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 in a semi-network precipitation, while the central structure is oriented. A small amount of acicular ferrite and fire stellite are distributed throughout the material. The fracture appears to be transgranular, and corrosion grooves caused by acid etching are visible at the fracture surface. The bar’s grain size is also 7, with a tempered sorbite structure and a small amount of dispersed acicular ferrite. The surface of the bolt has a zinc layer approximately 0.0375 mm thick, which was applied during the galvanization process. However, this zinc layer is not dense and has a clear boundary with the substrate. During installation and use, partial peeling was observed. The microhardness of the zinc-plated layer is around 346.99–88.44 HV, while the base material has a higher hardness of 388.44–4040.80 HV.
Chemical composition analysis revealed that the carbon content of the bolt material exceeds that of 40Cr steel, placing it within the range of 45Cr steel. This could be due to incorrect delivery from the steel mill or mixing after purchase. The test results indicate that the bolt and its bar material have high strength, but low section reduction and impact values, along with high hardness. From the fracture surface and microstructure, it appears that the bar was quenched and tempered for an extended period, leading to some ferrite precipitation. During tempering, the actual temperature was about 70°C, resulting in a high residual stress state. Machining introduced stress concentration at the thread roots, and the galvanizing treatment further exacerbated the stress imbalance, making the thread root the most vulnerable point.
Additionally, the presence of blocky silicate inclusions and numerous microscopic shrinkage cavities in the bolt material allowed the cleaning solution to penetrate through these defects, causing internal corrosion. Corrosion marks were found on the broken bolt sections, with the most severe damage reaching over two-thirds of the cross-section, confirming that surface cleaning had led to internal degradation. The corroded fracture surface showed no signs of deformation, and hydrogen embrittlement was not observed. However, the corrosion from the cleaning solution intensified the negative effects of metallurgical defects, contributing to brittle fracture under the combined influence of residual stress and external forces from heat treatment and galvanization.
The zinc-coated bolt exhibited metallurgical defects such as concentrated silicate inclusions and micro-shrinkage pores. When cleaned, the cleaning solution corroded the internal structure, weakening the bonding between different phases. Under the combined effect of applied loads and internal stresses 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, becoming the primary cause of failure. Improper heat treatment and metallurgical defects also played a significant role in the breakage.
To prevent future brittle fractures of zinc-plated bolts, it is recommended to: (1) Strengthen material inspection and management to avoid mixing and the entry of defective materials into production. (2) Strictly control the heat treatment process, including precise temperature and timing. (3) Adjust machining allowances based on the impact of the zinc coating and adhesive layers on thread dimensions, to mitigate the adverse effects of long-term cleaning prior to bolting.
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