General welding questions that dont fit in TIG, MIG, Stick, or Certification etc.
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I had some coupons go through tensile test recently (for procedure qualification).

1/4" 304l SS. TIG coupon, two tensiles, and MIG coupon, two tensiles. Both passed. The result you want to see is under "Failure mode", and should read, "Ductile--Weld"

All four coupons tested to an average of 90KSI, 20K stronger than 7018 is rated for. The range was 88.7 to 93.4 KSI.

I still don't understand two root fails from the MIG coupon that tested this high on tensile with the desired ductile failure. A fresh coupon is out for test now.

Steve
noddybrian
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    Thu Jan 24, 2013 12:13 pm

Well the way I see it is Mig is the hardest process to get 100% root penetration & if I understand the qualifying process across the pond you may have put in 2test pieces at either end of the accepted parameters so that anything in between & allowing some real world variations will pass - if there is even a microscopic line of root LOF seen by X ray it will usually fail - on a bend test even a few thou depth LOF ( or undercut ) will generally give a point that the weld breaks from which shows enough that it fails the test - on a tensile however that few thou at the root effects the cross sectional area of the total being tested so little that the results will not show any difference from a weld that passes all other inspections - the exact point of failure on a tensile will vary each test & the so will the exact figure - we used to see this in college where you had like 25 guys pulling welds apart every time we were in metallurgy class - I think the point of the different types of test are to catch not always the fact a given weld shows adequate strength one time - it's over time & cyclic loading / temperature cycling / unexpected bending forces etc that a small defect in say the root does'nt become the source of a crack later in it's life - we had locally an elevated walkway collapse that was part design flaw but weld defects that went unseen was the biggest issue - cost several lives so I do see why weld tests seem kinda harsh sometimes - the testers don't know the welds purpose - just if they find anything less than 100% it's going to fail as the it could later come back to haunt them.
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Ive seen an all weld metal tensile test welded. (not tested though) A test plate 50mm thick was welded with a big prep, maybe 25 degrees... then the weld is cut out with and a sample that comes from the centre of the specimen, this is machined into a sample that is totally weld metal, see pic.
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noddybrian
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    Thu Jan 24, 2013 12:13 pm

Yes - done similar at school many years ago - but the only reason was the machine doing the testing was very small with limited available force & holding ability - it was'nt used much as there were sample pieces kept on display with it for common materials used in the coursework - the big Denison at college was huge & would tear most stuff apart - had huge taper grip jaws that bit deeper the harder it pulled - only needed to measure pieces for cross section prior to testing to quantify results which was'nt that big a deal as it was more how & where the piece broke rather than the tonnage.
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