Hot-dip galvanized bolts are common components of transmission line towers and electric power fittings. The quality and performance of the hot-dip galvanized bolts play an important role in the safe operation of the line. Custom Deep Drawing Parts,Sheet Metal Stamping Anti Skidding,Metal Deep Drawing Cap,Deep Drawing Sheet Metal JIANGSU TONGDE INTERNATIONAL TRADE CO.LTD. , https://www.tongdetrade.com
During the operation of the transmission line tower, due to the heavy load or the action of the wind, the hot-dip galvanized bolts for fastening are not balanced. When the situation is serious, the bolts may even break and break, which will seriously threaten the transmission line. safe operation. The collapse of the transmission line tower facilities caused by the snow and ice disaster at the beginning of 2008 is largely related to the brittle and broken ends of the hot-dip galvanized bolts used for fastening.
Hot dip galvanized bolt current treatment
At present, the processing technology of bolts for transmission line towers is roughly divided into two types: one is a cold head forming processing process, and the other is a local heating hot forging forming process.
The cold heading forming process means that after cold drawing, the cold heading steel is directly subjected to upsetting, extruding, trimming, and reducing the diameter on the cold heading machine, and then adopting a thread rolling method to form the thread. The local heating hot forging process is that the material is cut into a material section after cold drawing, and is locally heated at a high temperature of 900 degrees Celsius to 1100 degrees Celsius on a high frequency machine, upset into a hexagonal head, and the rod portion is further subjected to cold working to reduce the diameter. Thread rolling processing.
The original process of heating hot forging is to heat the entire section of the coal in the furnace. Because this process can not meet the environmental protection requirements, and the product quality is difficult to control, the production efficiency is too low, and the cost is too high. Therefore, the processing technology adopted by the electric power industry bolt manufacturer is basically local heating and cold rolling forming. Ways.
It is worth noting that many manufacturers' bolt products are not subjected to stress relief annealing after being processed in the above two ways.
When the processed bolts are not subjected to stress relief annealing, the product quality parameters are as follows.
Rockwell hardness. The Rockwell hardness of the cold-formed bolt head exceeds the standard, and the hardness of the core of the locally heated molded bolt exceeds the standard. The reason is that the forging causes the plasticity of the material and the grain deformation to increase the hardness.
Stretching test. The bolts processed by the two processes are subjected to the tensile test, and the brittle fracture occurs when the maximum load is applied. The yield deformation is small, the extension and fracture shrinkage are not obvious, the fracture is flush, and the fracture position is at the root of the thread, and the fracture property is brittle fracture.
Necessity of performing stress relief annealing The above two processing processes are formed by cold extrusion, upsetting, and cold reduction. However, cold working causes the material to be plastically deformed, and the material structure and properties are unevenly changed. Due to the uneven deformation of the various parts, the bolts will generate residual internal stress. The existence of the internal stress of the bolt has the characteristic of time-delayed damage. Even if the bolt is installed and used, under the tensile stress below the yield strength (such as wind swing, pressure, low temperature), after a period of time, it may still occur due to fatigue. Sudden breakage, there is a big security risk.
After analyzing the residual stresses of the products processed by the above two processes, the results obtained are as follows.
1. Cold heading processing technology. The residual stresses include: residual stress formed by cold work hardening during cold drawing of raw materials; residual stress formed by folding of cold formed metal fibers during forming of bolt head; reduction in diameter and thread extrusion Residual stress due to cold work hardening.
2. Local heating hot forging process. Residual stresses present include: residual stresses due to cold hardening during cold drawing of raw materials; some materials are rapidly heated in a few seconds (above 900 degrees Celsius), thermoformed in the head, possible over-burning or Residual stress formed by under-burning; residual stress generated by the junction between heated and unheated; residual stress formed by cold hardening during shrinkage and thread extrusion.
To this end, the cold-worked bolts must be subjected to a stress-relieving annealing process to eliminate residual stress, restore the internal structure to equilibrium, adjust bolt strength and hardness, enhance bolt toughness, and obtain good mechanical properties (such as tensile and bending resistance, Shearing, anti-fatigue, etc.), and ultimately achieve the purpose of improving the life of the bolt.
American Standard ASTM A394-05 "Steel Galvanized Transmission and Transformation Tower Bolts and Nuts" stipulates that stress relief annealing must be performed before bolts are galvanized. In the German standard DIN 7990 "Hexagonal bolts and nuts for steel frame splicing", the bolts are also rated at 4.6 and 5.6. These standards have specified that the power tower bolts (with a strength rating below 8.8) before hot dip galvanizing must be subjected to stress relief annealing.
In order to improve the quality of hot-dip galvanized bolts and ensure the safe operation of transmission line towers, Guangdong Zisheng Power Equipment Co., Ltd. began to de-stress the annealing of galvanized bolts with strength grades below 8.8 since February 2008. . In the past year, the company's de-assured annealing bolts have not broken bolts during the installation and operation of the tower.
After the tensile test, all the index parameters of the stress relief annealing bolt can meet the standard requirements. There is obvious extension and section shrinkage during tensile fracture. The fracture breaks normally along the U-shaped groove, and the fracture is judged from the shape of the fracture surface. It belongs to normal ductile fracture.
Therefore, the author suggests that in order to ensure the long-term safe operation of the transmission line tower under dynamic load, all the hot-dip galvanized bolts below 8.8 for the iron tower should adopt the stress-relieving annealing process. (Chen Dingzhen)