We plan to fabricate some 304L stainless steel vessels from 0.156" thick (schedule 5) NPS 12" stainless steel pipe.
My tech spec says we must allow for, and address weld shrinkage (before contract award!). I don't have a problem in doing that for longitudinal shrinkage but am curious for the hoop shrinkage.
On thicker materials (1.0") we had very little hoop shrinkage (approximately 1/8"). We plan on using an orbital TIG and can probably cap out with no more thank 2 passes running at an average of 180 background DCEN amps and 8.9 volts with a travel speed of approximately 4.0 IPM and 0.035: ER 308L wire. Any thoughts? We just don't have the luxury of running mock-ups in this case.
If by hoop weld you mean the one that joins the vessel shell to the dished ends, then, from past experience, I'd say that you can expect a hoop shrinkage of about 1 millimeter.
Compare that figure with those that will be given by other frequentors of this site.
Depending on the vessel straight height, 2 millimeters (one for each hoop weld) are negligible.
Giovanni S. Crisi
Sao Paulo - Brazil
Professor Crisi; Thanks so much for your response.
The vessel's straight height is approximately 468mm. I am not concerned with lateral shrinkage, i.e., heighth, I am concerned with how much shrinkage will occur on the diameter itself at the weld joint and affected areas on each side of the joints ~ I have two circumferential full penetration welds to place. The effect on the diameter is what we commonly refer to as hoop shrinkage although the terminology may not be recognised. Any ideas on this?
jon,
Will you be trying to maintain the same dia. for the entire length? I'm not familiar with having to account for something like this ahead of time and was just curious what do you do?
John Wright
John;
Normally these things are "figurable" using rule of thumb methods. The best way, if you have time (and money) is to do a proto-type but obviously that method is expensive. If we intended to do dozens of these things it would be worthwhile but we may only do a few. It's not all that unusual in my industry for the customer to ask for anticipated shrinkage howver, on this particular item, I have very tight tolerancing to live with regarding the diameter shrinkage. I pretty much know how much length shrinkage to expect so that isn't my issue. Okay, a long-winded response to say yes, we must maintain a consistant diameter around the entire circumference of the vessels; I just need to know what to expect!
Don't know if this will help but I've worked in LNG plants that had 8" schd 5 that were GTAW and SMAW. The hoop shrinkage was so gross that when I first saw them I thought they were some kind of special joint. They were at least 1/4" smaller in dia than the rest of the pipe.
Hope I understood the question-Ted.
Ted; thanks. Yes, you understood the question and I can undertand why that much shrinkage occured using SMAW as part of the process. In my particular case, I'm using GTAW Orbital equipment which works very nice and has fairly low heat input but I still expect to get some degree of shrinkage, I just don't know how much!
As a purely unscientific 'rule of thumb', we usually fit the can so the longitudinal weld seam is 1/16" wider at the ends than it is in the middle section. This is for thinner shells- say, under 3/8" thick. For thicker shells, we don't make any allowances.
This is derived from past experience, having to cut and re-weld seams that didn't work out very well. Every now and then, nothing we 'remembered' works and have to go from there. Usually we have more than one 'can' to make so we can adjust as we go.
Hope that helps.
Chet Guilford
Chet,
I had figured using a rolled sheet rather than a pipe, this shrinkage could be accounted for in allowing more material at the ends to be welded, rather than in the middle.(hourglass shape) But with a pipe, I wasn't sure if there would be anyway to compensate. Sounds like a good challenge. I looked up some of the research surrounding figuring this "hoop shrinkage" and most of what I was reading was way over my head. There was alot of engineering language that made no sense to me at all.
Confused,
John Wright
Just a thought: Is your client trying to determine a change in yield strength as a result of weld shrinkage as it may relate to a stress corrosion cracking failure, based on your estimate of hoop shrinkage ?
I would think that the client's specs would call out a tolerance for roundness, allignment etc. or specify peaking measurments etc. if they are stricter than the construction code?
I dont believe my client is looking for a change in yield strength I think its more a matter of them determining that we can complete this project given the very tight tolerances we've been given, + or - 0.005" on the critical dimensions and 1/2 degree for angularity.
That is tight! Sometimes for me, the “back to the basics approach” can bring out some ideas. So here goes. Fit-up and alignment should be as accurate as possible. (so that misalignment is not confused with shrinkage). Tack welds should be uniform. Heat causes shrinkage, stainless is a great conductor of heat. What are the best adjustments to keep the amount of heat to a minimum? (travel speed, amount of weld deposited per pass, time between passes etc). Can you use an additional cooling method like a cooling pipe or air circulation on the inside face once the root pass is in? Too bad the mock-up is out of the question.
I am most familiar with angular distortion on pipe to flange welds. I have seen many fabricators bolt up flanges back to back prior to welding to minimize this effect.
Best of luck on some more advise………
Hi Jon
I would argue the matter as follows:
The hoop shrinkage really is just the same as a longitudinal shrinkage in any butt weld. It merely presents itself as a hoop shrinkage, because the butt weld is bent along a certain diameter. If you can then estimate the longitudinal shrinkage that you would get, if you were welding on a streight plate, you will be able to see this as the reduction in circumference of the pipe. There are a number of different equations around for calculating this longitudinal shrinkage, but they all leave something to be desired in accuracy. None-the-less, it would be a starting point.
Once you have calculated this reduction in circumference, you can compensate by "flaring" the ends of the pipes by this amount before welding commences.
Hope this helps
Regards
Niekie Jooste
Fabristruct Solutions
hello
i have a project about the welding of stainless steels... how can i get articles about subject?
thank you,
regards
Nahit BUYUKSAHIN
Hi Nahit
Here are a couple of links to try to see if it has the info you are after:
http://www.adtdl.army.mil/cgi-bin/atdl.dll/tc/9-237/toc.htm
http://www.lincolnelectric.com/knowledge/articles/
If you are looking for something in particular, ask the question here and you are sure to get an answer.
Regards
Niekie Jooste
Fabristruct Solutions
Jon,
I think you don't have the luxury of NOT running mockups in this case. I'm not aware of any tried and true calculations for hoop shrinkage as a function of heat input that would be likely to satisfy such a request from the customer. If you do use rule of thumbs to coinvince the customer in order to get the job, then have problems with too much shrinkage you will have lost the customer's trust. I think the best approach would be to include in your proposal a discussion of the welding process you plan to use (GTAW-machine), past experience with the use of low heat input GTAW techniques to control shrinkage, and include a plan to weld some mockups prior to the production welds to demonstrate control of hoop shrinkage.
Marty
Have you completed this project yet????
jon,
I was wondering about that myself.
John Wright
No, as it turns out another company was awarded the contract which is too bad I would have loved to have gotten it just to be able to answer my own questions. Sorry guy's but thanks for all the input and happy new year to all!
Forum
