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The microstructure and grain size fracture bolt grain size is 7 grades, the surface layer structure is tempered sorbite with orientation, some grain boundary has ferrite with semi-network precipitation, and the central structure is oriented back. Fire stellite and a small amount of acicular ferrite distributed, the bolt section is basically transgranular cracking, and the corrosion gully caused by acid etching can be seen at the fracture. The grain size of the bar is 7 and the structure is tempered sorbite. And a small amount of dispersed acicular ferrite, the surface of the bolt on the surface of the bolt is adhered to a zinc layer of about 0.0375 mm in the process of zinc infiltration. The zinc layer is not dense, and there is a clear boundary with the surface of the substrate. And the partial peeling phenomenon during installation and use, the microhardness of the zinc-plated layer is 346.996388.44HV, the base material is 388.444040.80HV, and the microhardness of the base metal is higher than that of the zinc-plated layer. Analysis and discussion From the results of chemical composition analysis, the carbon content of the bolt material is higher than 40Cr steel, which belongs to the composition range of 45Cr steel. The reason may be caused by the wrong delivery of the steel mill or the mixing after purchase. The above test results show that the strength of the fracture bolt and its bar material is high, the reduction rate of the section and the impact value are low, and the hardness value is high. According to the fracture surface and microstructure of the bolt, the bolt bar is longer in quenching and hollowing, which causes some ferrite to precipitate. In the tempering treatment, the actual tempering temperature is about 70e, so that the whole bar is at High stress state; after the bolt is machined, it causes stress concentration at the thread, especially at the root of the thread. The galvanizing treatment further exacerbates the stress imbalance of the bolt, making the root of the thread the most easily broken.
Because the silicate inclusions in the bolt material are concentrated in a block shape, and there are many microscopic shrinkage holes, the surface of the bolt is immersed and cleaned, and the void formed by the dissolution of the cleaning solution through the silicate and the microscopic shrinkage hole enter the interior of the bolt. Corrosion of its organization. There are different degrees of corrosion marks on the sections of the broken bolts, and the most serious corrosion area has reached more than 2/3, which proves that the surface cleaning has caused corrosion to the internal structure of the bolts. The corroded fracture surface is basically free of deformation, and no hydrogen embrittlement is observed in the section. The corrosion of the material by the cleaning solution further increases the adverse effect of metallurgical defects on the material properties, and then the brittle fracture of the bolt is triggered by the combined effect of the residual stress and the external force caused by the tempering treatment and the galvanizing treatment. The surface-zinc-plated stud bolt has metallurgical defects such as concentrated lumpy silicate inclusions and microscopic shrinkage holes in the material. When the surface is cleaned, the cleaning solution corrodes the internal structure of the bolt and reduces the bonding force between the tissues. Under the combined action of the applied external force and the material tissue stress during the assembly process, brittle fracture occurs at the root or middle portion of the bolt thread. The long-term cleaning treatment of the bolt surface before vacuum zinc infiltration increases the influence of the existing defects in the material on its comprehensive performance, which is the main factor for the bolt to break. Improper quenching and tempering process, material metallurgy defects are also the cause of bolt breakage. In order to prevent the occurrence of brittle fracture of zinc-plated bolts again, it is recommended to: (1) Strengthen the inspection and management of metal materials to prevent mixing and materials with serious defects from entering the production process. (2) Strictly implement the heat treatment process, and do a good job in heat treatment temperature control and timing. (3) According to the influence of zinc coating layer and adhesive zinc layer on the thread size of bolts, the appropriate machining allowance should be arranged during machining to prevent the adverse effects of long-term cleaning before bolting on bolts.
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