Stainless Steel Bolting Questions 1

[Guest102023]

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.

 

[MiketheEngineer]

How about buying new ones??

 

[Guest102023]

We will, but that doesn't answer the question.

These bolts were on a pressure-containing flange that did not contain pressure. We don't want that to happen again.

 

[SnTMan]

brimstoner, don't why there are so many varieties of SS bolting. If using 304 I will nearly always use B8CL2, good in small diameters, but stresses go downhill quickly as diamater goes up. As you noted the yields are pretty low. Not much you can do except to avoid yielding the bolting.

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

 

[TVP]

Here is an example of a simple stretch gauge that is used for measuring bolts before and after installation/preloading:

http://arp-bolts.com/pages/technical_installation.shtml

 

[Tmoose]

"These bolts were on a pressure-containing flange that did not contain pressure."

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.

 

[Guest102023]

The joint could have failed for any number of reasons. I've only just been called in to help, and that is one of the speculations reported to me. All I know is that no studs were broken.

 

[geesamand]

I assume the stretching took place on the threads. Couldn't a direct measurement of the thread pitch give a rough idea of strain?

David

 

[dinjin]

Why do you suspect that the studs elongated or stretched rather than the nuts backing off or loosening?

 

[Tmoose]

I worked for a company that had made a few hundred specialized hydraulic cylinders as part of a spindle drawbar release mechanism.
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.

 

[Screwman1]

Unless you knew the length of the parts before installation, I don't know of a way to check for yielding after use. The amount that they will have yielded will be so small that no thread gage will pick it up.
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.

 

[Guest102023]

Screwman1,
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.

 

[Guest102023]

Update:
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)?

 

[fegenbush]

brimstoner,

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.

 

[Justanothereng]

This is a common issue. First solution is to throw away the damn impact wrench!. Then, apply at least a modicum of "control" to your bolt tightening process. Although torque wrenches are notoriously inaccurate in terms of applied tightening force to bolt stress, perhaps in your situation the relative quantum leap over impact wrenching will be sufficent.
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

 

[Screwman1]

Just: With studs it is often not possible to measure stretch because one end of the fastener is not accessible. Torque angle is one alternative, but care must be taken to reduce (not eliminate) the effects of cross talk and interaction. Is a logical tightening pattern and multiple passes to bring the parts up to full load.

 

[Justanothereng]

"With studs it is often not possible to measure stretch because one end of the fastener is not accessible"

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'

 

[Tmoose]

Hi Justanotherengineer,

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/Photo-HD-FC_GA0275_BOLT.jpg

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

 

[Justanothereng]

@ Dan (TMoose)

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 . But, since even some of our Techs occaisionaly scrape their knuckles, we've made simple mods to our units to accomodate them.

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

 

[CoryPad]

The best ultrasonic measurement devices are from Micro Control Inc.:

http://www.mcrts.com/products/TransientRecorders/standardMC900.aspx

Other options with lesser capabilities are:

http://www.ge-mcs.com/en/ultrasound/bolt-extensometer.html

http://www.checkline-europe.com/product.php?id=126158&lang=en