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Up Topic Welding Industry / Technical Discussions / Interpass temperature
- - By shreemurali Date 05-09-2007 08:09
While doing circumferential weld overlaying of pipe id, what is the relevance of specifying interpass temperature? Suppose, we perform a two layer Inconel weld overlay by GTAW process inside a API 5L X60 pipe, as per API, the interpass has to be controlled to 175 deg C max. In practice, how do we do this? While the definition and practices of measuring preheat and interpass temperature are clear for groove, fillet and groove+fillet welds, the situation is not same while doing id overlay of pipes, especially while carrying out the overlay in circumferential manner. In this context, how does one specify a proper interpass temperature in WPS?

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Parent - - By Kulkarni (*) Date 05-12-2007 11:42
Murali,
1) What exactly is your first question? Are you saying that interpass temperature is not relevant to circumferential overlay but is to longitudinal  overlay? If you mean it is more difficult to measure and control the interpass temperature in circumferential overlay, I agree, but it is not impossible to ensure correct interpass. Only the methods may be indirect.
2) In practice, you could try cooling the pipe from outside with compressed air/ water spray during weld overlay. That is quite easy to rig up if the pipe is rotating. The exact amount of cooling would have to be deduced from trials. That should not be very difficult for you.
3) You specify the proper/required interpass temperature in the WPS and ensure the same through process controls. I presume you are talking about machine/automatic GTAW which should make developing and ensuring the requisite process control easier.

If you wish you may write to me at my office address.

dileep kulkarni
Parent - - By js55 (*****) Date 05-14-2007 14:22
I can see your difficulty.
When doing circumferential overlays the idea of interpass temperature is to a real extent non relevent, because you have no interpass (that is, if your doing your circumferentials right). Many people don't realize that interpass is exactly that, an 'inter'pass. Meaning that just because you have violated a specific temperature does not mean you necessarily need to shut down immediately (the reason why I have always opposed chart recorders). From a practical welding standpoint you can wait until the next pass is completed. But for circumferential ID overlays this idea breaks down. In my opinion what you need to do is to research the metallurgical reasoning behind the code requirement, and understand your material, and then develope a procedure that maintains the spirit of the metallurgical phenomena. I believe that the code requirement is probably not applicable to ID circumferential overlays (though I do not know API that well nor have a copy in my possession) but you cannot ignore the metallurgy of the material.
The problem is, if you stop your process at the exact temperature required you will never get done with your operation. And even worse, how do you restart accurately? Its not like you can physically see that well up inside that pipe. Unless you have some little bitty inspector you can send up in there. I believe this instance calls for some practical welding engineering. Becasue if you lose contracts to fabricators who claim they will adhere to the interpass, as such, absolutely, what the customer will probably end up with is a worse product because of all the starts and stops inside the pipe. Which IS NOT conducive to successful corrosive applications, what overlays are really all about.
Parent - - By js55 (*****) Date 05-14-2007 14:34
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.
Parent - - By js55 (*****) Date 05-14-2007 15:48
Wow. Don't know where that came from. Higher alloy content doesn't necessarily allow you to reduce dilution. It actually allows you to increase dilution, or at least increase deposit alloy content with maintained dilution levels.
Nothing like arguing with yourself. But when I'm wrong, I'm wrong. No I'm not. Yes I am. No I'm not.
Parent - - By jwright650 (*****) Date 05-14-2007 15:58
js...you OK?   LOL!
Parent - By js55 (*****) Date 05-14-2007 16:17
LOL!! Yeah. Just thought I'd jump in there and dismantle my own point before someone else did it for me. A guy stands little chance of slippin something by the folks in here. May as well fess up.
Parent - - By Fredspoppy (**) Date 05-15-2007 16:17
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.
Parent - - By js55 (*****) Date 05-15-2007 18:03
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.
Parent - - By Fredspoppy (**) Date 05-16-2007 12:50
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.
Parent - - By js55 (*****) Date 05-16-2007 14:38
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.
Parent - By Stephan (***) Date 05-18-2007 17:08
Gentlemen,

this is brilliant...

I have never seen "Interpass-Temperature" from this point of view!

Regards,
Stephan
Up Topic Welding Industry / Technical Discussions / Interpass temperature

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