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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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Failure analysis on quenching and tempering cracking of 42CrMo steel low-speed shaft thrust ring

Li Ziyan, Xu Hongxiang, Chen Yesheng, Liu Zhiqiang, Guo Jingqiang, Chen Yan, Zhao Shaofu

Heat Treatment of Metals 2023, 48 (9): 283-287. doi:10.13251/j.issn.0254-6051.2023.09.048

Abstract （96）

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42CrMo steel low-speed shaft thrust ring failed due to end face cracks after quenching and tempering treatment. The chemical composition, hardness gradient and microstructure of the fracture position were observed and detected, and the inclusions in the fracture were observed and analyzed by means of SEM and EDS. The results show that the cracks are caused by many factors, such as the inclusion in raw materials, the existence of rough tool marks on the workpiece surface, and the unreasonable structural design of the low-speed shaft part. The risk of quenching cracking at the thrust ring can be reduced by optimizing the rough turning process of machining, quenching auxiliary tooling and controlling the microstructure of raw materials.

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Cause analysis of carburizing quenching cracks of sun gear

Zhang Lei, Mi Pei, Ma Chunliang, Sun Yongpeng

Heat Treatment of Metals 2023, 48 (9): 293-296. doi:10.13251/j.issn.0254-6051.2023.09.050

Abstract （73）

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After carburizing and quenching, tooth cracks occurred in the sun gears of mining reducer, causing the products to be scrapped due to the deep cracks. In order to analyze the failure cause of the sun gear, the raw materials, heat treatment process, microstructure, grain size, and macro and micro morphological characteristics of the cracks were analyzed and discussed. The results show that the chemical composition of the material used for the sun gear conforms to the standard of the 18CrNiMo7-6 steel, and the cracks in the sun gear are typical intergranular quenching ones, the main reason of which is related to the coarse grain size. The coarse grains are not caused by the existing late heat treatment, but are mainly related to the improper heating temperature or forging process in the early process stages.

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Cracking failure analysis on high-speed intermediate shaft gear

Chen Shengchao, Chen Yesheng, Rong Zeyu, Wang Hongwei, Xu Hongxiang, Zhao Shaofu, Guo Jingqiang

Heat Treatment of Metals 2023, 48 (7): 287-291. doi:10.13251/j.issn.0254-6051.2023.07.048

Abstract （63）

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A 18CrNiMo7-6 steel high speed intermediate shaft gear of gearbox had abnormal vibration sound during the dynamic balance test after carburizing and quenching. After disassembly, it was found that the intermediate shaft gear had obvious axial crack at the axial diameter. After macro observation and multiple cutting, it was found that the crack source was located at the axle center of the large gear. The crack location of the main shaft was studied by chemical composition analysis and microstructure analysis. The results show that due to the incomplete dehydrogenation treatment of the large billet forgings, the hydrogen atoms gather to form hydrogen molecules over time, which causes the stress concentration inside the workpiece, leading to the delayed hydrogen embrittlement cracking of the gear shaft after carburizing and quenching, and resulting in the failure of the shaft gear.

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Cracking failure analysis of 18CrNiMo7-6 steel gear shaft

Zhang Bing, Mao Yimeng, Jiang Tao, Liu Xin

Heat Treatment of Metals 2024, 49 (2): 297-300. doi:10.13251/j.issn.0254-6051.2024.02.046

Abstract （61）

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A gear shaft cracked during static standing after carburizing, quenching and tempering. In order to find out the failure mechanism of the gear shaft, macro and micro morphologies were observed and local chemical composition of the fracture surface was analyzed. In addition, the microstructure of the shaft near the fracture was examined, and the hardness and hydrogen content were tested. The results show that the failure mode of the gear shaft is delayed brittle cracking induced by hydrogen embrittlement. The crack initiated from the inside of the shaft and there is a large-size nonmetallic inclusion at the source zone, leading to greater stress concentration, which is the main cause for the hydrogen embrittlement cracking. Therefore, the material quality of the gear shafts should be strictly controlled, and nondestructive detection should be carried out to detect the large-size nonmetallic inclusions in the gear shafts so to prevent such failure.

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Causes and improvement methods of induction quenching cracks in wind power inner gear ring

Zhang Haiying, Wen Chao, Bai Ling, Cai Yixin

Heat Treatment of Metals 2023, 48 (9): 288-292. doi:10.13251/j.issn.0254-6051.2023.09.049

Abstract （59）

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Induction hardening cracking of the inner gear ring of wind power was analyzed from the aspects of macro morphology, microstructure, decarburization layer and segregation. The analysis results show that carbon segregation and decarburization of forgings are important reasons for induction hardening cracking of the inner gear ring. When the carbon segregation degree of inner gear ring reaches above 0.04%, the risk of induction hardening cracking increases, and decarburization of tooth tip intensifies the risk of induction hardening cracking. The actual process improvement shows that the carbon segregation difference of the inner gear ring can be reduced to 0.02% by obtaining the blank with forging ratio greater than 7 by three-way forging, which can effectively avoid the carbon segregation. In the design of punching process, the segregation in the center of continuous casting slab can be eliminated by controlling the punching quality. When the blanking coefficient is greater than 0.019, the generation of micro defects and quenching cracks in the teeth can be avoided.

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Reason analysis and solution of rolling cracking of super duplex stainless steel S32750

Wang Hairui, Wang Jinyong, Liu Fuqiang, Qi Xilun, Gao Zhenglei

Heat Treatment of Metals 2024, 49 (2): 291-296. doi:10.13251/j.issn.0254-6051.2024.02.045

Abstract （59）

PDF （2323KB）（58）

Serious cracking occurred in the rolling process of a batch of pipes made of S32750 super duplex stainless steel. The chemical composition, non-metallic inclusions, microstructure and mechanical properties of the pipe materials were analyzed by means of spectrograph, metallographic microscope, scanning electron microscope, tensile tester and impact tester, and combined with rolling process and hot extrusion process. The results show that there are a lot of σ-intermetallic phases with Fe-Cr-Mo segregation in the microstructure before rolling, which is brittle and leads to cracking of the rolled steel pipe. After solution treatment at 1100 ℃ for 60 min before rolling, the intermetallic phase σ is fully dissolved, the strength and hardness are reduced, while the plasticity and toughness are improved, which can solve the rolling cracking problem.

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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）

PDF （3314KB）（44）

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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Strengthening and toughening mechanism of railway locomotive wheel

Jia Tuosheng, Liu Cuirong, Zhang Xiaofeng, Wei Huacheng, Zhang Lei, Gao Kang

Heat Treatment of Metals 2023, 48 (7): 283-286. doi:10.13251/j.issn.0254-6051.2023.07.047

Abstract （54）

PDF （2427KB）（55）

Effect of minor added alloying elements on improving the strength and hardness of railway wheel steel was studied by means of mechanical properties test and inclusions and microstructure analysis. The results show that the strength and hardness of the tested steel are high, but the impact property is poor, which cannot meet the standard requirements. The addition of alloying elements causes the formation of a large number of composite inclusions in the steel, which has an adverse effect on the toughness of the wheel. In addition, a small amount of abnormal microstructure in the wheel caused by the segregation of the microalloying elements may also have an adverse effect on the toughness of the wheel.

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Formation cause and elimination process of radial stripes in эи866 alloy forgings

Yang Shulin, Zhao Jiqing, Yan Lei, Yang Gang

Heat Treatment of Metals 2023, 48 (9): 279-282. doi:10.13251/j.issn.0254-6051.2023.09.047

Abstract （53）

PDF （2375KB）（25）

Formation reason of radial stripes in эи866 alloy forgings were analyzed by microstructure observation, mechanical properties and hardness testing. The heat treatment process to eliminate the radial stripes was also studied. The results show that the radial stripes are caused by the banded structure. A large amount of retained austenite remains after quenching, and transforms to secondary martensite in the tempering cooling process. The secondary martensite distributes alternately with the tempered martensite to form the banded structure, which results in the hardness nonuniformity and appears stripes after machining. Adding a secondary tempering after the performance heat treatment can make the martensite to decompose and recover, and the banded structure can be improved, but it is difficult to be completely eliminated. Adding preheat treatment before the performance heat treatment can raise the Ms point, make the martensite transformation sufficiently, and reduce the amount of retained austenite greatly after quenching, then a single uniform tempered martensite is formed, and the banded structure is eliminated completely. Thus, it is suggested to couple the preheat treatment with the performance heat treatment to improve the uniformity of the structure in the эи866 alloy forgings.

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Failure analysis of shield machine main thrust bearing ring caused by soft belt defects

Zhong Yunfei, Li Tianfu, Qu Shen, Zhang Zhefeng, Huang Hongjun

Heat Treatment of Metals 2023, 48 (12): 285-291. doi:10.13251/j.issn.0254-6051.2023.12.047

Abstract （52）

PDF （6431KB）（27）

After conducting testing on a certain type of shield machine, it was discovered that the spindle bearing had failed. Upon disassembly, large spalling pits and metal fragments were found in the part of the bearing raceway. The bearing ring, made of 42CrMo alloy steel, had undergone induction quenching to harden its surface. Analysis of the failure was carried out through macroscopic inspection, composition analysis, microstructure observation, and hardness analysis. The results sholo that the high temperature generated by contact fatigue in the soft belt area of the shield bearing ring results in the formation of a WEL (white etching layer) on the surface. The peeling of this layer further leads to severe abrasive wear on the raceway surface. As the wear particles transfer with the rolling body, they cause dense indentation pits in the hardened area, with some of the particles becoming embedded in the soft belt area. The increase of raceway surface roughness significantly impedes the bearing's normal operation, and finally leading to the failure.

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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）

PDF （2037KB）（39）

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）

PDF （4600KB）（15）

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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Analysis of unqualified mechanical properties of 1Cr12MoV steel forgings and process improvement

Li Wei, Liu Jing, Shao Zhuhao, Hao Hongjun, Tang Lin, Jiang Tao

Heat Treatment of Metals 2023, 48 (11): 300-304. doi:10.13251/j.issn.0254-6051.2023.11.049

Abstract （51）

PDF （5207KB）（57）

In order to study the cause of unqualified mechanical testing for 1Cr12MoV steel forgings, chemical composition analysis, optical microscopy and scanning electron microscopy were used to analyze the chemical composition, microstructure and fracture morphology of the forgings. The results show that large-sized and aggregated eutectic carbides are existed in the matrix which reaches level 5 and doesn't meet the requirement of below level 3, which is the main reason for the failure of mechanical performance testing. Through elongating the forging stock by free forging, increasing the deformation during the forging process, and adding a preliminary heat treatment process, the distribution of eutectic carbides is greatly improved, and mechanical properties meet the delivery requirements.

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Failure analysis and heat treatment process improvement of 40CrNiMoA steel upper joint

Liu Jinghong, Liu Jian, Liu Shuheng, Yao Mingfeng, Chen Jie

Heat Treatment of Metals 2024, 49 (2): 301-306. doi:10.13251/j.issn.0254-6051.2024.02.047

Abstract （50）

PDF （4863KB）（57）

In order to solve the fracture problem of the 40CrNiMoA steel upper joint, the main reasons for the fracture are the stress concentration caused by the small radius of variable diameter R, the over high hardness and the insufficient elongation via chemical composition analysis, metallographic analysis and mechanical property test of the workpiece fracture. The risk of crack fracture in heat treatment and use of the product can be reduced by optimizing the structural design and heat treatment processes. The results show that the mechanical properties of the workpiece material can meet the requirements of both product design and use by increasing the diameter rounded corner to R20, adopting intermediate heat preservation, controlling the heating rate to the quenching temperature of 810 ℃, cooling in slow oil, and subsequently, controlling the heating rate to the tempering temperature of 570 ℃ plus cooling in slow oil to prevent tempering brittleness. After testing in actual working conditions, no fracture accident occurs in the product life cycle.

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Analysis of crack defects on axle surface

Zheng Yi, Xue Zhenfeng, Zhao Xinglong, Li Shulin, Xu Junsheng, Pan jiayu

Heat Treatment of Metals 2023, 48 (12): 295-298. doi:10.13251/j.issn.0254-6051.2023.12.049

Abstract （50）

PDF （4348KB）（34）

In response to the surface magnetic detection cracks on railway freight car axles during maintenance, the chemical composition, mechanical properties, fracture morphology, microstructure, and microhardness of the axles were analyzed using spark discharge atomic emission spectrometer, electronic universal testing machine, scanning electron microscope, metallographic microscope, and energy dispersive spectrometer. The causes and mechanisms of surface cracks on the axles were studied. The results indicate that the nature of train axle cracks is fatigue cracking formed by martensitic transformation at the transition fillet, which belongs to the cracks caused by local burns during the grinding process of the axle surface due to improper grinding technology.

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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）

PDF （4585KB）（17）

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）

PDF （2649KB）（39）

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）

PDF （2450KB）（38）

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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