ASME IX Brazing Procedure Qualification: Tension Testing Question

[tc7]

Code brazing questions are few and far between on this site but here is one that hopefully can attract some interest.


Brazing procedure to be qualified IAW ASME Section IX;
Production joint will be a socket type with a 70/30 CUNI tube inserted into steel couplings;
Tube O.D. is .840";
Wall thk is 3/16";
Diametrical clearance will be .006" max;
Braze alloy will be BAg-1a (50% sliver).

Since the tube size is <3-inches my tension testing will be based on the full section sample Fig QB-462.1(e). The pass/fail criteria according to QB-153.1 is based on the specified minimum tensile strength of the base metal in the annealed condition and if the two base material in the joint are different alloys than the pass/fail criteria is based on the specified minimum tensile strength of the weaker of the two in the annealed condition.

What if the joint separates through the braze alloy ? Is this condition entirely unacceptable and considered a failure even if the peel test shows acceptable bond area coverage?

The question is based on real experience and I think that because of the extremely heavy wall thickness, the tube strength far exceeds the available joint strength. I can probably trick the tension test by using a thinner tube with wall thickness that is 1/2 of my production thickness and force the failure in the tube but would rather find a way to qualify based on braze material strength. For instance, if the manufacturer advertises the BAg-1a material has a shear strength of 25ksi, can I use this fact as a means satisfy my qualification when the failure is in the braze?

Thanks for any insight on this.

 

[metengr]

It fails, if it does not meet one of the conditions below regarding the calculated tensile value regardless of the location of failure. The following are conditions for tensile testing
2017 Edition of ASME Section IX

QB-153 ACCEPTANCE CRITERIA — TENSION
TESTS

QB-153.1 Tensile Strength. Minimum values for procedure
qualification are provided under the column heading
“Minimum Specified Tensile” of Table QW/QB-422. In
order to pass the tension test, the specimen shall have a
tensile strength that is not less than
(a) the specified minimum tensile strength of the base
metal in the annealed condition; or
(b) the specified minimum tensile strength of the
weaker of the two in the annealed condition, if base metals
of different specified minimum tensile strengths are
used; or
(c) if the specimen breaks in the base metal outside of
the braze, the test shall be accepted as meeting the requirements,
provided the strength is not more than 5%
below the minimum specified tensile strength of the base
metal in the annealed condition.
(d) the specified minimum tensile strength is for full
thickness specimens including cladding for Aluminum Alclad
materials (P‐No. 104 and P‐No. 105) less than 1/2 in.
(13 mm). For Aluminum Alclad materials 1/2 in. (13 mm)
and greater, the specified minimum tensile strength is
for both full thickness specimens that include cladding
and specimens taken from the core.

 

[CWEng]

If you fail in the braze metal below the strength of the base metal, and you had a sound joint, it generally means you need to increase your overlap. For a good reference, see this link:

http://www.sperkoengineering.com/html/articles/Brazing Article Final.pdf

The article actually talks about decreasing overlap (compared to solder joints), but is fairly relevant.

 

[tc7]

Thank you metengr for replying, I was not aware that the 2017 version was issued, however QB-153.1 does not appear to have changed at all.

Same to you CWEng, thank you for the reference to the Sperko article, I have been a fan of his for a long time. The idea of increased overlap is certainly an option but that drives me to deeper socket fittings which then becomes a special purchase.

But after re-re-re-rereading QB-153.1, I have a new point of view and I will rephrase my original question:
[li]The cross section of my production tube and my test specimen tube is .840" nominal diameter with (nominal) 3/16" wall thickness which equates to a cross sectional area of .384sq.in.
[/li]
[li]The specified minimum tensile strength of my annealed CUNI tube is 50ksi (the CUNI is the weakest material in this joint).
[/li]
[li]In order to pass my tensile test, I must withstand a breaking load of 50ksi x .384sq.in. = 19200 lbs.
[/li]
[li]The maximum load reached before the failure ensued (in the braze material) is 20,100lbs.
[/li]
Even though the failure was in the braze material, I think this test passes - yes or no?

 

[metengr]

Yes, because the load exceeded the minimum for the base material. In tensile testing the two factors are strength measured and location of failure.