Precautions for on-site welding of steel structure bolt balls

1. When it comes to welding, everyone is very worried. The welding of the ball handle is carried out in the factory, and it is the welding of the same base material, which is carried out according to the conventional process, which is no problem. On-site welding, such as the welding of the connecting plates of the rods, the welding of supports such as roofs, carriages, light bridges, and the welding of reinforcement parts when the grid is reinforced, etc., are welded after the grid is installed or during the use of the grid.
There are three concerns about on-site welding: first, whether the internal force will change after the grid components are welded and heated, resulting in structural instability and damage; second, whether the welding of different base materials of Q235B steel plate and 45 #steel bolt ball is possible; third, after the welding of components heats up, especially on the bolt ball, will the welding cause high-strength bolt annealing and the strength will decrease.

2. The first thing to solve is the welding temperature problem. When welding, the welding current should be reduced, and small-sized electrodes should be used instead of centralized welding. The welding method of broken welding and symmetrical jump is adopted. The purpose is to prevent the welding temperature from being high and achieve sufficient cooling. Due to discontinuous welding, the temperature dissipates heat quickly, so it will not have much effect on the stress state of the grid components.

For example, when a pressure-receiving rod is reinforced, it is usually wrapped with a large-size steel pipe outside the rod, or two angle steels are held and welded (see Figure 4). The weld is symmetrical, and the weld is broken, separated by 30cm, and welded by 5cm. It is symmetrical, and the welding is swapped back and forth, so that the rod will not overheat and cause problems.

3. Can the connecting steel plate of Q235B be welded with the bolt ball of 45 #steel? 45 #steel is not easy to weld, but it does not mean that it cannot be welded. If the Q235B is still welded with E43 mild steel electrode, the current is AC and DC, positive and reverse connection. When the temperature is low, the sphere is not preheated. After welding, the sphere will definitely crack. A piece of the sphere can be torn off with a hammer!

The correct way is to use E50 type low hydrogen medium carbon steel electrode, DC reverse connection, ball preheating 200 ℃ after welding, pay attention to take measures to reduce the welding temperature and sufficient heat dissipation.

4. What everyone is most concerned about is whether welding on the bolt ball will cause the annealing of high-strength bolts in the ball and reduce the strength? First, find out what temperature the high-strength bolts reach to anneal. The critical temperature for annealing is different for high-strength bolts of different materials. The data provided by the technical department of Shanghai Jinma High-strength Fastener Co., Ltd. is of great reference value (see Table 1). The table lists the annealing critical temperatures of the three materials of high-strength bolts at the three strength levels.

It can be seen that the alloy structural steel material of 40 Cr is still very good. As long as we take corresponding technological measures to control the welding temperature, the strength of high-strength bolts will not be reduced.

Jiangsu Bafang quality assurance center was commissioned to do a test. Two high-strength bolts were screwed into two adjacent M20 holes with a bolt ball of Φ120, and the hexagonal sleeve and sealing plate were normally nested on the bolts, and then a 10mm thick steel plate was welded in the middle, and welded according to the above process. After cooling, the two high-strength bolts were sent to (Xuzhou) Jiangsu Grid and Steel Structure Product Quality Supervision and Inspection Center (now approved to be listed as "National Grid and Steel Structure Product Quality Supervision and Inspection Center") for testing.

After testing, the hardness of the screw part did not decrease; after the destructive tensile test, the destructive tensile force of the two bolts was 268KN and 297KN respectively. M20 high-strength bolt, grade 10.9, tensile ultimate bearing capacity is 255~ 304KN. Obviously, as long as the corresponding welding process is adopted, the sense of responsibility is strengthened, careful construction is carried out, and welding on the bolt ball will not anneal the high-strength bolts in the ball and reduce the strength.