases, and M₂₃C₆ and MX precipitated phases are distributed in the prior austenite grain boundary and martensitic lath boundary. Under tensile test at room temperature, quasi-cleavage brittle fracture occurs in the joint as-welded, and ductile fracture occurs in the joints tempered at 760 ℃ and 780 ℃. The rupture position is transferred from the welding seam to the FGHAZ at the G115 steel side. Under 650 ℃ tensile test, the joints under different states all show ductile fracture, and the rupture position is transferred from G115 steel side base metal to G115 steel side FGHAZ. It is recommended that the PWHT temperature should be controlled at 780 ℃.

Effect of BASCA (β annealing+slow cooling+aging) treatment on microstructure and tensile properties of the Ti-55511 titanium alloy was analyzed by means of optical microscope (OM), scanning electron microscope (SEM), X-ray diffractometer (XRD) and electron universal testing machine. The results show that after BASC (β annealing+slow cooling) treatment, the size of the primary α phase in the microstructure increases to a certain extent compared to the α phase in the original microstructure of the alloy. The morphology of the primary α phase in the microstructure includes a large number of strip-shaped, a small amount of equiaxed and a few blocky. After BASCA treatment, the β transformation structure in the alloy is obvious, and the microstructure becomes more uniform and stable. BA (β annealing) temperature affects the size, morphology and content of the α phase in the microstructure. The strength of the alloy increases with the increase of BA temperature, while the plasticity shows the opposite trend. The tensile fracture morphology of the alloy treated by BASC process is mainly dimples, and the dimple is large and deep. With the increase of BA temperature, the tear ridge appearance in the fracture becomes more and more obvious. It is found that there are secondary cracks in the fracture after BASCA treatment, and the fracture presents a rugged morphology with high and low fluctuations.

Effect of solution treatment temperature (850, 820, 790, 760 ℃) on microstructure and mechanical properties of Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe corrosion-resistant titanium alloy plate aged at 550 ℃ for 4 h was studied by means of scanning electron microscope, transmission electron microscope, X-ray diffractometer and tensile testing machine. The results indicate that the microstructure of both the original and heat treated titanium alloy plate is composed of equiaxed α phase and retained β phase. As the solution treatment temperature increases, the content of α phase decreases while the content of β phase increases. The heat treatment reduces the internal stress and dislocation density of the titanium alloy plate, thus improves the elongation. As the solution treatment temperature increases, the tensile strength of the aged titanium alloy first increases and then decreases, while the elongation is opposite. The titanium alloy solution treated at 820 ℃ and aged has the highest tensile strength, with tensile strengths of 1387 MPa and 1357 MPa in the RD and TD directions, and elongation of 3.7% and 4.1%, respectively.

Based on 316L stainless steel, a high nitrogen stainless steel with nitrogen content of 1.1% was designed and prepared by high pressure melting and high pressure solidification method. After solution treatment at 1050 ℃ for 10 h and aging at different temperatures (600, 800 and 1000 ℃), the distribution of nitrogen content in the high nitrogen steel ingot and the mechanical properties and corrosion resistance of the high nitrogen steel after solution-aging treatment were studied by means of nitrogen content distribution detection, microstructure observation, energy spectrum scanning, mechanical properties test, immersion corrosion and electrochemical corrosion. The results show that the macro nitrogen content of the tested high nitrogen steel is evenly distributed, but the enrichment of N and Cr elements exists in the micro level. The enrichment on the acicular martensite strip can improve the corrosion resistance of the material, and the precipitation of chromium nitride can improve the strength of the material but reduce its corrosion resistance. The mechanical properties of the steel aged at 800 ℃ for 2 h are the best, with tensile strength of 771 MPa and yield strength of 658 MPa. The corrosion resistance of the steel after solution treatment is the best, and the corrosion resistance of the steel aged at 800 ℃ for 2 h is better than that of other aging temperature treatments. Comprehensively considering the mechanical properties and corrosion resistance, The optimum heat treatment process of the tested high nitrogen steel is the solution treatment followed by aging treatment at 800 ℃ for 2 h.

Microstructure of the 00Cr23Ni12Mn5Mo2N austenitic stainless steel under different preparation processes, as well as the mechanical properties and fracture morphology of the surface and core after high temperature homogenization, were studied by means of optical microscope, scanning electron microscope, tensile testing machine and impact testing machine. The results demonstrate that high temperature homogenization at 1190 ℃ for 24 h can effectively eliminate the σ-phase, effectively improve the high temperature properties of the tested steel and provide favorable conditions for forging. After high temperature homogenization, by forging and forming with two upsetting and two drawing methods, and controlling the forging temperature between 1210-950 ℃, the problem of insufficient deformation of the core can be effectively solved, the dynamic recrystallization of surface and core is complete, the microstructure is uniform and fine and the grain size of the surface and core can both reach grade 6 or above. The surface and core properties after forging are excellent. The tensile strength, elongation, percentage reduction of area, hardness and impact absorbed energy at 77 K on surface of the tested steel are 821 MPa, 41.0%, 65.0%, 252 HBW and 49.1 J, respectively, while the strength and hardness of the core are slightly lower than that of the surface, and the plasticity and toughness of the core are better.

Effect of aging treatment (150-210 ℃, 1 h) on the tensile properties and intergranular corrosion sensitivity of 2A97 Al-Li alloy after solid solution treatment(525 ℃×1 h) and 70% reduction cold rolling was investigated by tensile test, intergranular corrosion test, scanning electron microscopy and transmission electron microscopy. The results show that with the increase of aging temperature, the strength of the alloy increases first and then decreases, and the elongation decreases continuously. The corrosion type changes from uniform corrosion to pitting corrosion and the maximum corrosion depth gradually increases, which is related to the precipitation of T₁ phase and θ′ phase in the grains, and the intermittent distribution of precipitated phase at the grain boundary. When the aging temperature is 180 ℃, the alloy can obtain high strength(597, 540 MPa), high elongation(6.9%), and no intergranular corrosion sensitivity.

Solution treatment and incomplete annealing were carried out for 8Cr13 martensitic stainless steel, then the carbides refinement and its effect on the final properties of the steel after quenching and tempering was studied. The results show that high temperature solution treatment and incomplete annealing process can effectively refine the carbides in the 8Cr13 steel. With the increase of solution treatment temperature and the extension of solution time, the carbides gradually dissolve, and the volume fractions and sizes of carbides decrease. Compared with the specimen directly quenched and tempered without solution treatment, the specimen solution treated at 1100 ℃ for 4 h and incomplete annealed can obtain the best comprehensive properties after quenching and tempering. The yield strength, tensile strength and elongation are increased by 30.6%, 8.3% and 147.7%, respectively, the hardness is slightly reduced but meets the requirements, and the self-corrosion current density is reduced by one order of magnitude.

Microstructure and mechanical properties of the Al-Cu-Li alloy aged at 143 ℃ for different time were studied by means of tensile test, fatigue crack propagation rate test, scanning electron microscope(SEM) and transmission electron microscope(TEM). The results show that the strength of the alloy increases rapidly and shows a strong aging response when aged for 25-35 h, while the strength increases slightly and reaches a peak when aged for 65 h. The main precipitates of the Al-Cu-Li alloy after aging at 143 ℃ for 25, 35 and 65 h are T₁ phase. After aging for 25 h, T₁ phases precipitate with fine size, accompanying with precipitation of small amounts of δ′ phase. The alloy aged for 25 h shows a low fatigue crack growth (FCG) rate because the movable dislocations can cut through precipitated phases and undergo reversible slip, increasing fatigue crack deflection and crack closure effects, showing higher crack propagation resistance. With the increase of aging time, the size and number density of T₁ phase increase, and the FCG rate increases. After aging for 65 h, the size of T₁ phase greatly increases, the moving dislocation can only bypass these large phase, resulting in stress concentration, thus the FCG rate increases obviously, the fatigue property of the alloy is the worst.

Effect of different processes on microstructure and properties of 2219-T3 aluminum alloy plates was studied by metallographic observation, room temperature tensile and electron backscatter diffraction analysis. The analysis results indicate that, following by 1-5 days of room temperature parking, the solution quenched 2219 aluminum alloy plate achieves the T3 state after undergoing cold deformation, and its yield strength at room temperature progressively increases with the prolongation of natural aging time. Additionally, the yield strengths of the plates in the T3 state increase with the increase of cold deformation; however, after reaching a deformation of 7%-8%, further cold deformation has a limited effect on the increase in yield strength, and the elongation decreases. The specimens after room temperature parking for 3 days and then cold deforming show a better match between strength and plasticity. Further microstructure analysis shows that the specimens after cold deformation do not change the recrystallized structure formed in the solid solution, and KAM analysis shows that a more homogeneous distribution of dislocations is formed within the grains, which ensure the retention of the atomic clusters that play a greater role in strengthening in the T3 state.

Three kinds of solution treatment processes ((490, 500, 520) ℃×4 h+520 ℃×4 h) of 6061 aluminum alloy for hydrogen storage tank were developed, and the effect of solution process on the microstructure and mechanical properties of the alloy was studied. The results show that after solution treatment, recovery and recrystallization of the forged 6061 aluminum alloy occur in different degrees. Compared to the forged 6061 aluminum alloy without solution treatment and with directly aging, the strength and hardness of the 6061 aluminum alloy increase but the plasticity decreases after solution treatment and aging. The degree of second phase (AlFeSi related compounds and Mg₂Si phase) dissolution and the degree of recovery and recrystallization of the aluminum matrix have a significant impact on the strength of the alloy. As the degree of dissolution increases, the strength of the aluminum alloy increases accordingly, while as the degree of recovery and recrystallization of the aluminum matrix increases, the strength of the aluminum matrix decreases accordingly. Taking into account factors such as strength, hardness and plasticity, the optimal solution process is 500 ℃×4 h+520 ℃×4 h, with water cooling.

At laser power of 330 W, AlSi10Mg alloy was formed by selective laser melting (SLM) with and without a magnetic field (0, 150 mT), and the effect of magnetic field on the microstructure and properties of the SLMed alloy was studied. The results show that the alloy structure with and without magnetic field is composed of α-Al and Si phases, indicating that the introduction of magnetic field has no effect on the alloy phase. Under magnetic field of 150 mT, the damping effect in the molten pool reduces the flow velocity, suppresses the formation of pores, and increases the density of the alloy. The relative density increases by about 0.4%, and the hardness increases by about 11.8%. Compared with the SLMed AlSi10Mg alloy without magnetic field, the alloy has better ultimate tensile strength and ductility under 150 mT magnetic field, with an increase in tensile strength of about 55 MPa and an increase in elongation of about 0.8%.

Effect of aging treatment on microstructure, mechanical properties and damping properties of Mg-1.2Al-0.4Ca-0.3Mn-0.3Zn magnesium alloy plates after multi-pass rolling+annealing was investigated by means of optical microscope (OM), dynamic thermal analyzer, X-ray diffractor (XRD), tensile testing machine and scanning electron microscope (SEM). The results show that the average grain size of the annealed alloy is 4.82 μm with a small amount of twin, and the tensile strength and elongation are 212.09 MPa and 9.87%, respectively. When the aging temperature is 150 ℃, the average grain size is 4.95 μm, the tensile strength is 207.84 MPa, and the elongation is 6.58%. When the aging temperature is 190 ℃, the average grain size is 4.05 μm, the grains are fine and uniformly distributed and the second phases such as Al-Mn, Al-Ca and Mg₂Ca appear at the grain boundaries of the alloy, which results in good mechanical properties, with tensile strength of 224.21 MPa and elongation of 14.2%. In the low strain amplitude region (ε≤0.1%), the damping values of the alloy rolled, annealed, 150 ℃ aged and 190 ℃ aged are not significantly different. While, in the high strain amplitude region (ε>0.1%), the damping properties of the 190 ℃ aged alloy is the best. In the low-temperature range of 35-150 ℃, the damping peaks of the alloy in four states are all caused by dislocation slip, but in the high temperature range of 150-375 ℃, the damping peak of the rolled state alloy is the highest, and the damping peaks of the rolled, annealed and 150 ℃ aged alloys are caused by dislocation slip, and the damping peak of the 190 ℃ aged alloy is obtained by the superposition of the dislocation slip and the sliding of the grain boundary.

Effect of heat treatment on microstructure and mechanical properties of Mg-6Sn-3Zn-xAl (x=0, 1, 2, 3) alloys with different Al contents was studied by using XRD, OM, SEM, hardness test and tensile test. The results indicate that the as-cast Mg-6Sn-3Zn alloy is composed of α-Mg equiaxed dendrites and Mg₂Sn phase. The addition of Al element causes grain refinement of the as-cast alloy, and the Mg_x(ZnAl)_1-x quasicrystal phase is formed in the alloy. After 320 ℃×3 h+430 ℃×24 h solution treatment, the Mg_x(ZnAl)_1-x quasicrystal phase in the Mg-6Sn-3Zn-xAl alloy is completely dissolved, while the Mg₂Sn phase is mostly dissolved but has a small amount of residue. The aging hardening curve shows that during the aging process at 200 ℃, the Mg-6Sn-3Zn-xAl alloys all reach peak when aged for 216 h, and a large number of dispersed Mg₂Sn phases are precipitated in the peak aged alloy. The addition of Al element brings a certain strengthening effect, and both solution treatment and aging treatment improve the mechanical properties of the alloy to a certain extent. The improvement of mechanical properties of the alloy comes from the contribution of refined crystalline strengthening, solution strengthening and precipitation strengthening, respectively.

Effect of double annealing process on microstructure and mechanical properties of the TC18 alloy quasi-β forged piece was investigated. The results show that with the increase of first annealing high-stage temperature, lamellar α phases separate and spheroidize, the plasticity decreases slightly. With the decrease of cooling time from high-stage temperature (830 ℃) to middle-stage temperature (750 ℃) and with the increase of first annealing middle-stage temperature, the strength increases but the plasticity and the fracture toughness decrease. With the increase of first annealing middle-stage temperature, the quantity of untransformed β microstructure and dispersed secondary α phases increases. The second annealing temperature has a great effect on the morphology and content of the secondary α phases. With the increase of second annealing temperature, the secondary α phases aggregate and grow, and its dispersion degree decreases, the strength of the forging decreases, while the plasticity and fracture toughness increase.

Effect of Nb microalloying on the austenite grain size of the 17CrNiMo6 steel under different Nb addition was studied, and high temperature carburizing was carried out on the tested steel at different temperatures (910, 930, 940 ℃) by measuring the effective hardening layer depth and mechanical properties. The results show that the effect of Nb microalloying on the austenite grain refinement of the 17CrNiMo6 steel is significant, and the Nb content increases, the austenite grain grade increases, and the appropriate Nb addition range is 0.03%-0.04%. Compared with high temperature carburizing at 910 ℃, the effective hardening layer depth of the 17CrNiMo6 steel with 0.032%Nb is increased after high temperature carburizing at 930 ℃ and 940 ℃, and the carburizing rate is increased by 8.3%-11.9% and 13.3%-15.3%, respectively. However, there is no significant difference in mechanical properties and grain size. The carburizing rate of the microalloyed 17CrNiMo6 steel can be accelerated by appropriately increasing the carburizing temperature.

To address the issues of high pollution and high energy consumption in the current isothermal quenching process in bath furnaces, three alternative processes (vacuum quenching, step quenching and atmospheric furnace composite isothermal quenching) for 18Cr2Ni4WA steel were investigated. The microstructure, hardness and mechanical properties of the carburized layer and core of the tested steel were comparatively analyzed after carburizing and undergoing three kinds of quenching, deep cooling and tempering. The results indicate that the carburized layer microstructure of the 18Cr2Ni4WA steel treated with three quenching schemes is mainly tempered martensite, but the core microstructure is different. Under vacuum quenching and step quenching processes, the core structure is tempered martensite, while under composite isothermal quenching process, the core structure is tempered martensite. Consequently, under vacuum quenching and step quenching, the hardness and tensile strength are higher, while under composite isothermal quenching process, the plasticity and toughness are better. The performance indicators of the 18Cr2Ni4WA steel treated by three alternative processes can all meet the application requirements, but not using high-pollution and high-energy consumption alkali tank equipment, which can achieve the goal of green heat treatment after carburizing.

Based on FEM numerical analysis, the flow field in atmosphere-controlled sealed box furnace with different stirring fans was simulated, and by using transient and steady-state methods, the flow velocity, static pressure and dynamic pressure changes inside the controlled atmosphere furnace were analyzed under the conditions of a four blade stirring fan (Fan4) and a six blade stirring fan (Fan6). The airflow path inside the Fan4 furnace is relatively uniform, and they are all located in the upper non-working area of the furnace. The airflow path inside the Fan6 furnace is prone to forming vortices, which have a significant impact on the uniformity of furnace temperature and atmosphere. The dynamic pressure cloud diagrams inside Fan4 and Fan6 furnaces are basically similar, and the difference in dynamic pressure within the working area is relatively small. The static pressure inside the Fan6 furnace is generally higher than that of Fan4, and under the same conditions, a six blade fan will cause the furnace pressure to be too high.

Surface oxygen infiltration products and hardness of the pure tantalum used as artificial joints under different processes were studied by means of low pressure vacuum surface oxygenation technology, scanning electron microscope (SEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and nanoindentation technology. The results show that surface oxygen infiltrated layer with thickness of about 3.6 μm, which contains the phase of tantalum oxide Ta₄O, can be prepared under the condition of 500 ℃×2 h/273 mL(oxygen flow volume, the same below). The δ-(Ta, O) compounds can be prepared under the conditions of 550 ℃×2 h/273 mL, 550 ℃×5 h/1.3 mL, 550 ℃×10 h/0.013 mL and 650 ℃×3 h/0.013 mL. The thickness of surface oxygen infiltrated layer with δ-(Ta, O) compounds varies according to the parameters of oxygen infiltration temperature, time and oxygen flow volume. The surface oxygen infiltrated layer with the average thickness of nearly 60 μm is prepared under the condition of 550 ℃×1 h/273 mL, and the surface phase is tantalum oxides with polyvalent tantalum, the Vickers hardness and maximum nanoindentation hardness of the specimen are (160.27±2.80) HV5 and 7.92 GPa, respectively, which are significantly higher than those of the pure tantalum. This technology can be used to increase the abrasion resistance and lifespan of the Ta-based artificial joints.

In order to enhance the wear resistance of 42CrMo steel surface and improve the porosity and crack defects frequently generated in the clad layers, as well as the performance of the clad layer, TiC-Fe₃Al-Ni clad layer was prepared on the surface of 42CrMo steel via laser cladding, and mechanical vibration carrying different modes was introduced. The phase composition, microstructure and properties of the clad layer under different vibration directions were analyzed by means of SEM, XRD, microhardness tester and friction and wear tester. The results show that the porosities and cracks in the clad layer are significantly restrained due to the mechanical vibration. Furthermore, part of crystal arms of dendrites is broken by the mechanical energy coming from the vibration during the process of grain growth, and the dendrite growth is reduced. Therefore, the condition of equiaxial crystal growth is developed, and the compactness of the clad layer is strengthened. The clad layer primarily contains the phases of TiC, Ti₈C₅, Fe₃Al, FeO, as well as AlNi. Mechanical vibration has a little effect on changing the phase constituent of the clad layer. Vertical vibration significantly improves the microstructure of the clad layer. The mean hardness of the clad layer at vertical vibration reaches up to 1891.78 HV0.2, which is obviously higher than that without mechanical vibration, the wear resistance of the clad layer is optimal, with wear loss of 1.57 mg, and the wear process has small fluctuation.

22MnB5 hot-dip galvanized hot stamped steel was heated to different austenitizing temperatures for different time for hot stamping. The effect of austenitizing process for hot stamping on the microstructure and cracks of the galvanized layer on hot stamped steel was investigated through characterization of the microstructure, surface oxides and cracking of the galvanized layer by scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) analysis. The results indicate that the galvanized layer is primarily composed of α-Fe (Zn) and a small amount of brittle Fe₃Zn₁₀ after austenitizing. With an increase in heating temperature from 870 ℃ to 890 ℃, the galvanized layer thickness reaches the maximum, and there is a noticeable decrease in the presence of Fe₃Zn₁₀, leading to a reduction of cracks after hot stamping. However, the galvanized layer thickness slightly decreases at 910 ℃ due to Zn volatilization, resulting in the formation of a large amount of Mn, Zn and other elemental oxides on the surface, and the content of Fe₃Zn₁₀ in the galvanized layer significantly increases, leading to a large number of cracks after hot stamping. When the holding time at 890 ℃ is extended from 370 s to 400 s, the Fe-Zn reaction is promoted, more α-Fe(Zn) phases are formed, resulting in an increase of the galvanized layer thickness, but with no significant change of the surface oxidation degree.

Circlip broke when installed in the hollow shaft of the rotor component. Macroscopic and microscopic morphologies on the fracture surface were observed, chemical composition of the raw material was tested, and the microstructure and hardness of the material were investigated in order to find out the failure mechanism of the circlip. The results show that the failure mode of the circlip is hydrogen embrittlement fracture. The circlip has a high sensitivity to hydrogen embrittlement due to its high hardness caused by low temperature tempering, and during electroplating, the accumulation of hydrogen inside the material increases, resulting in hydrogen embrittlement fracture during installation. Therefore, the heat treatment process of the circlip can be controlled to reduce the material hardness, meanwhile, the electroplating process can be controlled to reduce the thickness of the galvanized layer on the circlip.

Bright white spot segregation on the surface of 42CrMo steel forgings used in wind turbine gearbox was observed and analyzed by OM, SEM, EDS and other technologies. The results show that the bright white spots segregating on the surface of the forging are tempered bainite. This is due to the existence of composition segregation of the 42CrMo steel ingot, which shows strip and spot bainite segregation after forging. After conventional post-forging heat treatment (normalizing+high temperature tempering), the chemical composition is not homogenized, and the bainite segregation evolves into aggregated granular Cr and Mo carbide segregation. After further quenching and tempering treatment, the microstructure is tempered bainite with varying sizes and uneven distribution in the tempered sorbite matrix.

A large number of micro-cracks were found after the finishing of 300M steel components. The cracking reasons and mechanisms of this batch of components were analyzed from the aspects of appearance, material chemical composition, microstructure and crack characteristics and hardness. The results show that these are grinding micro-cracks, which initiate in the near-surface layer, mainly exhibiting intergranular and quasi cleavage cracking characteristics. The reason for the cracking is that after chrome plating, the surface material undergoes plastic flow during the grinding process and residual tensile stress is generated under rapid cooling conditions. The surface and near-surface material are burned and the phase transition stress is formed. Meanwhile, the hardness of burned material decreases. After a long time, hydrogen atoms from chrome plating diffuse to near-surface and gather at the dislocations and grain boundaries, that induces the grinding crack to have delayed forming in near-surface under the action of residual tensile stress and microstructure tensile stress.

During the commissioning period of a nuclear power plant unit, the containment isolation valve did not close as expected. Upon disassembling the valve and inspecting it, it was found that there were cracks on the surface of gear shaft gear. The causes of valve gear cracking were analyzed by means of macroscopic inspection, chemical composition analysis, metallographic examination and hardness testing. The results show that the cracks are quenching cracks, which occur at the transition point between the tooth root and the outer edge of the gear shaft, and there are obvious rough machining tool marks at the transition point of the tooth root fillet. Due to the overall high hardness of the gear area and the hardening of all teeth, it does not meet the requirements of JB/T 6395-2010. The analysis suggests that the occurrence of cracks is mainly related to the incorrect selection of heat treatment methods and improper control of heat treatment processes. The obvious rough machining tool marks at the transition of the tooth root fillet are the contributing factors to the occurrence of cracks.