Preheat & PWHT Practice for Tube-Header Weld
Preheat & PWHT Practice for Tube-Header Weld
(OP)
I would like to know what is 'best practice' for repair of a single (or possibly several) tube-to-header welds.
I recently observed from a distance (I was only involved in the tube F/A) as a contractor proceeded to preheat only the end of the 12"Ø header where the repair was to be done, and with a propane torch. The repaired tube happened to be the last one, middle row; CSEF tube to conventional 2¼Cr-Mo header.
Firstly, I did not agree with oxyfuel preheating and temperature crayon monitoring - I thought now that we are in the 3rd millenium it would have been electric pads with TC control.
Second, I had doubts about thermal stresses resulting from PWHT only locally. I should think you would want to heat things up symmetrically.
Any experiences/practices anybody can share?
I recently observed from a distance (I was only involved in the tube F/A) as a contractor proceeded to preheat only the end of the 12"Ø header where the repair was to be done, and with a propane torch. The repaired tube happened to be the last one, middle row; CSEF tube to conventional 2¼Cr-Mo header.
Firstly, I did not agree with oxyfuel preheating and temperature crayon monitoring - I thought now that we are in the 3rd millenium it would have been electric pads with TC control.
Second, I had doubts about thermal stresses resulting from PWHT only locally. I should think you would want to heat things up symmetrically.
Any experiences/practices anybody can share?





RE: Preheat & PWHT Practice for Tube-Header Weld
For conventional Cr-Mo alloys joining a terminal tube to a header, local PWHT can be done with proper controls over the heating band and soaking band.
RE: Preheat & PWHT Practice for Tube-Header Weld
Agree with your thoughts and those of metengr. Do expect to make additional repairs to this repair! Oxy-fuel gas preheating has been shown to overheat the (assumed T91) tube. We have seen evidence whereby the T91 was heated above its lower critical temp. As a result, we prohibit its use for preheating T91/P91 weldments.
We would require electric resistance heating pads for preheating. While we would locally PWHT a single or a few repair welds near one another, we would control the temperature along the header to mitigate thermal stressing. A detailed PWHT procedure would be provided.
RE: Preheat & PWHT Practice for Tube-Header Weld
In the situation where only one end of the header is repaired, to what temperature would you PWHT the header; i.e., the rest of the header?
RE: Preheat & PWHT Practice for Tube-Header Weld
RE: Preheat & PWHT Practice for Tube-Header Weld
RE: Preheat & PWHT Practice for Tube-Header Weld
RE: Preheat & PWHT Practice for Tube-Header Weld
RE: Preheat & PWHT Practice for Tube-Header Weld
RE: Preheat & PWHT Practice for Tube-Header Weld
While I have not had the problem on T23 to P22 weld repairs, I had a very real problem when torch preheatinhg was done on some T22 to P22 tube to header welds. The hardness was near 350 BHN in the weld metal and P22 HAZ. After a leak was discovered during startup at a location not subject to the repair, the operator water quench cooled the system (thermal shock). From the failure investigation of the many cracked tube to header welds, it was believed that the actual PWHT hold temp was less than 1100F.
RE: Preheat & PWHT Practice for Tube-Header Weld
metengr - what form does the support take, simply some sturdy blocks between the header and the floor, or something more?
p.s., a tendency I notice with those who know 'a little' about welding metallurgy, but especially with heat treaters, is they feel PWHT of steel covers a multitude of sins and corrects all ills, especially incorrect preheat/interpass.
RE: Preheat & PWHT Practice for Tube-Header Weld
RE: Preheat & PWHT Practice for Tube-Header Weld
You can only check the weld hardness unfortunately and not the HAZ, but that should still give some useful indication of PWHT effectiveness.
On the subject of repair, would you guys advise surface grinding of the weld before PWHT to eliminate any sharp stress raisers? I am guessing this is not usual practice (in this case it is 100% GTAW and the weld profiles generally look pretty good.
Thanks again.