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Analysis of unqualified impact property of 30CrNiMo steel castings

Yin Yahao, Guo Lei, Su Jianfeng, Yue Xiaocan

Heat Treatment of Metals 2022, 47 (5): 262-265. doi:10.13251/j.issn.0254-6051.2022.05.046

Abstract （127）

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In view of the unacceptable impact absorbed energy of 30CrNiMo steel casting after quenching and tempering, the main influence factors of which were analyzed through fracture analysis, metallography, hardness testing and chemical composition analysis. The results show that there are intergranular fracture of impact specimen of the 30CrNiMo steel castings, and the quenched specimen appears mischcrystal structure. Furthermore, a large number of NbC phases segregate at the grain boundary, which impedes the movement of grain boundary effectively. At the same time, abundant NbC phases also consolidate the inheritance phenomenon of grain by the pinning effect, which enhances ductile-brittle transition temperature significantly after the austenite grain coarsening. Ultimately, brittle transgranular fracture and low impact property are obtained when the specimen is impacted at room temperature.

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Formation causes of white bright layer on surface of vacuum oil quenched martensitic stainless steel

Ding Yahong, Zhu Kai, He Jun, Zhu Ling, Wang Qiaoli, Li Jianfei

Heat Treatment of Metals 2022, 47 (10): 275-282. doi:10.13251/j.issn.0254-6051.2022.10.047

Abstract （119）

PDF （652KB）（42）

Surface of martensitic stainless steel nut was prone to brown yellow rust in the air environment. The surface rust of martensitic stainless steel was analyzed by means of optical microscope, scanning electron microscopy and chemical element detection. The results show that a uniform white bright layer structure is formed on surface of the martensitic stainless steel after vacuum quenched in oil. The corrosion resistance of the white bright layer is worse than that of normal martensite. The white bright layer is easy to rust in the air, and the rust does not penetrate the white bright layer. Through the measurement of carbon content and mechanism analysis, it is considered that the necessary conditions for the formation of white bright layer are vacuum heating and oil cooling. Vacuum heating causes the depletion of alloy element Cr on the surface of stainless steel, and carbon infiltration due to the oil decomposition leads to the carburization of the surface, which leads to no martensitic transformation on the surface of stainless steel during quenching, forming a white bright layer structure with poor corrosion resistance.

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Cause analysis and countermeasure of low surface hardness of SPCC steel after gas nitrocarburizing

Gao Xiaohui

Heat Treatment of Metals 2022, 47 (6): 274-276. doi:10.13251/j.issn.0254-6051.2022.06.049

Abstract （114）

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SPCC steel met the technical requirements in trial production, but the surface hardness was extremely low after mass production. After analyzing the incoming materials, manufacturing processes, and testing various process elements, the reasons were determined, and the problem of ultra-low surface hardness was resolved through consultation with customers. According to the verification and detection of the possible reasons, it is finally determined that the low hardness is caused by the mismatch between the product surface structure and the hardness testing method, that is, the turning tool cutting pattern on the product surface affects the accuracy of the inspection equipment. The correspondingly improved hardness testing is carried out by direct polishing without cutting and inlaying, and the detection position is only selected at the “wave peak” position.

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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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Typical fracture and cause analysis of 35CrMnSiA ultra-high strength steel

Wang Chunyan, Zhang Fa, Jie Ruihua, Wang Chunmei

Heat Treatment of Metals 2022, 47 (12): 284-288. doi:10.13251/j.issn.0254-6051.2022.12.047

Abstract （99）

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In order to investigate the fracture mechanism and fracture causes of the low percentage reduction of area of the 35CrMnSiA ultra-high strength steel tensile specimens, four common types of fractures were observed and analyzed by means of scanning electron microscope and energy dispersive spectrometer. The results show that the fish-eye defect caused by the combined action of a certain amount of hydrogen and tensile stress is the main reason for the low percentage reduction of area of conventional plastic fracture. Brittle-plastic mixed fracture is caused by hydrogen embrittlement around large inclusions, which leads to the decrease of the percentage reduction of area of the steel. The pencil-tip shaped fracture is mainly caused by large particle inclusions or hydrogen damage defects in steel. Brittle fracture is caused by mechanical damage to the material or the formation of micro-cracks caused by hydrogen accumulation on the damaged surface.

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Fracture analysis and process improvement of 65Mn steel spring leaf

Xu Feng

Heat Treatment of Metals 2022, 47 (12): 281-283. doi:10.13251/j.issn.0254-6051.2022.12.046

Abstract （97）

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A 65Mn steel spring leaf used in a certain type of product was found to break at the peak position when testing elastic force. Failure cause of the spring leaf was analyzed through the chemical composition, microstructure and microhardness test and the processing process analysis, and the then the process was improved. The results show that due to the high quenching temperature, high quenched hardness, and lower temperature tempering after quenching, the elimination of quenching stress is not sufficient, causing the spring leaf to produce microcracks under the combined effect of axial compressive stress and quenching residual stress during the installation of the fixture when the spring leaf is tempered at high temperature. The microcracks expand and transform during subsequent strong pressure treatment, resulting in brittle fracture. After adopting the improved process of 815 ℃ quenching+280 ℃ low temperature tempering+390 ℃ high temperature tempering, the hardness of the spring leaf can meet the technical requirement, the fracture is 100% ductile fracture, and no brittle fracture occurs again.

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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）

PDF （3243KB）（65）

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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Analysis of bright spot defect on surface of 590 MPa low-alloy high-strength galvannealed steel sheet

Zhao Chuanxiang

Heat Treatment of Metals 2022, 47 (6): 270-273. doi:10.13251/j.issn.0254-6051.2022.06.048

Abstract （94）

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Morphology and element content of bright spot defect on the surface of 590 MPa galvannealed high-strength steel sheet were investigated by SEM and EDS. The results show that the surface oxides produced by the selective oxidation of alloying elements can be reduced completely by the dissolved Al in the Zn bath when the oxides are small enough in the high-strength low-alloy steel. Compared with the normal position, a complete and dense Fe₂Al₅Zn_x inhibitory layer is not formed at the position of the precipitate particles. In the subsequent hot-dip galvanizing process, the Zn-Fe reaction and diffusion at this position are fast, and the coating grows thicker. During skin pass rolling, the compressed sheet-like structures formed at this position are connected with each other, and their macroscopic feature is the surface bright spot defect. By reducing the dew point in the furnace, the external oxidation of alloy elements on the surface of low-alloy high-strength steel can be inhibited, thereby preventing the occurrence of bright spot defects on the surface of the coating.

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Analysis of grinding cracks on 4Cr13 steel spindle sleeve for gyratory crusher after laser quenching

Shi Ruxing, Lin Yichou, Zhang Pei, Qi Guosai, Yu Wenping, Su Wenbo

Heat Treatment of Metals 2022, 47 (5): 257-261. doi:10.13251/j.issn.0254-6051.2022.05.045

Abstract （85）

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Several cracks were found in the grinding stage of the 4Cr13 steel spindle sleeve for gyratory crusher after laser quenching, and having angles about 15° with the outer circle axis. Through macroscopic analysis, chemical composition analysis, hardness, effective hardened layer depth detection and microstructure observation, it is confirmed that the cracks of the 4Cr13 steel spindle sleeve are grinding crack. The cracks are mainly caused by the movement of tangential and axial directions in the grinding process of the grinding wheel. The tangential rotation speed is far greater than the axial moving speed. The resultant force of grinding tensile stress in the two directions results in an angle of about 15° between the crack and the axial direction. At the same time, the radial columnar crystal produced by laser melting reduces the bonding force between crystals. In addition, the residual temperature self-tempering method after laser quenching makes the stress distribution of hardened layer more complex, which makes it easy to crack in the grinding process.

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Cause analysis of slag inclusion-like defects on stainless steel metallographic specimen

Tian Wei, Chang Song, Sun Xinyang, Zhou Changshen

Heat Treatment of Metals 2022, 47 (5): 270-274. doi:10.13251/j.issn.0254-6051.2022.05.048

Abstract （79）

PDF （590KB）（30）

Many irregular slag inclusion-like defects were found on the inspection surface of different stainless steels after grinding, coarse grinding, fine grinding and fine polishing for rating of the non-metallic inclusions. The morphology of defects was observed by means of optical microscope and scanning electron microscope, and the chemical composition of the defects was analyzed qualitatively and quantitatively by means of energy dispersive spectrometer. Meanwhile, the six elements of on-site quality management, namely human, machine, material, environment, method and measurement were used to conduct comparison and examination one by one in the whole process from steelmaking to specimen preparation, and to conduct the sandpaper comparison test with different materials. The results show that the slag inclusion-like defects mainly contain oxygen, aluminum and zirconium, and are caused by using zircon corundum sandpaper during grinding.

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Abnormal microstructure analysis of G12MnMo7-4 steel and quenching and tempering process optimization

Chang Youyu, Song Zhongming, Li Fuyan, Gou Chunlin

Heat Treatment of Metals 2022, 47 (6): 266-269. doi:10.13251/j.issn.0254-6051.2022.06.047

Abstract （79）

PDF （3242KB）（26）

Reasons for abnormal microstructure of quenched and tempered G12MnMo7-4 steel castings were analyzed by means of optical microscope (OM), scanning electron microscope (SEM), universal testing machine and micro Vickers hardness tester. And the quenching and tempering process was optimized. The results show that G12MnMo7-4 steel is a low-carbon alloy steel, due to high quenching temperature, overheating abnormal structure appears in some parts. After treated by improved quenching and tempering process (870 ℃×3 h rapid quenching+580 ℃×3 h tempering), the microstructure is fine tempered sorbite with good comprehensive properties, which meets the requirements of product technical conditions.

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Failure analysis on piston connecting rod mechanism of diesel engine

Jiao Li, Shang Haikun, He Jianfeng, Wang Guoliang, Feng Tao, Wang Jia, Hu Yutao

Heat Treatment of Metals 2022, 47 (7): 277-282. doi:10.13251/j.issn.0254-6051.2022.07.048

Abstract （78）

PDF （638KB）（39）

A malfunction of piston connecting rod mechanism of diesel engine occurred during use. After dismantling, various degrees of fractures were found that on the cylinder, piston and connecting rod. The comprehensive analysis was carried out by means of macro and micro examination of the fracture, chemical composition and microstructure analysis in combination with the production process. The results show that the connecting rod was the failure maker, and the cylinder and piston belong to secondary fracture in this cascading failure. The folding defects on the surface of the connecting rod caused by improper shot peening process, which induces the initial fatigue damage because of the stress concentration. Meanwhile the undesirable microstructure weakens the strength of the connecting rod in a certain degree. The connecting rod high cycle fatigue fracture and the cylinder and piston damage one by one under the action of cyclic stress and the inertia force.

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Distortion control of carburizing quenching for new energy vehicle motor shaft bar groove

Ouyang Zhifang, Chen Junjian, Hu Weifang

Heat Treatment of Metals 2022, 47 (7): 283-285. doi:10.13251/j.issn.0254-6051.2022.07.049

Abstract （77）

PDF （639KB）（31）

Distortion of the shaft bar groove of a new energy vehicle motor during heat treatment could not meet the dimensional accuracy requirements. By reducing the carburizing potential, increasing the quenching oil temperature, optimizing the quenching process and anti-distortion processing with a 20 μm conicity, the distortion of the motor shaft bar groove is successfully controlled within the range of dimensional accuracy requirements, and the distortion is not significant increase in small batch production, which indicates that the improved process can be applied to batch production.

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Fracture cause analysis of QBe2 beryllium bronze connector

Wei Caili, Zhang Shenjie, Du Gangfeng

Heat Treatment of Metals 2022, 47 (7): 286-288. doi:10.13251/j.issn.0254-6051.2022.07.050

Abstract （73）

PDF （642KB）（49）

A batch of connector made of QBe2 fractured in the course of service was fratured. Reason for fracture was analyzed by the methods such as microscopic morphology observation, chemical composition analysis, microstructure observation and hardness test; and contral mensuren was proposed. The results show that overheating of the connector caused in thermocouples fail lead to coarse grain in solution and aging treatment. The strength of the grain boundary becomes lower than the matrix, and lead to the fracture of connector when it is subjected to external force. Therefore, it is recommended to regularly monitor and maintain the thermocouple to ensure the normal operation of the equipment.

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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）

PDF （2495KB）（51）

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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Effect of hole defect on fracture behavior of γ-TiAl alloy

Liang Yuehui, Qi Wenjun

Heat Treatment of Metals 2023, 48 (2): 295-302. doi:10.13251/j.issn.0254-6051.2023.02.046

Abstract （70）

PDF （476KB）（40）

Molecular dynamics simulation was used to investigate the effect of hole defects on crack propagation of polycrystalline γ-TiAl alloy at different temperatures, hole locations and hole sizes. The results show that the polycrystalline γ-TiAl alloy with hole defect is of brittle cleavage fracture at 1-750 K, but of ductile creep fracture at 1000 K and 1200 K. The alloy is more likely to fail when the hole is located on the grain boundary and triple junction. Compared with the perfect crystal, the micropores increase the ductility of the polycrystalline γ-TiAl alloy. When the hole radius is larger than 1.0 nm, the yield stress and yield strain of the alloy decrease sharply, and the time of the alloy failure is advanced. The fracture mode is influenced by the hole size. Intergranular fracture occurs when the hole radius is less than or equal to 0.8 nm. When the hole radius is larger than 0.8 nm, the hole in the polycrystalline γ-TiAl alloy gradually expands and occupies the whole grain, and then transgranular fracture occurs.

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Cracking failure analysis of 18CrNiMo7-6 steel gear after heat treatment

Zheng Kai, Zhong Zhenqian, Cao Wenquan, Lin Shuangping, Liu Ming

Heat Treatment of Metals 2022, 47 (12): 275-280. doi:10.13251/j.issn.0254-6051.2022.12.045

Abstract （66）

PDF （643KB）（40）

A batch of gears after production and processing, quenching and low temperature tempering cracked during storage in the warehouse. The causes of gear cracking were analyzed by means of ultrasonic flaw detection, metallographic examination, chemical composition analysis, hardness test, fracture morphology observation, end quenching test and supplementary heat treatment. The results show that the high strength of the cracked gear leads to high hydrogen embrittlement sensitivity, and the hydrogen induced delayed crack at the sub-surface is the direct cause of rapid brittle cracking of the gear. The root cause of the gear cracking is that the hardening capacity and hardenability of the cracked gear are too high, which exceed the hardenability requirements of BS EN 10084-2008 for 18CrNiMo7-6+HH steel. Thus, the cracking can be effectively avoided by reducing the strength and hardness of the quenched and tempered gear through adjusting the quenching medium and reducing the cooling rate of the high temperature zone during quenching.

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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）

PDF （3187KB）（60）

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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Macro-segregation analysis and rescue measure of TC11 alloy large round precision forged bar

Zhao Minquan, Wang Yuan, Dong Jian, Yu Zhongliang, Xie Li, Guan Yulong

Heat Treatment of Metals 2022, 47 (5): 266-269. doi:10.13251/j.issn.0254-6051.2022.05.047

Abstract （60）

PDF （595KB）（23）

Speckles in the TC11 alloy large round precision forged bar were analyzed and studied by means of optical microscope, scanning electron microscope and EDS analysis. It is determined that the formation of low magnification speckles is mainly caused by the unreasonable setting of precision forging process, which belongs to a forging defect. The defect can be eliminated by high temperature diffusion at β phase zone and rolling into small round bars at α+β phase zone, which will avoid scrapping the material with such a defect.

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