Can we replace heat treatment PWHT by preheating?

Can we replace heat treatment PWHT by preheating?

Preheating is a commonly used technique in welding to reduce the cooling rate of the weld and surrounding base metal. It helps to minimize the risk of cracking and improve the overall weld quality. However, preheating alone is not a replacement for Post Weld Heat Treatment (PWHT) in all cases.

PWHT involves heating the welded component to a specific temperature and holding it at that temperature for a specified period of time, followed by controlled cooling. It is typically used to relieve residual stresses, improve toughness, and reduce the risk of hydrogen-induced cracking in high-strength materials or critical weldments. PWHT is often required by industry codes and standards for specific materials and applications.

Preheating, on the other hand, involves heating the base metal before welding to a temperature below the critical transformation temperature. It helps to slow down the cooling rate during welding, reducing the risk of cracking. Preheating is particularly beneficial for materials that are prone to hydrogen cracking or have high carbon equivalent values.

While preheating can provide some benefits in terms of reducing residual stresses and minimizing cracking, it does not provide the same level of control and uniformity as PWHT. PWHT involves specific time-temperature cycles, which are designed to achieve desired metallurgical and mechanical properties in the welded joint. It is particularly important for materials with specific heat treatment requirements, such as high-strength steels or certain alloy systems.

Does ASME Codes allow replacing PWHT by preheating?

The concept of replacing PWHT with preheating involves utilizing preheating of the weld joint and maintaining the temperature throughout the welding process.

This approach aims to achieve adequate grain refinement, thereby reducing the likelihood of cracking during the cooling phase after welding. However, it is important to note that while this technique provides some level of grain refinement, it falls short of the comprehensive benefits offered by full PWHT.

As a result, it is typically applicable to materials categorized as P1 (Grade 1, 2, 3) and P3 (Grade 1, 2), which inherently possess a low risk of cracking due to their low carbon content. However, materials like P2 Grade 2 steels that contain manganese and molybdenum are excluded from this method, as they have a higher susceptibility to cracking.

API 510 allows replacing PWHT by preheating using two approaches as described below: