About welding speed and quality

We should understand it dialectically, which is mainly reflected in the heating stage and crystallization stage.

Heating stage

In the working state, the billet edge of high frequency longitudinal welded pipe is heated from room temperature to welding temperature.During this period, the edge of the tube blank is completely exposed to the air without any protection, which is inevitably related too xygen and nitrogen in the air. Violent reactions occurred,significantly increasing nitrogen and oxides in the weld. According to the measurement, the oxygen content in the weld increases by 7 ~35 times and the nitrogen content increases by 20 ~ 45 times; At the same time, it is conducive to welding. A large number of manganese,carbon and other alloy elements are burned out and evaporated, which reduces the mechanical properties of the weld. It can be seen that the slower the welding speed, the worse the welding quality.

Moreover,the slower the welding speed, the longer the edge of the heated tube blank is exposed to the air, which will lead to the generation of non-metallic oxides in the deeper layer. These deep non-metallic oxides will be difficult to be used in the subsequent extrusion and crystallization process. After fully extruded from the weld,non-metallic inclusions will remain in the weld after crystallization, which will reduce the strength of the weld, destroy the continuity of the weld structure and form an obvious brittle interface. The welding speed is fast, so the oxidation time is short,and a small amount of non-metallic oxide is produced, which only exists in the surface layer. In the subsequent extrusion process, it is easy to be extruded from the weld, and there will not be too many non-metallic in the weld. Oxide residue, high welding strength.

Crystallization stage

In order to obtain high-strength weld, according to the principle ofmetallography, the grain of weld structure must be refined as much as possible, and the way of refinement is to produce enough nuclei in a short time. When they have not grown significantly, they contact each other and end the crystallization process. By accelerating the welding speed, the weld can leave the heating zone quickly and crystallize rapidly under the condition of high under cooling; When the under cooling degree increases, the growth rate increases less, and the nucleation rate can increase greatly, so as to refine the weld grain.

Therefore,under the basic welding conditions, the faster the welding speed, the better the quality.