measuring heat input/ Notch toughness.

Date 03-12-2007 01:10

I'm assuming AWS D1.5 as the basis for my opinion because you didn't say what code you are working to, and the majority of my PQR testing experience is with that code.

The CVN samples are cut from the center of the weld. Most likely, a high heat input root pass would not appreciably change the results of the CVN test. The problem I see is that if all passes are not within the range of parameters allowed, then the test is not valid. You should match your parameters and technique to weld all passes within the allowances. If you intend to make production welds with root pass parameters that are outside the essential variables qualified, then you should do another PQR that will qualify those ranges (min/max tests).

Will any of that really matter? Very few customers really scrutinize WPS's and supporting PQR's submitted for their work, likely because many have little to no welding experience. However, there are some customers who know what they are looking for. One that I deal with requires the data from every pass made on a PQR test before the records will be reviewed.
The other consideration is, what happens if the root does not have the mechanical properties required? (Cap passes are usually on the other end of the spectrum; low heat input due to faster travel speeds). Again, maybe that won't matter but no one yet has shown me that it doesn't, so I stay on the conservative side.

By js55

Date 03-12-2007 14:46

Most codified CVN requirements limit by maximum heat input. Therefore, it doesn't really matter what TS you use on your root as long as you maintain your HI under that max. And remember, the root will most likely be the pass most extensively tempered.

By GRoberts (***)

Date 03-13-2007 00:58

I mostly agree with both of you so far. A lot of times I see (and have done myself) the root pass, fill passes, and cap passes broken out seperately in the heat input section, so the WPS follows the heat input limits as qualified for each type of pass. I've also seen that requirement spelled out in customer specifications although no code I am familiar with requies it to be broken out so detailed.

However, on the root pass, I find that it is the pass that gets the least grain refiniement since it is thick and narrow as compared to other passes, which tend to be wider and thinner, aiding in grain refinement/bead tempering. That said, in my experience, I find the cap pass to have the lower of the two locations (cap vs root) CVN values more often that not. This is because the cap pass being the widest (using 2 passes/layer typically), has the highest heat input, and it has no subsequent grain refinement/bead tempering.

As far as specific codes go, each one has it's own specific requirements (ASME/AWS/MIL/etc), so you have to look at each one seperately.

By HgTX

Date 03-13-2007 11:33

To hijack this thread only slightly, what do you do when the passes are all over the place? It's one thing not to count the root and cap passes when considering the parameters that all the PQR tests are supposed to be evaluating, but what if each pass is different?

Hg

By js55

Date 03-13-2007 13:32

Thats probably why the codes don't refine the requirement more. A CVN PQR is allowed to be applicable to many thicknesses in which there will be great diversity in grain refining patterns. Bevel angles can change these patterns. Welder technique can change these patterns. Etc., etc.
There are a variety if ways to handle these things if there is a specific concern. One good way is to make sure that you have notches to represent the full thickness with allowance for unrefined root ID's and unrefined cap beads, as Greg spoke of. Though the cap can be covered by more careful temper beading type techniques (not so easy to really control in practice) or PWHT.
The general idea is to verify that you have an overall microsturcture capable of arresting fractures at a certain level. It isn't an either or situation. There are 'ALWAYS" trouble zones that can initiate cracks. Carbides, nitrides, grain triple points, microstructural discontinuities, intermetallics, etc., depending upon the alloy involved.
But if the general microstrucutre is 'tough' the weld will be viable.
The other thing is, as always, 'sound engineering practice'. If you have to 'refine' your evaluation too much, perhaps you should go with a more forgiving alloy. For example Ni steels if your pushing the envelope with C steel, or even austentics if the ferritics are risky.
The problem here is everybody is trying to save money, so sometimes they try to put the burden on the backs of a CVN PQR. This is not without risks because there are just too many variables inthe real world.

By CHGuilford (****)

Date 03-15-2007 10:25

That's basically my point. If the parameters are outside the allowable ranges qualified by a PQR, then, in my opinion, the welding is not qualified and tests should be run in the ranges needed. Or the welding parameters should be adjusted so that they are within the ranges.

One common problem I have run into is when a single pass fillet weld can't be run within the groove PQR test ranges. The fillet needs enough heat input to penetrate to the root particularly 5/16" 2F. Yet a groove weld can tolerate a wider range and still get adequate fusion.
Unfortunately, I believe many 5/16" 2F FCAW fillets do not fuse to the root, but we cannot see that in production. I have given many fillet tests and found the average welder (better than 90%) can't do that size on the first attempt, but almost all believe they can.
Some say that doesn't prove anything because production welds that size are in multiple passes- but then the heat input is out of range and we're back to unqualified welding.

Again, I am speaking from my experience with D1.5 and no other codes. And I know that test conditions often do not duplicate actual production situations, but they should be nearly the same because that is the point of the testing.

As far as not counting root and cap passes, I have not seen anything saying it is OK to do that. Will that matter? I can't say but it seems to me that a weld in service usually starts cracking from the outside and not the insde.

One thing that would be nice is if AWS could address PQR testing in a more definitive manner. There is a huge amount of information about the requrements of testing, but very little about how to actually conduct the testing.

By HgTX

Date 03-19-2007 19:22

The PQR specimen is set up basically for doing the AWMT. That's why it's so big, that's why it needs to be a groove, etc. Most of the tests don't look at what's happening at the fringe of the weld--and most of those problems are dependent at least in part on weld geometry, and the PQR isn't supposed to be testing a particular weld geometry. That's why the cap and root passes can be in good conscience eliminated. But I've seen variation of 100% (or 50%, depending on how you look at it) from pass to pass within the rest of the weld ON THE PQR TEST ITSELF! Meanwhile I've been using Table 5.3 (which only officially kicks in for 5.13 qualification) as a rule of thumb.

Hg