Post Weld Heat Treatment is a type of thermal treatment which is employed to material primarily after welding. This technique is a common
practice, which is exercised in industries to adjust the mechanical properties
of weldment. The required welded material is allowed to heat, usually at
lower temperature in order to prevent any phase change. The intended use of
post weld heat treatment is;
Great care is required in selection of temperature required for weld-able steels because due to wrong adjustment of parameters, the weld outcome may suffer one of the following problems;
- to relieve unwanted level of residual stresses
- to soften the hard weldment.
Great care is required in selection of temperature required for weld-able steels because due to wrong adjustment of parameters, the weld outcome may suffer one of the following problems;
- Phase Change likely to occur
- Material gets more harder
- Depletion of weld strength may happen
Below is the description of primary advantages that make post
weld thermal treatment an exceptional process in welding industries.
1. Softening
After welding, the material (particularly HAZ) gets harder
and excess hardness is nearly useless for a particular service application. For
this PWHT is the best tool, primarily used by many industries, in order to temper
the hard heat affected zone. This implies strong protection and provides adequate resistance against
brittle fracture failure during service.
2. Stress Relieving
Residual stresses during welding may occur due to high thermal gradient. During thermal cycle of welding process, expansion cause to
happen in some areas due to heat while in cooling, also the localized contraction may occur, this whole produces undesirable residual stresses. In order to reduce the level these
stresses, thermal treatment is applied which involves the baking of material
(by uniform heating) at specific temperature and then brought back to ambient
temperature at particular cooling rate. See Thermal Cycle of Post Weld Heat treatment.
3. Prevention of SCC
What is SCC? Stress corrosion cracking is referred to as SCC
which occurs due to the intensification of present cracks in the material due to applied stress or load. PWHT facilitates the prevention of stress corrosion cracking.
4. Ease in machining
Well it is stated earlier that post weld thermal treatment
tends to reduce the degree of hardness, due to which the welded components can
easily be machined to required dimensional conditions. Softening of welded
parts allow easy grinding, machining and other surface treatments.
5. Prevention of brittle fracture
The primary role of post weld heat treatment is to improve
the mechanical characteristics of weldment and make it outlast for particular
service conditions. Again what is demonstrated in above paragraphs; PWHT helps
to reduce the residual stress level and hardness, imparting the required level
of ductility in the weldment. This improves the strength and resistance to
brittle fracture and increase the lifespan of material for its service
conditions.
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Post weld heat treatment is a type of thermal treatment
which is applied to weld-able materials (primarily steels) after welding and
joining. In this technique the material is heated below the lower critical
temperature line (where no phase change occurs) and then allow to furnace cool with
controlled monitoring by using thermo-couples. There are some exceptional cases
when the welded joints are required to get normalized, for the treatment of
heat affected zone but normalizing is not a recommended practice and should be
employed with strict precautions, when there is no other option left.