& STRESS RELIEF
An Overview of Stress Relief & Heat Treatment Processes
Post-weld heat treat is used to relax the weld and adjacent base metal. This prevents the materials from becoming brittle and increases resistance to stress corrosion cracking and hydrogen induced cracking, which can be caused by high levels of hydrogen released during the welding process.
After welding, the material (particularly HAZ) gets harder, and excess hardness is nearly useless for a particular service application. Post weld heat treatment (PWHT) is the best tool to avoid this and is primarily used by many industries to temper the hard heat affected zone. It builds strong protection and provides adequate resistance against brittle fracture failure during service.
- Stress relieving
Residual stresses during welding may occur due to high thermal gradient. During the thermal cycle of the welding process, expansion happens in some areas while cooling and a localized contraction may occur. Both of these can produce undesirable residual stresses. In order to reduce the level these stresses, thermal treatment is applied, which involves baking the material (by uniform heating) at a specific temperature and then brought back to ambient temperature at a particular cooling rate.
- Prevention of stress corrosion cracking (SCC)
PWHT helps prevent stress corrosion cracking. Stress corrosion cracking is referred to as SCC, which occurs when present cracks intensify in the material due to applied stress or load.
- Ease in machining
Post weld thermal treatment tends to reduce the degree of hardness, so the welded components can easily be machined to required dimensional conditions. Softening welded parts allows for easy grinding, machining and other surface treatments.
- Prevention of brittle fracture
The primary role of post weld heat treatment is to improve the weldment’s mechanical characteristics and make it outlast for particular service conditions. PWHT helps to reduce the residual stress level and hardness, achieving the required level of ductility in the weldments. This improves strength and resistance to brittle fracture and increases the material’s lifespan.