Research On Relationship Between The Processing Technology Of Tower Bolt And The Residual Stress In The Bolt

Cold-drawing, cold heading or hot forging, stress relief annealing and hot dip galvanizing(HDG) are important forming processes of the technology of the tower bolt. In this paper, relationship betwen forming technology of 6.8 grade tower bolt and residual stress distribution in sub-surface of bolt shank, structure of the coatings and corrosion resistance of hot-dip galvanizing and hot-dip Zn-5Al in different temperature were investigated. The effect of stress relief annealing and hot-dip galvanizing of cold headed bolt and hot forged bolt on residual stress in sub-surface of bolt shank were compared. The distribution and the mechanism of residual stress in sub-surface in different depths of 6.8 grade bolts were analyzed by OM, SEM, EDS, X-ray Residual Stress Tester and Electrolytic Polishing Machine. The corrosion resistance of hot-dip galvanized and hot-dip Zn-5Al coatings was studied by neutral salt spray test and electrochemical polarization method. The results show that:Different residual stress distribution in sub-surface layer of the bolt can be detected in the each process of the technology of tower bolt. When the bolt was cold-drawn, large residual tensile stress appeared in sub-surface of the bolt shank. Before stress relief annealing, the axial and tangential surface residual stresses were characterized by tensile stress in the joint of the bolt shank and bolt head, the joint of bolt shank and screw thread in cold headed bolt and the residual stress increased within the depth in the scope of 0.02~0.2 mm in the direction of radius of the bolt. The central position of bolt shank existed smaller residual compressive stress. After stress relief annealing, the residual stress value reduced obviously. SEM results showed that the structure of the bolt in no change before and after the stress relief annealing. High temperature annealing can strengthen the space movement and rearrangement of dislocation, reduced the vacancy and dislocation density, eliminated in the crystal lattice distortion within the structure.The risks of the residual stress existed in the service of cold headed bolt will be reduced after stress relief annealing.Before stress relief annealing, the axial and tangential sub-surface residual stresses were characterized by compress stress in the joint of bolt shank and bolt head in hot forged bolt. The reason is that the temperature of hot forged bolt in head forming process was beyond the recrystallization temperature. The residual stress of bolt was result from interactions among heat and deformation. After stress relief annealing, the part of residual stress was reduced.The mechanism and rule of hot-dip galvanized and hot-dip Zn-5Al on residual stress in sub-surface of bolt shank were similar. In the process of hot-dip galvanizing, the reaction between iron and zinc and the formation of the Fe-Zn alloy layer on the surface of the bolt may generate internal stress, which made the residual stress in the joint of bolt shank and bolt threads change obviously and the residual stress showed a large of compressive stress. The stress relief annealing before hot-dip galvanizing has little effect on the residual stress of the galvanized bolt.When the bolt material was hot-dip galvanized at 460℃, a compact and continuous coating was formed and consisted of ?, δ, δ and the outmost layer of ε phase.When the temperature was 520℃, the coating was consisted of ?, δ, δ, without ε, and δ phase was grown rapidly. When the temperature was 580℃,the thin coating was consisted of Г, δ, ε, without δ. When bolt material was dipping in Zn-5Al, the coating was thin and contained two layers: the inner was intermetallic compound of Fe-Zn-Al and the outer was Zn-Al eutectic organization. Compared to hot-dip galvanized coating, the corrosion resistance of hot-dip Zn-5Al coating was better and the technology of hot-dip Zn-5Al will be a good alternative technology of hot- dip galvanining of the bolt.