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.

 

[Justanothereng]

The "best"? Perhaps but the "best" unit as described doesn't read bolt stress !! You cannnot use typical UT thickness guages or even UT flaw detectors for this specialised application. Attempts to do so have gotten more than one well-known NDE company into very hot water!

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!

 

[Screwman1]

I agree with Cory, the Micro Controls is the best unit. We have used several of them over the years and they are versatile and tough as a brick. You can set them up for pretty much any type of fastener data acquisition that you need. We use a tensile pull to calibrate the load / deflection curve for a specific part, since that is the most accurate way of determining stress in the fastener. At that point the MC unit works like a charm. (if you have a trained operator; I'm still partial to Tq/ Angle for the vast majority of big bolt applications because of historic issues related to operator error with US).
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.

 

[CoryPad]

Justanothereng,

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.

 

[Justanothereng]

Ahh, I stand to be corrected.

 

[tothepoint]

Throw out your used bolts, install new B8 bolts (preferably strain hardened class 2- and make sure that your supplier really does supply class 2), calculate the assembly torque required staying below 60% of yield if possible, then use controlled torquing following the legacy (star pattern) procedure outlined in ASME PCC-1-2010.
Problem solved.

 

[dik]

Can a sample be cut, polished and etched for a microphotograph? Looking for Luders Bands. This may show that the material has slipped... doesn't solve the problem, though.

Dik

 

[unclesyd]

I will have to agree and not to agree with the above posts.
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.

 

[dik]

Can you cobble together a device that has a dial gauge and fits over the thread end of the bolt and check the extension from snug to whatever pre-tension you require?

Dik

 

[unclesyd]

Here is an example of many small bolts in lieu of a few large bolts. In this area we use Gortex Gasket material and by using this we have very good sucess with this approach. All manways, tanks and vessels are bolted this way. The mechanics like this approach as they don't have to drag big tools around. We use C-5A on all studs.. We also allow impact wrenches as the tightening procedure uses the parallel face method.