The importance of preheating welded pipe before welding

The importance of preheating welded pipe before welding

Preheating refers to heating the workpiece to be welded to room temperature before or during welding. According to the application standard of materials, several preheating temperature levels are usually required in the current specification.

Preheating is required by pre welding and post welding specifications. However, other preheating methods can be used under certain conditions. Preheating has the following advantages regardless of whether preheating is required.

1.Reduce the shrinkage stress between the weld and the adjacent base metal, suitable for the weld with high stress value.

2. Slow down the cooling rate of the weld in the critical temperature range, prevent excessive hardening, and reduce the ductility of the weld and heat affected zone.

3.At 400 ° F temperature range, slow down the cooling rate, let hydrogen escape from the weld, prevent hydrogen induced cracks.

4.Remove pollutants.

The preheating temperature range is usually suitable for various weld groove sizes and constraints. Although the minimum preheating temperature is specified, a lower preheating temperature and a higher preheating temperature will be used in some cases.

We should also pay attention to the practical operation skills to avoid the softening of materials caused by preheating. We should select the welding process and welding rod that rarely introduce hydrogen. Some technologies can reduce or reduce the residual stress. Careful monitoring can ensure the correct use of preheating methods. The following description is very important for the successful application of these skills.

Size and skill of welding groove

The skills in welding process have great influence on welding shrinkage, residual stress, heat input control and crack avoidance. Compared with long weld, the longitudinal shrinkage of short weld is smaller. Backhand welding or special welding sequence can also be used to reduce residual stress and control or reduce heat input.

Reduce cracks

Craters and welding cracks can be reduced or removed by using proper manufacturing process.

The welding seam of circular section has less cracks than that of thin and wide section. Sudden welding or stop should be avoided. The welding seam formation is controlled by the up and down slope welding technology or the electrical method of welding power supply. There should be enough deposited materials to avoid welding shrinkage or cracks caused by normal welding.

Preheating method

Flame heating, electronic induction heating, resistance heating and other methods can be used for preheating in workshop or field. No matter what method is used, the preheating should be uniform. Unless there are special requirements, the preheating needs to penetrate the whole thickness of the weldment.