thank you for your response...meaning i have to stop pre-heating/welding after completion of the root pass and cool down for RT, and continue pre-heating /welding again until the completion of the entier weld. i wonder will it affect the quality /property of the weld and base metal as this is a P91 material??
Are you sure this is what your contract technical documents are stating? It is usually considered bad engineering practice to leave a weld such as that just in it's rooted state.
I have been in similar situations where client requirements meant X-ray/Gamma of root but only when the weld was say, a third full.
Dilution of the root weld metal and the strain/lack of thru thickness metal, Hydrogen, removal/application of pre-heat etc would mean your root deposit would most likely crack.Even more so when using P91 material.
You might want to take another look at your spec. Cooling down prior to 25% or 3/8" fill is a code violation(assuming B31.1).
And it can definately be problematic. It may contribute to the very thing they are trying to prevent, cracking. IMO its stupid.
If you want to shoot 'chip outs' as we called them (and I have done it many times on heavy walls) make sure you have code fill first.
However, if it doesn't crack it may actually help improve properties since you will have a complete martensitic transformation to then be tempered with subsequent beads. But there is generally too much stress on roots in the hardened condition to cool to room temp.
One other thing, if you shoot a chip out do not move the assembly. This may cause a problem with RT and a shop full of people especially if you're using Co (I'd use Ir even though it may take longer), but don't move the assembly. Shoot it in place.
Also, finish the weld as soon as possible and keep it dry and warm or you may experience yet another cracking issue, Stress Corrosion Cracking.
Preheating is typically performed at 200 °C (400 °F) minimum for all welding processes except GTAW. Lower preheat
temperature of 149 °C (300 °F) is acceptable for root pass and thin wall components where GTAW is utilized [37, 42].
Other references indicated the need for higher preheat temperature in the range of 200 °C to 250 °C (392 °F to
482 °F) to prevent cracking in Grade 91 welds with the lower temperatures are appropriate for thinner sections and
the higher temperatures are applicable to thicker sections [60, 61]. The metal temperature is typically maintained at
the preheat temperature until the weld is completed. It is prudent to use electrical resistance or electrical induction to
provide better temperature control and heat distribution for preheating [42].
API TR 938 B
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