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Creep property and microstructure evolution at 700 ℃ of a novel Fe-Ni based superalloy

Jiao Chunhui, Pan Yanjun, Li Shengzhi, Bai Du, Li Bei, Deng Ge, Jia Xiaoshuai

Heat Treatment of Metals 2025, 50 (2): 1-7. doi:10.13251/j.issn.0254-6051.2025.02.001

Abstract （90）

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A novel Fe-Ni-based superalloy, intended for ultra-supercritical thermal power generating units, was evaluated under constant load conditions at 700 ℃ with varying stress levels of 250 MPa and 200 MPa. The service limit and creep life of the alloy were predicted, and the microstructure evolution during creep was analyzed. The results indicate that the creep life of the alloy at 700 ℃/250 MPa and 700 ℃/200 MPa is 2378 h and 12 716 h, respectively. Based on the Larson-Miller equation, the alloy can withstand stresses of approximately 152 MPa after 100 000 h and 134 MPa after 260 000 h at 700 ℃, fully meeting the service requirements (stress of 35 MPa, creep life of 100 000 h). Microstructure analysis reveals that high-density dislocations are distributed in the 700 ℃/250 MPa specimen, whereas fewer dislocations are observed in the 700 ℃/200 MPa specimen. The MC carbides with larger size within the grains predominantly exhibit blocky or rod-like morphologies, with faster growth rates under higher stress conditions. The smaller M₂₃C₆ carbides at grain boundaries precipitate primarily in chain form, and their width increases with prolonged creep exposure. The γ′ phase within the grains remains spherical but undergoes coarsening during creep. Notably, some grain boundary γ′ phases exhibit abnormal growth, forming PFZs/DCZs, which adversely affect the alloy's high-temperature creep performance.

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Hot deformation behavior of Inconel617 alloy

Ding Zuojun, Ren Wenhao, Zhang Guo, Zheng Yue, He Xikou

Heat Treatment of Metals 2025, 50 (2): 8-14. doi:10.13251/j.issn.0254-6051.2025.02.002

Abstract （35）

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Hot compression simulation tests of Inconel617 alloy were carried out using Gleeble-3800 thermal simulation test machine in the range of deformation temperature of 950-1200 ℃ and strain rate of 0.001-1 s^-1. The hot deformation behavior under different deformation conditions were analyzed. The results show that the Inconel617 alloy exhibits obvious work hardening characteristics at the initial stage of deformation. As the strain increases, the flow stress increases to the peak value and then gradually decreases, showing dynamic softening dominated by dynamic recrystallization. The hot deformation constitutive equation of the Inconel617 is constructed based on the Arrhenius model modified by the hyperbolic sine function. The hot deformation activation energy Q is 418.4 kJ/mol. The hot working map of the Inconel617 alloy is constructed, and the appropriate hot working range is determined: deformation temperature of 1080-1170 ℃, strain rate of 0.012-1 s^-1.

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Creep rupture property at 550 ℃ and microstructure evolution of 316H stainless steel forging for generation IV reactor

Zhang Zhifeng, Zhao Jiqing, Wang Xiaofang, Wang Yunhai, Yang Gang

Heat Treatment of Metals 2025, 50 (2): 15-22. doi:10.13251/j.issn.0254-6051.2025.02.003

Abstract （28）

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Creep rupture strength of 316H stainless steel forging was tested by rupture test machines at 550 ℃, and the fracture morphology and microstructure of the fracture specimens were analyzed by optical electron microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The results indicate that the 316H stainless steel forging has high creep rupture strength at 550 ℃, the creep rupture stress after 10 000 h determined through the isothermal extrapolation method is 314 MPa, which significantly exceeds the ASME design criteria. The fracture morphology of the 316H stainless steel forging consists of both dimple and intergranular fracture patterns. As the fracture duration increases, the proportion of intergranular fracture progressively rises, and the origin of the fractures transitions from non-metallic inclusions into second-phase particles at the grain boundaries. The 316H steel forging displays good structural stability throughout the 550 ℃ rupture test. As the applied stress reduces and fracture duration extends, the grain size progressively increases, the grain boundary area diminishes, and only a small amount of M₂₃C₆ and σ phases are precipitated at the grain boundaries

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Influence of Mn on high-temperature mechanical properties of low Ni duplex stainless steel

Yang Yuheng, He Jianguo, Song Zhigang, Feng Han, Lü Jiesheng, Wu Xiaohan, Gu Yang, Zhu Yuliang

Heat Treatment of Metals 2025, 50 (2): 23-28. doi:10.13251/j.issn.0254-6051.2025.02.004

Abstract （20）

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High-temperature tensile tests were conducted on 17Cr-2Ni-2Mo-0.2N-xMn (x=2, 3, 4) duplex stainless steel at 900-1300 ℃ and 1 s^-1, and thermal compression tests were conducted with 50%, 60%, and 70% deformation at 900-1300 ℃ and 1 s^-1, respectively, to investigate the effect of Mn content on its high-temperature mechanical properties. The results indicate that at a deformation temperature of 900 ℃, ferrite undergoes dynamic recrystallization, while austenite only undergoes partial dynamic recrystallization. At a deformation temperature of 1200 ℃, ferrite mainly undergoes dynamic recovery, while austenite undergoes dynamic recrystallization. All specimens exhibit varying degrees of dynamic recovery or dynamic recrystallization during high-temperature deformation, with secondary hardening occurring in the final stage of the thermal compression test. In the thermal compression test, when the Mn content increases from 2% to 4%, the dynamic recrystallization behavior of 17Cr duplex stainless steel first intensifies and then slows down in the temperature range of 900-1100 ℃. However, at deformation temperatures of 1200 ℃ and higher, the effect of Mn content on the dynamic recrystallization behavior is relatively small. At the temperature range of 1150-1200 ℃, the thermal tensile deformation resistance decreases initially and then increases with the rise of manganese content, with 3% manganese providing the best hot working performance.

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Microstructure and hot deformation behavior of U-50%Zr alloy for nuclear fuel

Li Yanfeng, Guo Hong, Li Mingyang, Hu Bingkun, Liu Jiancheng

Heat Treatment of Metals 2025, 50 (2): 29-35. doi:10.13251/j.issn.0254-6051.2025.02.005

Abstract （25）

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U-50%Zr alloy for nuclear fuel was prepared by high-frequency induction melting, and its microstructure and phase constituent were analyzed using metallographic microscope and X-ray diffractometer. The Gleeble-3800 thermal simulation test machine was used to conduct hot compression tests at different deformation temperatures (500, 550 and 600 ℃) and strain rates (0.01 s, 0.1 and 1 s^-1) to investigate its hot deformation behavior. The results show that, except for a deformation temperature of 600 ℃ and a strain rate of 0.01 s^-1, the U-50%Zr alloy exhibits significant work hardening characteristics under other deformation conditions. At the same strain rate, the peak stress decreases with the increase of deformation temperature, especially at 600 ℃, where the peak stress decreases significantly compared to 500 ℃ and 550 ℃. At the same deformation temperature, the peak stress increases with the increase of strain rate. After hot deformation, no phase transformation occurs in the microstructure, all of which are δ-UZr₂ phase. As the hot deformation temperature increases, the grain size of the alloy initially remains basically unchanged and then increases. At 500 ℃ and 550 ℃, the grain size is around 250 μm, and at 600 ℃, the grain size significantly increases to 493 μm. Based on peak stress, the constitutive equation of U-50%Zr alloy is established using the hyperbolic sine function Arrhenius model and temperature compensation factor Z parameter, with a hot deformation activation energy Q=694.9 kJ/mol. A hot working map is constructed using DMM dynamic material model, and the appropriate processing range is determined as follows: deformation temperature above 540 ℃, strain rate between 0.01 s^-1 and 1 s^-1.

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High-temperature fatigue property of 30Cr1Mo1V rotor steel at 540 ℃

Na Risu, Zhang Yuetao, Zhang Shuo, Meng Weiran, Wang Xiao

Heat Treatment of Metals 2025, 50 (2): 36-42. doi:10.13251/j.issn.0254-6051.2025.02.006

Abstract （22）

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High-temperature fatigue tests were conducted on 30Cr1Mo1V steel under different stresses (450$\rightleftharpoons$0, 400$\rightleftharpoons$0, 350$\rightleftharpoons$0, 350$\rightleftharpoons$233, 350$\rightleftharpoons$175 MPa) at 540 ℃. The results indicate that the fatigue life of the 30Cr1Mo1V steel decreases rapidly as the peak stress exceeds 350 MPa. Under a stress of 350 MPa, the ferrite does not change too much, which maintains the initial polygon-like shape, and the ultimate failure feature of the steel is fatigue cracking. Nevertheless, when the stress rises to 400 MPa, the ferrite deforms remarkably and shows a wrinkled river-like shape after fatigue failure, meanwhile, the pores also appears at the intersection location of the ferrite bands. In addition, subjecting the long-term high stress level like at 350$\rightleftharpoons$233 MPa and 350$\rightleftharpoons$175 MPa are harmful to the fatigue life of the 30Cr1Mo1V steel, as continuous stress can significantly increase the number of fatigue pores inside the material. In summary, the 30Cr1Mo1V steel is relatively safe under service conditions of 540 ℃/350 MPa, with a fatigue life of 12 887 cycles, and a one-time attrition rate of approximately 0.007 76% which is less than the requirement of 0.01%.

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Influence of martensitic multi-level structure and TiN inclusions on fracture toughness of 20CrMnTi steel

Long Shaolei, Zhu Dandan, Luo Xianglan, Yi Yanliang, Yang Ming, Lu Yemao, Liang Yilong

Heat Treatment of Metals 2025, 50 (2): 43-51. doi:10.13251/j.issn.0254-6051.2025.02.007

Abstract （17）

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Fracture toughness (K_IC) of 20CrMnTi steel was tested by using three-point bending specimens, and it was found that the value of K_IC decreases with grain coarsening, which contradicts the relationship between the K_IC and the grain size of other low carbon lath martensitic steels. Based on this, the fracture behavior of the 20CrMnTi steel was analyzed by means of OM, EBSD, SEM and thermodynamic model, fracture mechanics model. The results show that the relationship between the multi-level microstructure of the 20CrMnTi steel and K_IC established based on the Hall-Petch equation reveals that the martensitic lath block is the effective grain for fracture toughness. Still, the fracture morphology analysis reveals that the effective grains in the coarse or fine grain states of the 20CrMnTi steel are different. Further analysis reveals the presence of a certain number of irregular TiN inclusions on the fracture, and thermodynamic calculations show that the TiN inclusions are formed in the liquid-phase region with coarse size, which can easily lead to crack initiation. The crack propagation is analyzed with the help of SEM and fracture mechanics calculations, revealing that the crack propagation in the fine grain state of the 20CrMnTi steel is zigzagging and cross-cutting the lath martensite packet, which leads to a better fracture toughness, whereas the crack path in the coarse grain state is straight and shows a poor toughness. Therefore, the formation of TiN inclusions is the key factor leading to the abrupt change in fracture toughness of the 20CrMnTi steel, and the results of this study are of great significance in guiding the preparation of the high-performance 20CrMnTi steel.

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Microstructure refinement and properties of as-cast Ti-6Al-4V titanium alloy

Sun Hao, Meng Xun

Heat Treatment of Metals 2025, 50 (2): 52-60. doi:10.13251/j.issn.0254-6051.2025.02.008

Abstract （22）

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Microstructure of the as-cast Ti-6Al-4V titanium alloy was refined by deformation combined with heat treatment. Firstly, the as-cast Ti-6Al-4V titanium alloy was subjected to solution treatment at 1100 ℃ and water cooling to transform the microstructure into martensite. Then, the microstructure was refined by repeated rolling at 750 ℃. The results show that the principles of microstructure refinement are the deformation twinning, dislocation rearrangement, crystal rotation and discontinuous dynamic recrystallization.Through tensile test, it is found that compared to the as-cast Ti-6Al-4V titanium alloy, the alloy treated with microstructure refinement has a better strength-plasticity matching relationship. The improvement of alloy strength is the result of the combined effect of fine grain strengthening and dislocation strengthening. The improvement of plasticity is due to the formation of a plastic zone at the crack tip during crack initiation and propagation, which hinders crack propagation and enhances the plasticity of the material. In addition, dislocations can complete slip transmission between the α and β phases, which can better coordinate the deformation between the α and β phases and further enhance the plasticity of the material.

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Effect of Zr element on microstructure and properties of Cu-6Ni-3Ti alloy

Jing Qingxiu, Yang Xueqing, Wei Dandan, Wei Miao, Huang Xiaodong

Heat Treatment of Metals 2025, 50 (2): 61-68. doi:10.13251/j.issn.0254-6051.2025.02.009

Abstract （20）

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Effect of Zr content on the microstructure, mechanical properties and electrical conductivity of Cu-6Ni-3Ti-xZr alloy was investigated. The results show that zirconium elements mainly segregate at grain boundaries during solidification, forming zirconium-rich atomic clusters, slowing down the grain growth and inhibiting dendritic segregation. During the aging process, zirconium accumulates around the precipitated phases of the alloy, inhibiting the growth of the precipitated phases, and improving the hardness and its resistance to softening. When the content of zirconium elements is increased, Cu₄Zr is formed in the Cu-6Ni-3Ti-0.5Zr alloy. After aging at 450 ℃ for 2 h, the Cu-6Ni-3Ti-0.3Zr alloy has best mechanical and electrical properties, with tensile strength of 807 MPa, hardness of 230 HV2, electrical conductivity of 50%IACS.

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Analysis of continuous cooling transformation of 4Cr13 steel for plastic mold

Zhao Zhengrong, Zhang Yunfei, Zhao Yingli, Fan Mingqiang, Bai Lijuan, Liu Lijun

Heat Treatment of Metals 2025, 50 (2): 69-73. doi:10.13251/j.issn.0254-6051.2025.02.010

Abstract （21）

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4Cr13 steel for plastic mold was prepared by vacuum induction furnace, its phase transition points and continuous cooling transformation (CCT) curve were measured by using a DIL805A quenching thermal dilatometer, and its continuous transformation process was analyzed by combining microstructure observation and hardness test. The results show that the phase transition points of the tested 4Cr13 steel are Ac₁=857 ℃, Ac₃=937 ℃, and there are only pearlite transformation zone and martensite transformation zone on the CCT curve. The 4Cr13 steel has a good hardenability, and the martensitic transformation temperature range is 67-397 ℃.When the cooling rate is 0.01-0.05 ℃/s, the main product of phase transformation of the 4Cr13 steel is pearlite, and the hardness is low, which is 172-193 HV. When the cooling rate is 0.1-1 ℃/s, the microstructure is martensite and retained austenite, and the hardness is increased to 528-688 HV. When the cooling rate is ≥1 ℃/s, the martensitic transformation is completely finished.

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CCT and TTT curves of a carbide-free bainitic steel

Yang Siyuan, Li Aiguo, Luo Ping, Zhang Wenliang, Li Xianjun, Zhang Minghao, An Weicheng, Wang Kaize

Heat Treatment of Metals 2025, 50 (2): 74-80. doi:10.13251/j.issn.0254-6051.2025.02.011

Abstract （15）

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Continuous cooling transformation and isothermal transformation tests of undercooled austenite were carried out on a carbide-free bainitic tested steel by using the DIL-805A phase transformation tester. The CCT and TTT curves of the tested steel were obtained. The results show that the starting and ending transformation temperatures of austenite of the tested steel are 760 ℃ and 860 ℃, respectively, and the starting transformation temperature of martensite is approximately 300 ℃. When the cooling rate is between 0.1-15 ℃/s, the undercooled austenite mainly transforms to bainite+martensite complex structure. When the cooling rate is between 15-50 ℃/s, the undercooled austenite transforms into martensite. The microhardness of the tested steel increases with the increase of cooling rate, up to 742 HV0.5. When the tested steel undergoes isothermal transformation, the bainite transformation temperature range is 300-400 ℃, and the "nose tip" temperature is about 360 ℃.

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Differences and optimization practice of banded structure in gear steels

Zhang Zhixing, Guo Ziqiang, Wang Hailong, Qin Xuan

Heat Treatment of Metals 2025, 50 (2): 81-85. doi:10.13251/j.issn.0254-6051.2025.02.012

Abstract （17）

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A comprehensive study on the formation mechanisms and differences in the banded structure of commonly used automotive gear steels, namely SCr420H, 16MnCr5, 20CrMnTiH, SAE8620H and 22CrMoH, which possessed varying alloy element compositions but underwent similar smelting and rolling processes, was conducted. In addition, the process was optimized. The findings reveal that SCr420H, 16MnCr5 and 20CrMnTiH steels exhibit relatively less pronounced banded segregation, whereas 22CrMoH and SAE8620H steels display distinct banded structures. The alloying elements Mo and Ni in 22CrMoH and SAE8620H steels affect the microstructure transformation during rolling and cooling, resulting in the development of an abnormal microstructure, bainite, in the final rolled steel. By adjusting the continuous casting superheat to a range of 15-30 ℃, maintaining a consistent pulling speed, intensifying the crystallizer and secondary cooling, achieving a homogeneous rolling heating temperature of 1200-1240 ℃, ensuring a diffusion duration of at least 90 min, and maintaining a final rolling temperature of 910-960 ℃, the banded structure in SAE8620H round steel is effectively improved to a grade of ≤2.0.

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Effect of segregation on surface microstructure and hardness of induction hardened inner gear ring

Chen Yongxiang, Lü Hesheng, Yang Gang, Li Yong, Wang Chunli, Zhong Boying

Heat Treatment of Metals 2025, 50 (2): 86-89. doi:10.13251/j.issn.0254-6051.2025.02.013

Abstract （14）

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Segregation phenomenon of 42CrMoA steel inner gear ring was characterized, and the microstructure and hardness gradient of the induction hardened layer of the inner gear ring were analyzed. The results show that the microstructure of the 42CrMo steel inner gear ring forging is uneven, with severe banded structure and compositional segregation, which leads to uneven induction hardened layer structure, and the distribution of banded structure still exists. The banded structure and compositional segregation of inner gear ring forgings have a significant impact on the hardness gradient of the induction hardened subsurface and transition zone. The hardness gradient of the induction hardened subsurface and transition zone of the inner gear ring fluctuates greatly and exhibits a steep rise and fall phenomenon.

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Effect of partial austenite reverse transformation process on microstructure and mechanical properties of Fe-8Mn-0.2C-3Al medium Mn steel

Liu Mingzhu, Ding Hua, Zou Yuming

Heat Treatment of Metals 2025, 50 (2): 90-95. doi:10.13251/j.issn.0254-6051.2025.02.014

Abstract （16）

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Microstructure and mechanical properties of Fe-8Mn-0.2C-3Al medium Mn steel treated by partial austenite reverse transformation (PART) process with two-step annealing were studied by means of field emission scanning electron microscope, X-ray diffractometer and universal testing machine. The results show that for the tested steel treated by PART process with the increase of first step annealing temperature, the austenite content gradually decreases from 49.7% to 21.6%, and the microstructure is lath-like, the yield strength gradually increases, while the tensile strength and elongation gradually decrease. After treated at 755 ℃ for 15 min and 620 ℃ for 30 min, the tested steel obtains optimal mechanical properties, with the tensile strength of 1087 MPa, elongation of 43.4%, and the product of strength and elongation of 47.2 GPa·%, which is attributed to that the moderate austenite stability can fully utilize the TRIP effect during the tensile process and improve the comprehensive mechanical properties of the steel.

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Inheritance and refinement of coarse grains in hot forging rings of a Cr-Ni-Mo-Ti maraging stainless steel

Ding Yali, Gao Qi, Wang Ao

Heat Treatment of Metals 2025, 50 (1): 169-172. doi:10.13251/j.issn.0254-6051.2025.01.026

Abstract （27）

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In order to solve the problem of coarse grains remained in hot forging rings of a Cr-Ni-Mo-Ti maraging stainless steel, the temperature range of inheriting of forged coarse grains was investigated, and the recrystallization temperature for grain size refinement was determined. Moreover, the effect of repeated solution treatment on the grain size refinement and mechanical properties was determined. The results show that the forged coarse grains are inherited in a wide temperature range above Af of the steel. After solution treatment near 950 ℃, the austenite grains with completely closed grain boundaries and obvious refinement are formed, indicating that the recrystallization temperature T_re is approximately equal to 950 ℃, moreover, further increasing the solution temperature leads to grain growth. Repeated heating and cooling slightly below 950 ℃ lead to large grain boundary migration and forming the fine grain with open boundries. The repeated heating and cooling at 950 ℃ can still fine the grains, while the effect is weakened with the further increase of temprature, untill the effect vanish at 1050 ℃. The grain refinement by repeated heating and cooling at 875 ℃ significantly improves the impact properties, but the tensile strength and yield strength decrease, showing that the strength decreases with the grain refinement. The strength and toughness when heating and cooling once at 925, 975 ℃ (near T_re) are close to those of repeated heating and cooling for 3 cycles at 875 ℃, while the grains are slightly refined with the increase of repeated heating and cooling times, and there is no significant change in mechanical properties.

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Isothermal quenching phase transformation characteristics and mechanical properties of 300M steel

Zhao Xiaoyu, Zhang Zheng, Yang Huijun, Shi Xiaohui, Zhang Min, Qiao Junwei

Heat Treatment of Metals 2025, 50 (1): 173-179. doi:10.13251/j.issn.0254-6051.2025.01.027

Abstract （31）

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Effect of isothermal quenching process at 240-320 ℃ near Ms on the phase transformation characteristics and mechanical properties of 300M steel was studied by means of thermal dilatometer, color metallography, scanning electron microscopy and transmission electron microscopy. The results show that the isothermal quenching below Ms result in athermal martensite produces first and the incubation period of bainite transformation shortened of the 300M steel. Compared with the isothermal quenching process at 320 ℃ above Ms, the isothermal quenching process at 260 ℃ below Ms makes the retained austenite transform from blocky M/A islands to film between bainite laths, the average width of bainite lath decreases, the tensile strength increases from 1550 MPa to 1930 MPa and the total elongation increases from 7.7% to 9.2%, showing that the strength and plasticity increase simultaneously.

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Effect of tempering on microstructure and properties of air cooled bainite steel containing rare earth for oil casing

Han Qiang, Gao Zhimin, Jia Xin, Li Tao, Sun Zhaoqi, Dai Congwei

Heat Treatment of Metals 2025, 50 (1): 180-186. doi:10.13251/j.issn.0254-6051.2025.01.028

Abstract （29）

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Effects of addition of rare earth Ce and tempering process on microstructure, precipitation behavior of the second phase and hardness of air-cooled bainite steel for oil casing were compared and studied. The phase transition points of the rare earth and non-rare earth bainite steels were determined by thermal dilatometer, the microstructure and the second phase after tempering were observed and analyzed by using metallographic microscope and TEM, the precipitation behavior of the second phase was simulated by thermodynamic calculation software, and the hardness of the tested steels was tested by using Rockwell hardness tester. The results show that after tempering at 200, 450 and 650 ℃, respectively, for different time, the microstructure of the non-rare earth bainite steel is tempered martensite, tempered troostite and tempered sorbite, respectively, however, the microstructure of the rare earth bainite steel is tempered martensite, tempered troostite and tempered troostite+tempered sorbite, respectively. When tempered at 650 ℃ for 60 min, the second phases in the bainite steels are (Fe, Cr)₃C and Cr₂₃C₆ carbides. With the increase of tempering temperature and time, the hardness of the rare earth and non-rare earth bainite steel decreases overall, while the secondary hardening phenomenon is found when tempered at 450 ℃ for 120 min, and the hardness of the rare earth bainite steel is higher than that of the non-rare earth bainite steel. In conclusion, the addition of rare earth Ce can increase the temperature of the phase transition point, delay the pearlite transformation and improve the hardness of bainite steel.

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Microstructure and mechanical properties of 9Cr heat-resistant steel strengthened by TiC nanoparticles

Zhu Xiaolong, Wang Zhenghui, Wang Wenyan, Xie Jingpei, Diao Xiaogang, Zhang Feiyang

Heat Treatment of Metals 2025, 50 (1): 187-194. doi:10.13251/j.issn.0254-6051.2025.01.029

Abstract （34）

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In order to improve the high temperature strength of heat-resistant steel for steam turbine, 9Cr heat-resistant steels with different contents of TiC nanoparticles (mass fraction of 0%, 0.01%, 0.05% and 0.1%) were prepared. The microstructure of the tested steel with 0.01%TiC nanoparticles was analyzed by using high temperature laser scanning confocal microscopy (LSCM), optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the mechanical properties were studied by tensile and impact tests. The results show that the addition of TiC nanoparticles can significantly enhance the high-temperature strength at 600 ℃ of the 9Cr heat-resistant steel, and when the addition amount is 0.01%, the maximum tensile strength is 356 MPa, and the strain at fracture is 38%. The martensite structure is obtained for the tested steel with 0.01% TiC nanoparticles under air-cooling conditions, the second-phase carbide category is M₆C, and the distribution of TiC nanoparticles in the grains plays a role in dispersive strengthening. The tested steel with an addition of 0.01%TiC nanoparticles has excellent comprehensive mechanical properties after annealing at 1040 ℃+normalizing at 990 ℃+tempering at 740 ℃, with the hardness, tensile strength, yield strength, elongation and reduction of area of 177 HBS, 734 MPa, 538 MPa, 15.45% and 31.15%, respectively, and a large number of dimples are observed in the tensile fracture, indicating ductile fracture. The impact absorbed energy at room temperature is 116 J, and the ductile-brittle transition temperature FATT₅₀ is -15 ℃.

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Effect of nitrogen on microstructure and mechanical properties of cold-rolled 022Cr18Ni8N metastable austenitic stainless steel

Liu Jinrun, Lang Yuping, Chen Haitao, Feng Hanqiu, Gao Zhijun, Zhang Zhengfu

Heat Treatment of Metals 2025, 50 (1): 195-200. doi:10.13251/j.issn.0254-6051.2025.01.030

Abstract （22）

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Microstructure chang of metastable austenitic stainless steel 022Cr18Ni8N with varying nitrogen contents (0%, 0.02%, 0.05% and 0.14%) before and after cold rolling was investigated, and the effect of nitrogen on the microstructure and properties was characterized by using OM, EBSD, XRD, and room temperature tensile tests.The results indicate that nitrogen can improve the stability of austenite of the tested steel and reduce the formation of deformation induced martensite during cold deformation. The strain-induced martensite nucleates at the intersections of deformation bands, within solitary deformation bands, and at the intersections of deformation bands and grain boundaries. An appropriate nitrogen content endows metastable austenitic stainless steel with enhanced comprehensive mechanical properties through the regulation of two strengthening mechanisms, namely phase transformation strengthening and solution strengthening.

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Effect of grain size on 650 ℃ tensile properties of GH4720Li nickel-based alloy

Guo Jing, Wen Xiaocan, Wang Qiang, He Xin, Lü Shaomin, Xie Xingfei, Qu Jinglong, Du Jinhui

Heat Treatment of Metals 2025, 50 (1): 201-205. doi:10.13251/j.issn.0254-6051.2025.01.031

Abstract （34）

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GH4720Li alloy bars were solid solution treated at different temperatures of 1100, 1090, 1080, and 1070 ℃ to obtain four different microstructure with average grain sizes of 24, 18, 15 and 3-5 μm, respectively. The effect of grain size on the high-temperature tensile properties of the alloy at 650 ℃ was investigated. The results show that with the reduction of grain size, the 650 ℃ strength of the GH4720Li alloy increases, accompanied by a decrease in plasticity. When the grain size is further reduced to 3-5 μm, the plasticity begins to increase, and at this time, the alloy exhibits the optimal comprehensive tensile properties at 650 ℃. This is attributed to the pinning effect of primary γ′ strengthening relative to grain boundary, resulting in a fine grain structure that not only provides a large number of grain boundaries and improves strength, but also disperses plastic deformation, reduces stress concentration, and improves plasticity. Simultaneously, an appropriate amount of secondary and tertiary γ′ strengthen phase can further ensure the strength of the alloy.

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