Radiography, W/ or W/O Backing bar?

Date 10-23-2003 12:35

I believe you are talking about two different things(apples and oranges).

(apples)-The Figures in Section 4 require the steel backing to be 3" wide and a min of 1/4" thick, if the welder's test coupon is to be RT'd. If it is not RT'd then 1" wide backing is OK.

(oranges)-Then you mentioned a RT procedure in Section 6 which is talking about removal of backing, if required by para.5.10(use of other than steel backing) or other provisions of the contract docs. I have had FEMA jobs that require Seismic Full Pen Moment Connection joints to have the backing removed at the bottom flange only. These are the cases that para.6.17.3.2 is talking about. Where it is "specified" that the backing shall be removed. I don't believe you "must remove" backing just for RT or UT. I UT joints with backing quite frequently. I'm not all that familiar with RT. We have had welder's test coupons RT'd and no one ever removed any backing for those.

If I'm way off on the interpetation of this, someone please help me out.
John Wright

By mbrcic Date 10-23-2003 13:38

Thanks for your expediant response, where I am obviously confused is, (and I am only referring to R/T) in para. 6.17.3.2, yes, I am going to paraphrase it due to the confusion..."When (steel backing is) required by Para 5.10, the steel backing SHALL be removed, and the surface shall be finished flush by grinding prior to radiography" It does not leave room for an option.

What I have is, an old Andrex (200 kVp) machine/tubehead (5 MA, due to limits of tubehead) but I have a hard time getting through 1" coupons with a 1/4" backing bar. During my research, (of which I am quite certain I CAN remove the backing bar without hindering the speciman and keep within the AWS standards) I came across this issue regarding "in service" radiographs of welds on site. My cohorts here seem to believe no requirement exist to remove the backing. While I understand the excess labor involved in removing the plate may not justify it, when you may have a live source that can easily punch through it...but I must say...WHAT IS REQUIRED BY THE BOOK...and what has been done for the pass 21 years is the apple and oranges here.
thanks again,,,,,mike

By jwright650 (*****)

Date 10-23-2003 14:27

The reason I was saying that it (steel backing) can remain in place for RT'ing unless specified in contract documents or by 5.10 that it must be removed. Read 5.10.4 and 5.10.5 where it says it need not be removed unless the backing is running transverse to the computed stresses in the joint in cyclically loaded structures or specified by the EOR. Now if it needs to be removed then you must finish the weld as you described by grinding flush(5.24.4.1). Also read the Commentary on this in C16.17.3 that describes the intent of the code writers.

If you need to remove the backing to RT because of the limitations of your equipment, then the parties involved with the fit-up and welding of the joint to be RT'd, should be notified and made aware of this. So they can add the extra labor involved, with removal of the backing and finishing of the welds, to thier bid to the customer. Or add an "extra" to thier contract.
Just my two cents,
John Wright
Erratum data...Please correct the "C16.17.3" to "C6.17.3"

By mbrcic Date 10-23-2003 14:52

thank you for clarification, I believe I am getting the big picture, your quote of the commentary Para, I believe, was suppose to be C6.17.3 (but it got me in the right church, wrong pew)

V/R....

mike (Chief)

By jwright650 (*****)

Date 10-23-2003 14:55

Yeah Mike,
Fat finger virus struck again! :)
John Wright
Hope I helped

More like confused finger virus for me though.

By thcqci

Date 10-23-2003 19:21

Another reason to have a 3" wide backing bar on welds to have RT is to keep the edge out of the area of interest. Must keep the edges out of area of interest to make sure it does not interfere with possible HAZ discontinuities.

By jwright650 (*****)

Date 10-23-2003 19:32

Removal of weld tabs is a must, for the same reason.
John Wright

By thirdeye (***)

Date 10-23-2003 20:28

If we're talking qualification coupons, the 3" backing bar is required so that the edge of the bar is not shown in the weld area on the radiograph. Tabs can be left in place since 1 1/4" at each end of the coupon are excluded from the radiographic evaluation.

In production joints, with approval from engineering, can't tabs remain until radiography is complete?

By - Date 10-24-2003 07:39

That's correct. Check out paragraph 6.17.3.1. Specific approval is required by the Engineer for the performance of RT prior to the removal of tabs.
Mankenberg

By cawelder (**)

Date 11-05-2003 22:32

The backing bar can stay on the plate, but the RT guy will have to use an IQI for the size of the plate only. The backing bar can not be counted for plate thickness. And if using hole type IQI's. The pennys (IQI's) must be positioned over the backing bar. This is in order not to have the use shim the thickness of the bar. So a three inch wide bar will give you enough room to put the pennys 3/4" from the weld, but still be over the bar. Basicly your using the bar as shim. Hope I havent confused you.

Thanks
Chuck

By thirdeye (***)

Date 11-06-2003 00:53

The hole type pene(s) should be placed a minimum of 3/8" from the toe of the weld, which would still place it (or them) over a 3" wide backing bar. You are correct about excluding the thickness of the backing bar for the calculation of the correct pene. But, if weld reinforcement is not removed, then the pene is selected based on the material thickness plus the allowable reinforcement. For this condition, a shim must be used which equals the thickness of the reinforcement.

If the weld is a production joint and a 1" backing bar is used, the image
of the pene may not be over the backing bar. For this condition, the thickness of the shim must equal the thickness of the backing bar and the allowable reinforcement. The correct pene is still based on the material thickness plus the allowable reinforcement.

Now the problem is density of the pene. For single film exposures, it can't be any lighter than 1.8 H&D (x-ray) or 2.0 H&D (gamma ray). I've always prefered the use of the wire penes for this reason.