Stainless Steel Bolting Questions
Stainless Steel Bolting Questions
(OP)
I have A193 Grade 8 (low yield stress) stainless steel studs which may have been stretched. What kind of gauge would detect that and who might have one? I kind of doubt that metallography would capture it, but all opinions and experiences are welcome.
In general, how is torquing of low YS bolting done? Personally I would avoid them altogether by going to a higher YS variety, resizing if necessary.
Perhaps someone could provide some explanation for the myriad variations on Grade 8 bolts in A193.
In general, how is torquing of low YS bolting done? Personally I would avoid them altogether by going to a higher YS variety, resizing if necessary.
Perhaps someone could provide some explanation for the myriad variations on Grade 8 bolts in A193.





RE: Stainless Steel Bolting Questions
RE: Stainless Steel Bolting Questions
These bolts were on a pressure-containing flange that did not contain pressure. We don't want that to happen again.
RE: Stainless Steel Bolting Questions
I just about never use SS for external bolting unless it is known to be in an external corrosive environment. External bolting is generally not considered to be exposed to the process.
Don't know any way to detect yielding by physical measurement unless overall lengths were know precisely beforehand.
Regards,
Mike
RE: Stainless Steel Bolting Questions
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RE: Stainless Steel Bolting Questions
The flange leaked? they may have yielded, but If the bolts were not torqued to apply clamping force sufficient to resist the force the joint will open.
RE: Stainless Steel Bolting Questions
RE: Stainless Steel Bolting Questions
David
RE: Stainless Steel Bolting Questions
RE: Stainless Steel Bolting Questions
One day the design changed to require a little extra hydraulic pressure. On the test bench when the higher pressure was reached according to the test tech, the cylinder "farted" and made a big oily mess. A check of a few pressure/area/force//clamp numbers confirmed the lightly torqued bolts' clamp preload was likely exceeded, leaving the stiffness of the bolts (not the stout main housing) left alone to controll the cap position. The name brand US made socket head cap screws stretched elastically (not yielded) a few thousandths of an inch which was enuff to unload the o-ring. The "solution" was to torque the existing bolts tighter than the customary "2 hands on a T-handle allen wrench". Loctite would not have changed what happened at the low torque.
RE: Stainless Steel Bolting Questions
I agree with others that it is likely that some other factor caused the problem. I would look at the gasket stress to see if it has the correct resilience and I would also look at the tightening sequence; flange are notorious for bolt cross-talk issues.
RE: Stainless Steel Bolting Questions
I agree a thread gage can only hope to pick up deformation that is probably already obvious in other ways. At the size I am looking at, the gage covers only 3 threads anyway. My best hope is to assume the pitch was correct to begin with and try to capture elongation over as much length as possible.
Installation was done by impact wrench, almost certainly uncalibrated, following a procedure no more sophisticated than that used at your local tire shop.
BTW I don't permit my tire installer to use an impact wrench. I have been stuck on the highway with a flat I could not change because the nuts had been so severely reefed.
RE: Stainless Steel Bolting Questions
As expected, my brand new thread gauge did not detect anything. It does not help that in the larger thread sizes the gauge only covers 3-4 threads.
However, I have found one stud to be bent by about 0.050", located at where one of the nuts seated. What does this say about the flange assembly practice (or rather, malpractice)?
RE: Stainless Steel Bolting Questions
I recommend using a thread pitch micrometer. Check the potentially yielded bolt against a new one, preferrably from the same manufacturer. The yielding action would have necked down the thread pitch. It is not usually uniform across all threads, but centered on one area.
RE: Stainless Steel Bolting Questions
In any critical bolting application where leaks canot be tolerated, even greater control must be applied to the process. A key factor is acheiving the proper bolt stress. This cannot be done by torquing alone since the relationship between torque and bolt "tightness" is highly dubious and capricious. To overcome this, one must measure each fastener's length before tightening and then again after tightening. Any fasteners which are thus found to be too loose or too tight are then torqued or tensioned more (or less) until they're within spec.
The above takes the "guess work" out of the torquing process
RE: Stainless Steel Bolting Questions
RE: Stainless Steel Bolting Questions
Actually, this isn't the case. In fact, it's as easy to measure the relative elongation of studs as it is bolts. Furthermore, this is true regardless if either is in a blind hole or not. Our Technicians use specialized ultrasonic devices for this purpose. The instruments are effectively glorified "fish finders" working on a pulse-echo principle: A transducer is placed on the top of the fastener, it is pulsed and then "catches" the return echo a few nano seconds later. The time-of-flight is converted into a length measurement (after temperature and stress have been taken into account). Cool system and highly effective
We've found that "Torque-angle" is fraught with innacuracies since it depends on the accuracy of the initial torquing step. If the former is 'out', the subsequent angle step is also 'out'
RE: Stainless Steel Bolting Questions
Do you know the make and model?
How much does that ultrasonic device cost?
I'm wondering about a few things.
Does the end of the threaded end of a stud need to be finished better than these typical bolts ? -
http://www.lesker.com/newweb/flanges/jpg/photo/Pho...
Does it need to be calibrated for different sized bolts, or different configurations of bolt heads, or fully threaded bolts?
You mentioned stress is taken into account. A bolt is stressed along nearly its entire length. After the first few threads a stud is barely stressed at all. Can you enter a stress gradient?
How much does it weigh, and how long does it run on fully charged batteries? Ot does it need to be plugged in?
Is it sealed well enough to be used atop a piece of equipment inside a cement factory or a coal fired power plant or in a pumping station where a little water might splash or drip unexpectedly?
Are the cables oil resistant and tough enough to be pulled around machinery and occasionally stepped on?
Is the case "hardened" so it would withstand a few bumps as it was being slung from a shoulder strap as a technician climbed a ladder? Or, worse, a 3 foot drop?
thanks,
Dan T
RE: Stainless Steel Bolting Questions
Our Techs use these things in some pretty nasty conditions. However, speaking of "conditions", the Techs are also conditioned to have considerable respect for the devices! At approximately 7-8 K per instrument (including the necessary bits'n'pieces) we don't want them to get banged up. We find that they're fairly-well protected by the nylon working-cases. This also enables them to be slung around the neck. As such, they're quite portable and convenient to use. Based on which part of the globe a particular project might be, our guys simply tuck the units into a pocket in their coveralls when not using the units.
The parts that one has to be particularly careful with are the cable connections. Step on them? No way! Sorry, but these are precise instruments. They're not made for knuckle-draggers
Yup: The bolt ends have to be prepped! They must be flat. Otherwise, one can't place the transducer properly. In fact, if one were to try to shoot fasteners with raised markings, one would be a very frustrated Tech indeed!
Oh yes, they run on 3 AA batteries.
You have to realize that these things require significant expertise to operate properly. We've encountered a number of situations where somebody simply bought one, tried to read the manual and then began to screw up the project. After facing significant frustration, it was obvious (after reviewing their recorded signals) that they "cooked" the numbers (easy to do if the Client or Inspection Agency doesn't know what they're looking for!). Keep in mind: Any monkey can buy a scalpel and read a manual but, would you entrust that ape to take out your appendix?
Here's a link: http://www.heviitech.com/Hevii_UT.html
RE: Stainless Steel Bolting Questions
http://www.mcrts.com/products/TransientRecorders/s...
Other options with lesser capabilities are:
http://www.ge-mcs.com/en/ultrasound/bolt-extensome...
http://www.checkline-europe.com/product.php?id=126...
RE: Stainless Steel Bolting Questions
The other two units mentioned are proper units. Just bear in mind that since these are (I'll repeat) precision instruments requiring particularly skilled operators (your standard NDE Techs don't make the grade because this is a very distinct and highly specialized process quite unlike what they're familiar with), make sure that wherever you buy your units the supplier knows what he's talking about. You'll surely run into situations in the field where you'll need some quick on-the-mobile-phone assistance. A simple salesman is not going to be much help to you in those inevitable under-the-gun situations!
RE: Stainless Steel Bolting Questions
Just one thing to note about using US on studs; you will have to do the end prep yourself if the parts are to one of the recognized specs, since they will have to be supplied by the mfg. with end markings per the spec.
RE: Stainless Steel Bolting Questions
Compared to an MC900 system, all the other units are glorified ultrasonic thickness gauges. The MC900 system certainly can read bolt stress, from ultrasonic sensors and strain gauges.
RE: Stainless Steel Bolting Questions
RE: Stainless Steel Bolting Questions
Problem solved.
RE: Stainless Steel Bolting Questions
Dik
RE: Stainless Steel Bolting Questions
The majority of our fasteners in the intermediates area are 304 SS or 316 SS.
We use impact wrenches to tighten all flanges using the parallel face method, Just make sure the faces come together in parallel. The impacts are for Ingersoll Rand and routinely refurbished and calibrated. The mechanics determine this point in time. They also change the torque based on fastener size up to 1" studs. Most of our fasteners are stud bolts Class 2 up to 1 1/4".
We have a design philosophy of many small instead of a few large on all flanges where possible..
Gore-Tex gaskets on Class 125 flanges. spiral wound can only be used on class 300 or above.
A lubricant is used on all studs, normally C-5A copper.
All studs for a job are reassembled in teashop and any problems the stud or nut is tossed, in other words no one tries to seat a bad stud/nut.
The mechanics will try one wrap with the backup wrench if one sticks, and if it doesn't go it is tossed.
No opened end tooling, if we can catch it.
We reuse all that we can, normally a pretty high recovery level.
The last time I looked at some problems the source we in the nut. In the case the lead was off and there were root/tip interference.
There have been various attempts to impart some trying o tightening requirements over the years and all have been a failure. All types of gauges have been tried to no avail. We even bought a Raymond UT bolt extensometer and an instrument by Branson, neither were practical in the field and laboratory work showing such a scatter that there was no practical correlation
There is a process that will reduce the scattered of SS fasteners and I hope to be able to work on it. In the near future, this will be an independent effort on my part. It worked well enough on the first passes that the statistician was able to calculate a correlation coefficient that matched what I me measuring by micrometer.
RE: Stainless Steel Bolting Questions
Dik
RE: Stainless Steel Bolting Questions