Aiming at the problem of high energy consumption in conventional quenching and tempering process of high chromium cast irons, the subcritical heat treatment tests of three kinds of high chromium cast irons were carried out. Moreover, the effect of the process on the microstructure, hardness and surface wear resistance was studied. The results show that the microstructure of high chromium cast irons is primarily composed of retained austenite, M₇C₃ carbide, (Fe, Cr)₂₃C₆ carbide, Mo₂C carbide and pearlite when subjected to holding temperatures ranging from 500 ℃ to 650 ℃ for 1 h to 14 h. A peak value in hardness is observed for the three high chromium cast irons when holding at 550 ℃ to 600 ℃ for 6 h to 12 h, with the highest peak hardness being 57.4 HRC. The hardness is most significantly influenced by the Mo element. During the insulation process, the pearlite phase transformation of austenite is promoted by the precipitation of secondary carbides in high chromium cast irons. The effects of pearlite transformation strengthening and precipitation strengthening of secondary carbides gradually decrease with prolonged holding time. The eutectic high chromium cast iron with the highest carbon content exhibits the best wear resistance when holding at 600 ℃ for 8 h. It can be seen that the first step to improve the wear resistance of high chromium cast irons is to increase the amount of eutectic carbides, and then to add elements such as Mo to enhance the precipitation strengthening effect.

A new heat treatment process was designed for low carbon medium manganese steel (0.2C-5Mn-0.5Si-1.5Al) to introduce preparatory microstructure. The effect of the preparatory microstructure on the microstructure and properties of the medium manganese steel was studied by means of SEM, EPMA, XRD and tensile test. The results show that by introducing pearlite phase into hot-rolled steel, a multiphase preparatory microstructure composed of ferrite, martensite and pearlite is successfully established. This achieves a differentiated enrichment of C and Mn elements within the preparatory microstructure. Further cold rolling and intercritical annealing treatments result in two types of retained austenite (blocky and lath) with differentiated morphology and size. These exhibit a synergistic transformation induced plasticity effect over a large strain range in the tensile test. The specimens with preparatory microstructure achieve optimal mechanical properties through intercritical annealing process at 700 ℃ for 5 min. The tensile strength exceeds 1000 MPa, the elongation after fracture reaches 48%, and the product of strength and elongation is close to 50 GPa·%. Compared with the annealed specimens without introduction of a preparatory microstructure, the heat treatment regime used in this study not only makes the distribution of retained austenite stability more reasonable, but also significantly increases the elongation considerably while maintaining the level of high strength, resulting in an excellent product of strength and elongation.

Solution treatment for as-forged 022Cr22Ni5Mo3N duplex stainless steel was carried out, and the effects of solution temperature (1000-1100 ℃) and time (1 h and 2 h) on the microstructure and -46 ℃ low temperature impact properties were studied. The results show that with the increase of solution temperature, the austenite content of the 022Cr22Ni5Mo3N stainless steel decreases, the phase boundary of austenite tends to be smooth and the number of needle-type and island austenite in ferrite decreases gradually. The low temperature impact properties increase first and then decrease with the increase of solution temperature. When the solution temperature is 1050 ℃, the low temperature impact absorbed energy of the 022Cr22Ni5Mo3N stainless steel is higher, which is about 260 J. The solution treatment can change the austenite phase morphology and adjust the phase content. Serrated phase boundaries(below 1050 ℃) and lower austenite phase content(above 1050 ℃) promote the brittle fracture of dual-phase steels at low temperatures, which is the main reason for the reduction of low-temperature toughness.

Direct aging treatment (720 ℃×8 h＋620 ℃×8 h, air cooling) and solution treatment (950, 1000, 1100, 1200, 1250 ℃×1 h, water cooling) were conducted on the Inconel 625 alloy arc additive deposited by CMT (cold metal transfer). The effects of different heat treatment processes on the microstructure and properties of the deposited Inconel 625 alloy were analyzed by mechanical properties test, electrochemical potentiodynamic polarization test, optical microscope, field emission scanning electron microscope (SEM) and EDS. The results show that the effect of aging treatment on the Laves phase in the deposited alloy is not obvious. As the solution treatment temperature increases, the microstructure changes from deposited dendrites to equiaxed grains, and the harmful Laves phase dissolves and disappears. Aging treatment can significantly improve the hardness and tensile strength of the alloy, compared with that of the deposited state, the hardness and tensile strength are increased by 35.1 HV0.5 and 176.4 MPa, respectively, but the plasticity is reduced. Solution treatment at 1200 ℃ can significantly improve the plasticity of the alloy, compared with that of the deposited state, the percentage total extension at fracture is increased by 49.6%. Aging treatment has no obvious effect on the corrosion resistance of the alloy, but solution treatment at 1200 ℃ can significantly improve the corrosion resistance of the alloy as the self-corrosion potential increased by 22.8% and the self-corrosion current density reduced by 46.9% compared with that of the deposited state.

Effect of cryogenic treatment on the tensile properties and electrical conductivity of 6201 aluminum alloy was investigated using orthogonal test methods. The results show that the tensile strength of the 6201 aluminum alloy is 399.3 MPa through the cryogenic treatment with cooling rate of 1 ℃/min, cryogenic temperature of -120 ℃, holding time of 12 h and cycles number of 2 times, which is improved by 17.96% relative to the untreated group. Through the cryogenic treatment with cooling rate of 5 ℃/min, cryogenic temperature of -196 ℃, holding time of 12 h and cycles number of 1 time, the electrical conductivity of the 6201 aluminum alloy is 48.38%IACS, improved by 1.75% relative to the untreated group. The cryogenic treatment mainly affects the tensile properties and electrical conductivity of the alloy by influencing the size, number and distribution uniformity of the precipitated phases, and the cryogenic treatment further affects the electrical conductivity by influencing the vacancy changes.

Microstructure evolution and mechanical properties of the H13 steel during cryogenic treatment for different time (0-4 h) were studied. The phases and microstructure were analyzed by XRD and SEM, and the mechanical properties were characterized by hardness, impact and tensile tests. The experimental results show that the cryogenic treatment can refine martensite laths, promote the transformation of retained austenite to martensite and the precipitation of banded carbide. The increase of C content in martensite results in increasing in lattice constant and in lattice distortion effect, and a shift to the left in the XRD diffraction peak. With the prolongation of cryogenic treatment time, the strength and hardness of the steel show an upward trend, while the toughness and plasticity gradually decrease. Under the condition of maintaining the same hardness, the plasticity of the H13 steel after cryogenic treatment is comparable to that of the normal heat treatment, and its strength and toughness are improved.

Effects of different heat treatment processes on mechanical properties and microstructure of large size bulb flat steel were researched by means of OM, SEM, EBSD, TEM and other methods. The results show that compared with the hot rolling and tempering process, the impact absorbed energy at -40 ℃ at ball head center of the bulb flat steel after induction quenching and tempering process is increased by about 112 J, the crack propagation work is increased by about 125 J, the dynamic tearing energy and the fracture fiber ratio are significantly increased, and the ductile-brittle transition temperature is reduced from about 10 ℃ to below -60 ℃. After induction quenching and tempering process, the microstructure at ball head center of the bulb flat steel is ferrite+martensite+bainite, of which average grain size is significantly refined compared with the hot rolling and tempering process, and the number of carbides is more and the size is smaller. After hot rolling and tempering process, the microcracks on impact fracture are formed at the carbides on the pearlite and ferrite grain boundaries, while after induction quenching and tempering process, the impact fracture are characterized by ductile fracture with obvious dimples. Compared with the hot rolling and tempering process, the induction quenching and tempering process greatly improves the low temperature toughness of the large size bulb flat steel.

In order to investigate the phase transformation law of high carbon steel wire rod during isothermal quenching in salt bath, the cooling curves of high temperature wire rod after entering molten salt was measured by taking 92Si steel as the research object, and the convective heat transfer coefficient between the wire rod and molten salt was calculated by using Deform software, and then the cooling law of wire rod in molten salt was analyzed. In addition, the effects of heating temperature and salt bath isothermal temperature on microstructure and mechanical properties of the wire were studied. The results show that the phase transformation heat is difficult to be transferred quickly during isothermal quenching, which leads to the continuous non-isotermal phase transformation of wire rod in molten salt. The heat transfer capacity of the molten salt can be improved by reducing molten salt temperature, and then the cooling effect of the wire rod can be improved. The tensile strength of the wire rod after salt bath isothermal quenching is mainly determined by the molten salt temperature, and reducing the molten salt temperature in the pearlite transformation temperature range leads to smaller pearlite lamellar spacing and higher tensile strength. The plasticity of the wire rod is mainly determined by the heating conditions before phase transformation. With the increase of heating temperature, the prior austenite grain size becomes larger, the size of pearlite colony and block in the wire rod after phase transformation increases, and the percentage reduction of area of the wire rod decreases.

In order to investigate the effect of quenching media on microstructure and impact property of the ion-nitrided 42CrMo steel, PAG aqueous solution and quenching oil was used respectively for quenching at 850 ℃ for 1 h and tempering at 580 ℃ for 1.5 h, and then ion-nitriding at 540 ℃ for 60 h was carried out. The microstructure, strength and toughness before and after ion-nitriding were studied by means of optical microscope (OM), scanning electron microscope (SEM), tensile test, impact test and microhardness tester. The results show that the microstructure of the 42CrMo steel is all martensite after quenching with PAG aqueous solution, while the oil quenched microstructure is a mixture of martensite and lower bainite, however, both of the microstructure of the steel after quenching and tempering with two quenching medias is fine and uniform tempered sorbite. After ion-nitriding, the nitriding layer of the 42CrMo steel with PAG aqueous solution quenching is thicker than that with oil quenching, and the microstructure at the core is not different from that after quenching and tempering, while the microstructure at the core of that with oil quenching shows obvious free ferrite. The impact absorbed energy at -20 ℃ of the 42CrMo steel after quenching and tempering with oil quenching is higher, while the impact absorbed energy at -20 ℃ of the nitrided 42CrMo steel with PAG aqueous solution is 13.8% higher than that with oil quenching.

Effect of tempering temperature (300, 400, 500, 600, 700 ℃) on yield ratio and residual stress of 550 MPa high strength steel was studied by means of microstructure analysis, tensile test and stress measurement. The results show that with the increase of tempering temperature in the range of 300-500 ℃, the amount of dispersed and fine precipitates increases, the M/A constituents decompose and the dislocation density decreases gradually. When the tempering temperature rises up to 600 ℃, the amount of the precipitates reaches the maximum, while the M/A constituents completely decompose and only a small amount of dislocations exist. When the tempering temperature is 700 ℃, partial recrystallization of bainite occurs with the formation of more ferrite, and the precipitates aggregate and coarsen and dislocations are almost invisible. With the tempering temperature increasing from 300 ℃ to 700 ℃, the yield ratio first increases and then decreases, while the residual stress decreases gradually. An excellent combination of yield ratio and residual stress is obtained after tempering at 500 ℃, with yield ratio of 0.85 and residual stress of 13.98 MPa.

The solution argon gas quenching and aging treatment of TC4 titanium alloy were carried out by using a vacuum heat treatment furnace. Effect of argon gas quenching pressure on the microstructure and mechanical properties of the alloy was studied, and the fracture surface was analyzed. The results show that after solution treatment at 955 ℃ for 1 h and aging treatment at 550 ℃ for 5 h, the strength of the TC4 titanium alloy first increases and then decreases with the increase of argon gas quenching pressure, the elongation and percentage reduction of area gradually increase, the impact absorbed energy increases, and the hardness first increases and then decreases. The higher the volume fraction of β phase, the smaller the width of α phase and β phase, and the better the comprehensive mechanical properties of the alloy. The comprehensive mechanical properties of the alloy are optimal when the argon gas quenching pressure is 4 bar, with the tensile strength of 1043.3 MPa, the yield strength of 961.3 MPa, the elongation of 12.9%, the percentage reduction of area of 38.5%, the impact absorbed energy of 47.0 J, and the Rockwell hardness of 37.6 HRC.

Friction stir welding was conducted on 2195-T8 Al-Li alloy with 8 mm thickness under different welding heat input, and the welded joint was subjected to post-welding heat treatment at 205 ℃ for different aging time (6, 12, 18, 24 h). The microstructure and properties of the welded joint were analyzed by means of OM, TEM, SEM and universal tensile testing machine. The results show that when the heat input of friction stir welding is 1250 J/cm, the comprehensive mechanical properties of the welded joint are excellent, and the microhardness distribution of the joint is U-shaped. After aging at 205 ℃, in weld nugget zone, T₁, θ′ and δ′ precipitates increase with the aging time prolonging, and T₁ phase is the main phase, and then the precipitates coarsen with the aging time extend. In heat affected zone, based on the original distribution of a small number of T₁ phases, T₁, θ′ and δ′ phases increase gradually, and then with the increase of aging time, the density of precipitated phase decreases, T₁ and θ′ phases grow and coarsen, and some θ′ phases transform into θ phases. The mechanical properties of the welded joint reach the peak after aging for 12 h, the tensile strength of the joint is 497 MPa, which is 88.6% of the strength of the base material. However, the plasticity of the joint decreases, the fracture mode is intergranular fracture, and the microhardness of the joint increases from the original average 126.8 HV0.05 to 168.0 HV0.05.

Direct aging treatment of GH4169 superalloy obtained through hammering directed energy deposition (HDED) process was carried out. The results show that the microstructure of the specimen after HDED process is characterized by alternating distribution of equiaxed and columnar grains, and the content of Laves phase is reduced. The redissolution of the Laves phase promotes the precipitation of the γ″ phase and δ phase during direct aging process, while the small equiaxed grains and the work hardening effect of the specimen are retained after direct aging treatment. The small equiaxed grains produce fine grain strengthening effect, and the precipitation of the γ″ phase provides precipitation strengthening effect, by which the mechanical properties of the specimens are significantly improved after direct aging treatment, where the hardness is increased to 457.1 HV0.2, and the transverse and longitudinal tensile strengths are increased to 1205 and 1140 MPa, respectively.

TC4 titanium alloy plate was vacuum lap brazed with B-Ti57CuZrNi solder, and the effects of brazing temperature and time on microstructure and mechanical properties of the brazed joint were studied. The results show that the microstructure of brazed joint is mainly composed of solidification microstructure of B-Ti57CuZrNi solder and needle-like microstructure of diffusion transition layer. With the increase of brazing temperature, the width of weld and needle-like layer increases, and the width of solidification microstructure decreases. With the increase of brazing time, the width of solidification microstructure decreases, and the width of weld and needle-like layer increases. When the brazing temperature below 930 ℃, the room temperature tensile properties will decrease as the maximum breaking force at 910 ℃ is only 5.1-5.5 kN. When the brazing time is longer than 45 min, the room temperature tensile properties will also decrease. Considering the mechanical properties of the brazed joint, the brazing temperature of 930-950 ℃ and brazing time of less than 45 min are suitable.

Solution and aging treatments of 15-5PH stainless steel were carried out by using vacuum hydrogen annealing furnace on the basis of H485, H495, H550, H580 and H620 processes in GJB 8268-2014 Specification for precipitation-hardening stainless steel bars for aviation, and the impact and tensile properties at room temperature and -50 ℃ low temperature were studied. The results show that with the increase of aging temperature, the tensile strengths of the 15-5PH steel at room temperature and low temperature decrease, and the impact absorbed energy and elongation increase gradually. The low temperature impact property of the 15-5PH steel under H485 process is the worst, and the lowest impact absorbed energy is about 10 J. The low temperature impact property under H550 process is significantly increased, the impact absorbed energy is increased to 72 J, and the low temperature tensile strength is about 1200 MPa, which shows good comprehensive mechanical properties.

Effect of secondary forging deformation on microstructure, precipitation and impact properties of Cr-Mo-V die steel was studied by means of metallographic microscope, scanning electron microscope, high resolution transmission electron microscope and impact testing machine. The results show that compared with conventional upsetting and drawing deformation, the microstructure and network carbides of the die steel after spheroidizing annealing under secondary radial forging and secondary upsetting and drawing deformation processes are significantly optimized, the impact absorbed energy are improved by 12% and 58%, respectively, and the precipitated carbides increases in quantity and is dispersed.

Effects of deformation temperature and deformation rate on microstructure of the 7N01 aluminum alloy were studied through high-temperature compression test and metallographic observation. Based on these studies, extrusion process parameters were formulated, and the microstructure and fatigue properties of the T5-state 7N01 aluminum alloy profiles under different extrusion processes were observed and tested by using scanning electron microscope (SEM) backscattered diffraction technology and fatigue test lifting method. The results indicate that the alloy undergoes dynamic recovery and recrystallization during high-temperature compression. And with the increase of deformation temperature, the degree of dynamic recrystallization in the alloy increases. Meanwhile, with the increase of the deformation rate, the dislocation density of the alloy increases sharply, but the dynamic recovery in the alloy is impeded, which resulting in the degree of dynamic recrystallization decreasing first and then increasing. During extrusion, increasing the extrusion temperature and extrusion rate results in an increase in the recrystallization fraction within the 7N01-T5 profile, leading to a decrease in the median fatigue strength of the profile. However, when the extrusion temperature is 485 ℃ and the extrusion rate is 1.0 m/min, the recrystallized grains and subgrains within the profile are alternately arranged in a wide layered structure, effectively inhibiting the propagation of fatigue cracks, and the fatigue strength of the alloy reaches maximum.

In order to solve the problem of severe distortion of the SAE5120 steel bearing ring after carburizing and quenching, an induction fixture hardening process was carried out. The results show that, compared with conventional carburizing and hardening, the surface and core martensite of the bearing ring are refined, the hardness is increased, and the distortion of the bearing ring is significantly reduced. It is possible to reduce the grinding allowance, save the production cost and improve the production efficiency.

Non-magnetic drilling steel is 200 series austenitic stainless steel, its high strength characteristics are obtained by strengthening deformation, without heat treatment, resulting in poor grain uniformity after hot forging, can not meet the requirements of grain size difference within level 2. The effect of heating temperature and soaking time of intermediate billet on grain uniformity of non-magnetic drilling steel was studied by means of increasing the heating process of intermediate billet before strengthening deformation of non-magnetic drilling products. The results show that when the intermediate billet is heated at 1100 ℃ for 2-6 h before strengthening deformation, the grain size and uniformity are ideal, the grain size is above grade 3, and the grade difference is within level 2. Although the heating temperature of the intermediate billet is too high, it will make the grain size more uniform, but it will lead to the trend of mutual merging of grains resulting in coarse grains, and the heating temperature is too low, nucleation and static recrystallization energy is insufficient, and the purpose of grain uniformity cannot be achieved.

TC25 titanium alloy forged in the β phase region was subjected to double annealing treatment at (T_β-40) ℃+540 ℃, and the effect of first annealing cooling methods (box cooling, air cooling, forced air cooling) on the microstructure and properties of the TC25 titanium alloy forgings was studied. The results show that the microstructure of the alloy forgings is basket weave consisting of lamellar α phase and β transformed structure under different first annealing cooling methods. As the cooling rate increases, the size of lamellar α phase reduces, in the meantime the tensile properties improve, but plasticity and impact properties decrease. The TC25 titanium alloy forgings have good room temperature and high temperature strength and toughness matching under air cooling.

The texture and formability of IF steel with different cold rolling reduction rates after annealing at 800 ℃ for 300 s were studied. The microstructure and texture of annealed specimens at different cold rolling reduction rates were measured by metallographic microscopy and electron backscatter diffraction (EBSD). The tensile tests of the IF steel specimens were carried out by means of universal tensile testing machine. The experimental results show that the recrystallization grain size decreases gradually with the increase of cold rolling reduction rate. The texture uniformity along the gamma-orientation line increases first and then decreases with the increase of cold rolling reduction rate. When the reduction rate is 75%, the uniform shape of the texture is optimal. By comparing the plastic strain ratio (r value) and anisotropy Δr of the different IF steel specimens, it can be seen that IF steel has the largest r value and the smallest anisotropy Δr when the cold rolling reduction rate is 75%. Therefore, it has excellent formability.

TC4 alloy third-stage rotor blade for aeroengine was annealed at different temperatures (600-950 ℃) with different cooling methods (air cooling and water cooling). The microstructure was analyzed by OM, SEM and EBSD. The microhardness and mechanical properties at room temperature were tested by Vickers hardness tester and tensile testing machine. The effect of annealing treatment on the microstructure and mechanical properties of the TC4 titanium alloy blades was studied. The results show that the volume fraction of α phase changes little with the increase of annealing temperature below 850 ℃, and the grain size of α phase is about 12 μm. When the annealing temperature exceeds 850 ℃, the acicular α phase appears in the microstructure. When the annealing temperature is below 900 ℃, the hardness of the air-cooled specimen is 5-10 HV0.2 higher than that of the water-cooled specimen. When the annealing temperature exceeds 850 ℃, the content of acicular α phase increases and the microhardness rapidly increases. The maximum strength appears at the annealing temperature of 600 ℃, but with relatively low elongation. Under air cooling conditions, at annealing temperature of 800 ℃, the tensile strength reaches 1033.7 MPa, the elongation is 23.2%, and the comprehensive mechanical properties of the blade are optimal.

Effect of intercritical annealing time on microstructure and mechanical properties of Fe-0.16C-0.2Si-4.78Mn-0.24Mo medium manganese steel was studied by means of scanning electron microscope, electron backscatter diffraction, X-ray diffraction and transmission electron microscope, room temperature tensile test and impact test at 0 ℃. The results show that the microstructure of the hot-rolled steel is mainly composed of martensite, while the microstructure after annealing for different time is composed of martensite and reverse austenite. With the increase of annealing time, the content of reverse austenite increases linearly, reaching a peak value of 12.74% after annealing for 30 h. At this time, in addition to film-like austenite and strip austenite, large sized blocky austenite also appears in the microstructure. The yield strength and tensile strength of the tested steel gradually decrease with the increase of annealing time, which are mainly attributed to the increase in reverse austenite content and the decrease in dislocation density. With the increase of annealing time, the impact absorbed energy at 0 ℃ gradually increases, and the elongation first increases and then decreases, which is due to the TRIP effect of reversing austenite improving the impact property and plasticity, but the stability of austenite decreases after annealing for 30 h, leading to a decrease in elongation. After annealing at 600 ℃ for 30 h, the tested steel exhibits the best TRIP effect.

Remarkable hardness difference was found in the cross-sectional regions of low outside surface hardness in P92 main steam pipe after serving for 70 000 h. Microscopic and phase analysis, and room-/high-temperature tensile property tests were executed to confirm the microstructure characteristics and mechanical properties of the specimens in different hardness regions. The results show that for the low hardness specimens, widening and decomposing of martensite lathes is severe, lath martensite is gradually transferred to tempered martensite. The M₂₃C₆ and Laves-phase particles precipitated along the prior austenite grain boundaries and martensite lathes are further coarsened and distributed in semi-continuous chains. Meanwhile, the precipitated phase particles dispersively distributed in the martensite and (or) ferrite matrices are also expanded. The room and high temperature tensile strengths of the low hardness specimens are low too, especially, room and high temperature yield strengths of them are lower than the indexes required by the relevant national standards, which could be used for the key mechanical property index in aging state assessment of the as-served P92 main-steam pipe.

Evolution of the metallographic structure, recrystallization, second phase, aging precipitated phase, precipitation free zone and microhardness of the 7050-T7451 aluminum alloy thick plate with different thickness layers along the thickness direction were studied by means of metallographic microscopy, field emission scanning electron microscopy, transmission electron microscopy and hardness meter. The results show that the 80 mm thick 7050 aluminum alloy plate has different degrees of recrystallization during heat treatment. The average grain size and area fraction of recrystallization from the surface to the center show a trend of first decreasing and then increasing along the thickness direction, and the recrystallization area fraction of each thickness layer is less than 5%, while the average size of second phase increases gradually, the area fraction is smaller in the surface layer and relatively larger in the center. The density of intragranular precipitates is basically consistent in each thickness layer, but the precipitates at grain boundaries are more discontinuous and larger in size in the center compared to other thickness layers, while the spacing and size of the surface layer are smaller. In addition, there are obvious precipitate free zones(PFZ) in different thickness layers, and their widths vary with the size of precipitates at grain boundaries. The width of the precipitate free zone in the center is relatively wide, about 94 nm. The hardness values of each thickness layer show a trend of first increasing and then decreasing from the surface to the center, and are related to the degree of recrystallization and metastable η′ phase.

CoCr_0.4NiSi_0.5 medium entropy alloy was prepared by vacuum suspension melting, and the effect of annealing process (annealing temperature of 1000 ℃, 1200 ℃, and holding time of 1 h) on microstructure and properties of the alloy was investigated. The results show that the phase structure of the annealed alloy is still a face-centered cubic solid solution compared to the as-cast alloy, and the microstructure is still a typical dendrite structure, with lamellar eutectic structure distributed among the dendrites, and a small amount of Cr-rich particles are precipitated in the dendrites. The hardness of the annealed alloy decreases, and decreases obviously when the annealing temperature is high, which is caused by the increase of interdendrite eutectic structure, the growth of dendrite and the decrease of σ phase content. After annealing treatment, the compression properties of the alloy are obviously improved, the compression yield strength and compression rate are increased, but the compression strength is decreased.

Microstructure and properties of SLM formed specimen and after heat treatment of a new high thermal conductivity injection mould steel 1CrMoVNb were analyzed by means of optical microscope, X-ray diffractometer (XRD), scanning electron microscope (SEM). The results show that the SLM formed specimen is well lapped, with mainly cellular crystals in the cross-section and columnar crystals in the longitudinal section, and the microstructure is mainly ferrite. After quenching at 1250 ℃ for 60 min, WQ and tempering at 600 ℃ for 120 min, AC, the lap line dissolves, the microstructure is more uniform and mostly transforms to tempered martensite. The mechanical properties of the SLM formed specimen are anisotropic, the hardness in transverse section is greater than that in longitudinal section, the transverse tensile strength and yield strength is better than the longitudinal, while the transverse elongation is lower than the longitudinal. After heat treatment, the mechanical properties and thermal conductivity of the specimen are greatly improved, and the anisotropy is improved, which can well meet the requirements of injection mold steel.

Three types of Cu wires with different deformation strains were prepared by the cold drawing process. The relationship between the strength, microstructure and strains of the Cu wires was studied. The results show that during the cold drawing process, the strength of the Cu wire shows a trend of rapid increase in the early drawing stage and slow increase in the later drawing stage. Microstructure observations reveal that the grain width of the Cu wire decreases continuously, and the grain orientation gradually rotates from <001> to <111>, forming a high proportion of <111> texture. Additionally, the dislocation density first increases and then decreases with the increase of drawing strains. Further established quantitative relationship between the microstructure and the strength reveals that the grain boundary strengthening and dislocation strengthening are the fundamental reasons for the significant increase in strength of Cu wires during the initial drawing stage. While, the main mechanism for the slow increase in strength of Cu wires in the later stage of drawing is the increase in fine grain strengthening and texture strengthening, as well as the significant decrease in dislocation strengthening.

Microstructure evolution and mechanical properties of Mn-10Cu-4Cr alloy after solution treatment at 850 ℃ for 40 min and aging at 400-600 ℃ for 8 h and at 440 ℃ for 0-120 h were studied. The results show that the matrix of the alloy after solution treatment shows stripe morphology with width of about 1.5 nm under TEM, and there is a copper-rich region without stripe morphology, indicating that the occurrence of spinodal decomposition leads to the inhomogeneity of alloy composition. Moreover, there is also α-Mn precipitated phase in the microstructure. With the increase of aging temperature and the extension of aging time, the grain size of the alloy changes little, while the hardness and strength increase significantly and the elongation decreases, which is due to the formation, growth and coarsening of Cu-rich nanoparticles, and the formation of the α-Mn phase within and between grains caused by aging treatment.

Dynamic recrystallization behavior of 20Cr2Ni4A carburized steel for gears was studied by means of single isothermal compression test with deformation temperature of 800-1050 ℃ and strain rate of 0.01-5 s^-1 at the strain variable of 0.8 on Gleeble-3500 thermal simulation machine. According to the experimental results, the flow stress curve was drawn. The results show that the flow stress curve can be divided into dynamic recovery type and dynamic recrystallization type according to the degree of work hardening, dynamic recovery and dynamic recrystallization under different deformation conditions. At low strain rate (0.01-0.1 s^-1), the dynamic recrystallization occurs more obviously at 950-1050 ℃. The critical strain ε_c and peak strain ε_p of dynamic recrystallization are determined by the work hardening rate, and the relationship model between ε_c and ε_p is established. Based on the experimental data, a dynamic model of dynamic recrystallization is established, and the analysis of microstructure shows that the model is reliable.

To enhance the plastic processing performance of WE43 magnesium alloy, the as-cast specimens were homogenized at temperature ranging from 480 ℃ to 540 ℃ and holding time of 4, 8, 12 and 16 h, respectively. The effects of the homogenizing temperature and time on the microstructure of the WE43 magnesium alloy were investigated by using metallographic and X-ray diffraction analysis testing techniques. The results show that the optimal homogenizing temperature is 520 ℃ and the holding time is 8 h. Then hot compression test was carried out on the homogenized WE43 magnesium alloy by using a thermal simulation testing machine with deformation temperatures of 350-475 ℃, strain rates of 0.001-1 s^-1 and deformation of 60%, the flow stress-strain curves of the tested alloy are obtained, based on which the constitutive model for WE43 magnesium alloy is established.

Influence of bismuth on solid-state phase transformation and microstructure of ultra-low carbon steel during cooling was studied by means of optical microscope (OM), scanning electron microscope (SEM-EDS) and differential scanning calorimetry (DSC). The results indicate that the addition of 0.06% bismuth to the tested ultra-low carbon steel delays the austenite-to-ferrite phase transformation during cooling and increases the activation energy of the transformation. The phase transformation start and end temperatures are decreased by 0-16 ℃ and 0-31 ℃, respectively, under different cooling rates, and the activation energy of the phase transformation is increasesd by 1364 kJ/mol. During cooling, bismuth tends to segregate at the austenite and ferrite grain boundaries, refining the austenite and ferrite grains. As the cooling rate increases, the ferrite grain size after transformation in the tested steel gradually decreases, with the average grain size reducing from 77.64 μm at 10 ℃/min to 66.03 μm at 20 ℃/min. Additionally, compared to the bismuth-free steel, the ultra-low carbon steel containing 0.06% bismuth has smaller ferrite grain size after transformation at the same cooling rates, with the average grain size reductions ranging from 3.28 μm to 13.74 μm.

Effects of deformation amount of single pass deformation and double pass deformation and temperature waiting time on austenite recrystallization and grain growth of X80 pipeline steel were studied by thermal simulation experiments. The results show that the austenite grain size decreases with the increase of single pass deformation amount. When the deformation amount is 25%-30%, the average grain size of austenite of the tested steel is about 22 μm. For the X80 pipeline steel with Nb content of about 0.08%, the austenite can still be fully recrystallized after the deformation at 980 ℃ and then holding for 1 min. The growth trend of austenite grain is not obvious with the increase of temperature waiting time, with the grain size is 22-28 μm. According to the calculation of the recrystallization temperature model, the recrystallization temperature of the tested steel is about 946 ℃. According to the calculation of the temperature drop equation of the slab, the temperature waiting time after single pass deformation should not exceed 200 s.

To study the microstructural evolution of GH3625 alloy during hot deformation, hot compression tests were conducted on Gleeble-3800 thermal simulation machine under various deformation conditions (deformation temperature of 1030-1130 ℃, strain of 10%-40% and strain rate of 0.01-1.0 s^-1). The strain softening and strain hardening coefficients of the GH3625 alloy were determined by stress-strain curves, leading to the development of Laasraoui-Jonas (L-J) dislocation density model for the GH3625 alloy. The microstructural evolution of the GH3625 alloy during hot deformation was simulated using the cellular automata (CA) module integrated within DEFORM-3D, and the results were compared with the experimental microstructural observations. The results indicate that the flow stress of the GH3625 alloy decreases with the increase of deformation temperature and decrease of strain rate. Analysis combining microstructural observations and CA simulation results shows that significant dynamic recrystallization occurs during hot compression of the GH3625 alloy. As the strain increases, the deformation temperature rises and the strain rate decreases, the volume fraction of dynamic recrystallization increases and the microstructure becomes finer and more uniform. The developed L-J dislocation density model accurately predicts the microstructural evolution of the GH3625 alloy during hot deformation.

As a class of emerging advanced metal materials in recent years and based on the unique multi-principal element alloy design concept, high-entropy alloys (HEAs) are expected to break through the performance limit of traditional single-principal element alloys. Among them, FeMnCoCrNi HEA is one of the most widely studied systems with excellent mechanical properties and broad application prospects. The unique “four core effects” of HEAs were firstly introduced, including high entropy effect, lattice distortion effect, hysteresis diffusion effect and cocktail effect. Secondly, the research status of FeMnCoCrNi high entropy alloy in high temperature tensile properties, high temperature creep properties and corresponding deformation mechanisms was reviewed.

Effect of V content on tempering hardness, microstructure and carbides of Cr5-type hot work die steel was studied by scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that high V content reduces the quenching hardness of the steel, enhances its resistance to tempering softening and has no significant effect on the secondary hardening peak. At the same time, it promotes the formation of M₇C₃ type carbides with a size of 50-100 nm. In addition, high V content can make those large-sized MC type V-rich carbides not completely dissolved into the matrix during austenitizing retain to the tempered state.

In order to develop new alloy materials with excellent high temperature oxidation resistance, Al and Ta was added in a synergistic manner into the base substrate of CoCrNi alloy to prepare (CoCrNi)_96-xTa₄Al_x(x=3, 6, 12) high entropy alloy, and the effect of Al content on microstructure, phase structure, high temperature oxidation resistance and oxidation kinetics at 1200 ℃ of the alloy was studied. The results show that with the increase of Al content, the matrix of the (CoCrNi)_96-xTa₄Al_x(x=3, 6, 12) high entropy alloy gradually transforms from FCC phase to BCC phase, and the (CoCrNi)₉₀Ta₄Al₆ alloy forms FCC+BCC dual-phase. The (CoCrNi)_96-xTa₄Al_x(x=3, 6, 12) high entropy alloy exhibits good oxidation resistance after high temperature cyclic oxidation at 1200 ℃. Moreover, with the increase of Al content, the oxidation resistance also increases, and the oxidation kinetic curve and oxidation rate constant are stable. The surface oxide layer of the (CoCrNi)_96-xTa₄Al_x(x=3, 6, 12) high entropy alloy after oxidation is mainly composed of Cr₂O₃, Al₂O₃ and a small amount of Ta₂O₅. The complete and dense oxide layer formed at high temperature effectively protects the alloy, so that the high temperature oxidation resistance of the alloy is very excellent.

Continuous cooling phase transformation curves of the DX51D+Z low carbon steel with and without rare earth element Ce were drawn by Formaster-F automatic phase transformation instrument combined with microstructure analysis. The effects of rare earth on the continuous cooling phase transformation points and microstructure of the DX51D+Z steel at different cooling rates were studied. The results show that the addition of trace rare earth Ce (9×10^-6) reduces the Ac₁ transformation point temperature of the DX51D+Z steel by 3 ℃ and increases the Ac₃ transformation point temperature by 9 ℃, increasing the phase transformation interval from α-Fe→γ-Fe during slow heating. When the cooling rate is 0.05-2 ℃/s, the trace rare earth Ce has little effect on the the starting and ending temperature of phase transformation of the tested steel. When the cooling rate is greater than 5 ℃/s, the trace rare earth Ce reduces the phase transformation termination temperature, expands the phase transformation range of ferrite+pearlite and ferrite+bainite, and shifts the static CCT curve to the lower left. Within the cooling rate range of 0.1-20 ℃/s, the grain size grade of the tested steel with rare earth Ce is 0.2-0.4 higher than that without rare earth, and the grain size distribution is more uniform.

AlCr_xCuFeNi (x=0, 1.0, 2.0) high-entropy alloys, which named as Cr₀, Cr_1.0 and Cr_2.0, was prepared by using arc melting technology. The effect of Cr content on microstructure and microhardness of the as-cast high-entropy alloys and after heat treatment at 600 ℃ was studied. The results indicate that the phase structure of the as-cast Cr₀ alloy is mainly composed of (Fe, Ni)-rich FCC solid solution phase and AlCu₃ phase, while both Cr_1.0 and Cr_2.0 alloys are composed of Cu-rich FCC2 phase, (Cr, Fe)-rich BCC phase and AlNi phase. After heat treatment at 600 ℃, the phase structure of the alloy remains largely unchanged, indicating good phase stability, meanwhile, the element segregation degree of the high-entropy alloy is significantly improved. The microhardness of the as-cast Cr₀ alloy is 348 HV10, which slightly increases to 370 HV10 after heat treatment at 600 ℃. As the Cr content increases, the hardness of the high-entropy alloys also increases, and the microhardness of the as-cast Cr_2.0 alloy and after heat treatment at 600 ℃ is the highest, reaching 435 and 462 HV10, respectively. The increase in hardness of the high-entropy alloys after heat treatment is mainly related to the mechanisms of solid solution strengthening, precipitation strengthening and fine-grained strengthening.

FeCoCrNiAl_0.3Mo_0.1 alloy coating was prepared on the surface of 304 stainless steel, and the effect of laser power (800, 1000, 1200 W) on the coating quality and microstructure was studied. The results show that the coating prepared at 800 W and 1000 W laser power has cracks and stoma defects, and no obvious defect is found at 1200 W. The coating structure at different laser powers is columnar, but after comparative analysis, the microstructure of the coating at 1200 W is finer and more uniform. The composition analysis of the coating finds that the Fe content is much higher than the nominal value, but the Al content is far lower than the nominal value. From bottom to top, the average content of Fe and Cr gradually decreases, and the content of other elements gradually increases. It is also found that the content of Mo and Cr in intergranular is higher than that in intragranular.

Nitrided layer was prepared on the surface of Ni15Cu6Cr2 austenitic gray cast iron by glow plasma nitriding at different temperatures (380, 540 ℃). The cross-section structure, element distribution and phase composition of the nitrided layer were analyzed by Zeiss optical microscope, scanning electron microscope, electron probe and X-ray diffractometer. The tribological properties of the nitrided layer were analyzed by wear scar morphology and three-dimensional profile. The corrosion resistance of the austenitic gray cast iron before and after nitriding was investigated by electrochemical test. The results show that after nitriding at 380 ℃ for 10 h, a compound layer composed of ε phase and a small amount of γ′ phase with a thickness of about 2 μm is obtained on the surface of the austenitic gray cast iron, and the surface hardness is about 330 HV0.05. When the nitriding temperature rises to 540 ℃, the surface hardness increases to about 900 HV0.05, the thickness of the compound layer reaches about 10 μm, the ε phase gradually transforms into the γ′ phase, and the thickness of the whole nitrided layer is about 45 μm. The wear mechanism of the austenitic gray cast iron matrix is abrasive wear, which gradually transforms into adhesive wear after nitriding. The width of the wear scar gradually narrows and the depth gradually becomes shallower, and the wear resistance is significantly improved. After nitriding at 540 ℃, the corrosion resistance is significantly improved. Compared with the matrix, the self-corrosion potential increases from -0.337 V to -0.217 V, the self-corrosion current density decreases from 1.51×10^-6 A/cm² to 3.07×10^-7 A/cm².

Effect of different boronizing temperatures (800, 850, 900 ℃) on thickness, hardness, fracture toughness, brittleness and corrosion resistance of boronized layer on 45 steel was studied. The hardness, crack morphology of indentation and elastic modulus of boronized layer were tested by Vickers hardness tester. The elastic modulus of boronized layer was measured by nanohardness tester, and the fracture toughness and brittleness of boronized layer were characterized quantitatively. The corrosion resistance of boronized layer was characterized by electrochemical workstation. The results show that after boriding at different temperatures, as the boronizing temperature increases, the thickness and hardness of the boronized layer increase, the fracture toughness decreases and the brittleness increases. After boronizing at 800 ℃, the self-corrosion potential of the boronized layer is the highest, the self-corrosion current density is the lowest and the corrosion resistance is the best. Comprehensively, the optimal boronizing process for 45 steel is boronizing at 800 ℃ for 4 h.

In order to improve the abrasion resistance and corrosion resistance of multi-stage step-down control valve spool, and shorten the manufacturing cycle of such parts, Stellite12 alloy was prepared on the surface of valve spool by laser cladding and gas tungsten arc welding(GTAW). The microstructure, microhardness, friction and wear properties, uniform corrosion properties of the surfacing welded Stellite12 alloy layer were compared and studied. The results show that the surfacing welded alloy layer prepared by both methods forms a good metallurgical bond with the substrate, and the microstructure is mainly composed of dendrite structures of plane crystal, coarse columnar dendrites, fine and dense equiaxed dendrites. Compared with that of GTAW, the laser clad alloy layer has more dense and fine dendrite structure, and more uniform distribution of dendrite composition. At the same time, the laser clad alloy layer has higher microhardness and better abrasion resistance. In addition, the laser clad alloy layer has lower uniform corrosion rate and more uniform corrosion behavior than the GTAW alloy layer.

Alternating current field(ACF) enhanced pack aluminum-carbonitriding was carried out at 750 ℃ for commercial pure titanium TA2. The microstructure, phase structure, thickness and microhardness distribution along the case depth of the treated specimens were analyzed by means of optical microscopy, X-ray diffraction and microhardness test. The results show that the case obtained with the help of the ACF with a current of 2 A consists of TiN, Ti₂N and a small amount of AlTi₃. As the ACF current gradually increases from 2 A to 6 A, Ti₃AlN and C_0.3N_0.7Ti are found in the surface layer, and the amount of TiN and Ti₂N disappear, while AlTi₃ always exists. No aluminum-containing phase is detected in the surface layer of TA2 treated by conventional pack aluminum-carbonitriding with the same agent, temperature and time used for the ACF enhanced treating. The ACF has a significant promoting effect on the pack aluminum-carbonitriding. The promoting effect increases with the increase of the ACF current. Applying an electric field with a current of 6 A can produces a case with a thickness about 40 μm. The ACF also increases the case's peak hardness and overall hardness along the case's depth.

Causes of pitting defects on hot-rolled galvanized steel sheet were studied by means of systematic surface examination, detailed microstructure analysis and precise chemical composition testing. The results show that aberrant dew point conditions within the annealing furnace lead to an insufficiently reducing atmosphere, which induces the oxidation and formation of iron oxide on the surface of the steel sheet, so that the surface wettability of hot-rolled substrate is insufficient during galvanizing, thereby impeding the formation of the Fe-Al inhibition layer and leading to pinhole leakage in the zinc coating. These pinhole leakages disrupt the flow of the zinc liquid after the steel sheet is discharged the zinc pot, eventually manifesting as visible pitting defects on a macroscopic scale.

Taking the properties of 1.5 GPa grade hot stamping steel as the benchmark, the strength and toughness of 2 GPa grade hot stamping steel were accurately evaluated from multiple perspectives as elongation, strength, ultimate cold bending angle and crack nucleation work by means of uniaxial tensile test, three-point bending test and notch tensile test, and the strength and toughness were characterized by the method of weighted calculation of comprehensive score. The results show that after baking, the yield strength, elongation, ultimate cold bending angle, crack nucleation work which characterize the fracture toughness of the 1.5 GPa, 2 GPa and 2 GPa+AS hot stamping steel specimens are improved. Compared with the 1.5 GPa steel, the 2 GPa and 2 GPa+AS steels have higher safety performance within their tensile strength range, however, once holes or microcracks are formed in the specimen, the microcracks will rapidly expand to crack. Through weighted calculation of the comprehensive score, it is concluded that the comprehensive evaluation results of the strength and toughness of the tested hot stamping steels from high to low are 2 GPa>1.5 GPa>2 GPa+AS.

Water marks occurred on the surface of cold-rolled nickel-saving stainless steel strip after continuous annealing, which would evolve into corrosion and cause serious loss. The surface morphology, composition, microstructure and corrosion resistance of the stainless steel strip specimens with and without water marks were compared and analyzed by means of metallographic microscopy, scanning electron microscopy and EDS analysis, and the forming mechanism of water marks was studied in combination with rolling process. The results indicate that during the cold rolling process of stainless steel, the middle part of the cold-rolled nickel-saving stainless steel where the water marks forms is in a high speed rolling state and is at a higher plastic deformation rate than the head and end without water marks. The high speed rolling results in numerous deformation bands within the microstructure of the strip which compromise the corrosion resistance of cold-rolled stainless steel, ultimately leading to the formation of numerous micro-pitting water marks.

Shape of the steel plate is a key quality indicator during the quenching process. In order to solve the problem of plate shape prediction of steel plates during the quenching process, a two-stage shape prediction method for steel plates in roller-hearth machine quenching process based on convolutional neural network and long short-term memory network (CNN-LSTM) was proposed. This method was divided into two stages. Firstly, the CNN was used to extract the plate shape features and capture the spatial information of the plate shape. Secondly, using quenching process parameters and historical plate shape characteristics as inputs, a plate shape prediction model was established through LSTM. Finally, by concatenating these two stages, both spatial and temporal information of the plate shape could be considered simultaneously. Based on the experiments with actual production data, the results show that the proposed method reduces the root mean squared error of the prediction is reduced from 0.0471 to 0.0264, which represents a 43.9% reduction in prediction error, achieving the goal of improving the plate shape prediction accuracy.

By analyzing the macrostructure of 42CrMoA marine gear box input shaft forging and the microstructure of its crack region, the causes of quenching cracks were studied with the combination of numerical simulation of quenching process. The results show that the shape of the marine gear box input shaft forging is complex and with sharp changes in its cross section, thus the stress concentration occurs easily at the step shaft transition position and results in transverse quenching cracks during quenching.

Longitudinal cracks were found on the 1-2 teeth of the tail end of the 30CrMnSiNi2 bolt thread through magnetic particle testing. Combining with the bolt production process, the causes of microcrack formation were analyzed by means of macro and micro morphology observation of the microcracks and microstructure and hardness examination of the bolt. The results show that due to improper cooling during grinding, secondary quenching occurred at the 1-2 teeth of the thread end tail tooth, resulting in fine needle-like martensite appears, causing grinding burns and quenching cracks under secondary quenching stress. In order to avoid such problems recurrence, it is recommended reinforcing the grinding cooling detection and related personnel training and improving the equipment maintenance system.