As an addenda to assist, with overlays you can often increase the alloy content in the filler, which allows you to reduce dilution and increase the possibility of travel speed in order to decrease the heat input and therefore decrease the number of time you have to stop.
Pulsing with GTAW could also be something to consider.
js
I would respectively disagree with the comment that interpass temperature is "non relevent". There is a substantial useage of API 5L pipe (of various yield strengths) for critical service where weld overlay of the ID is done for corrosion control. The concept of interpass temperature is fairly straight forward, i.e., the metal temperature (either base or weld) at any position, immediately prior to the deposit of a subsequent weld pass. For circumferential overlays, adherence to a maximum interpass temperature may be more difficult than for overlay in the longitudinal direction, considering pipe diameter, wall thickness and overlay process. Continuous overlay may have the effect of raising the temperature of the base metal with each rotation of the pipe. Depending on pipe diameter/wall thickness, preheat, heat input of the welding process and chemistry of the pipe, the mechanical properties of the base metal may be degraded substantially without control of the interpass temperature.
Most welding specifications will take into consideration the use of the finished component and will dictate minimum mechanical properties (usually that of the pipe itself, i.e., X60, X65, etc) and maximum hardness allowed in the HAZ/base metal. As welded (overlayed) mechanical properties are dependent upon many factors, one of them being interpass temperature.
Fredspoppy,
No problem. I suppose some correction in my thought is due. Is the intent of API that a single rotation could, with the violation of the interpass maximum, be considered a 'pass'? Metallurgically and mechanically this is sound. Corrosively I think its problematic. For say, a 20' pipe thats a lot of stops and starts (locations of selective attack) on a surface exposed to a corrosive medium. It would almost make you wanna two pass everything to ensure you had adequate overlap on the stops and starts. Or use vision systems to ensure cleanliness and accuracy in start location.
It really would drive you to longitudinals instead. Which, corrosively would be more sound since you eliminate any mid pipe stops and starts. Its just that in my experience warping with longitudinals is more unpredictable.
My thoughts are really exploratory on this, since I've done a lot of ID corrosion overlays, both longitudinal and circumferential but never on anything requiring interpass control. It was always CS substrates. And in so many instances if pressure was problem they just increased wall thickness.
JS
Preferably, corrosion resistant overlays, done circumferentially, utilize a spiral welding technique. Travel of the welding head, down the ID of the pipe, is slaved to the rotation of the pipe so that there are no stops and starts. As an option, a step over technique is used where the travel of the welding head is indexed each time the pipe makes a complete revolution. I also would not recommend straight rotation of the pipe with a stop/start for each pass. You are correct.....lots of room for weld defects. The rotational method substantially reduces distortion concerns.
Fredspoppy,
You are exaclty right about indexing spiralling/rotation. I spent a little time with a company that specialized in pipe ID ovelays, both hardfacing and corrosion (the President of the company was the son of an old Linde guy that actually invented the open arc process for FeC hardfacing overlays).
I would agree with you that in order to maintain mechanical properties for those sensitive alloys some form of interpass may be necessary. But I believe some engineering is in order here. Perhaps heavier substrate walls and more lenient interpass requirements to emphasize the critical corrosion application.
I don't know.
My advice, as scant as it was, was consistent with this thinking.
Gentlemen,
this is brilliant...
I have never seen "Interpass-Temperature" from this point of view!
Regards,
Stephan