Effect of cold rolling and aging process on microstructure and properties of Cu-1.8Ni-0.9Co-0.641Si alloy sheet and strip was studied. The effects of liquid nitrogen cold rolling and room temperature cold rolling on the properties of the alloy sheet were compared, and the mechanism was analyzed by X-ray diffractometer (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that when the aging time is 60 min, the hardness of alloy sheet reaches the maximum after cold rolling at room temperature and low temperature, which are 285.53 HV0.3 at 450 ℃ and 307.2 HV0.3 at 400 ℃, respectively. It is also found that the conductivity of the alloy increases with the increase of aging temperature. After aging at 500 ℃, the conductivity of the low temperature cold rolled alloy is the highest, reaching 60.11%IACS. During the aging process, the dislocation density in the sheet decreases, and a large number of fine and uniform nanoscale (Ni,Co)₂Si ordered phases appear in the grains and on the grain boundaries. Compared with cold rolling at room temperature, the microstructure of the Cu-Ni-Co-Si alloy after low temperature cold rolling is finer and the dislocation density is higher.

In order to improve strength and simultaneously reduce hydrogen embrittlement sensitivity, a low-alloy high-strength steel with 0.11wt%Nb and 0.55wt%V was developed. The effect of three different tempering temperatures (560, 600, 640 ℃) on the microstructure, hydrogen diffusion behavior and hydrogen embrittlement sensitivity of the tested steel was characterized and studied by using optical microscope (OM), X-ray diffraction (XRD), transmission electron microscope (TEM), hydrogen permeation technique of double electrolytic cells and slow strain rate tensile test. The results show that with the increase of tempering temperature, the ferrite grain size increases while the dislocation density reduces, and the carbides precipitated become coarse, hence the tensile strength decreases and the elongation increases. Meanwhile, with the increase of tempering temperature, the density of hydrogen reversible traps reduces and the activation energy of hydrogen diffusion decreases, as well as the hydrogen diffusion coefficient increases, hence the hydrogen embrittlement sensitivity becomes lower.

Room temperature tensile test and the stress rupture test at 975 ℃/196 MPa were conducted in K424 alloy after solution treatment at different conditions (1200-1240 ℃×4 h, air cooling). The results show that the uniform fine γ' precipitates with high cubicity are reprecipitated in the K424 alloy after completely solution treatment at 1220, 1230 and 1240 ℃. The room temperature strength and plasticity of the K424 alloy after complete solution treatment at 1220-1240 ℃ for 4 h are slightly lower than that of incomplete solution treatment at 1200-1210 ℃ for 4 h, but the stress rupture life is higher. K424 alloy performs the highest stress rupture life (97.8 h) after treatment treatment at 1220 ℃ for 4 h, which is the optimal solution treatment process. The intragranular dislocation and grain boundary are nailed by irregular big γ' precipitates, which are responsible for the improvement of room temperature strength and plasticity of the K424 alloy after incomplete solution treatment. The higher mismatch, cubicity and γ' phase volume fraction, as well as the dissolved γ/γ' eutectics are responsible for the highest stress rupture life of the K424 alloy after complete solution treatment at 1220 ℃ for 4 h.

NM500 wear-resistant steel was toughed by intercritical quenching. The results show that after intercritical quenching at 790 ℃ and tempering at 170 ℃, the microstructure of the tested steel contains 7.3% volume fraction of ferrite, and the impact absorbed energy is increased from 40 J after complete quenching to 58 J, with the best wear resistance. Complete austenitization cooling to below Ac₃ followed by intercritical quenching reduces wear properties of the steel due to the distribution of ferrite, while impact absorbed energy does not change a lot. Intercritical quenching increases the carbon content of martensite, a small amount of needle-like ferrite in the structure can improve the toughness of the steel, and wear resistance is better than that of the complete quenched.

Traditional spring steel 50CrVA was taken as the research object, and the bainite transformation was realized by austempering at 350, 400, 450 ℃ in the furnace after water mist +air fast cooling. The effect of austempering temperature on microstructure and properties of the 50CrVA steel was studied by means of thermal simulation, SEM, XRD, tensile and impact tests. The results show that the bainite transformation rate first increases and then decreases with the increase of austempering temperature, and the transformation rate is the fastest at 400 ℃. The bainite transformation quantities at 350 ℃ and 400 ℃ are the same and higher than that at 450 ℃, this is due to the precipitation of carbides, which reduces the carbon content in the undercooled austenite, the bainite transformation is completed, thus obtaining full bainite microstructure. With the increase of austempering temperature, the bainite, cementite and M/A structure become coarser, resulting in a significant reduction in tensile and impact properties, and the fracture mode gradually changes from quasi-cleavage fracture to cleavage fracture.

Hot rolling with different deformations for high aluminum 316L stainless steel was carried out, and the effects of which on microstructure, tensile properties and fatigue properties of the 316L steel were studied using SEM and TEM characterization, as well as tensile and fatigue testing methods. The results show that the hot rolled high aluminum 316L stainless steel has austenitic and ferritic microstructure, while the strength and fatigue life of the tested steel can be improved by hot rolling deformation. With the increase of rolling deformation, the ferrite volume fraction and dislocation density in the tested steel microstructure increase gradually, the tensile strength and fatigue life increase obviously, while the elongation decreases. The tensile strength and fatigue life of the tested steel with 75% hot rolling deformation reach their maximum values, which are 937.6 MPa and 3.8 ×10⁷ times, respectively, while the elongation reachs the minimum value of 42.7%.

Secondary cold rolling process was implemented for the trial production of high-grade non-oriented silicon steel, which had a thickness of 0.25 mm and a Si content of 3.5%. The evolution of microstructure and texture of the non-oriented silicon steel at different intermediate annealing temperatures (850, 950 and 1050 ℃) was studied by means of OM, EBSD, XRD and square coil magnetic properties testing. The results show that with the increase of intermediate annealing temperature, the average grain size of the finished annealed sheet increases first and then decreases, and the strength of the cube and Goss textures of the finished annealed sheet increases first and then decreases, the magnetic induction strength B₅₀₀₀ of the finished annealed sheet increases first and then decreases, while the iron loss value P_1.0/400 decreases first and then increases. When the intermediate annealing temperature is 950 ℃, due to the larger average grain size and the best grain uniformity, the number of shear bands after two-stage cold rolling is the highest, providing a large number of nucleation sites for cube and Goss textures during the annealing process of the finished sheet, resulting in texture strengths of 13.56 and 5.03, respectively, resulting in the highest magnetic induction intensity, with B₅₀₀₀ of 1.691 T, and the iron loss is the lowest, with P_1.0/400 of 13.36 W/kg.

Effect of tempering process on microstructure evolution and mechanical properties of HT9 steel was studied by SEM, TEM, phase analysis, tensile and impact tests. The results show that the tempering process can affect the mechanical properties of the tested steel by influencing the precipitates and dislocations. After tempering, the microstructure of the HT9 steel is lath martensite, and the precipitated phase is M₂₃C₆ carbide, and no other precipitated phase is observed. With the increase of tempering temperature, the matrix gradually recovers, the dislocation tangles are gradually normalized and thinned into dislocation networks, and forming subgrains. The M₂₃C₆ carbide undergoes Ostwald ripening, the precipitation strengthening and dislocation strengthening effects are weakened, the strength and hardness decrease, while the toughness increases. Tempered at 770 ℃ for 1-12 h, as the tempering time prolongs, the matrix gradually recovers and the M₂₃C₆ phase gradually ripens. The strength and hardness gradually decrease with a certain time dependence. According to the mechanical properties standards, the strength and toughness match well when the heat treatment system of the HT9 steel is quenching at 1040 ℃ for 1 h and tempering at 770 ℃ for 6 h.

Effect of normalizing temperature on the microstructure, texture and magnetic properties of 3.2wt%Si high magnetic induction orientation (Hi-B) silicon steel was studied by means of EBSD technique and single-chip measurement method. The results show that with the increase of normalizing temperature, the columnar grains of the normalized plate are completely recrystallized, and the texture types are similar to α texture, λ texture and Goss texture. After decarburizing annealing, the texture mainly consists of {114}<481>, γ texture and Goss texture, and the primary recrystallized grain size increases first and then decreases with the increase of normalizing temperature (maximum at 900 ℃ , up to 20.3 μm). After normalizing annealing at 850 ℃, the area fractions of {114}<481> texture and Goss texture, which are beneficial to magnetic properties, reach the highest (26.1% and 4.5%, respectively) in the primary recrystallized specimen. When the normalizing temperature is 850 ℃, the Hi-B steel can obtain the best magnetic properties, B₈=1.938 T, P_1.7/50= 0.828 W/kg.

In order to effectively repair the surface damage of FV520B steel and improve the corrosion resistance of the repaired surface, the method of surfacing followed by laser surface treatment was applied, i.e., surfacing was first performed by open-arc swing cladding method with martensitic-type flux-cored wires on the base material of the FV520B steel, and laser surface treatment was performed on the surfaced layer with two types of lasers (semiconductor and fibre optic lasers), respectively. The microstructure and hardness of the surfaced layer, the elemental distribution between the surfaced layer and the base material, as well as the microstructure, hardness, corrosion and depth of laser effects of the surfaced layer after laser surface treatment were investigated. The results show that the surfaced layer consists of martensite, δ-ferrite, retained austenite and carbide, and the hardness is about 480 HV0.3, which is about 33.33% higher than the base material, and the metallurgical bonding between the surfaced layer and the base material is well established. After laser surface treatment, the surfaced layer is categorised into the original surfaced zone, the heat affected zone and the remelted zone. In the heat affected zone, the hardness decreases, which is related to the tempering effect of the laser surface treatment, in the remelted zone, the hardness is comparable to the original surfaced zone. The results of the immersion experiments show that the laser surface treatment improves the corrosion resistance of the surfaced layer. As the scanning speed decreases, the depth of laser action increases, but the hardness does not change much. Compared with the fibre laser, the semiconductor laser has a greater depth of effect, but the surface hardness after both laser treatments is basically the same.

In order to further improve the performance of 9Cr18Mo steel, the Quench-Polish-Quench (QPQ) rare earth nitriding agent was composed of self-made nitriding base salt as nitriding agent, potassium nitrate as oxidizing agent, lanthanum carbonate, yttrium carbonate and cerium carbonate as rare earth accelerator, and then the QPQ salt bath composite treatment was carried out for the 9Cr18Mo steel at 610 ℃. The effects of rare earth on the microstructure and properties of the 9Cr18Mo steel were studied by means of SEM, EDS, XRD, microhardness test, electrochemical performance test and wear resistance test. The results show that the surface of the QPQ salt bath composite treated specimen is mainly composed of oxide layer, nitriding layer and diffusion layer. The main component phases are Fe₄N, CrN, α_N, Fe_2-3N, Li₂Fe₃O₄ and Fe₃O₄. Compared with that without rare earth addition, the addition of rare earth on the QPQ salt bath composite treatment can increase the microhardness by 90 HV0.2 on average, and also improve the corrosion resistance and wear resistance.

Compared with ordinary high speed steels, PM60 high-speed steel prepared by powder metallurgy contains a large amount of Co element, which can promote the precipitation of secondary carbides. The PM60 high-speed steel was quenched at 1120, 1150, 1180 and 1210 ℃, and oil cooling, and then tempered three times at 540, 560 and 580 ℃. After the heat treatment, the microstructure, precipitated phases, and hardness of the PM60 high speed steel after heat treatment were analyzed by metallographic microscope, scanning electron microscope, and Rockwell hardness tester. The types, morphology, composition, distribution, and precipitation behavior of the carbides in the samples before and after heat treatment were investigated by using JMatPro thermodynamic calculation software. The results show that the original precipitation phases are MC, M₆C, M₂₃C₆ and M₇C₃ phases, after heat treatment, the precipitation phases are MC and M₆C phases. By energy spectrum analysis, two types of carbides are found: the dark V-rich carbides, and the bright W and Mo-rich carbides. In the quenching process, part of the carbides is dissolved, and in the tempering process, there is fine dispersive secondary carbides precipitated out. Under the condition of quenching at 1180 ℃+tempering at 540 ℃, the hardness of the tested steel is the highest.

16Cr25N ferritic stainless steel was nitrided by a high-temperature solution nitriding process in a self-designed high-temperature and high vacuum nitriding furnace, and then the microstructure and properties of the nitrided layer were analyzed by means of X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, metallographic microscopy, microhardness tester, and oxygen nitrogen analyzer. The results show that an austenitic nitrided layer with a thickness of more than 1 mm is formed on the surface of the tested steel, with the maximum single-side thickness of the nitrided layer being more than 2 mm and the maximum nitrogen content reaching 1.502%, and the original ferrite microstructure transforms into austenite, which is equivalent to obtaining a high-nitrogen and nickel-free stainless steel. and its hardness after the nitriding treatment is greatly increased to 320 HV0.5. The activation energy of nitrogen diffusion during solid solution nitriding of the 16Cr25N stainless steel is calculated by the depth of the infiltration layer, which is 176 kJ/mol under the condition of nitrogen pressure of 0.3 MPa, insulation time of 24 h, and nitriding temperature of 1200 ℃. The nitrogen content model for high temperature solution nitrided layer of the 16Cr25N ferrite stainless steel is established by using the infiltration layer hardness and diffusion theory, which has certain guiding impact.

A machine-learning approach based on gaussian process regression (GPR) was proposed to optimize the processing window of laser power (P) and scanning speed (v) in the IN625 alloy fabricated by laser powder bed fusion (LPBF) using the experimental data of relative density, crystallography orientation and shape aspect ratio of columnar grains. The effect of laser power-scanning speed combinations and laser energy density (ED) on the relative density and microstructure of the LPBF specimens was investigated as well. The results show that the applied laser ED≥55 J/mm³ is prerequisite for fully dense LPBF specimens with relative density ≥99%. The optimized L-PBF processing window for manufacturing fully dense IN625 alloy is pear-shaped, and the selectable scanning speed range can be wide in the case of high laser power. The LPBF specimens possess the columnar grains with the preferred orientation <001> parallel to build direction (BD). The preferred orientation intensity and the shape aspect ratio of the columnar grains tend to increase with the laser ED but marginally decrease after a high ED value. The leave-one-out cross validation reveals that the GPR model predicting the optimized LPBF process window based on relative density and microstructure features is reliable and can be readily applied to multi-objective optimization of laser additive manufacturing process in other metals and alloys.

An extra-deep drawing steel for enameling was annealed at different temperatures using continuous annealing simulator. The mechanical properties, recrystallization texture, microstructure and second phase particles of the annealed steel were studied by tensile testing machine, X-ray diffractometer, optical microscope and transmission electron microscope (TEM). The results show that the recrystallization texture of the tested steel after annealing is mainly distributed in γ orientation line, the texture types are mainly {111}<110> and {111}<112>, which contributes to high r value. With the increase of annealing temperature, the r value increases, which is attributed to the increase in density of the {111}<110> and {111}<112> textures with the increase of annealing temperature. Among them, the {111}<110> texture is increased significantly in density, while the density increment of {111}<112> texture is very small or even reduced, where the former significant increment is attributed to the weakening of grain pinning effect on {111}<110>texture orientation caused by TiC particle coarsening. The coarsening of TiC particles is a necessary condition for the full development of {111}<110>oriented grains during the annealing process, and is an important factor for the steel plates to achieve excellent formability. Meanwhile, as the {111} texture density increases, the strength of the steel plate decreases, the elongation after fracture increases, and the formability improves.

After annealed at 800-950 ℃ for 8-48 h, the changes of microstructure and hardness of the hot-rolled 420U6 nitrogen-containing martensitic stainless steel under different annealing processes were studied by means of laser confocal microscope and Vickers hardness tester. The results show that the annealed microstructure is composed of ferrite matrix distributed with carbide, and the size and morphology of the carbide differ obviously under different heat processes. When annealed at 850 ℃ for 16-24 h, the carbide exhibits spheric shape and its size is generally uniform with average diameter of 0.4-0.5 μm. With the increase of annealing temperature and prolonging of holding time, the hardness of the tested 420U6 steel decreases at first and then increases slightly. The occurrence of lamellar carbide is responsible for the increase in hardness.

Effects of different spinning temperatures and different water tank water amount distributions (high-medium-low, high-low-high) on the microstructure and properties of SWRH82B high carbon steel wire rod were investigated. The results show that when the spinning temperature is increased to 910±10 ℃ and the water in the water tank is distributed according to high-low-high, the temperature difference between the core and the surface of the wire rod increases from 18 ℃ to 30 ℃, the cooling rate of the core of the wire rod is improved, the formation of network cementite is reduced, the detection rate of network cementite is reduced from 70% to 5%, and the grade of network cementite is reduced from grade E to grade C. At the position of 1/4 diameter of wire rod, the lamellar spacing of the sorbite decreases from 153 nm to 138 nm, and the tensile strength of wire rod increases from 1174 MPa to 1187 MPa. For continuous drawing the rod from Ø6.5 mm to Ø2.1 mm, the breaking rate is reduced from 9 times /100 t to 1 time /100 t, which effectively improves the drawing performance of hot rolled wire rod.

Aiming at the technical problem that the mechanical properties of the ZL201A aluminum alloy could not meet the design and acceptance requirements during the trial production of a certain type of product, combining with the performance characteristics of the ZL201A binary alloy and the actual production experience, through the formulation of a reasonable heat treatment process plan, the effect of different heat treatment temperatures and time on the microstructure and properties of the ZL201A aluminum alloy under the condition of two-stage solid solution was investigated. The results show that with 535 ℃×9 h+545 ℃×9 h two-stage solid solution treatment and 160 ℃×6 h aging, the ZL201A aluminum alloy demonstrates best comprehensive mechanical properties with tensile strength 389 MPa, yield strength 342 MPa and elongation 11.5%. The testing results not only meet the technical requirements of product design acceptance, but also accumulate effective practical data for the subsequent material engineering application and system research.

Effect of tempering temperature on the microstructure and mechanical properties of a deep well oil casing steel quenched at 920 ℃ was studied by means of optical microscope (OM), scanning electron microscope (SEM) and tensile testing machine. The results show that the experimental steel obtains tempered martensite structure after tempered at 500-600 ℃, which has high product of strength and elongation and high toughness, where the product of strength and elongation is ranged from 20.5 to 22.1 GPa·%, and the impact absorbed energy is ranged from 94.6 to 100.3 J. When the tempering temperature is 550 ℃, the deep well oil casing experimental steel has the best comprehensive mechanical properties, with tensile strength of 978 MPa, yield strength of 935 MPa, and product of strength and elongation of 22.1 GPa·%, impact absorbed energy is 100.3 J.

Conventional annealing process (long time at low temperature) and rapid annealing process (instantaneous at high temperature) were compared and analyzed from four aspects of microstructure, grain size distribution, grain orientations and mechanical properties of cold rolled ultra-low carbon steel by means of OM, SEM, EBSD and uniaxial tensile test. The results show that after rapid annealing at 650 ℃ for 15 s, the yield strength and tensile strength of the tested steel are 445.38 MPa and 494.07 MPa, respectively, which are obviously increased from that of conventional annealed (i.e., the yield strength and tensile strength being 274.35 MPa and 388.99 MPa, respectively), while the elongation is still guaranteed up to 24.7%. The grain refinement and the typical γ orientation are the main reasons for the difference of properties between the conventional and rapid annealed specimens, where the grain refinement is the dominant. The rapid annealing has a higher grain refinement strengthening effect, but it deteriorates the work hardening ability and leads to a decrease in elongation. In addition, the rapid annealing inhibits the precipitation of the second phase particles while refines them. By comparison, the rapid annealed specimens show a typical γ orientation, which is more conducive to improving the rolling performance, but that by conventional annealing show a near γ orientation which has less such effect.

By simulating the continuous annealing process of a cold rolled 980 MPa dual-phase steel by a Vatron Voestalpine thermal simulator, the effect of soaking temperature, slow cooling temperature, overaging temperature and the strip speed in continuous annealing on microstructure and mechanical properties of the dual-phase steel was studied by means of tensile testing machine, optical microscope and scanning electron microscope. The results show that with the increase of the soaking temperature in the range of 820, 840 and 860 ℃, the proportion of ferrite decreases and the yield strength increases. A certain amount of bainite appeares in the microstructure when the soaking temperature is increased to 860 ℃, which leads to the tensile strength reach the lowest value 1020 MPa. The slow cooling temperature can change the volume fraction of martensite and affect the mechanical properties of dual-phase steel. When the overaging temperature varies among 250, 270, 290, 310 and 330 ℃, with the overaging temperature increases, the degree of softening of the martensile increases, the tensile strength decreases and the total elongation of the tested steel increases. With the increase of overaging temperature above 310 ℃, the decomposition of martensite is promoted, and at the same time the precipitations of second phases are increased and the yield strength decreases first and then increases. Comprehensively considering the mechanical properties control of the cold rolled dual-phase steel under different annealing processes, the optimal annealing parameters are selected as soaking temperature of 840 ℃, slow cooling temperature of 680 ℃, overaging temperature of 250 ℃ and the strip speed of 120 m/min, under which the maximum value of the product of strength and elongation is 15.6 GPa·%.

Vacuum heat treatment furnace was used to test the dimensional distortion rate of SLD-MAGIC and DC53 cold working die steel specimens under three heat treatment conditions of quenching at 1030 ℃, tempering at 210 ℃ and tempering at 520 ℃, and combined with X-ray diffractometer (XRD), scanning electron microscope (SEM), high-precision differential dilatometer, JMatPro software to analyze the microstructure, characteristic parameters of martensite, the content of retained austenite and the content of secondary carbides. The results show that the volumes of the SLD-MAGIC and DC53 steel specimens expand after the three heat treatments, and their dimensional distortion rates range from 0.02% to 0.12%. The average dimensional distortion rate of the DC53 steel is about 1.9 to 2.2 times that of the SLD-MAGIC steel under the same heat treatment conditions. The average thermal expansion coefficient of the SLD-MAGIC steel from 25 ℃ to 700 ℃ is 0.15×10^-6℃^-1 lower than that of the DC53 steel, the carbon content and lattice constant of the martensite of the SLD-MAGIC steel are 1.34% and 2.8894×10^-10 m, respectively, which are smaller than that of the DC53 steel. The content of retained austenite and the content of secondary carbide of SLD-MAGIC steel after heat treatment are less than that of the DC53 steel. Therefore, the size expansion effect of the SLD-MAGIC steel is smaller than that of the DC53 steel, and the SLD-MAGIC steel has the characteristic of heat treatment microdistortion.

Alloying elements in PCrNi3MoV high-strength steel can improve the hardenability and mechanical properties, but also lead to banded structure by their segregation. Microscopic characteristics of banded structure in the PCrNi3MoV steel in different heat treatment processes were studied by means of optical microscope, scanning electron microscope, transmission electron microscope and electron probe microanalysis. The results show that the banded structure presents at different heat treatment stages of the tested steel, though the microscopic composition and characteristics are different. In the quenched state, the banded structure is characterized by alternating distribution of large and small size grains. In the tempered state, the size of martensitic lath in fine-grained region of the banded structure is smaller, the precipitation density of the carbides is higher, and the microhardness of the fine-grained region is higher than that in the coarse-grained region, which is attributed to the segregation of C and Cr, Mo, Mn and V elements in the fine-grained region, the grain strengthening and precipitation strengthening effects are more pronounced than those in the coarse-grained region.

Inconel 718 alloy was prepared by selective laser melting (SLM), and the as-deposited alloy was double aged (DA) and solution+double aged (SA). The precipitation behavior and tensile properties of the alloy under different heat treatment states were studied by means of scanning electron microscope, electron backscattered diffraction, transmission electron microscopy, and universal testing machine. The results show that aging alone cannot eliminate the Laves phase. After direct aging, the Laves phase still exists in the alloy, but the content of γ' and γ″ strengthened phase is higher than that of the deposited state. The strength of the material significantly increases, and the plasticity significantly decreases. After high-temperature solid solution treatment at 1150 ℃ followed by double aging, the alloy is completely recrystallized, and the substructure disappears. The brittle Laves phase dissolves completely, and the content of the strengthened phases γ' and γ″ phases increases significantly. The strength increases and plasticity decreases compared to the as-deposited specimen. The tensile strength, yield strength and elongation of the alloy reaches 1330 MPa, 1110 MPa and 16.73%, respectively. The solution+double aging process, while enhancing the material's strength, ensures plasticity, which makes the Inconel 718 alloy to have excellent comprehensive mechanical properties.

Hydrogen traps, hydrogen diffusion coefficient and hydrogen embrittlement sensitivity at the surface, quarter and half thickness of EH36 steel were investigated by means of hydrogen penetration test and slow tensile test. The results show that the microstructure of the surface and the quarter thickness is mainly bainite, the microstructure of the surface is relatively fine, and the half thickness is mainly ferrite and pearlite. The density of reversible and irreversible hydrogen traps decreases sequentially from the surface to the half thickness, and the hydrogen diffusion coefficient increases accordingly. As the hydrogen charging current density or time increases, the yield strength, tensile strength, and elongation of tensile specimens at various thicknesses decrease to varying degrees, and the hydrogen embrittlement sensitivity increases accordingly. The tensile fracture morphology gradually transitions from ductile dimples with ductile fracture characteristics to river like patterns with brittle fracture characteristics. The hydrogen embrittlement sensitivity is the smallest at the surface, and the hydrogen embrittlement sensitivity is the highest at the half thickness. Some hydrogen induced cracks are observed on the tensile fracture surface at the half thickness.

In order to meet the strength and toughness requirements of low-carbon microalloyed steel for coiled tubings, a 5.3 mm thick hot-rolled plate for coiled tubing was prepared by designing controlled rolling and controlled cooling production process. The low cycle fatigue property of the hot rolled plate was analyzed by low-cycle fatigue test and the low-cycle fatigue fracture was observed. The mechanical properties and microstructure of the hot rolled plate were analyzed. The results show that the fatigue life of the tested steel is 3784 cycles when produced at final rolling temperature of 810 ℃, cooling rate of 52 ℃/s, and coiling temperature of 450 ℃. The morphology of the transient fracture zone of the low cycle fatigue fracture is mainly dimple, and the fracture mode belongs to ductile fracture. The fitted formula for the relationship between low cycle fatigue strain and fatigue life has a good fitting effect and can be used to predict fatigue life under different strain amplitude conditions. The mechanical properties of the tested steel meet the requirements, and the microstructure consists of ferrite, bainite, and M/A islands. The volume fraction of ferrite is about 74%, and mainly exists in the form of acicular ferrite, making the tested steel have good fatigue property.

Microstructure and mechanical properties of SCM435 steel quenched at 870 ℃ and tempered at 350-650 ℃ were studied by means of SEM, TEM and tensile test. The results show that when tempering temperature is 350 ℃, the structure is martensite lath and a small amount of carbide. With the increase of tempering temperature, the lath morphology of martensite gradually disappears, and carbides precipitate and grow along the direction of lath. The microstructure tempered at 525 ℃ still has obvious martensite lath morphology and short rod cementite is dispersed. By controlling the mechanical properties of the SCM435 steel within the studied tempering temperature range, it can meet the mechanical performance requirements of 8.8-12.9 level fasteners. The fatigue properties of the steel quenched at 870 ℃ and tempered at 525 ℃ is experimentally verified, with a median fatigue limit of 425 MPa, indicating good fatigue properties.

A novel low alloy ultra-high strength steel 40CrMnSi2Mo with a 2 GPa grade tensile strength was developed by adding Si and small amount of Mo in the traditional 35CrMnSiA steel. The effects of quenching temperature (875-975 ℃) and tempering temperature (200-300 ℃) on the mechanical properties of the 40CrMnSi2Mo steel were systematically investigated. The results show a higher martensite starting temperature (Ms) decreases the formation tendency of twinning martensite structures, which benefits the toughness of the 40CrMnSi2Mo steel. A small amount of Mo can effectively inhibit the temper brittleness, which makes the impact absorbed energy of the steel tempered at 300 ℃ only decrease 7.7%, while tempering at 275 ℃ and 300 ℃ reduces the retained austenite quantity which leads to the decrease of fracture toughness. In general, a well combination of tensile strength (over 2 GPa) and fracture toughness (over 85 MPa·m^1/2) of the 40CrMnSi2Mo steel can be obtained when the studied steel quenched at 950 ℃ and tempered at 200 ℃ and 225 ℃, which outperform that of AISI 4340 steel and 300M steel. It is expected that this novel low-cost steel has a significant application prospect.

Primary α phase microstructure of Ti62A titanium alloy solution treated at 910 ℃ for different time was characterized by means of OM, SEM and EBSD techniques, to investigate the evolution characteristics of the primary α phase and the spheroidization mechanism during the solid solution process. The results show that the initial microstructure of the forged Ti62A titanium alloy consists of the primary α phase, lamellar α phase and residual β phase. During the solid solution process at 910 ℃, the lamellar α phase dissolves rapidly, and the volume fraction of primary α phase first decreases and then remains unchanged with the increase of solution time, but the axis/diameter ratio is basically maintained at about 2.1. During the solution treatment, a large misorientation appeares inside the grains of the primary α phase, producing α/α grain boundaries with different orientations. With the growth of solution time, the grain is prone to grain boundary separation and the formation of new equiaxed primary α phase when the misorientation of α/α grain boundaries within the long strip primary α phase is greater than 10°.

Through orthogonal test and grey correlation degree analysis, the primary and secondary relationship and rule of influence of nitrided layer of 1Cr11Ni2W2MoV martensitic stainless steel on wear performance were studied. The microstructure of nitrided layer was observed by optical microscope, and the thickness of nitrided layer was measured by metallography. The surface phase of the nitrided layer was analyzed by means of XRD diffractometer. The surface hardness and hardness gradient of the nitrided layer were measured by Vickers hardness tester. The friction and wear properties of nitrided layer were measured by SRV friction and wear testing machine, the maximum wear depth and wear volume were measured by white light interference 3D surface profilometer, and the wear mechanism was analyzed by scanning electron microscope. The influence of surface hardness, hardness gradient and thickness of nitrided layer on wear properties was studied by grey correlation analysis. The results show that the wear failure of stainless steel nitrided layer is mainly caused by abrasive wear, adhesive wear and local fatigue spalling. The surface hardness, hardness gradient and thickness of nitrided layer have significant effects on the wear performance under the condition of constant large load 200 N, and the thickness of nitrided layer has the most significant effect on the wear performance, and the wear performance decreases with the increase of the thickness of nitrided layer.

An ultra-low carbon bainitic steel was tempered in the dual-phase region for different time after quenching. The evolution of reversed austenite and the distribution of Ni and Mn elements in the matrix and in the reversed austenite were studied by means of scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and X-ray stress meter. The results show that the matrix of the quenched specimen is lath bainite with a small amount of retained austenite. With the extension of tempering time, reversed austenite appears in the matrix, and the content increases first and then decreases, and the morphology changes from film-like to strip-like. When the tempering time is up to 240 min, the reversed austenite in the matrix is the most stable and the content is the highest, about 15.6%. The content of Ni and Mn in the reversed austenite is 15.23% and 1.42%, respectively, which is much higher than that of the matrix. When the tempering time is 1440 min, the content of reversed austenite decreases to 5.50%, and the partially reversed austenite is unstable and transforms into new martensite. The contents of Ni and Mn are 12.58% and 1.21%, respectively, which is slightly lower than those in the most stable reversed austenite.

In order to obtain the precipitation law of Laves phase and carbides of Inconel718 superalloy, the phase composition of the alloy in equilibrium was calculated using the nickel-based database in Thermal-Calc software. The microstructure and energy spectrum of the alloy were observed by means of optical microscope, scanning electron microscope and field emission electron probe microanalyzer, and the element segregation in the precipitated phase was explained. Finally, the melting and solidification process of the Inconel718 nickel-base superalloy was studied by using high-temperature laser confocal microscope (HT-CLSM). The results show that the phase transformation sequence of the Inconel 718 alloy during cooling is liquid phase, L+γ phase, L+γ+MC phase, L+γ+MC+δ phase, γ+MC+δ+η phase, γ+MC+δ+η+σ phase, γ+MC+δ+η+M₂₃C₆ phase; Ni, Cr and Fe elements in the alloy are mainly distributed in the dendrite trunk, showing negative segregation, while Nb element is mainly distributed in the interdendritic, showing positive segregation. The solidus temperature of the alloy is 1380 ℃. During the solidification process, with the increase of cooling rate, the temperature at which the alloy begins to solidify gradually decreases, the time required to achieve complete solidification decreases, and the liquid fraction also decreases with the increase of temperature. In addition, the whole solidification process is divided into initial, stable and final solidification stages according to the precipitation phase and microstructure changes during the solidification process. At the same time, the secondary dendrite spacing in the alloy is proportional to the third power of the cooling rate after solidification.

After rapid heating quenching and austenite reverse phase transformation (RH-ART) treatment, the strengthening mechanism and carbide precipitation behavior of Nb-Mo microalloyed medium manganese steel used for self-made mobile umbrella type crossing frame truss were investigated. The results show that (Nb, Mo)C in the microalloyed experimental steel not only precipitates inside the ferrite grains and on dislocation lines, acting as a barrier to dislocation movement, thereby improving the strength of the experimental steel; but also precipitates at the grain boundaries between austenite and ferrite, playing a dragging effect role and suppressing the austenite grain coarsening. Meanwhile, after rapid heating quenching and austenite reverse phase transformation treatment, a large amount of nanoscale cementite precipitates, which is beneficial to increase the retained austenite content in medium manganese steel. In addition, when austenite reverse phase transformation is carried out at a temperature of 690 ℃, there is a certain amount of metastable carbide particles in the matrix, which helps to improve the nucleation rate of austenite and also reduces the average diffusion distance of C, which increases the growth rate of austenite and obtains more retained austenite content. The tested steel exhibites excellent comprehensive mechanical properties with a retained austenite content of more than 49%, a tensile strength of 779 MPa and an elongation at break of 77.7%.

By studying a V-N micro-alloyed non quenched and tempered steel using thermal simulation testing machine, the strain induced precipitation behavior was analyzed and the Precipitation-Temperature-Time (PTT) curve was drawn. A thermomechanical treatment was designed, the microstructure was characterized by means of OM, SEM and TEM. The mechanical properties was tested by universal tensile testing machine. The results show that the precipitation temperature of V(C,N) at the nose tip is around 800 ℃ during the stress relaxation process, and it begins to precipitate after 12 and ends after 90. During the deformation process at 800 ℃, the yield strength and tensile strength gradually increase by 30 MPa and 45 MPa respectively, while the deformation amount increases from 15% to 30%. Elongation varies within the range of 11%-12%. The impact absorbed energy increases from 19.5 J to 25 J. With the increase of isothermal deformation, the dislocation density inside the matrix grains significantly increases, the number of precipitates increases, and the distribution becomes more dispersed. Precipitated phases with lengths ranging from 50 nm to 150 nm and widths ranging from 20 nm to 50 nm are found both within the grain and at the grain boundaries.

Chemical composition of Ni₃Al-based alloy was designed based on the “Cluster-plus-glue-atom model” to improve its room temperature properties, the microstructure, mechanical properties and corrosion resistance of the alloys were investigated. The results show that the alloys before and after annealing are composed of β phase, α phase and matrix phase γ'. After annealing at 1050 ℃ for 24 h, β phase and α phase are evenly distributed in the matrix of Ni₅Co₅Cr₂Al_2.5Ti_1.5 alloy, which improves not only the mechanical properties (maximum compression rate of 29.35%, yield strength of 1796 MPa, hardness of 546.35 HV), but also induces galvanic corrosion, enhances the passivation effect and improves the corrosion resistance (impedance of 7.74×10⁴ Ω, corrosion potential of -0.055 V(vs SCE), corrosion current density of 5.93×10^-7 A/cm²) of the alloys. The alloys designed in this paper show significant improvements in mechanical properties and corrosion resistance compared to Ni₃Al. This work provides a new avenue for research in the composition design of high-temperature alloys.

Microstructure and mechanical properties of the high nitrogen martensitic stainless steels with 0.32%V and 0.80%V (named as 0.3V and 0.8V steels) were studied by means of optical microscope, SEM, hardness test, impact test and salt spray corrosion test. The results show that carbide bands appear in both the two as-annealed steels. For the 0.8V steel, the carbide bands are more pronounced, the annealed hardness is 257.0 HBS, which is 7% higher than that of the 0.3V steel, and the increase of V content can reduce the austenite grain size and retained austenite content after quenching. After the same heat treatment (quenched at 1050 ℃ for 30 min, cryogenic treatment at -73 ℃ for 2 h, and tempered at 250 ℃ for 2 h), the hardness and impact absorbed energy of the 0.3V steel are 58.4 HRC and 9.4 J, respectively, while those of the 0.8V steel are 54.4 HRC and 12.7 J, respectively, which implies that the increase in V content reduces the hardness of the tested steel and improves the toughness. After salt spray corrosion for 120 h, the corrosion rate of 0.3V steel is 0.0235 g·m^-2·h^-1and there is no obvious corrosion pit on the steel surface, while the corrosion rate of the 0.8V steel is 0.0258 g·m^-2·h^-1, and there are obvious corrosion pits on the surface, which indicates that the corrosion resistance of the 0.3V steel is slightly better than that of the 0.8V steel, meaning that the corrosion resistance slightly decreases with the increase of V content.

Mg-2Zn-0.4Ca-0.2Mn(wt%) alloy with high thermal conductivity and elongation was prepared by near solidus temperature extrusion method, and the effect of trace Ce element on microstructure and properties was explored. The results indicate that the grain size in the extruded alloy exhibits a bimodal distribution, and the average grain size is significantly decreased compared with that of as-cast state. The addition of Ce not only transforms the second phase in the alloy from Ca₂Mg₆Zn₃ phase to (Mg, Zn)₁₁Ce phase, but also reduces the unrecrystallized area fraction from 5.4% to 1.6%, with an average grain size of 5.29 μm dropping to 3.68 μm, and the basal texture intensity of the alloy is significantly weakened. Due to the grain refinement strengthening effect and that of the second phase containing Ce, the yield strength of the alloy is significantly increased from 163.67 MPa to 237.44 MPa, while the elongation is reduced from 27.45% to 19.58%. The addition of Ce increases the volume fraction of the second phase in the microstructure, but it also reduces the solid solubility of Zn in the matrix, so the thermal conductivity of the alloy is not significantly reduced.

In order to investigate the effect of SiC particle size on the microstructure and mechanical properties of SiC_p/Al composites, three kinds of SiC_p/2024Al composites containing 55%(volume fraction) SiC particles with size compositions being 100% 10 μm, 25% 10 μm+75% 76 μm and 100% 76 μm were fabricated by hot isostatic pressing and labelled as S, G and L, respectively. The results show that all the three SiC/2024Al composites contain SiC, Al and Al₂Cu phases. The flexural strength of the tested composites can be enhanced by increasing the proportion of fine SiC particles. The flexural strengths of S, G and L composites are 515, 489 and 422 MPa, respectively, and the corresponding fracture strains are 0.25%, 0.30% and 0.22%, respectively, indicating that the SiC particle gradation is an effective method to improve the density and the fracture strain of the composite. After annealing at 490 ℃, the flexural strengths of the three composites decrease to 494, 470 and 368 MPa, respectively, and the corresponding fracture strains increase significantly to 0.43%, 0.39% and 0.37%, respectively.

Continuous cooling transformation curves of the Nb-V-Ti-N microalloyed steels were conducted on a DIL-805 dilatometer, and the effect of Nb content on the microstructure and properties was investigated. The results show that when the cooling rate increases from 0.1 ℃/s to 30 ℃/s, the undercooled austenite successively transforms into ferrite, pearlite, bainite and martensite, and the corresponding cooling rate ranges are 0.1-20, 0.1-5, 1-30 and 10-30 ℃/s, respectively. The transformation temperatures of ferrite, pearlite and bainite decrease with the increase of cooling rate, while that of martensite increases. As the Nb content increases from 0.05% to 0.10%, the solid solution of Nb in austenite increases, the CCT curves are slightly downward, and both Ac₁ and Ac₃ temperatures increase. With the increase of cooling rate, the microhardness is enhanced, and the corresponding value in the range of 1-20 ℃/s for the tested steel containing 0.10%Nb is higher than that for the steel with 0.05%Nb, which is largely related to the percentage of hardening phases, the degree of microstructure refinement and the precipitation of carbonitrides. In addition, the submicron Nb-rich carbonitrides precipitated at the high temperature region of austenite can not only pin grain boundaries but also act as heterogeneous nucleation sites to induce the formation of intragranular ferrite, which effectively refine the microstructure and consequently improve the strength and toughness.

Though investigating and comparing the microstructure and stress corrosion cracking (SCC) behaviors at different locations of large-size Z-shaped sections of 2224 aluminum alloy in T3511 state, the effects of grain morphology on the SCC resistance and cracking modes were discussed. The results show that the specimens with fibre-like grains have lower stress corrosion sensitivity and are fractured in 52 days at stress of 250 MPa, while the specimens with nearly equiaxed grains are fractured in only 42 days. The nearly equiaxed grains have larger grain size and large-angle grain boundaries with higher interfacial energy, along which the stress corrosion crack is easy to propagate due to the lower grain boundary strength. The cracking mode of the specimens at different positions is all intergranular cracking, and the conductivity value in different positions of the Z-shaped section remains stable.

Effect of vanadium content on the microstructure and properties of duplex stainless steel TDVS2001 was investigated. The precipitates and microstructure were characterized by metallographic microscope, SEM, EDS and TEM. The results indicate that when the vanadium content is less than 0.11%, there is no vanadium precipitate in the metallographic structure and the mechanical properties are slightly improved. When the vanadium content is 0.27%, a large number of nano-scaled vanadium precipitates occur in the ferrite phase, the yield strength of the material increases by 68 MPa, while the impact absorbed energy has no obvious change. The vanadate is confirmed to be VN precipitation by transmission electron microscope. The smaller grain size and nano-scaled VN precipitation are the main reasons for the improvement of strength.

Effects of Si, V and Ti alloying elements on the microstructure and mechanical properties of 40Cr13 martensitic stainless steel were studied by means of optical microscope, scanning electron microscope, room temperature tensile test and room temperature impact test. The results show that the addition of V and Ti alloy elements to the 40Cr13 martensitic stainless steel effectively inhibit the precipitation of (Fe, Cr)₂₃C₆ carbides. The hardness and impact property of the 40Cr13 martensitic stainless steel are effectively improved by increasing the content of Si element. The addition of 0.21%V improves the strength and hardness of the 40Cr13 martensitic stainless steel, but the addition of 0.21%Ti reduces the strength and hardness of the 40Cr13 martensitic stainless steel. The addition of V and Ti alloy elements at the same time reduces the tensile strength and yield strength of the 40Cr13 martensitic stainless steel slightly, but improves the impact toughness of the 40Cr13 steel effectively.

By designing the specimens with/without CoNiCrAl coatings as stepped ones to exclude other environmental factors, the effect of stress on the creep microstructure and elemental diffusion of DZ411 superalloy at 950 ℃ was studied, and the protective effect of the coating on the specimen was analyzed. The results show that the coating can significantly increase the creep life of the specimen. It is find that the greater the stress in the specimen, the higher the degree of degradation of the γ' phase and the higher the degree of compactness, especially the γ' phase in the high-stress zone is completely rafted, and that in the rest zones shows different degrees of spheroidization. The coating of the coated specimen in the high-stress zone is seriously damaged, while the coating in the low-stress and the non-stress zones only undergo slight oxidation. The degree of oxidation on the surface of the uncoated specimen increases with the increase of stress, though that in the high stress zone is more serious and with defects such as cracks and looseness, while the surface of the low stress position is still kept more complete. In addition, the element diffusion also changes with the stress, and different oxides are formed on the surface of the specimens, among which the Ti element content of the coated specimen gradually decreases from the matrix to the oxide layer surface, and decreases with the decrease of stress, while the Cr element content continues to increase from the surface to the inside and then decreases.

TiC coatings were cladded on the surface of 45 steel by a CO₂ constant current laser using Ni60+TiC gradient layer structure. The effect of process parameters(laser power, scanning rate and overlap rate) on the microstructure and properties of TiC coatings was investigated. The results show that the surface cracks of the coating decrease and then increase with the increase of laser power and scanning rate. The surface cracks disappear when the laser power is 4.0 kW and the scanning rate is 400 mm/min. With the increase of the overlap rate, the surface cracks of the coating increase and then decrease, and the surface cracks of the coating disappear when the overlap rate is 50%. The optimal process parameters are laser power 4.0 kW, scanning rate 400 mm/min, and overlap rate 50%. The thickness of TiC gradient coating is about 1.2 mm after the best process, and it is metallurgically bonded with the base layer. The microstructure of the most superficial layer is dendritic TiC and a small amount of Ni-based solid solution, the microstructure of the gradient layer is TiC+Ni-based solid solution, TiC content from the surface to the inside is in a gradient decrease, Ni-based solid solution content from the surface to the inside is in a gradient increase. The hardness of TiC coating decreases from 1968 HV0.2 to 240 HV0.2. In addition, the wear rate of the TiC coating is 1.757×10^-15 m³/(N·m).

Based on the electromagnetic induction heat treatment experiment of 100 mm thick-walled cylinder of Ø1900 mm with external diameter, a step-type temperature rise control method was designed, and the influence of different induction heating parameters on temperature field of local heat treatment of cylinder was analyzed, and the optimal electromagnetic induction heating parameters were obtained. The results show that, according to the technical requirements of the holding temperature of 595-620 ℃, the best designed parameters are that the 20 mm diameter induction cable is selected with the induction coil number of 16, the width of the outer wall insulation is 1460 mm, the inner wall is fully insulated, and the turn spacing of 32 mm is the best design parameters. The larger the current frequency of the induction heating, the faster the heating rate of the cylinder surface. In order to maintain temperature uniformity, it is recommended to choose a smaller frequency of 2 kHz within the appropriate range.

For the problem of long tubular shape coefficient in the current heat treatment standard, a standard sample library consisting of 21 simulated temperature curves with Ø20-Ø40 mm diameter of the rod was established by using the Heat Transfer analysis module provided by ABAQUS simulation software and by uniform adopting environmental radiation heat transfer. Using simulation and data analysis technology, the “thickness comparison method under equivalent circular rod diameter condition” is proposed. The results show that the short tube shape coefficient is 2, and the long tube shape coefficient is 2+2R_inside/R_outside.

In view of the prominent problems of poor cooling capacity and uniformity of the rail air-quenching technology, supersonic profiling nozzle (SP nozzle) was designed to improve the cooling capacity through supersonic jet and improve the cooling uniformity through the profiling structure of the rail head, and then the effect of heat transfer characteristics of SP nozzle on the rail was studied by using the numerical simulation combined with experiments. The results show that the cooling capacity of the SP nozzle is 54.22%, 13.88% and 47.01% higher, and the cooling uniformity is 31.11%, 31.11% and 6.06% higher than that of the existing circular constant velocity nozzle, circular supersonic nozzle and constant velocity profiling nozzle, respectively. In the production, the following measures are beneficial for improving production efficiency while ensuring the performance of the rail: adjusting the inlet pressure of the SP nozzle to cool the rail at a rate close to but not greater than the critical cooling rate of the pearlite transformation before cooling to the end temperature of phase transformation (about 500 ℃), and then increasing the inlet pressure as much as possible.

With the implementation of China's marine development strategy, the demand and development of marine engineering steel will become a new growth point in the domestic steel industry. The composition design, microstructure design, and production process research of marine engineering steel have become hot areas of focus for scholars. Due to significant differences in cross-section, there are certain differences in the arrangement of processing technology between steel plates and H-beams used in marine engineering. This paper summarizes the current research status and progress of marine engineering steel, introduces its composition design characteristics of marine engineering steel and alloying mechanism, analyzes the microstructure design characteristics of different intensity levels, and expounds the production process characteristics of marine engineering steel plates and H-beams from three aspects: heating temperature, rolling and cooling process. It also summarizes and prospects the research focus of marine engineering steel production technology.

7××× series ultra-high strength aluminum alloys with high specific strength, good toughness and corrosion resistance have important application potential and development prospect in bogie components of rail vehicles to meet the major application requirements of green, safe, efficient and intelligent in the manufacturing of new generation rail vehicles. The development trend of the 7××× series ultra-high-strength aluminum alloys is reviewed. In view of the new manufacturing and service requirements of the 7××× series ultra-high-strength aluminum alloys for rail vehicles, the performance characteristics and research status of two typical 7××× series ultra-high strength aluminum alloys (7050 and 7085 alloy) have been introduced and compared. The problems that need to be studied and solved in the application and service of the 7××× series series ultra-high strength aluminum alloy with high efficiency, safety and long life under complex conditions are also prospected. It is expected to provide a guideline for the design, material selection, manufacture and application of bogie components of new generation rail transit vehicles.

Heat-treatable aluminum alloys can obtain higher mechanical properties through aging strengthening, but the aging strengthening behavior of the alloys is affected by many factors. The effects of temperature, pre-deformation, thermomechanical treatment, applied stress and applied electric field on alloy precipitation kinetics and strengthening behavior are summarized. Through reviewing and analyzing the influencing factors, it is hoped to provide the theoretical basis for the design and preparation of high-strength aluminum-lithium alloys.