The microstructure of the bolt shows a grain size of 7, with a surface layer composed of tempered sorbite that exhibits orientation. Some grain boundaries contain ferrite with semi-network precipitation, while the central structure is more randomly oriented. A small amount of acicular ferrite and martensite are distributed throughout the material. The fracture surface primarily displays transgranular cracking, and corrosion grooves caused by acid etching are visible. The bar also has a grain size of 7, with a tempered sorbite structure and some dispersed acicular ferrite. The bolt’s surface is coated with a zinc layer approximately 0.0375 mm thick due to the galvanization process. However, this zinc layer is not dense, and there is a clear boundary between the coating and the base material. During installation and use, partial peeling of the zinc layer was observed. The microhardness of the zinc-plated layer is around 346–388 HV, while the base material has a higher hardness of 388–404 HV, indicating that the base metal is harder than the coating.
Chemical analysis reveals that the carbon content of the bolt material exceeds that of 40Cr steel, placing it within the composition range of 45Cr steel. This may be due to incorrect delivery from the steel mill or mixing after purchase. The test results indicate that the bolt and its bar have high strength but low section reduction and impact values, along with elevated hardness. From the fracture surface and microstructure, it appears that the bar underwent excessive quenching and tempering, leading to partial ferrite precipitation. The actual tempering temperature was approximately 70°C, leaving the entire bar in a high-stress state. Machining further induced stress concentration at the thread roots, especially at the bottom of the threads. The galvanizing treatment exacerbated this stress imbalance, making the thread root the most vulnerable point for failure.
Additionally, the presence of block-shaped silicate inclusions and numerous microscopic shrinkage cavities in the bolt material allowed cleaning solutions to penetrate during surface treatment. These voids enabled the corrosive solution to reach the internal structure, causing corrosion. Corrosion marks were found on various sections of the broken bolts, with the most severe damage reaching over two-thirds of the cross-section. This confirms that the cleaning process led to internal corrosion. The corroded fracture surfaces showed little deformation, and no hydrogen embrittlement was observed. The corrosion caused by the cleaning solution worsened the effects of metallurgical defects, ultimately leading to brittle fracture under the combined influence of residual stresses from heat treatment and galvanization, as well as external forces during assembly.
The zinc-coated stud bolt contains metallurgical defects such as concentrated silicate inclusions and micro-shrinkage cavities. When cleaned, the cleaning solution corroded the internal structure, reducing intergranular bonding. Under the combined effect of applied loads and material stress during assembly, brittle fracture occurred at the thread root or middle portion. Long-term cleaning before vacuum galvanization increased the impact of existing material defects on the overall performance, making it the main cause of bolt failure. Improper quenching and tempering processes, along with metallurgical defects, also contributed to the breakage. To prevent future brittle fractures, the following measures are recommended: (1) Strengthen material inspection and management to avoid mixing or using defective materials in production. (2) Strictly control the heat treatment process, ensuring accurate temperature and timing. (3) Adjust machining allowances based on the effects of the zinc coating and adhesive layers on thread dimensions to minimize the negative impacts of long-term cleaning before bolting.
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