Heat Treating

Why Pre Heat

Welding Pre Heat can be defined as the process of raising the temperature of the parent material before welding begins. Preheating is applied to ferritic steels principally to reduce the probability of
Hydrogen Induced Cold Cracking (HICC).

Welding Pre Heat is used to slow down the cooling rate of the parent material. The welding Pre Heat allows more hydrogen to diffuse from the weld area by extending the time the weld is at an elevated temperature. The reduction in hydrogen reduces the risk of Hydrogen Induced Cold Cracking. The welding Pre Heat slows the cooling rate of the weld and local base metal, potentially resulting in weld metal and heat affected zone microstructures that are less hard.

Why Post Weld Heat Treat

Post Weld heat treatment (PWHT) is a post welding heating process. The PWHT process is used to improve the properties of the weldment. PWHT following the welding may be carried out for one or more of the following reasons:

• To achieve dimensional stability to maintain tolerances during machining operations.
• To produce specific metallurgical structures to achieve the required mechanical properties.
• To reduce the risk of in-service issues such as stress corrosion or brittle fracture by reducing the residual stress in the welded component.

Most austenitic stainless-steel weldments do not require post weld heat treatment (PWHT). However, heat treatment is sometimes used to improve corrosion resistance or to relieve stresses, or both. Corrosion issues associated with austenitic stainless steels welds often are localized in the Heat Affected Zone. If the service environments are known to attack sensitized areas containing intergranular carbides or if maximum corrosion resistance is required, a PWHT solution heat treatment at the annealing temperature can be used to redissolve the carbides.