Processing Difficulties of Power Tower Bolts

At present, the common strength grades of bolts used in power towers in China are 4.8, 6.8, 8.8, and 10.9. Among them, 6.8 grade and 8.8 grade are the most used. According to the processing technology, there are two main categories: cold heading forming and warm forging forming. There are also differences in material applications. For grade 4.8 bolts, low carbon steel Q215, Q235, ML08AL and 1010 are generally used. 6.8, 8.8 grade bolts, due to the supply situation of the material market, generally use SWRCH35K, 35, ML35 steel, etc.; There are 8.8 grade bolts if warm forging forming, using SWRCH45K, 45 steel, etc. The 10.9 grade cold heading forming or warm forging forming bolts, most of which are made of 40Cr, 35CrMo or SCM435 steel.

The rationality of bolt processing of power tower is the basis of ensuring the quality of bolt products. So far, the power tower bolt U-turn accidents have occurred, now on the product processing process processing difficulties and the resulting stress analysis is as follows.

1. Cold heading forming process

① Residual stress due to cold work hardening during cold drawing of raw materials.
The head cold forming process due to the unreasonable forming process parameters, resulting in excessive folding of metal fibers, or even broken and the formation of residual stress.
③ Residual stress due to cold hardening in the process of reducing diameter and thread extrusion forming.
④ Residual stress of high strength bolts above grade 8.8 after heat treatment.

2, Warm forging forming process

① Residual stress due to cold work hardening during cold drawing of raw materials.
The local material in a short period of time within a few seconds of rapid heating (900 ℃ or more), but also includes the possibility of over-burning or under-heating phenomenon, and the formation of residual stress.
③ Residual stress generated at the junction of heated and unheated parts.
④ Residual stress due to cold hardening in the process of reducing diameter and thread extrusion forming.
⑤ Residual stress caused by excessive folding of metal fiber due to unreasonable forming process parameters in the process of head forming.
⑥ Residual stress of high strength bolts above grade 8.8 after heat treatment.

3. Fracture shape

① There is a large residual stress, resulting in brittle fracture. Basically perpendicular to the bolt axis, the tensile yield deformation is small. The fracture diameter basically has no surface shrinkage phenomenon, the fracture position is all located at the root of the thread, and the fracture is flush, the grain is coarse, and the fracture surface morphology belongs to brittle fracture.
② normal tensile fracture, fracture when there is obvious extension and section shrinkage phenomenon, the fracture position in the thread, the fracture surface and the bolt axis into a certain angle, the fracture along the thread V-shaped groove fracture, the fracture surface morphology belongs to ductile fracture.

4. Quality defects

① Due to defects such as impurities and fine folding cracks in the raw materials, and unreasonable processing technology (such as eliminating the need for spheroidizing annealing or incorrect parameters of the spheroidizing annealing process), the cold hardening degree of the product is serious, resulting in quality defects such as bolt head bursting.

The head forming process parameters are unreasonable, and the metal fiber of the head is over-folded or even broken during the forming process of the product, resulting in stress concentration. Serious quality problems caused by bolt U-turn.

③ Also due to the unreasonable process parameters of the bolt, the metal fiber is over-folded or even broken during the head forming process and the unreasonable heat treatment process, resulting in quenching cracking when the high-strength bolt is tempered.

In short, the existence of residual stress in the process of power tower bolts is the main cause of quality problems of bolts, especially in the actual use process, due to the tension of transmission line cables, wind pendulum and other effects, resulting in uneven local stress of bolts, resulting in early failure of bolts, and even sudden brittle fracture, there is a greater security risk. More and more attention has been paid to the research on the importance of eliminating residual stress in the process of power tower bolts. For example, the American standard ASTMA394-2005 steel galvanized transmission and transformation tower bolts and nuts and the German standard DIN7990-2008 hexagon bolts with hexagon nuts for steel structures all stipulate that the strength grade of bolts is 4.8 and 6.8, that is, the power tower bolts (strength grade below 8.8) before hot galvanizing need to be de-stressed and annealed to reduce and eliminate residual stress, improve product toughness. After the cold heading is completed, the stress relief annealing is generally cooled slowly at 550-650 ℃ for 2-4h.