What are them?
Preheating before welding and postweld heat treatment are heat treatment processes that can improve the quality, mechanical properties and integrity of welds. They are typically used for welding thick or high-strength materials, or in applications where protection from cracking or deformation is required.
Preheating involves heating the base metal around the weld or the entire component to a specified temperature before welding. This reduces the cooling rate of the weld and drives out moisture, helping to prevent hydrogen build-up and cracking. Preheating can also reduce residual stress and distortion in the weldment. Normally, preheating before welding will heat the groove and the 100 mm area at both ends of the groove to a predetermined temperature. For thick plates or materials with a greater tendency to harden, the role of preheating is particularly significant. In actual operation, the preheating temperature will be adjusted according to the thickness, volume, and other factors of the weldment to ensure temperature stability and weld quality during the welding process.
Preheating before welding helps improve the mechanical properties and corrosion resistance of welded joints, and is crucial to ensuring welding quality and safety.
Post-weld heat treatment (PWHT) involves heating the weldment or the entire component to a specified temperature for some time after welding, to improve the mechanical properties and corrosion resistance of the material by changing its crystal structure. This allows for metallurgical changes in the weld metal and heat-affected zone (HAZ) that can improve the mechanical properties, ductility, and toughness of the weld. PWHT also relieves residual stresses and reduces the risk of brittle fracture.
Preheating and PWHT There are different heating methods such as induction, open flame, resistance heating, and convection ovens. Each method has its advantages and disadvantages, depending on the application, materials, size and shape of the part, temperature and duration required, and available equipment and personnel.
The necessity of preheating and PWHT depends on several factors, such as the type and thickness of the base material, welding process, and parameters, service conditions, and welding codes or standards. The Welding Procedure Specification (WPS) for this job will outline minimum and maximum preheat temperatures, interpass temperatures, PWHT temperatures and durations, and 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.
