Not logged inAmerican Welding Society Forum
Has anyone come across in there career a requirement to inspect tack welds used on fasteners to keep them from spinning? I have recently come across in our quality plan where it has been asked of the CWI (me) to inspect 100% welds on a particular product. The fabrication codes are AWS D1.1 and AWS D1.6. In saying that, is there any verbiage one has come across to keep a subcontractor from having to have their CWI stamp off on tacking a nut to a bolt? Prime contractor said they would wave the tack welds if we the sub could find anything within the spec that spoke to this. Were talking a spot tack the diameter of a pencil on a 3/8" bolt and nut. The drawing just says to tack weld nut. Basically I'd be signing off on the fact it was done because I don't have anything else to go off of. Thoughts?
What is the base metal and is there a WPS and supporting PQR?
Why don't they just snug up each assembly and foul the bolt threads with a centerpunch so that the nut can't back off?
Interesting proposal for maybe the next project, but right now the approved drawing specifies for the nuts to be tack welded and it would be like pulling teeth to get it changed.
They are 304L Stainless bolts and nuts. Yes, we do have a WPS and a supporting PQR.
You could always use the wire wraps on the threads projecting through the nut to make sure the nuts don't come off even if they do loosen. Many fabricators are going that route.
But, you alter the strength and properties of the nut and bolt with even the slightest tack weld and it normally is not recommended though we have all seen it done.
I did a job at a shop that had PQR's and WPS's for tacking nuts inside HSS for a bolt together structure. Even with documentation that the product would hold to all products designed strength they were still challenged on it several times.
He Is In Control, Have a Great Day, Brent
Are you talking about something like aircraft hardware where there is a hole in the bolt and nut that wire goes through, and they always wire at last 2 bolts together in such a way that if they were to loosen, the wire would be pulling on the nut in the wrong direction for it to loosen? The application is going inside a intake housing for a Natural Gas power plant turbine. They are concerned about anything coming loose and getting into the turbine rightfully so, but there is a filter bank in between the our unit and the turbine. Not sure anything that isn't permanently fixed to the weldment would fly.
If the base metal is 304L stainless, there should be no issue with the weldability. The process has been validated and properly documented with the PQR and if there is an approved WPS, all should be good.
I would be concerned that the purchasing requirements are very clear on the materials used for the fastener. I have encountered a problem in the past where the purchasing department ordered stainless fasteners as 18-8 stainless steel. As you know, 18-8 is generic and is often used to specify fasteners that need to be corrosion resistant. Ordering generic 18-8 is less expensive, thus saves the purchaser money. The 18-8 family includes many of the austenitic stainless, i.e., 304, 308, 316, etc., that exhibit good weldability, but the family also includes AISI 303 free machining stainless steel. The free machining stainless contains sufficient sulfur to cause a cracking problem. The purchase order should include a requirement that all fasteners be shipped with CMTRs to ensure AISI 304L is provided by the supplier. The suppliers often views all 18-8 fasteners to be equivalent. They may not be aware the assembly is going to be welded.
The one problem I would give some thought to is the size of the weld. I assume the fasteners are tightened to a minimum preload to ensure they are tight. A small tack weld should not be a problem, but a large weld would be sufficient to heat the bolt and allow it to lose the preload and the proper tightness. A small tack weld is all that is needed to ensure the assembly doesn't loosen. Since there is little danger of forming a martensitic microstructure, you could consider using a heat sink to reduce the heat transferred to the body of the bolt.
Best regards - Al
I've always wondered and sometimes argued about this..
1. When does a tack weld stop being a tack weld and start being an arc strike.
2. The tack weld in this scenario is permanent, therefore subject to 100% visual inspection just like any other D1.1 or D1.6 weld/weldment.. By what criteria is this final weld evaluated? Size, throat? What objective measurement is used on a tack the size of a pencil led?!?!?!?!
3. Is the washer being tacked to the nut within the base metal thickness limitations of the governing standard at all ?
4. Is it a fillet weld? Sounds like it to me.... But how oh how to evaluate
5.18 Tack Welds and Construction
5.18.1 General Requirements
(1) Tack welds and construction aid welds shall be
made with a qualified or prequalified WPS and by qualified
(2) Tack welds that are not incorporated in final
welds, and construction aid welds that are not removed,
shall meet visual inspection requirements before a member
This is one of those cases where the Engineer has to step up to the plate and provide the design criteria and the inspection criteria. The Engineer can modify, add to, or delete specific requirements in both D1.1 and D1.6.
It isn't the inspector's job to determine what inspection criteria to apply.
Best regards - Al
You are right of course Al.
I just felt the need to yank on the chain of Mr. Kix
He does not visit us enough and needs to be punished.
In the original post I said that the drawing only states to tack weld all nuts to the bolt it is fastened to. There is no fillet weld symbol with a size requirement. I have nothing else to go off of and could really only sign off on that there is a tack weld securing the nut and bolt currently. I like the verbiage you supplied in paragraphs 5.18 and 5.18.1 however, I didn't see anything pertaining to hardware. Tying into a threaded bolt without overlapping in between all the thread and not getting the bolt really hot is tricky. Measuring undercut, fillet size and throat should also be pretty tricky. Those paragraphs you supplied also leave me little hope in trying to get out of the requirement.
I will have to go to engineering and have them give me objective criteria in writing while also working with them on the potential issues the welders may come across during the tacking process. I'm also trying to hang out in here a bit more, but I'm being abused where I work currently and do not have a lot of time to partake with and learn from the big dogs on the porch anymore.
Thanks for the help.
I wanted to use that statement for the engineer to try and wave the requirement, but they didn't think it would hold water. In the end it's getting done and I have to add 100 nuts to my weld map and sign off that each meet whatever the engineer comes up with them having to meet. About your experience with 18-8 hardware, I find that information very useful and will put it in the ol noggin to be stored. I am also going to review the P.O. requirements we are flowing to our vendor and the material certs on record to see what we actually have. What is you opinion on welding 18-8 stainless hardware even if it ends up being AISI 303 with ER308L filler? The customer did approve one of our ASME Sec IX qualified procedures for P8 materials to perform the tacking. In saying that, one PQR welding 304L to 304L with ER308L filler covers you for the material ranges of 18-8 you could get seeing as from your experience they're all a type of 300 series stainless right? This being of course only if we yield acceptable results with no cracking if it turns out we got AISI 303? Does that sound about right? That last question is more for WPS and PQR coverage for 18-8 bolts.
It isn't a matter of waiving the requirements, it a matter of tweaking the inspection criteria to fit the purpose of the tack weld, i.e., ensure the nuts cannot back off. Essentially this application simply involves destroying the threads of the bolts so they no longer function. If the fasteners were made from 316 or 316L, no filler metal would be required. However, since they are 304 or 304L, the low ferrite could result in a cracking problem and your WPS is qualified with filler metal.
Again, this is a situation where the engineer has to give a little thought as to the amount of weld required and what visual criteria is appropriate. Welding on a threaded fastener is more than likely to produce undercut and unfilled weld craters simply because the threads are melting. Basically, if the weld isn't cracked, it should fulfill it's purpose. Even if the weld is cracked, it would do it's job, but it would give too many people the heebie-jeebies. Heck, clean the stainless fasteners in alcohol before installation and tighten them a little past snug and the galling will keep them from loosening. Smack the threads with a small cold chisel and the nut is never going to come off! You could cut the bolt flush with the nut and center punch the end of the bolt and the nut will never come off. The latter method gets my vote. Run a couple of tests to demonstrate the technique works and go for it. Welding; too many variables that causes every pin head on the job to start looking at the weld with a magnifying lens.
As for the 303 stainless, cracks are almost guaranteed due to the high sulfur content. Once again, the last method of upsetting the end of the bolt gets my vote, especially considering the diameter of the fasteners involved.
Best regards - Al
Thanks Al. What I meant by waiving the requirements was to get out of having to perform a full inspection to code criteria on a welded nut to a bolt. We already have quality inspections in place to check that every nut is tight and tacked, but now the customer wants a CWI to sign off that the tack weld meets what the engineer will now have to come up with as criteria and me to sign off on it. We never perform full blown inspections on the nut welds and it's not estimated into the job, but they have us locked because the quality plan says 100% inspection of all weld. They want their nut welds included in that 100%. So needless to say, I'm getting ready to be a pretty busy guy. They did give us a chance to get out of it by telling us to find something within the spec that would present a good argument as to why it shouldn't be done. On another not, learned a lot from you guys as always so that's a plus.
First, I now have a better picture in my mind as to what you are doing.
Second, no, no hole in the bolt. But, that won't matter with your explanation. It would be even more of a risk of coming loose as it only wraps the OD of the bolt and is twisted and pulled tight by a tool that completes this process.
Lawrence and Al make some good points about inspections and other criteria that need to be in the Job/Inspection Specifications in order to make sure it is handled in a proper way.
I like Scott's idea but it would take some time to change your procedures from what you have already been doing.
Powered by mwForum 2.29.2 © 1999-2013 Markus Wichitill