Knowledge about preheating before welding

Update: 22-09-2021

Preheating method In the workshop or in the field, flam […]

Preheating method

In the workshop or in the field, flame heating (air fuel or acetylene fuel), resistance heating, electronic induction heating and other methods can be used for preheating. No matter what method is used, the preheating must be uniform. Unless there are special requirements, the preheating must penetrate the entire thickness of the weldment. Figure 1 is a device that uses resistance (no insulation, applied later) and induction heating.

Warm-up monitoring

Many devices can be used to measure and monitor temperature. The components or weldments to be welded should be preheated until the heat completely penetrates the material. If possible, the degree of thermal soaking should be tested or evaluated. Generally, for most welding applications, monitoring the temperature from a certain distance from the edge of the weld is sufficient. Must not cause the welding groove pollution caused by monitoring or reading the temperature value.

Temperature stylus

These stylus or pencil-like tools melt at a certain temperature value and can be used to simply and economically determine the minimum temperature reached by preheating, that is, the temperature at which the stylus melts. The disadvantage is that if the temperature of the workpiece is higher than the melting temperature of the stylus, it will not work. When the temperature of the workpiece is too high, it is necessary to use more stylus with different melting temperature.

Electronic temperature monitoring

For preheating and welding operations, some direct measurement devices such as contact pyrometers or direct reading thermocouples (with analog or digital readings) can also be used. All measuring equipment must be calibrated, or use some method to verify their ability to measure the temperature range. Because the thermocouple can continuously monitor and store data, it can use a curve recorder or data acquisition system for preheating or PWHT operation. AWS D10.10 provides a variety of solutions and examples of thermocouple placement.

"Indigenous Law" Monitoring

Many “indigenous methods” have been used for decades to determine whether the preheating temperature is sufficient. Of course, one is to spray water or smoke liquid directly on the workpiece. The size of the "sound" when the saliva is sprayed on is the temperature indicator. Although not very precise, many "veterans" use it.

Another more accurate way to determine the preheat temperature is to use an acetylene torch. The flame is adjusted to a high degree of carbonization, and a layer of smoky gray is gathered in the area that needs to be preheated. Then, adjust the welding torch to mid-smoke to heat the smoke-gray area. When the smoky gray disappears, the surface temperature can reach more than 400°F (200°C).

Make sure that the entire thickness of the workpiece and weldment area has reached the preheating temperature. Most monitoring is only for the outer surface of the workpiece. The recommended practice of AWS D10.10 is to provide useful guidance for the soaking zone. When the pipe is welded, the entire workpiece thickness is heated.

Care must be taken during preheating to avoid overheating of the preheated base material, especially when applying resistance heating or induction heating methods. Many shippers now require thermocouples to be placed under each resistance heating plate or induction coil assembly to monitor and avoid overheating.


Regardless of whether preheating is required, no matter what preheating method is used, preheating brings the following benefits: reducing the shrinkage stress of the weld and adjacent base materials, which is especially beneficial for highly constrained welded joints; slowing down the workpiece in the critical temperature range The cooling rate prevents excessive hardening of the workpiece and reduces the softening of the weld and HAZ; slows down the cooling rate when the workpiece passes through the 400°F (200°C) temperature range, so that hydrogen has more time to diffuse from the weld and adjacent base materials , To prevent hydrogen-induced cracks; to remove pollution; it is best to uniformly heat the entire welding thickness at the specified preheating temperature during preheating. Too much local heating may cause material damage, try to avoid it.


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