What are them?
Preheating before welding and post-welding heat treatment are thermal processes that can improve the quality and integrity of welds. They are often required for welding thick or high-strength materials, or for applications that need to prevent cracking or distortion.
Preheating involves heating the base metal around the weld joint or the entire part to a specified temperature before welding. This reduces the cooling rate of the weld and drives out moisture, which helps prevent hydrogen buildup and cracking. Preheating can also reduce residual stresses and distortion in the weldment.
Post-welding heat treatment (PWHT) involves heating the weldment or the entire part to a specified temperature after welding and holding it for a certain time. This allows the weld metal and the heat-affected zone (HAZ) to undergo metallurgical changes that can improve the mechanical properties, ductility, and toughness of the weld. PWHT can also relieve residual stresses and reduce the risk of brittle fracture.
There are different methods of applying heat for preheating and PWHT, such as induction, open flame, resistance heating, and convection ovens. Each method has its advantages and disadvantages depending on the application, the material, the size and shape of the part, the required temperature and duration, and the available equipment and personnel.
The need for preheating and PWHT depends on several factors, such as the type and thickness of the base material, the welding process and parameters, the service conditions, and the welding code or standard. The welding procedure specification (WPS) for the job will outline the minimum and maximum preheat temperatures, the interpass temperature, the PWHT temperature and duration, and the temperature verification methods.
Why use induction heating for them?
- It is fast and efficient, reaching the desired temperature in minutes.
- It produces a uniform and consistent heat distribution, avoiding hot spots or cold spots.
- It is safe and clean, as it does not generate sparks, flames, or smoke.
- It is flexible and versatile, as it can be used for different applications and materials.
What are the applications?
- Welding thick or high-strength steel pipes or plates that require a large temperature range to prevent cracking or distortion.
- Welding flat plates or other part geometries that need to be heated uniformly to avoid thermal stresses or warping.
- Welding shrink-fit components that need to be heated quickly to avoid deformation or damage.
- Welding coated or painted parts that need to be removed before welding.
