Abstract: A batch of CRMO alloy steel KV gas-insulated metal-enclosed switchgear (GIS) mechanism electrogalvanized stop bolt fracture failure occurred in the closing test adjustment, and the cause of bolt fracture was analyzed by chemical composition analysis, fracture analysis, hardness test, hydrogen embrittlement evaluation test, metallographic inspection and other methods. The results show that the fracture of the stop bolt is caused by hydrogen embrittlement, and the hydrogen embrittlement fracture of the stop bolt is caused by the improper dehydrogenation process after electrogalvanizing of the bolt. Finally, the improvement measures to prevent hydrogen embrittlement fracture of bolts are proposed. Keywords: stop bolts;Fracture;Hydrogen embrittlement;CLC Number: TG TH Literature Symbol Code: B Article Number: A KV gas insulated switchgear (GIS) intelligent transformer was tested for factory mechanical characteristics, and after operation, it was found that the three-phase closing time of the circuit breaker of GIS was not within the management value range (ms). Use nm torque wrench to tighten the closing time bolt of the adjusting mechanism, the first adjustment fails to meet the requirements of the management value, and when adjusting again, it is found that the C phase closing adjustment bolt is loose, and the bolt is found to have broken when unscrewing out, and the morphology of the broken bolt is shown in the figure. The stop bolt of the hole in the fracture zone is a Toshiba import bolt, the specification is m, the strength grade is H, the material is CRMO steel, and the surface treatment process is electrogalvanized (F e B) and dehydrogenated (P F).In order to find out the cause of the fracture failure of the bolt, the author has inspected and analyzed the fractured bolt and the bolt of the same specification through a series of physical and chemical test methods, and put forward the corresponding preventive measures. 
Physical and chemical testing Chemical composition analysis The fracture bolt matrix was sampled, and the chemical composition analysis was carried out by Shimadzu PDA direct reading spectrometer, and the results are shown in the tableIt can be seen that the element content of the fractured bolt meets the technical requirements of GB T alloy structural steel for CRMO steel composition. 
Macroscopic analysis of the perforated stop bolt under the disassembly of the GIS mechanism has not been completely broken and separated, as shown in Figure a). Tear off the last connecting part by hand, and there is no big difference between the upper and lower fractures visually, and the fractures are relatively flat. The fracture was observed using a KEYENCE VHX digital microscope, as shown in Figure B), the fracture started at the bottom of the screw, the bolt was subjected to tensile stress before fracture, and the fracture gradually expanded from the bottom of the screw to the entire section until the final fracture, and there was no obvious plastic deformation at the fracture, and the section was perpendicular to the axis, showing the characteristics of brittle fracture. Microscopic analysis of the fracture of the stop bolt after repeated cleaning of acetone, the microscopic morphology of the fracture source, expansion zone and terminal fracture area of the bolt fracture in Fig. B) was observed by JSM A-type scanning electron microscope (SEM) of Nippon Electronics, and the results are shown in Fig. Fig. a) shows the morphology of the grain along the fault source, and the fault is dominated by the typical rock sugar morphology along the grainFig. b) shows the morphological characteristics of the crystal plane along the crystal fault at the fault source, and it can be seen that there are chicken feet textures distributed on the crystal planeFig. c) shows the microscopic morphology of the propagation zone at the fracture core, and it can be seen that there are intergranular secondary cracks accompanied by microporesFig. d) shows the mixed fault topography of the terminal fault zone, with dimples and along the grain faults on the fractures. The results of microscopic fracture analysis show that the fracture of the stop bolt shows typical hydrogen embrittlement fracture characteristics. 
The Vickers hardness test uses the HV type microhardness tester to test the surface and core hardness of the fracture stop bolt with a test load of n( gf ) and a loading time of s, the results are shown in the table. The surface and core hardness of the visible bolt conform to the mechanical characteristics of fasteners made of carbon steel and alloy steel Part II: Fastening screws not less than tensile strength and similar fastening screws Technical requirements for H strength grade bolts. 
Hydrogen embrittlement evaluation test Torsional strength testAccording to the strength test method in the mechanical properties of the JISB, parts are randomly selected from the same batch of bolts, and other batches of bolts are removed from the normal operation equipment for torsion test. The trial was performed as shown in the figure in the JISB. The test results show that the bolts of the same batch of the same batch are broken when bearing the nm torque, while the bolts normally used on the equipment can withstand the nm torque and do not break, indicating that the strength of the bolts of this batch can not meet the torsion test requirements of the specification m and the strength grade h. When the hydrogen content in the bolt reaches a certain level, the ultimate load of the bolt will be reduced [ ] Observe the bolt fracture flush, which is a brittle fracture, so it is judged that there is a risk of hydrogen embrittlement in this batch of bolts. The slow strain tensile test was carried out on the same batch of M bolts and other batches of M bolts that were normally used in other batches, and the slow strain tensile test was carried out to quickly evaluate its resistance to hydrogen embrittlementThe results of the slow strain tensile test are shown in the table, and the test curves are shown in a). As can be seen from Figure a), the bolt number and number of the same batch are fractured before yield, showing brittle fracture, and there is no obvious yield;The bolt is ductile fracture with obvious yield. The tensile strength of bolts is greater than that of bolts and bolts, because with the increase of hydrogen content in bolts, their tensile strength decreases and brittle fractures occurSimilar to the fracture morphology of the fracture bolt, the number and bolt also have chicken claw textures along the grain fracture surface, and there are local secondary cracks along the grain accompanied by micropores, as shown in Fig. d) and Fig. e).The fracture shape of the bolt is shown in Figure f), and there are a large number of dimples on the fracture, which is a ductile fracture. The results of the slow strain tensile test show that there is a serious risk of hydrogen embrittlement in this batch of bolts. 
Metallographic inspection Non-metallic inclusion inspection Longitudinal sampling of broken bolts, after grinding and polishing, use OlympusGX metallographic microscope to observe the non-metallic inclusions and defects in the polished state of the bolts. The results showed that there were no obvious non-metallic inclusions and band-like segregation in the magnified center of the bolt throwing photo. The decarburization layer test is to cut the broken bolt along the axial direction, prepare metallographic samples, and carry out the spiral decarburization layer inspection. After grinding and polishing, it was eroded with a nitric acid alcohol solution (volume fraction), and observed with an OlympusGX metallographic microscope, and the micromorphology of the threads is shown in Fig. According to the regulations of the metallographic determination of the bolt decarburization layer in JISB, the height of the thread without decarburization is measured E mm (technical requirements E mm), the depth of the full decarburization layer of the thread G m (technical requirements G m), and the inspection results of the thread decarburization layer meet the technical requirements of JISB. 
The fractured bolts and the fractured bolts in the slow strain tensile test were selected for microstructure comparison in the microstructure test. The two bolts were sampled laterally, ground and polished with a (volume fraction) nitric acid alcohol solution and observed by an OlympusGX metallurgical microscope. As shown in the figure, the microstructure of the two is basically the same, both are tempered sostenite, and there is a small amount of massive ferrite in the fracture bolt, which is the microstructure of normal quenching and tempering heat treatment [ Comprehensive analysis of the above physical and chemical test results show that the chemical composition, hardness and microstructure of the fracture stop bolt meet the relevant standard technical requirements. The fracture analysis results show that there is no obvious plastic deformation on the macroscopic surface of the fracture bolt fracture, the cross-section is flat, and the microscopic morphology of the fracture is dominated by rock sugar-like fracture along the crystal, and there are a large number of chicken claw textures along the fracture surface, and there are micropores on the crystal plane accompanied by intergranular secondary cracks, showing typical hydrogen-embrittlement fracture characteristics. The results of the bolt hydrogen embrittlement evaluation test show that there is a serious risk of hydrogen embrittlement in the fractured batch of bolts. Hydrogen embrittlement in bolts is mainly related to the high hydrogen content in the steel, but it is also related to the carbon content, microstructure, strength and stress on the parts in the steel. The study found that if the tensile strength of the steel reaches MPa or the hardness reaches HRC or above, the hydrogen embrittlement sensitivity will be very high, and if the dehydrogenation process is not appropriate after surface treatment (electroplating, pickling) of the parts, the hydrogen elements introduced are very easy to induce hydrogen embrittlement cracking. 
The strength grade of the fracture stop bolt is H, which is a high-strength bolt, and its hardness reaches more than HRC, so the hydrogen embrittlement sensitivity is strong. The surface of the bolt has been electrogalvanized, and the electrogalvanizing process is as follows: degreasing, washing, pickling, electrolysis, oil removal, washing HCL (volume fraction) activation, water washing, electrogalvanizing, washing light, passivation (CRO, Hso, HNO) washing, drying, aging, it can be seen that there are pickling processes and hydrochloric acid activation processes in the electrogalvanizing process, and it is very easy to introduce hydrogen when these two processes are carried out. If the dehydrogenation process is not done properly or the dehydrogenation process is missed after electroplating, hydrogen will remain in the bolt, resulting in delayed fracture caused by hydrogen embrittlement during the use of the bolt. Generally, hydrogen embrittlement fracture will occur when the hydrogen content in steel is in mass fraction, and the critical hydrogen content is much lower for high-strength steel. The hardness of the fractured bolt and the results of the hydrogen embrittlement evaluation test indirectly verify the excess hydrogen content in the bolt, and the bolt has the danger of hydrogen embrittlement. The stop bolt is mainly affected by static stress and hydrogen element when working, and after the operation test of the product assembly and assembly, it is speculated that a crack appeared on the stop bolt, and finally the final fracture occurred due to the action of torsional force when making adjustments. Therefore, the combined effect of hydrogen and stress is the main reason for the hydrogen embrittlement fracture of the stop bolt of this batch [ Conclusion and improvement measures ( ) The stop bolt fracture of the KV GIS mechanism of this batch belongs to the typical hydrogen embrittlement fracture along the crystal. (The bolt introduces hydrogen elements in the electrogalvanized surface treatment process, and the improper dehydrogenation process causes hydrogen element residue, which leads to the delayed cracking of the bolt under the joint action of hydrogen and stress in the use process, wherein excess hydrogen is the root cause of the hydrogen embrittlement fracture of the batch of stop bolts. (It is recommended that bolts with tensile strength greater than MPa or hardness higher than HRC should be dehydrogenated in time after electrogalvanizing, and the dehydrogenation process specifications should be strictly implemented to avoid hydrogen embrittlement fracture caused by improper dehydrogenation process;.)In addition, measures such as appropriately reducing the carbon content of alloy steel and increasing the tempering temperature can be appropriately increasedFor parts that are sensitive to hydrogen embrittlement to be transported by sea, they should be packaged and protected during transportation to avoid local hydrogen seepage of parts caused by long-term exposure to sea fog. References:[ Cui Zhemin Research on Hydrogen Embrittlement Detection and Evaluation Methods for High-strength Fasteners[D] Hangzhou: Zhejiang University of Technology, General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China, Standardization Administration of the People's Republic of China Corrosion Stress Corrosion Test of Metals and Alloys Part I: Slow Stress Rate Test: GB T S] Beijing: Standards Press of China [ General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China, Standardization Administration of the People's Republic of China Mechanical properties of fasteners Bolts, screws and studs: GB T Beijing: China Standards Publishing House [ Li Jionghui Metallographic Atlas of Metal Materials[M] Beijing: Machinery Industry Publishing House, Han Kejia, Zhao Xiaohui, Li Hongwei Analysis of fracture failure of high-strength bolts of CRMO steel[J] Physical and chemical test (physical fascicle), Japan Heat Treatment Technology Association, Japan Metal Heat Treatment Industry Association New Edition Introduction to Heat Treatment Technology[M] Yao Zhongkai, translated Beijing: Machinery Industry Press, Liu Delin, Tao Chunhu, Liu Changkui, et al. New Phenomena and New Understanding of Steel Hydrogen Embrittlement Failure[J] Failure analysis and prevention, Wang Ting, Li Zhenhua Fracture analysis of galvanized screws[J] Physical and chemical testing (physical fascicle), article**—Materials and testing network.