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Cause analysis on crack of bolt thread tooth

Fan Kailun, Song Wenjun, Liu Wencheng, Pei Lieyong, Dai Aili, Zhang Renyin, Liu Hongyan, Li Aiping, Zhang Yongxiang

Heat Treatment of Metals 2024, 49 (10): 306-310. doi:10.13251/j.issn.0254-6051.2024.10.050

Abstract （111）

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

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Fatigue fracture failure analysis and heat treatment process improvement of high carbon steel spring component

Wang Yameng, Xu Zhiwen, Gao Jinfeng, Xu Jinling

Heat Treatment of Metals 2024, 49 (6): 295-300. doi:10.13251/j.issn.0254-6051.2024.06.046

Abstract （57）

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High carbon steel spring component of wireless headphone charger cracked in the fatigue test, for which the fracture mode and cause were determined through observation of the fracture and microstructure, hardness test, and combining with the heat treatment process and fatigue test conditions. The results show that the fracture mode is low stress fatigue fracture, the cause of which is that a lot of undissolved carbides exist in the microstructure after austempering and tempering, leading to stress concentration in the circular arc transition zone of the elastic arm of the spring component in the durability test, then leading to crack initiation, propagation, and finally resulting in fracture. By improving heat treatment process according to the actual production conditions, the optimum heat treatment process is found as: quenching at 830 ℃ for 20 min with carbon potential 0.75%+salt bath austempering at 305 ℃ for 35 min+tempering at 305 ℃ for 90 min. After heat treatment with the improved process, the Vickers hardness of the high carbon steel spring component is 584 HV0.2, and the durability test life is over 90 000 times.

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Fracture failure analysis of hydraulic equipment bolts

Song Quan, Cao Huihui, Zhang Tianqi, Dan Ting, Ma Fang, Zhang Ying

Heat Treatment of Metals 2024, 49 (6): 301-304. doi:10.13251/j.issn.0254-6051.2024.06.047

Abstract （57）

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Three hydraulic equipment bolts fractured during hydraulic tests. The fracture mode and causes of the bolts were analyzed by SEM fracture morphology observation, chemical composition detection, metallographic observation and hardness test. The results show that the bolts are made of different materials. The material of two of the bolts is 30CrMnSiNi2A steel, which do not meet the design material requirements and do not match with the heat treatment, thus the bolts exhibit tempering brittleness. The material of another bolt is 30CrMnSiA steel, the microhardness and microstructure are abnormal after heat treatment, so the bolt cannot withstand the assessment pressure and fractures during the hydraulic test process.

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Failure analysis of 20CrMnTi steel rotary drilling rig square head

Zhang Junhua, Zhou Dapeng, Yu Yonghao, Kang Xueqin

Heat Treatment of Metals 2024, 49 (8): 301-304. doi:10.13251/j.issn.0254-6051.2024.08.050

Abstract （52）

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20CrMnTi steel rotary drilling rig square head was fractured after use for 30 days. Failure analysis was conducted through fracture morphology observation, chemical composition analysis, mechanical properties and microstructure analysis. The results show that both the design criterion and the heat treatment process are not so rational, which leads to decreased strength and hardness of the square head. Overload fracture is easy to occur at the stress concentration area of the square head under the action of torsional stress during the using. When the P element content exceeds the standard requirement, the material has a tendency towards cold brittleness, showing brittle fracture. The quality inspection of raw materials should be strengthened to meet the standard requirements. Quenching and low-temperature tempering should be adopted to ensure that the strength and hardness of the square head material meet the technical requirements.

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Analysis on surface pitting defects of hot-rolled galvanized steel sheet

Li Shiming, Zhang Jie, Liu Xin, Zhang Ziyue, Chen Binkai, Zhang Qifu

Heat Treatment of Metals 2024, 49 (10): 278-283. doi:10.13251/j.issn.0254-6051.2024.10.045

Abstract （52）

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

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Fracture failure analysis of 60Si2MnA steel circlip

Chen Hehe, Liu Chunjiang, Jiang Tao, Guo Chen

Heat Treatment of Metals 2024, 49 (11): 302-307. doi:10.13251/j.issn.0254-6051.2024.11.047

Abstract （49）

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

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Cause analysis of steel foot cracks

Bi Lian, Liu Dongjie, Xue Feng

Heat Treatment of Metals 2024, 49 (6): 305-307. doi:10.13251/j.issn.0254-6051.2024.06.048

Abstract （49）

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Cracks of steel foot were analyzed by means of metallographic microscope, Brinell hardness tester and inductively coupled plasma emission spectrometer. The results show that the steel foot has fewer non-metallic inclusions, the metallographic structure is quenched and tempered sorbite, which is consistent with the heat treatment process, and the chemical composition meets the requirements of GB/T 3077—2015.The fracture morphology is fine porcelain like, without any fatigue cracking characteristics. No decarburization, oxidation or folding morphology is observed on both sides of the entire crack, indicating a typical quenching crack morphology. The reason for cracking is that the large and small workpieces are treated in the same furnace, causing the cooling rate of small workpieces to be too fast and the stress to be too high, leading to crack.

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Failure analysis and improvement of hydrogen removal process for 17-4PH steel screw

Guo Junlei, Han Yunzhao, Han Wenkai, Li Guosheng

Heat Treatment of Metals 2024, 49 (5): 278-281. doi:10.13251/j.issn.0254-6051.2024.05.047

Abstract （49）

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In response to the fracture of 17-4PH stainless steel screws during using, macroscopic and microscopic microstructure observation, energy spectrum composition analysis and hydrogen content comparison analysis were conducted on the 17-4PH steel fractured screws. Based on the testing results of the fracture hardness, strength, microstructure and other aspects of the fractured screw, it is analyzed and determined that the main cause of screw failure is hydrogen induced fracture. The hydrogen removal process is optimized, and applied to production with which good results have been obtained.

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Design of special-shaped inductor during induction hardening process

Zhou Shengli, Shen Hui, Lei Delang

Heat Treatment of Metals 2024, 49 (6): 308-313. doi:10.13251/j.issn.0254-6051.2024.06.049

Abstract （48）

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In order to meet the heat treatment technical requirements of some parts with complex geometric shapes and high technical requirements, such as universal joint transmission shaft R, King Pin parts and transmission shaft (stub shaft) ears, etc, high-properties digital induction heating power supply and microcomputer programming automatic control medium frequency special-shaped inductor were used for induction hardening. For different parts, special-shaped inductor are different from circular inductor in terms of structural design, which is simple structure, and low production cost. The quenched parts can meet the technical requirements of the effective hardened layer depth, and also have a small range of deviations in microstructure and mechanical properties, stable quality. By using the special-shaped inductor during induction hardening, the problem of R hardened layer and coarse grain of thin-wall of the transmission shaft fork is solved, greatly improving the service life of the parts.

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Detection and identification of network carbide defects in steel based on improved CAM-YOLOx-DeepSORT algorithm

Huo Jinliang, Cai Ying, Tong Haisheng

Heat Treatment of Metals 2024, 49 (6): 282-286. doi:10.13251/j.issn.0254-6051.2024.06.044

Abstract （45）

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In the detection and identification of network carbides defects in mold steels, algorithms based on image processing and machine learning are commonly used, which can achieve automation to a certain extent, while the identification accuracy still has limitations in cases where the number of network carbides is large and the morphology is complex. To address the limitations of low recognition rate mentioned above, an improved CAM-YOLOx-DeepSORT algorithm is proposed, which utilizes the CAM attention mechanism and combines with the YOLOx object detection algorithm and DeepSORT object tracking algorithm to realize automatic detection and identification of the network carbides in mold steels. The results show that the algorithm can efficiently and accurately detect network carbides,the detection accuracy reaches 99.1%, thus providing guidance for quality control of mold steels.

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Comprehensive evaluation of strength and toughness of hot stamping steel

Wang Qiuyu, Xu Kuan, Li Liming, Liu Shuying, Zhang Saijuan

Heat Treatment of Metals 2024, 49 (10): 284-289. doi:10.13251/j.issn.0254-6051.2024.10.046

Abstract （45）

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

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Failure analysis for grinding delayed cracking of 300M steel components

Yue Haiyan, Jia Min, Zeng Zhiqiang, Sun Xinming, Cheng Sheng

Heat Treatment of Metals 2024, 49 (11): 315-319. doi:10.13251/j.issn.0254-6051.2024.11.049

Abstract （44）

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

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Fracture failure analysis of 20Cr1Mo1VTiB steel bolt

Liu Fei, Zhang Xuechao, Sun Changming, Dong Yunhe, Liu Yang, Zheng Jianjun

Heat Treatment of Metals 2024, 49 (12): 306-311. doi:10.13251/j.issn.0254-6051.2024.12.049

Abstract （44）

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A high-temperature fastening bolt (20Cr1Mo1VTiB steel) for the high-stress main steam valve of a steam turbine unit cracked after serving for 1.29×10⁵ h. The cracking failure cause of the high-temperature fastening bolt was analyzed by means of chemical composition examination, SEM and TEM microstructure observation, XRD detection, and mechanical property testing. The results show that the fracture of the bolt is primarily due to creep damage from long-term high-temperature and high-stress conditions of serving. Additionally, the accumulation of creep damage is mainly due to two types of creep voids forming in microcrack zones near coarse inclusions sharp corners and along the interface of Laves phase. With the increase of running time, the number of creep voids in the 20Cr1Mo1VTiB steel bolt matrix continuously rises and aggregates, eventually forming crack sources that lead to fatigue rupture.

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Failure behavior of a low carbon alloy carburized steel roller

Lü Ben, Wan Xunzhi, Liu Cheng

Heat Treatment of Metals 2024, 49 (5): 272-277. doi:10.13251/j.issn.0254-6051.2024.05.046

Abstract （43）

PDF （4219KB）（26）

Fracture surface observation, microstructure observation, hardness distribution test and the basic stress analysis of a low carbon alloy carburized steel roller failed in roll forming process for manufacturing automobile flywheel, were investigated by means of optical microscope, scanning electron microscope, microhardness tester and finite element analysis. The results show that the carburized layer of the failed roller is mainly composed of acicular tempered martensite, small carbides and a few retained austenite. The microhardness decreases from the roller notch surface to the core. The distribution of microhardness near the failed roll notch is not uniform, the case depth of carburizing layer is lower than the design specification and low hardness microzones are formed in the carburized layer. Microcracks initiate from the roller notch bottom, then joint into a main crack and propagate from the notch bottom surface to the inner. The propagation direction of the main crack basically agrees with the result of finite element analysis. Both the heat treatment process and the complex loads in the service environment are attributed to the early failure of the roller.

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Plate shape prediction method in two stage based on CNN-LSTM for quenching process with roller-hearth quenching machine

Liu Ai, Zhang Tinghu, Wang Zhongliang

Heat Treatment of Metals 2024, 49 (10): 295-300. doi:10.13251/j.issn.0254-6051.2024.10.048

Abstract （43）

PDF （1776KB）（12）

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

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Formation reason analysis of spot segregation in 42CrMo steel forgings

Xu Quan, Chen Mintao, Dang Shue, Zhang Yingfan, Jiao Yongxing, Bai Hongliang

Heat Treatment of Metals 2024, 49 (11): 308-314. doi:10.13251/j.issn.0254-6051.2024.11.048

Abstract （42）

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

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Cause analysis and solution of crack of hydraulic support column cylinder

Cui Leilei, Ma Ke, Lan Zhiyu, Wang Piaoyang, Deng Xiangtao

Heat Treatment of Metals 2024, 49 (8): 289-294. doi:10.13251/j.issn.0254-6051.2024.08.048

Abstract （42）

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Cracks in cylinder of a large hydraulic support column during service were analyzed by means of mechanical property detection, microstructure analysis and crack morphology characterization. The results show that the cracking of the cylinder of hydraulic support column is not directly related to the microstructure and properties of the matrix material, but is caused by the cladding defects. The cracks occur at the interface of the stainless steel clad layer and the matrix, and then expand to the matrix side and the clad layer side under the action of long-term load, finally, resulting in cracking failure. By improving laser cladding parameters, laser cladding defects can be effectively avoided.

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Fracture behavior of 06Cr25Ni20 austenitic heat-resistant steel at high tensile temperature

Wang Chu, Ai Fangfang, Pan Kaihua, Yan Bingyu, Hu Haiyang, Gao Tianyu, Wang Yong

Heat Treatment of Metals 2024, 49 (12): 289-294. doi:10.13251/j.issn.0254-6051.2024.12.046

Abstract （40）

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Static and dynamic microstructure evolution and fracture behavior of the 06Cr25Ni20 austenitic heat-resistant steel at different high temperature tensile temperatures were analyzed by means of optical microscope, scanning electron microscope, transmission electron microscope and high temperature laser confocal microscope. The results show that the yield strength of the tested steel decreases from 180 MPa to 40 MPa, the tensile strength decreases from 450 MPa to 50 MPa, and the elongation after fracture increases from 44% to 56% with the increase of tensile test temperature in the rang of 600-1000 ℃.The fracture is ductile. The number of dimples gradually decreases with the increase of temperature, and the size and depth gradually increase. The recrystallization phenomenon occurs in the microstructure, and the secondary phase black carbide and intragranular substructure decrease or even disappear with the temperature increasing. The formation of cracks and fracture are mainly generated at the interface between austenite and ferrite and high energy grain boundaries when the tested steel is stretched at high temperature.

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Failure analysis of gear cracking of containment isolation valve for a nuclear power plant

Tang Heng, Li Guoqiang, Xu Yangyang, Deng Jiangyong

Heat Treatment of Metals 2024, 49 (11): 320-324. doi:10.13251/j.issn.0254-6051.2024.11.050

Abstract （37）

PDF （3263KB）（13）

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

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Brittle fracture characteristics and fracture behavior of hot rolled 60Si2Mn strip steel

Tian Yaqiang, Yao Zhiqiang, Nian Baoguo, Zhang Xiaolei, Xue Qihe, Song Jinying, Zhang Mingshan, Chen Liansheng

Heat Treatment of Metals 2024, 49 (8): 281-288. doi:10.13251/j.issn.0254-6051.2024.08.047

Abstract （37）

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Decarburization, inclusions, segregation and brittle fracture of hot rolled 60Si2Mn steel strip were analyzed by means of optical microscope, scanning electron microscope and electronic tensile testing machine. Microstructure and mechanical properties of the specimens near the brittle fracture and away from the fracture were characterized, and the brittle fracture characteristics and fracture behavior of hot rolled 60Si2Mn steel strip were investigated. The results show that there is no obvious decarburization, inclusion and segregation at the brittle fracture, and the fracture presents brittle fracture. The content of proeutectoid ferrite in the specimens near the brittle fracture and away from the fracture is 12.45% and 0.66%, respectively, and the pearlite lamellar spacing is 578 nm and 236 nm, respectively. The main cause of brittle fracture of hot rolled 60Si2Mn steel strip is that the content of proeutectoid ferrite in the brittle fracture is higher than that in the position away from the fracture, and the pearlite lamellar spacing is larger, which leads to poor resistance to deformation and is prone to uneven deformation during the rolling process. Additionally, the stress difference between the coarse ferrite and cementite layers can easily lead to brittle fracture, and the cracks propagate along the two-phase interface. However, the plastic deformation of fine pearlite is relatively uniform, and cracks propagate through the layers of pearlite.

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