technology

During the fabrication process, welding is the most commonly used method of joining items together. The welding processgenerally involves melting and subsequentcooling, and the result of this thermal cycleis distortion if the welded item is free tomove, or residual stress if the item issecurely held. There comes a point when theamount of residual stress can create potential problems, either immediately or during thelife of the welded structure, and it needs to be reduced or removed. Post weld heattreatment is the most widely used form of stress relieving on completion of fabrication of welded structures. The principle is that as the temperature is raised, the yield stress andthe elastic modulus of the material fall. A point is reached when the yield stress no longer supports the residual stresses and some localised plastic deformation occurs.

Advantages of Post Weld Heat Treatment – Stress Relieving

Metallurgical structure is improved.
A much greater dimensional stability is obtained and maintained
The potential of stress induce cracking is reduced.
Strength of the Material and Life of the equipment is enhanced
Reduction in the risk of brittle fracture can be achieved.

WHY DO WE NEED THE POST WELD HEAT TREATMENT?

Residual Stress
The development of residual stresses approaching or even exceeding the yieldstress is possible when welding thick sections. For certain industry sectors, eg.Petrochemical,Chemical, Oil and Gas, etc.the existence of residual stress of this magnitude is completely unacceptable. With pressure equipment operating at 200 degree and below a variety of stress corrosion cracking mechanisms under the general term“environmental cracking” become prevalent.There is also the problem of fatigue to be considered and the effect that residual tensile stresses have in that regard.

Tempering Effect
Post weld heat treatment will generally result in a modification of the microstructure of both the weld metal and heat affected zone. With the exception of the 9Cr1Mo and12Cr1MoV materials, the microstructure of all other materials should contain a mixture of ferrite and iron or alloy carbide. The effect of short-term (1 to 2 hours) post weldheat treatment on the carbide is generally beneficial, whereas longer times result in areduction in toughness due to spheroidising effects. The normal microstructure for the parent, weld and HAZ for the 9Cr1Mo and12Cr1MoV materials is martensite, and postweld heat treatment is absolutely essential inthese materials to temper the martensite phase.

Effect on Mechanical Properties
As a series of very general statements, the following are the consequences of post weldheat treatment compared with the as-weldedcondition:
Yield strength is decreased slightly, the effect falling off fairly rapidly with time.
The tensile strength is decreased.
The ductility is increased.
Hardness levels are reduced.
Toughness is slightly reduced at shorttimes but the effect can be significant over longer times

Effect on Creep Properties
For creep resisting material, post weld heattreatment is required in order to fully develop the creep strength. This is especially true for thicker components such as headers.There has been a tendency in recent years to allow waiving of the post weld heat treatment stage for thinner materials used typically for superheater and reheater coilsin the Power Generation industry, but avariety of conditions have to be met.

Other benefits
Improving the diffusion of hydrogen out of weld metal
Softening the heat affected zone and thusimproving toughness (although not weldmetal toughness)
Improving dimensional stability duringmachining.
Improving ductility.
Improving the resistance to stresscorrosion cracking.
Reducing the effects of cold work.