Sandip,
PWHT is a broad based term which applies to all welds. For C.S and LAS the PWHT's involved in common fabrication are :-
- Normalizing -- the heating temperature is typically above upper critical temperature AC3
- Annealing - heating temperature is typically close to lower critical temperature AC1
- Stress relieving- most common for all fabrications in C.S and LAS. Heating temperature is well below the subcritical temperature. Look at UCS-56 for the recommended temperature ranges for various C.A and LAS.
The first two heat treatments causes changes of microstructures and while the Stress Relieving does not.
The intent of any is PHD is to improve resistance to brittle fracture. Stress Relieving in particular attempts to improve notch toughness and relax residual stress.
Residual stresses are generated due continuous heating and cooling cycle involved during welding. It is the locked up stress within the weld/HAZ, which is usually tensile in nature. The sheer volume of the weld metal adds up to this residual stress. Lesser the volume of weld , lower is the residual stress, example Narrow Gap bevel vs. conventional bevel. Residual stress is primarily caused by the compressive yielding that occurs around the molten zone as the material heats and expands during welding.
Lower heat input or control of interpass temperature or balanced welding etc all are factored to reduce residual stress in the weld.
Hope the above answer clarifies your doubt, which is the very basic of any post weld heat treatment.
Thanks.
Pradip Goswami, P.Eng,IWE
Welding & Metallurgical Specialist
Ontario, Canada
pgoswami@quickclic.net
pgoswami@sympatico.ca
-----Original Message-----
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of sandeep monody
Sent: Friday, June 28, 2013 12:26 PM
To: materials-welding@googlegroups.com
Subject: [MW:18041] metallurgy of weld joint PWHT
Dear experts.,
Recently I came to know that PWHT does not change the micro structure of weld seam or HAZ.
Please reply me regarding metallurgy of PWHT of weld seam and HAZ in C.S and low alloy steel.
Please clarify why PWHT required for C.S and low alloy steel.
What is residual stress?
How do we remove residual stress during PWHT?
Sandeep.M.V
QA/QC Engineer
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