SEM, EBSD, TEM and tensile tests were used to investigate the influences of various rolling temperatures on the microstructure and mechanical properties of Sc microalloyed Al-3.2Cu-1.5Li alloy. The results indicate that the grain structures in hot rolled and T6 treated specimens are primarily composed of substructures. The fine and stable Al₃(Sc, Zr)/Al₃(Li, Sc, Zr) dispersed phases precipitated along dislocations or sub-grain boundaries are the reason that the Sc-containing Al-3.2Cu-1.5Li alloy has higher recrystallization resistance. Meanwhile, the precipitation of nanoscale Al₃(Sc, Zr)/Al₃(Li, Sc, Zr) particles efficiently prevents the growth of recrystallized grain during hot rolling and T6 heat treatment. Furthermore, as the rolling temperature increases, the tensile strength of the alloy first increases and then decreases, while the elongation has a rising tendency and then reaches the stabilizing stage. The alloy rolled at 420 ℃ possesses excellent overall properties, with a tensile strength of 480.4 MPa and an elongation of 13.0%, which can be attributed to the synergistic effects of fine grain, precipitation, dispersion and dislocation strengthening.

Effect of aging on the microstructure and properties of hot-extruded Mg-8Gd-4Y-1Sm-0.5Zr (GWS841) alloy was studied by means of scanning electron microscope (SEM), transmission electron microscope (TEM), electron backscatter diffraction (EBSD), hardness test and tensile test. The results show that the hardness of the hot extruded GWS841 alloy reaches the highest after aging at 200 ℃ for 96 h, 225 ℃ for 24 h, and 250 ℃ for 12 h, respectively. The peak hardness and tensile strength reach their maximum values after aging at 200 ℃ for 96 h. During the aging process at 200 ℃, rare earth atoms first precipitate along 〈12$\bar{1}$0〉_α crystal direction in α-Mg matrix to form a single black contrast stripe, β′ phase, which is in the under aged state. As the aging time increases, the β′ phase expands along 〈10$\bar{1}$0〉_α crystal direction, and the hardness of the alloy gradually increases. After aging for 96 h, β′ phase precipitates simultaneously in the (11$\bar{2}$0)_α、($\bar{1}$2$\bar{1}$0)_α and (2$\bar{11}$0)_α crystal planes, which is in the peak aged state. After aging for 120 h, the β′ phase in the matrix grows and forms a shuttle like shape, and sub-micron β phases are generated at grain boundaries, which is in the overaged state. The tensile fracture mechanism is quasi-cleavage fracture, and the fracture structure consists of cleavage planes, tearing edges, and dimples.

Microstructure and mechanical properties of super-martensitic stainless steel SMSS 00Cr13Ni5Mo quenched at 880 ℃ and tempered at different temperatures (530, 580, 630 ℃) were studied. The changes in microstructure during tempering were analyzed using electron backscatter diffraction analysis (EBSD) and scanning electron microscopy (SEM), and the tensile properties were tested using an electronic universal testing machine. The results show that the tempered microstructure of the tested steel consists of a ferrite-like phase derived from the decomposition of tempered martensite, along with a small amount of austenite. As the tempering temperature increases, the ferrite-like phase evolves from a granular to a banded structure, eventually forming continuous bands. The grain size also increases from 25.76 μm to 29.45 μm. When the tempering temperature increases to 580 ℃, Ni enrichment leads to the formation of reversed austenite, reducing the martensite phase content from 88.7% to 75.8%. Further increasing the tempering temperature to 630 ℃ causes excessive Ni content in the Ni-rich phase, which inhibits the formation of reversed austenite, resulting in a recovery of martensite phase content to 79.6%. As the tempering temperature is increased, the change in reversed austenite leads to a decrease in the microhardness and yield strength initially, followed by an increase (hardness: 274.19 HV0.1→239.16 HV0.1→242.45 HV0.1; yield strength: 857.63 MPa→792.91 MPa→823.51 MPa). The elongation first increases, then decreases (20.67%→22.87%→20.80%). When the tempering temperature is 580 ℃, the steel exhibits the highest austenite content, which results in the best plasticity of the tested steel.

GH738 nickel-based superalloy was subjected to three deformation levels (10%, 20%, 30%) of cold deformation, followed by heat treatment consisting of annealing at 1040 ℃ for 0.5 h, aging at 800 ℃ for 8 h, and further aging at 600 ℃ for 8 h. The changes in microstructure and mechanical properties of the alloy under different processing conditions were investigated. The results show that the GH738 nickel-based superalloy primarily consists of γ′, γ, TiC, and Cr₂₃C₆ phases. As the deformation level increases, the grain size of the heat-treated GH738 nickel-based superalloy gradually decreases from 44.41 μm in the solution-treated state to 22.03 μm. The γ′ precipitates gradually redissolve into the matrix, with their size becoming more uniform and decreasing from 165 nm to 102 nm. Cold deformation and heat treatment significantly enhance the tensile strength, yield strength, and hardness of the GH738 nickel-based superalloy, and to some extent, improve the alloy's ductility and toughness.

0.86%C-0.045%Nb high-carbon steel specimens were quenched and then tempered at 200 ℃ under different pressures using a CS-IB type hexagonal top press. The effects of high pressure on the microstructure and properties of the niobium-containing high-carbon steel after quenching and tempering were compared with those of specimens tempered at atmospheric pressure. The results show that after quenching at 1100 ℃ and tempering at 200 ℃ under pressures of 1, 3 and 5 GPa, the microstructure of the tested steel consists of tempered martensite with a small amount of retained austenite. Compared to the atmospheric pressure tempering condition, the NbC particles are observed in the tempered martensite matrix. The solution Nb content in the tempered specimens under different pressures determined by ICP-OES reveals that the solution Nb content in the steel matrix decreases with the increase of pressure. This shows that high pressure effectively promotes the low-temperature precipitation of NbC. As the pressure increases, the tensile strength and percentage total extension at fracture of the tested steel initially increase and then decrease. The best mechanical properties of the tested steel are obtained after tempering at 200 ℃ under 1 GPa high pressure, with a tensile strength of 1556 MPa and percentage total extension at fracture of 6.45%. Compared with the atmospheric pressure tempering condition, the tensile strength and percentage total extension at fracture are increased by 280 MPa and 1.12%, respectively.

Effects of extrusion temperature and speed on microstructure, mechanical properties and dissolution properties of the AZ80NC degradable magnesium alloy were investigated. The results show that the as-cast microstructure of the AZ80NC alloy consists of α-Mg matrix and secondary phases including β-Mg₁₇Al₁₂, MgAlNi and MgAlCu. During hot extrusion at temperatures above 300 ℃, complete dynamic recrystallization occurs in the extruded rods of the AZ80NC alloy, resulting in an equiaxed grain microstructure. Under low temperature and low strain rate extrusion, partial recrystallization occurs at the edges of the extruded rods, forming a bimodal grain microstructure. As the extrusion temperature and extrusion rate decrease, the yield strength and tensile strength of the AZ80NC alloy gradually increase, while the elongation and corrosion rate decrease. However, when a bimodal grain microstructure is formed under low temperature and low strain rate extrusion, the alloy exhibits both high strength and high ductility, with higher corrosion rate reaching a maximum of 31.18 mg/(cm²·h).

In order to further study the effects of adding rare earth and microalloying elements and heat treatment process on the fatigue life of the pearlitic rail steels, the high-cycle fatigue test was conducted on BG400 rail steel in rolled and heat-treated states and the contrast design of microalloyed rare earth tested steel, the fatigue fracture was also carefully observed and analyzed. The results show that the heat treatment process of austenitizing at 900 ℃ for 15 min, followed by isothermal quenching at 540 ℃ for 60 s, significantly improves the fatigue life of the rail steels, and the fatigue property of the microalloyed rare earth tested steel is better than that of BG400 steel, making the overall S-N curve move to the right. The reason is that rational heat treatment process reduces the lamellar spacing of pearlite. The addition of rare earth elements and microalloying elements effectively refines the grain size. The combining effect of these two factors means that cracks need to consume more energy when traversing grains of different orientations and lamellae. This enhances the ability to resist fatigue crack propagation, reduces the spacing of fatigue striations, and slows down the fatigue crack growth rate, thereby significantly improving the fatigue life of the steel.

Effects of cooling rate (2-20 ℃/s) after hot deformation at 830 ℃ and tempering temperature (150-600 ℃) on the microstructure and hardness of direct-quenched ultra-high strength steel for construction machinery were studied by MMS-200 thermomechanical simulator, optical microscope(OM), scanning electron microscope(SEM) and Vickers hardness tester, and the microstructure and hardness of the steel under direct quenching and tempering process were compared with that under reheated quenching and tempering process. The results show that granular bainite and upper bainite disappear and the content of auto-tempered martensite decreases with the increase of cooling rate after hot deformation, and the microstructure is a mixture of lath martensite and auto-tempered martensite when the cooling rate exceeds 5 ℃/s. As the cooling rate increases, the hardness of the tested steel gradually increases, and the maximum hardness is 509 HV at the cooling rate of 20 ℃/s. When the tested steel is directly quenched and tempered, with the increase of tempering temperature, the precipitation position of carbides transits from the inside of martensite lath to the prior austenite grain and martensite lath boundaries, and the carbides continuously aggregate and coarsen. The hardness of the tested steel decreases with the increase of tempering temperature, and the maximum hardness is 510 HV at the tempering temperature of 150 ℃. Compared with reheated quenched and tempered steel, the width of martensite lath in direct quenched and tempered steel is smaller, and the hardness of matrix is higher under the same tempering process.

By means of SEM and EBSD observations as well as hardness tests, the variations in microstructure, texture and hardness of 403 martensitic stainless steel after quenching at different temperatures (900, 975, 1050 ℃) and tempering at 700 ℃ were studied. The results show that with the increase of quenching temperature, the microstructure of the tested steel is lath martensite, and the lath width gradually decreases. The EBSD test results indicate that as the quenching temperature rises, the texture intensity of the tested steel after heat treatment increases, the KAM value and Schmid factor decrease, the volume fractions of recrystallized grains and deformed grains increase, while the volume fraction of substructure grains decreases. The results of hardness tests show that the microhardness gradually increases with the increase of quenching temperature, which is mainly caused by the decrease of average grain size and the decrease of Schmidt factor.

Repair welding was performed on 4Cr5MoSiV die steel by using TIG welding, followed by post weld heat treatment of quenching and quenching and tempering. The microstructure, hardness, and tensile properties of the joints in three states (welded, quenched and quenched and tempered) were observed and measured by means of scanning electron microscope, microhardness tester, and tensile testing machine. The results indicate that the weld seam microstructure in as-welded state is mainly composed of coarse martensite, austenite, and precipitated carbides. After quenching the weld seam microstructure is mainly martensite, while its microstructure in the quenched and tempered state is mainly tempered martensite and a small amount of precipitated carbides. The average hardness of the weld zone in the as-welded specimens is about 648.7 HV0.1. The hardness of the weld zone in the quenched specimens increases, with an average value of about 670.1 HV0.1, and the hardness is uniform. The hardness value of the weld zone in the quenched and tempered specimens decreases to about 481.7 HV0.1, which is close to that of the base metal zone, and the hardness is also uniform. After post weld heat treatment, both the tensile strength and elongation of the joints are significantly improved. Among them, the tensile strength and elongation of the quenched and tempered joint reach 1270 MPa and 11.3%, which are 29% and 30% higher than those of the as-welded joint, respectively.

The effect of quenching process on impact property of the carburized 18Cr2Ni4WA steel shearer walking wheels was investigated. After carburizing heat treatment, the 18Cr2Ni4WA steel shearer walking wheels were oil-quenched at 780, 800 and 830 ℃, respectively, and finally cryogenic treated at -60 ℃ for 2 h, then tempered at 180 ℃ for 15 h. After the heat treatment, the mechanical properties and microstructure of the specimens were tested. The results show that the hardness of the core of the walking wheel is significantly reduced, while the impact property of the core is greatly improved when the quenching temperature is reduced. By adopting intercritical quenching process, when the quenching temperature is 780 ℃, the shearer walking wheel can meet the surface hardness requirements and improve the impact property of the core, and the microstructure is composed of lath martensite, bainite, ferrite and retained austenite, and the impact property of the core is greatly improved with impact absorbed energy of 108 J, achieving the improvement of the core of the impact property of the carburized and quenched walking wheel.

Production process of a non-oriented silicon steel is primary cold rolling→intermediate annealing→secondary cold rolling→final recrystallization annealing. The effect of intermediate annealing temperatures (400, 650, and 800 ℃) on the texture of the tested non-oriented silicon steel during the preparation process was investigated. The results show that with the increase of intermediate temperature, the grain size gradually increases, resulting in an increase in grain size of the cold-rolled sheet in the secondary stage, which further leads to an increase in the final recrystallized grain size. The texture intensity of the intermediate annealed specimen decreases, the λ texture components disappear, but the γ texture components gradually become the dominant texture. Strong Goss texture is formed in the specimen intermediate annealed at 650 ℃, and after the second stage rolling, a part of the Goss texture component is retained and the γ texture with the peak at {111}<112> is formed. In the initial stage of subsequent recrystallization annealing, both η and λ texture oriented grains nucleate at their shear bands. With the extension of the annealing time, the deformed matrix is consumed. After annealing for 1 h, the recrystallization texture of the final recrystallized specimen usually exhibits strong Goss texture.

To explore the influence of programable ion permeation (PIP) processes on the properties of FN04Mo steel prepared by metal powder injection molding technology (MIM), the MIM FN04Mo steel was subjected to surface modification treatments under four different PIP time, resulting in four types of specimens with compound layer thicknesses of 5, 10, 15, and 20 μm, respectively. The microstructure, surface hardness, hardness distribution, brittleness, tensile properties, wear resistance, and corrosion resistance of the infiltration layer were characterized and analyzed. The results show that the surface hardness of MIM FN04Mo steel before treatment is 372.99 HV0.1. After being treated by the PIP process, the average surface hardness reaches 510 HV0.1, with an increase of 37%, and the brittleness rating is grade 1. After PIP treatment, with the increase of the thickness of compound layer, the yield strength and tensile strength slightly decrease, while the wear resistance gradually increases. The neutral salt spray test of the specimens after PIP treatment shows that the protection rating R_p is grade 10. The electrochemical test indicates that the impedance of the specimens increases, the self-corrosion potential rises, the self-corrosion current density decreases, and the corrosion resistance improves.

A novel cemented carbide WC-CrFeNiCu_0.5 (HEA) was successfully fabricated by substituting CrFeNiCu_0.5 high-entropy alloy for the conventional metal binder (Co, Fe, and Ni) and vacuum hot-pressing sintering method. The microstructure evolution of the WC-HEA cemented carbide sintered at different temperatures (1050, 1100, 1200, and 1300 ℃) and its influence on the friction and wear properties were systematically investigated. The results show that the phases of the WC-HEA cemented carbide sintered at different temperatures mainly include WC and FCC phases, as well as a small amount of Cu-Fe alloy and metastable carbide M₃W₃C. When the sintering temperature is 1100 ℃, the WC-HEA carbide shows the highest densification and the lowest wear rate, with a porosity of only 5.15%±0.6% and a wear rate of 0.96×10^-6 mm³·N^-1·m^-1, resulting in the best wear resistance. The wear mechanisms of the WC-HEA cemented carbides sintered at different temperatures are mainly abrasive wear, accompanied by slight adhesive wear.

Effect of solution temperature on microstructure and properties of QBe1.9 and QBe2 beryllium copper alloys was studied by solution treatment at 780, 790, 800 ℃ and aging treatment at 320 ℃. The results show that QBe1.9 beryllium copper alloy requires lower driving force for complete solution treatment, has smaller grain size and more uniform distribution. With the increase of solution temperature, the hardness, tensile strength and conductivity of the QBe1.9 and QBe2 beryllium copper alloys after solution treatment decrease continuously, while the elongation increases continuously. After aging at 320 ℃ for 3 h, the tensile strength and hardness of the QBe1.9 beryllium copper alloy are much higher than that of the QBe2 beryllium copper alloy, and the conductivity and elongation are lower than that of the QBe2 beryllium copper alloy. When the solution temperature is 800 ℃, the two alloys exhibit the optimal comprehensive properties. The hardness of the QBe2 beryllium copper alloy is only 327 HV0.5, the tensile strength is 1032.0 MPa, the elongation is 8.9%, and the conductivity is 31.80%IACS. The hardness of the QBe1.9 beryllium copper alloy is 404 HV0.5, the tensile strength is 1336.8 MPa, the elongation is 7.3%, and the conductivity is 26.12%IACS. It provides a solution to the problem of improving the mechanical properties of the QBe1.9 beryllium copper alloy.

Effect of tempering time on the microstructure and mechanical properties of steel used for deep-well tubing was investigated by using optical microscope, scanning electron microscope, transmission electron microscope, tensile testing at room temperature, and Charpy impact testing at 0 ℃. The results show that when the tested steel is tempered at 550 ℃ with a tempering duration of 35-75 min, a tempered martensite microstructure is obtained. With the increase of tempering time, the microstructure gradually recovers, the lath morphology gradually disappears. Within this tempering time, the mechanical properties of the tested steel remain stable, with tensile strength ranging from 955 to 958 MPa, yield strength from 903 to 908 MPa, product of strength and elongation of 18.7 to 19.1 GPa·%, and impact absorbed energy at 0 ℃ of 128.7 J to 133.3 J. Optimal comprehensive mechanical properties are achieved when the tempering time is 75 min, with tensile strength of 957 MPa, yield strength of 908 MPa, product of strength and elongation of 19.1 GPa·%, and impact absorbed energy at 0 ℃ of 133.3 J. Both the tensile fracture morphology and impact fracture morphology of the tested steel exhibit ductile fracture characterized by dimples.

Based on the one-step quenching and partitioning (Q&P) process conditions, the effect of partitioning time (2,30 min) on the microstructure and properties of a tested one-step Q&P high-strength steel was studied by using universal tensile machine, scanning microscope, electron backscatter diffraction and X-ray diffractometer. The results show that the excellent comprehensive mechanical properties are obtained when the partitioning time is 2 min. The yield strength, tensile strength and percentage total extension at fracture are 965 MPa, 1288 MPa and 11.1%, respectively. The dislocation density in martensite increases and the retained austenite content decreases as the partitioning time increases. The difference in yield strength of the specimens under different partitioning time is mainly attributed to the different strengthening increments of solution strengthening, fine grain strengthening and dislocation strengthening, among which fine grain strengthening and dislocation strengthening contributions are dominant. The theoretical yield strength calculation results are in agreement with the tested results.

Stress relaxation aging tests were conducted on 2195 Al-Li alloy under a loading stress of 250 MPa, with aging at different temperatures (160, 170 and 180 ℃) for 17 h. The effect of aging temperature on the stress relaxation behavior, mechanical properties, and precipitates of the alloy was investigated by using MTS CMT5205 universal material testing machine and scanning transmission electron microscope (STEM). The results show that the stress relaxation curves exhibit two distinct stages: a stage of decreasing stress relaxation rate followed by a stage of constant stress relaxation rate. As the aging temperature increases, both the initial stress relaxation rate and creep strain are enhanced. Specifically, the creep strain after stress relaxation at 180 ℃ for 17 h is approximately 62% higher than that at 160 ℃, indicating that increasing the aging temperature can accelerate the stress relaxation process and the accumulation of creep strain. Furthermore, the tensile strength of the alloy increase with the rising of aging temperature. Under the aging condition of 180 ℃, the yield strength and tensile strength of the alloy are approximately 20% and 10% higher, respectively, than those at 160 ℃. Meanwhile, the elongation after fracture decreases as the temperature rises, and that under the aging condition of 180 ℃ is about 27.6% lower than that at 160 ℃. At different aging temperatures, the precipitated phase in the alloy is always the T₁ phase. Moreover, as the aging temperature increases, the T₁ phase becomes more dispersedly distributed, with a larger quantity and smaller size, which is beneficial for enhancing the strength of the alloy.

Compared with the off-line quenching (Q) process, the effect of direct quenching (DQ) on the microstructure evolution and mechanical properties of a 590 MPa Cu-bearing HSLA steel was studied. The microstructure of the specimens was analyzed by OM, SEM, EBSD and TEM. The strength and toughness were tested by tensile test and impact test. The results show that the DQ process retains the large dislocation density and fine grains obtained through controlled rolling. The microstructure of the DQ tested steel exhibits a flattened and elongated state, while the microstructure of the Q tested steel consists of equiaxed grains. The effective grain size of the DQ tested steel is smaller than that of the Q tested steel, but there is little difference in strength and low-temperature toughness. In the subsequent tempering process, Cu-rich phase and carbide precipitation are produced, and the yield strength of the tested steel is improved by the enhancement of dislocation, fine grain and precipitation, while the reduction of toughness caused by the increase of dislocation density and the flattened structure is made up by fine grain strengthening. After direct quenching+680 ℃ tempering, the tested steel has the best comprehensive properties, and the yield strength is 805 MPa which is 159 MPa higher than that of off-line quenching+680 ℃ tempering and the impact absorbed energy at -84 ℃ is 248 J.

In order to predict the low-temperature impact performance at -45 ℃ of normalized 35CrMoA steel test bars after subcritical quenching and high-temperature tempering, the actual state parameters of low-temperature impact property at -45 ℃ under different heat treatment temperatures were used as learning specimens to train and predict the low-temperature impact property using a three-layer back propagation artificial neural network (BPANN). The results show that the BPANN can predict the low-temperature impact property at -45 ℃ of the 35CrMoA steel specimens subcritical quenched at different temperatures and tempered at different temperatures, with an error range of 5% to 9%. The predicted values of the BPANN are lower than the actual values, and the prediction accuracy can be improved by increasing the training specimens. The trend of the predicted data is the same as that of the measured data, and it can achieve a meaningful prediction. This study can reduce the number of tests in actual production through prediction, save test costs, and is helpful for the research on the low-temperature impact property prediction of the 35CrMoA steel at other subcritical quenching temperatures.

Variation laws of mechanical properties, electrical conductivity and microstructure of extruded 7C04 aluminum alloy solution treated at 450, 460 and 470 ℃ for 1 h, respectively, and solution treated at 470 ℃ for 0.5, 1 and 2 h, respectively, and aged at 120 ℃ for 18 h were investigated by means of hardness, electrical conductivity, tensile test and microstructure analysis. The results show that when the solution treatment temperature is between 450-470 ℃ and the solution treatment time is 1 h, with the increase of solution treatment temperature, the soluble second phase particles in the aged state alloy gradually dissolve, the strength, elongation and hardness increase, and the electrical conductivity decreases. When the solution treatment temperature is 470 ℃ and the solution treatment time is between 0.5-2 h, with the extension of solution treatment time, the dissolution degree of soluble second phase particles in the aged state alloy increases, reaching a supersaturated state after 1 h. If the solution treatment time is further extended, the number of second phase particles does not change significantly, the strength increases first and then decreases, the elongation and hardness continuously increase, and the electrical conductivity first decreases and then increases. Under the condition of solution treatment at 470 ℃ for 1 h and aging at 120 ℃ for 18 h, the mechanical properties of the alloy can reach a tensile strength of 645.3 MPa, a yield strength of 585.0 MPa, a hardness of 182.53 HV3, and an electrical conductivity of 27.59%IACS.

Effects of two different quenching processes (delay quenching, intercritical quenching) on the microstructure and mechanical properties of a ferrite/martensite dual-phase steel after the same tempering treatment were studied by means of optical microscope, scanning electron microscope, transmission electron microscope and tensile and impact tests. The results show that the microstructure of the as-tempered steel quenched by the two different processes consists of ferrite, tempered martensite and M₂₃C₆ precipitates. Polygonal dual-phase structure is obtained by the delay quenching+tempering process, while fine lath structure is obtained by the intercritical quenching+tempering process. The yield strength and tensile strength of the as-tempered tested steel quenched by the two different processes decrease, while the yield ratio, elongation and impact absorbed energy increase. Compared with the delay quenching process, the yield strength and tensile strength of the as-tempered steel under intercritical quenching process are lower, while the yield ratio, elongation and impact absorbed energy are higher, which show that the intercritical quenching+tempering process can significantly improve the plasticity and toughness of the tested steel.

In order to eliminate the streamlined structure of the most severely distorted part from the core to 1/2 radius of the forged billet of 20CrMoH steel gear with a large forging ratio, the effects of different normalizing processes on the microstructure transformation and hardness change of the most severely distorted part were studied. The results show that when the heating temperature is 880-920 ℃ and the heating time exceeds 2 h, the isothermal temperature is above 600 ℃ and the holding time is 1 h, the microstructure heredity of the forged billet can be eliminated, and the deformation streamline can be eliminated. The ferrite grain size can reach grade 9, and the hardness can be greatly reduced from 249 HBW to 146-165 HBW. With the extension of heating and isothermal time, the normalizing effect do not increase significantly. When the heating time in the range of 880-920 ℃ is shortened to 1 h, the isothermal temperature needs to reach 630 ℃ to achieve the same effect, and more energy can be saved and production costs can be reduced.

High chromium alloy 3J40 was subjected to solution treatment at different temperatures (ranging from 1020 ℃ to 1200 ℃ with a holding time of 1 h) and aging treatment (at 700 ℃ and 800 ℃ with a holding time of 24 h). Effect of solution and aging processes on microstructure and properties of the 3J40 alloy was studied by using optical microscope, scanning electron microscope, universal tensile testing machine, and hardness tester. The results show that a large number of granular α-Cr phases are distributed in the matrix of the 3J40 alloy. With the increase of the solution temperature, the grains gradually become coarser, and the granular α-Cr phases gradually dissolve back into the matrix. When the solution temperature is 1200 ℃, all the α-Cr phases are dissolved. After aging at different temperatures, the supersaturated Cr element in the matrix of the 3J40 alloy precipitates again in the form of strip-shaped dendrites or lamellar structures. Meanwhile, as the precipitation amount of the lamellar α-Cr phase increases, the tensile strength and hardness of the alloy at room temperature are significantly improved, but the plasticity is significantly reduced.

Effect law and mechanism of molybdenum (Mo) element on the microstructure and room-temperature mechanical properties of Fe-Co-Cr bearing steel were investigated. The results show that the addition of Mo increases the variety of major secondary phases in the tested steel. In the 0%Mo steel, the primary secondary phase is M₂₃C₆. In the 2%Mo steel, the primary secondary phases change to M₂₃C₆ and Laves phase. In the 4.6%Mo steel, the primary secondary phases become M₂₃C₆, Laves phase and M₆C. The addition of Mo element increases the volume fraction of retained austenite and dislocation density, and refines the width of martensite laths. As the Mo content increases, the strength, impact property and hardness of the tested steel show upward trends.

Microstructural characteristics such as morphology, dislocation density and crystal orientation were studied by using metallographic microscope, transmission electron microscope, electron backscatter diffraction technology and X-ray diffraction in a high strength low alloy steel with high vanadium content under both hot-rolled and quenched-tempered states, respectively. At the same time, the behavior of hydrogen diffusion and crack propagation were also studied by means of electrochemical hydrogen permeation technique and fatigue crack growth test. The aim was to establish the relationship among the microstructure, hydrogen diffusion and crack growth rate. The results show that compared with the hot-rolled state, retained austenite decomposes, the dislocation density decreases, and a large number of vanadium carbides precipitate, leading to an increase of irreversible hydrogen traps in the quenched-tempered state. Consequently, the hydrogen diffusion coefficient decreases, and the diffusion activation energy increases, making hydrogen diffusion more difficult and reducing the susceptibility to hydrogen embrittlement. In air and under low hydrogen charging current density conditions, the presence of retained austenite in the hot-rolled state provides superior crack growth resistance compared to the quenched-tempered state. However, at higher current densities, hydrogen embrittlement becomes dominant, resulting in an accelerated crack growth rate.

Evolution of the normalizing microstructure and texture of 50W400 non-oriented silicon steel after adding rare earth element Ce was analyzed by means of metallographic observation and SEM-EBSD instruments. This investigation aimed to clarify the trend of changes in grain size and texture strength of the tested steel after normalizing.The results indicate that the addition of Ce can delay the complete recrystallization time during normalizing. Specifically, with rare earth Ce adding, the time for maintaining a uniform grain size distribution at a uniform temperature of 970 ℃ can be postponed from 155 s to 215 s, and the grain size is smaller. Furthermore, in the thickness direction of the longitudinal section of the tested steel, the proportion of favorable surface textures {100} and {110} increases, while the proportion of the unfavorable surface texture {111} decreases.

Tensile and fatigue crack growth tests were conducted on high temperature martensitic steels PCrNi3MoV, 25Cr3Mo3NiNbZr and 20CrNiMo2VNb at 25 ℃ and 700 ℃ to investigate the fatigue crack growth behavior of pressure vessel steels, respectively. The results show that at room temperature, the strength of the three tested steels is fairly the same. When the stress intensity factor level (ΔK) is 60 MPa·m, the fatigue crack growth rates (da/dN) are 1.05×10^-3, 8.80×10^-5, and 7.73×10^-5mm/cycle, respectively. The fatigue crack growth rate of the 20CrNiMo2VNb steel at room temperature is the lowest, which is attributed to its largest proportion of substructure high-angle grain boundaries, reaching 81.46%. At 700 ℃, when the stress intensity factor level ΔK is 30 MPa·m, the fatigue crack growth rates of the three tested steels are 1.58×10^-3, 3.78×10^-4, and 3.89×10^-5mm/cycle, respectively. Through the comparative analysis of the microstructure and high temperature strength, it is found that the highest tensile strength of the 20CrNiMo2VNb steel at 700 ℃ is the main reason for its low fatigue crack growth rate. Based on the Paris model, a quantitative relationship between the fatigue crack growth rate and mechanical properties of high temperature resistant martensitic steel is established, which provides theoretical support for the subsequent research on the fatigue crack growth of the 20CrNiMo2VNb steel.

Fe-18Mn-8Al-1C lightweight steel was annealed at 950 ℃ for 1 h and tensile deformation with different tensile strain (ε=5%, 15%, 25%, 35%) was conducted. The microstructure, mechanical properties and deformation mechanism of the Fe-18Mn-8Al-1C lightweight steel were investigated by means of optical microscope, electron backscattered diffraction, electron channeling contrast imaging and transmission electron microscope. The results show that an excellent combination of high strength and adequate total elongation is obtained of the tested steel annealed at 950 ℃. A typical deformation mode, i.e., planer glide, is observed in the tested steel. The deformation mode can be concluded as: dislocation gliding in single/multiple slipping system →Taylor lattices →microband. The strain hardening ability can be significantly enhanced as a result of the increase in the geometrically necessary dislocation, while the effect of microband induced plasticity can be activated by the development of microband, leading to a simultaneous enhancement in the strength and elongation of the tested steel.

A comparative study between GH4145 alloy bolt served for about 60 000 h at 537 ℃ and standard heat treated (unserved) GH4145 alloy bars was carried out. The microstructure and precipitated phase characteristics of the both specimens were observed and analyzed by means of optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The Brinell hardness, tensile properties at room temperature and high temperature, impact properties and endurance properties were tested. The results show that the microstructure of both specimens is typical austenite, with twins and carbides in the grains. The average grain size of the serviced bolt is not significantly larger than that of the unserviced bar. The size and volume fraction of γ′ phase at the thread and polished rod of the serviced bolt are basically the same. The size and the volume fraction of γ′ phase in the serviced bolt are larger and higher than those in the unserviced state. In terms of mechanical properties, in addition to the high hardness value at the polished rod of the service bolt, the room temperature tensile and high temperature durability properties of the serviced bolt and the unserviced bolt both meet the standard requirements. The yield strength at the polished rod is higher than that at the thread, and the hardness and strength of the serviced bolt are higher than those of the unserviced state.

Effect of high-temperature aging at 675 ℃ on microstructure evolution and room temperature mechanical properties of large-diameter thick-walled pipes of G115 steel was investigated by using optical microscope, field-emission scanning electron microscope, field-emission transmission electron microscope, room temperature tensile and impact tests. The results show that the matrix structure of the G115 steel in the original normalized and tempered state is tempered martensite, and the precipitated phases are M₂₃C₆ carbide and MX carbonitrides. During the long-term aging process at 675 ℃, the matrix structure remains tempered martensite, however, a slow recovery of the martensite laths occurs. The M₂₃C₆ and MX phases grow slowly, and the Laves phase precipitates rapidly and coarsens. Compared with the properties in the original state, both the tensile strength and yield strength of the G115 steel show a monotonous decreasing trend, while the elongation first increases and then decreases. And the impact property of the G115 steel decreases in the initial stage of aging, but as the aging progresses, the impact property as a whole shows a trend of first increasing and then decreasing.

Isothermal thermal compression tests of metastable β titanium alloy Ti-38644 with the strain rate of 0.01-50 s^-1and reduction of 60% at the temperature between 800-1000 ℃ were carried out by a Gleeble-3800 thermal simulation machine. The flow stress curves under different deformation conditions were analyzed to establish the Arrhenius constitutive model and processing maps of the Ti-38644 titanium alloy, and the effect of deformation parameters on the microstructure evolution was systematically studied. The results show that with the increase of strain, the flow curves successively present work hardening and dynamic softening, and the discontinuous yielding occurs after the peak stress. The flow stress decreases as the deformation temperature increases and the strain rate decreases. Combined with the hot processing map, it is found that the instability region is mainly concentrated in the high strain rate area, and the peak efficiency of power dissipation (η) region occurs in the temperature range of 840-1000 ℃ and the strain rate of 0.01-0.1 s^-1, meanwhile the microstructure shows dynamic recrystallization characteristics.

Effect of grain size on hydrogen damage behavior in 99% pure iron was investigated by controlling grain size through different annealing processes. Two types of pure iron with different grain sizes were subjected to electrochemical hydrogen charging. The hydrogen-induced damage was characterized by using scanning electron microscopy, while thermal desorption spectroscopy was employed to analyze hydrogen desorption behavior. The X-ray diffraction was utilized to examine lattice distortion caused by hydrogen charging. The results show that hydrogen-induced cracks primarily initiate at grain boundaries in the surface layer. As the cracks propagate, the surface region is extruded, forming hydrogen blisters. The size of hydrogen blisters show a positive correlation with grain size, whereas the amount of hydrogen desorption decreases with the increase of grain size. The X-ray diffraction analysis reveals that hydrogen charging causes a shift in diffraction peaks towards lower angles, indicating lattice expansion, which may significantly influence hydrogen damage in pure iron. These findings elucidate the effect of grain size on hydrogen damage behavior in pure iron and provide experimental evidence for improving hydrogen embrittlement resistance.

Focusing on Cr8Mo2SiV ledeburite cold working die steel with high carbon and medium chromium content, the evolution of eutectic carbides (M₇C₃) and secondary carbides (M₇C₃) during the process of heating, holding and cooling was in-situ observed by using a high-temperature laser scanning confocal microscope. The results show that when the heating temperature exceeds 1180 ℃, the low-melting eutectic phase undergoes remelting, leading to pronounced overheating and overburning phenomena around the grain boundaries. Furthermore, The higher the holding temperature is, the higher the precipitation temperature of secondary carbides during the cooling process is. As the holding temperature rises from 1160 ℃ to 1200 ℃, Mo-rich particulate metastable carbides located at the grain boundaries dissolve, and the alloy elements enriched in the elongated dendritic secondary carbides continuously diffuse into the matrix. The difference in alloy element content between the matrix and the carbides is reduced gradually.

Hot compression deformation behavior of a low-temperature high-Mn steel for welding materials was studied using a Gleeble-3800 thermal simulation testing machine within the temperature range of 800-1000 ℃ and strain rate range of 0.01-10 s^-1. Friction correction and temperature correction were applied to the stress-strain curves. The Arrhenius constitutive equation and the BP neural network constitutive model of the tested steel were established using the corrected flow curves. Additionally, hot processing maps at true strains of 0.2, 0.4, and 0.6 were plotted. The results show that the correlation coefficient between the predicted and tested values of the flow stress in the Arrhenius constitutive model is 0.976. The constitutive model based on the BP neural network has a higher prediction accuracy for the flow stress, with a correlation coefficient of 0.996. The optimal hot processing zone for the tested low-temperature high-Mn steel is as follows: the deformation temperature ranges from 930 ℃ to 1000 ℃, and the strain rate ranges from 0.01 s^-1 to 0.1 s^-1. Combined with the microstructure analysis, it is found that under the process parameters of a deformation temperature of 950 ℃ and a strain rate of 0.01 s^-1, complete dynamic recrystallization can occur in the tested steel, resulting in a uniform and fine equiaxed grain structure.

The surface decarburization behavior of 36MnVS4 and 46MnVS5 microalloyed medium carbon steels for fractured splitting connecting rod was carried out by observing metallographic structure and quantitative analyzing decarburized layer. The results show that the surface decarburization behavior of the 36MnVS4 and 46MnVS5 steels heated at 700-1250 ℃ for 60 min is the same. The decarburized layer increases first and then decreases with the increase of heating temperature, with the maximum of 795.4 μm and 898.3 μm at 1200 ℃, respectively. Compared to the 46MnVS5 steel, the 36MnVS4 steel has a lower C content of 0.08% and a higher V content of 0.128%, and the decarbonization sensitivity decreases by 5%-25%. By accelerating the heating speed and cooling speed, or hot forging at the temperature away from the decarbonizing sensitive temperature, the decarburization can be controlled. During heating billets in rolling and forging connecting rod, it's necessary to avoid peak temperatures and reduce residence time.

Continuous cooling dilatometric curve of drilling steel 23CrNi3MoA was tested by using a Gleeble-3800 thermal simulator at cooling rate of 0.05-20 ℃/s, then the static continuous cooling transformation(CCT) curve of the 23CrNi3MoA was plotted. The microstructure transformation law of the drilling steel 23CrNi3MoA under different cooling rate conditions was analyzed. The results show that the Ac₁ phase transformation temperature of the 23CrNi3MoA is 713 ℃, and the Ac₃ is 774 ℃. When the cooling rate is 0.05-0.5 ℃/s, the microstructure of the tested steel is all bainite. When the cooling rate is 0.5 ℃/s, martensite appears. Starting from 1 ℃/s and over, the microstructure is all martensite. According to the phase transformation temperature of the drilling steel 23CrNi3MoA, the annealing process ranged at (Ac₁±30) ℃ can be formulated to guide the production, and a good as-annealed state structure can be obtained, so as to achieve a good hardness value range of the product.

The Fe-Mn-Al-C steel has become the development direction of lightweight and high strength steel for automotive applications due to its low density and excellent mechanical properties. Compared with high manganese steels with a conventional manganese content exceeding 20%, medium manganese lightweight steels have significant advantages in cost control and processing. Based on relevant research achievements at home and abroad, the effects of different alloying elements and precipitates on the microstructure and properties of the Fe-Mn-Al-C low-density steel, as well as the strengthening mechanisms and tensile properties of different types of low-density steels were deeply analyzed and summarized. In addition, based on the characteristics of Q&P process, it was proposed to use Q&P process to treat medium manganese low-density steel, and its application prospects were also discussed.

Research status of medium-Mn steels at home and abroad were reviewed from three aspects: chemical composition, strengthening and toughening mechanism, and heat treatment processes. The effects of elements such as Mn, C, and Al on the microstructure and properties of medium-Mn steels were analyzed and summarized. Regarding the strengthening and toughening mechanism during the deformation of medium-Mn steels, the transformation-induced plasticity (TRIP) effect and its influencing factors were emphasized. Considering that the microstructure and properties of medium-Mn steels were closely related to heat treatment processes, the heat treatment processes such as austenite reverse transformation (ART), quenching and tempering (Q&T), quenching and partitioning (Q&P), and their effects on the microstructure and properties were introduced in detail. Finally, the future research directions of medium-Mn steels was prospected.

An important core course for majors in metal materials, “Metal materials and heat treatment” has gradually revealed some problems during the teaching process under the traditional teaching mode, making it difficult to meet the higher requirements for talent cultivation in the new era. The rapid development of artificial intelligence has brought new opportunities and challenges to higher education teaching. Integrating artificial intelligence with course teaching has become a new direction for the reform of college course teaching in the new era. The course content and teaching status of “Metal Materials and Heat Treatment”were analyzed, and the key reform points including optimizing the course knowledge system based on artificial intelligence, innovating teaching methods, strengthening practical teaching, and constructing a precise teaching evaluation system were put forward. It conducts a preliminary exploration of the integration of artificial intelligence and the teaching of the “Metal Materials and Heat Treatment” course. The aim is to promote the application of artificial intelligence in course teaching, improve teaching effectiveness, and meet the needs of talent cultivation in the new era.

After carburizing treatment with theoretical case depths of 1.1 and 1.3 mm on BG801 gear steel, the microstructure, room temperature hardness, 500 ℃ high temperature hardness, residual stress, and 500 ℃ high temperature rotating bending fatigue of this two specimens were tested and characterized, and the main factors affecting fatigue property were analyzed. The results show that in the carburized zone, the carbide content, hardness, and residual compressive stress of the specimens with a theoretical case depth of 1.3 mm are higher than those of the specimens with a theoretical case depth of 1.1 mm. The rotating bending fatigue strength of the specimens with theoretical case depths of 1.1 and 1.3 mm at 500 ℃ is 675 and 850 MPa, respectively. In the carburized zone, the specimen with a theoretical case depth of 1.3 mm has relatively high hardness, which can suppress the initiation of microcracks to a certain extent. In addition, combined with the effect of residual stress and carbide size, the fatigue crack propagation driving energy of the specimen with a theoretical case depth of 1.3 mm is smaller than that of the specimen with a theoretical case depth of 1.1 mm under the same external applied cyclic stress, which is another reason why the fatigue property of the specimen with a theoretical case depth of 1.3 mm is better.

Effect of adding different concentration of Ti nanoparticles in silicate electrolyte on the morphology, structure and corrosion resistance of AZ91D magnesium alloy micro-arc oxidation film were studied by SEM, EDS, electrochemical workstation and hydrogen evolution test. The results show that with the addition of 0.25-1.00 g/L Ti nanoparticles, the number of micropores decreases, the film thickness increases, the compactness increases, and the corrosion resistance firstly increases and then decreases with the increase of concentration of Ti nanoparticles. In the process of micro-arc oxidation, TiO₂ is generated to fill some holes in the micro-arc oxidation film and improve the quality of the film. When the concentration of Ti nanoparticles is 0.50 g/L, the corrosion resistance of the Ti micro-arc oxide film can be increased by more than 3 orders of magnitude in 0.6 mol/L sodium chloride solution, and the hydrogen evolution rate can be reduced greatly.

The Fe-based amorphous coating, Fe₇₈Si₉B₁₃, was successfully prepared on the surface of ZK60A magnesium alloy using the cold spray method. The coating was then heat-treated below its crystallization temperature (494 ℃) at different temperatures (300, 350, 400 and 450 ℃) for 30 min. The hardness, corrosion resistance and corresponding mechanisms of the coatings was investigated by using Vickers microhardness testing, electrochemical workstation and scanning electron microscope(SEM).The results show that the Fe₇₈Si₉B₁₃ coating has a dense structure, with a porosity of 2.73% and an amorphous fraction of 92.6%. The Vickers hardness of 836.2 HV0.2 is much higher than that of the magnesium alloy (68 HV0.2). In the SBF solution, the corrosion rate of the coating (0.55 mm/a) is significantly better than that of the ZK60A magnesium alloy (9.15 mm/a). After heat treatment, the corrosion resistance of the coating first increases and then decreases with the increase of heat treatment temperature. The coating treated at 350 ℃ has the best corrosion resistance, with self-corrosion potential of －0.684 V and corrosion rate of 0.26 mm/a. After soaking in SBF for 30 days, the ratio of Ca to P elements on the surface of the coating is very close to that in hydroxyapatite, indicating a certain biological activity.

By magnetron sputtering metal Ti thin film on the TC21 titanium alloy surface, and then the alloy with pre-fabricated Ti film was plasma nitrided at 580 ℃, the plated-nitrided Ti-N layer on the TC21 titanium alloy surface was prepared, and the effects of different N₂/H₂ ratios during the plasma nitriding process on the structure and properties of the plated-nitrided Ti-N layer were studied. The results indicate that the pre-deposited Ti metal film promotes the effective diffusion and reaction of nitrogen atoms, achieving low-temperature plasma plating-and-nitriding composite strengthening on the surface of the titanium alloy. The plated-nitrided Ti-N layer is mainly consisted of ε-Ti₂N, TiN and ɑ-Ti phases, and the relative content of brittle phase ε-Ti₂N decreases with the increase of N₂/H₂ ratio, resulting in the improvement of adhesion strength. The microhardness and wear resistance of the titanium alloy are significantly enhanced by the plating-and-nitriding composite strengthening. When the N₂/H₂ ratio is 3∶1, the hardness of the plated-nitrided Ti-N layer reaches a maximum of 725 HK0.05.

To improve the wear resistance of the thermal power boiler water wall, desulfurization system desulfurization circulation pump and ash removal system pipeline, Ni25WC and Ni35WC coatings were deposited on the surface of 2205 stainless steel by high velocity oxygen fuel (HVOF) spraying and vacuum heat treatment. The microstructure and phase of the coatings, as well as the key properties, such as abrasive wear resistance and erosion wear resistance, were studied. The results show that heat treatment enhances the bonding between the hard phase and the adhesive phase, and the pores change from lamellar shape to spherical shape. The hard phases in Ni25WC and Ni35WC coatings do not flake off during abrasive wear test, and the wear mass loss of the Ni25WC and Ni35WC coatings are 42 mg and 30 mg, respectively, which less than 4% of the mass loss of 2205 stainless steel. Both coatings mainly contain Ni, W₂C, Cr₃C₂ and C. The hardness of Ni25WC and Ni35WC coatings is basically the same, both more than three times that of the 2205 stainless steel. During erosion wear test, the sand repeatedly impacts the surface, causing the hard phase peeling off. As a results, the erosion wear resistance of the two coatings is only about 30% higher than that of 2205 stainless steel. According to results of abrasive wear and erosion wear, the performance of Ni35WC coating is better than that of Ni25WC coating.

Effect of ultrasonic shot peening on NiCoAlFeCrMoSi high entropy alloy coating prepared on the surface of magnesium alloy was investigated in order to enhance the corrosion and wear resistance of the coating. The results show that with the increase of ultrasonic shot peening time (0-800 s), the surface roughness of the coating decreases and then increases, the porosity decreases, and the corrosion resistance and wear resistance show a trend of first increasing and then decreasing. When the ultrasonic shot peening time is 400 s, the coating has the best corrosion resistance and wear resistance. Compared with the coating that is not shot peened, the average surface hardness is increased by 50 HV0.2, the surface roughness reduced by 53.82%, the corrosion current density decreases from 3.012×10^-5 A/cm² to 9.80 μA/cm², and the wear volume is reduced by 26.8%.

In the actual production process, cracks are found in the grinding surface of the parking ratchet after carburizing and quenching, and it would lead to the scrap of the parts. In order to find out the cause of surface cracking after heat treatment and grinding of the parking ratchet and subsequent control measures, the crack parts were tested and analyzed by using microhardness tester, scanning electron microscope, energy spectrometer, X-ray stress analyzer and other characterization means, and the reproducibility test was conducted with normal parts and relevant indicators were compared. The results show that the carburizing and quenching microstructure of the cracked parts of the parking ratchet is normal, the surface residual stress is normal, there is no oxidation decarburization and carburization at the crack, there is no burn in grinding, and there are metal oxide inclusions on the surface of the parts. The root cause of surface cracks is the defects of oxide inclusions in the raw materials. This study provides a feasible direction for failure analysis. Some control measures to prevent cracks are put forward, such as monitoring the inclusions on the surface of raw materials, shortening the interval time of quenching and tempering, controlling the microstructure of the surface of heat treatment and optimizing the grinding parameters.

An 1Cr17Ni2 stainless steel connection bolt of a device broke during routine maintenance. The chemical composition, metallographic structure, phase structure, fracture morphology and hardness of the bolt were analyzed by means of physical and chemical test. The result show that failure mode of the bolt is stress corrosion. The main causes of stress corrosion cracking are that the large-sized chromium-rich carbides in the material reduce its corrosion resistance and the hardness of the material exceeds the standard requirement, which makes it highly sensitive to stress corrosion.

The abnormal coarse pearlitic colonies observed in the microstructure of EA1N axle steel after conventional normalizing treatment was discussed and the causes of these abnormal coarse pearlitic colonies, as well as the effects of normalizing temperature and subsequent cooling rate on the microstructure of the EA1N axle steel were analyzed. The results show that the formation of coarse pearlitic colonies is related to carbon segregation in the EA1N axle steel, normalizing temperature, and subsequent cooling rate. The higher the normalizing temperature, the coarser the abnormal pearlitic colonies become. The slower the cooling rate after normalizing treatment, the easier it is to form pearlitic colonies, and the more severe the coarsening of the colonies.

Cracks were found during the machining process, on the surface of some forged track links of 35MnB steel produced in a certain batch. In order to find the cause of crack formation and prevent further problems in subsequent production, macroscopic analysis and microscopic morphology observation were conducted on the surface cracks of the 35MnB forged track links, and composition analysis was performed using an energy spectrometer. The chemical composition, non-metallic inclusions, and microstructure of the defective and normal parts were compared and analyzed. The results indicate that the heating temperature before forging is too high, which results in coarse austenite grains and the precipitation of manganese sulfide particles on the grain boundaries, and leads to poor bonding force at the grain boundaries. The main reason for the formation of such defects is grain boundary cracking during forging. Burning occurs due to excessive heating temperature, which is an irreversible defect. Properly selecting the heating temperature or shortening the residence time in the high temperature range can avoid the formation of such defects.