Got hit with a question today that I should have know the answer to, but didn't. We were talking weld design and I stated that your weld could not be thicker than the base material thickness you were welding on. Of course the guy asked me why and boom I was at a loss. It is just something I have practiced for so long I can't remember why it is done. Someone please help me out here. Also do the same rules generally apply to all material....stainless, exotics, etc.?
Your help is appreciated.
On a fillet weld, it is considered full strength if the leg is as long as the thinnest piece is thick. D1.1 does have limits on maximum convexity. On a groove weld, if it is thicker than the material, you will have excessive reinforcement and that is also limited. Check out D1.1 part 5.24.1, 5.24.2, 5.24.3, 5.24.4, 5.24.4.1 and Figure 5.4
Respectfully,
Mike Sherman
Shermans Welding
There was an article in the Journal by the previous D1.1 chair a few years ago on this very subject. He noted that the maximum fillet weld size was determined by the Engineers' calculations and that it was a common misnomer that these welds should not exceed the material thickness.
It is very common to oversize fillet welds, I have seen this condition often on various types of heavy equipment and now with weld overlays on SMRF's. The fillet welds were much larger than the thinner material joined. Fillets with unequal leg sizes with the larger-leg being greater than the material thickness are common as well. I have recent photos of high-strength steel with "oversized" fillet welds. The structure failed, but not due to the weld sizes. This was a serious piece of lifting equipment, I think it may have been overloaded (that's someone elses job to determine.)
Then there's the issue of assembly where the leg size increases with the root opening increase (D1.1:2002, 5.22.1).
My own opinion is that an oversized fillet weld can be useful as a "gusset" for certain applications. The mindset of the weld not being greater than the material thickness would apply to a weld in tension or compression.
What do you think? Good question on this post!
DGXL, I never new you stuttered! lol.
Seems to me the only reason you would limit the size of the weld after you equaled the thinnest leg would be distortion and undue stresses.
Not being an engineer, I was under the impression, you simply could not use the extra weld in your calculations when designing the joint. I have seen the extra large fillet welds called for on the drawings too.
I tend to agree with DXGL in that many times I have seen fillet sizes required to be larger than the metal thickness. I would say it's up to the engineers to determine the size required, although I also have heard of the same rule-of-thumb.
Recently another inspector told me that D1.1 has a provision disallowing oversized fillets and showed me Figure 2.1 as the basis for his statement. I pointed out that we weren't dealing with lap joints, these were tee joints. But I have run into this before, so I have to wonder if the "weld not larger that the material thickness" rule stems from Fig 2.1 being misapplied?
CHGuilford
Sometimes in a skewed plate (T) joint, the fillet weld will be thicker than the base metal to acheive the required throat through the fillet weld. (- see Fig. 3.11 "Z" loss) I also agree with CHGuilford with regards to Fig. 2.1 being mis-applied. If you will notice in previous versions of D1.1 (prior to 2002) the words under the figure read Details for prequalified Fillet Welds and referred you to para. 2.4.5., Now in 2002 the Fig. 2.1 reads like this..
Maximum Fillet Weld Size Along edges in Lap joints and refers to para 2.3.2.9 (which explains it's intent).
John Wright
If a fillet weld is applied that is larger than the base metal thickness, don't you have to worry about excessive heat input causing a brittle heat affected zone, possibly inducing toe cracks?
Also, especially in a T joint where an oversized fillet is applied to both sides of the plate, would'nt that be a good way to cause lamellar tearing?
Just thinking out loud...
Tim
I recently had a job in here that had 3/4" fillet welds (BS) of 5/8 thick stiffener plates in the 9/16" thick web of columns (plates were full pen at the flanges and fillet welds in the web). I think the Fig. 2.1 had been used for every fillet weld in the past and now it only applies to lap joints.
My guess, since it has been changed from previous issues of D1.1.
Could write AWS and wait for a ruling of the interpetation, if you have time to wait for the answer.
just a thought,
John Wright
I wouldn't think a larger weld will necessarily cause cracking in the heat affected zone if tested and proven weld parameters are followed. That doesn't mean there couldn't be problems but that risk is always there, so we rely on NDE to reduce the risk. (I think this is where the phrase "appears to be acceptable" comes from)
As far as lamellar tearing, I would think preheating would help prevent that. But the tearing could happen with any tee joint, especially in thick material. Thinner material would tend to stress relieve (distort) more. Again, certainly not impossible. But normally welds wouldn't be monstrously oversized, and as fabricators, we would want the engineers to review anything like that. We don't want to put in large welds if we don't have to for economic reasons; to reduce weld costs and distortion control/correction costs.
Chet Guilford
There are usually limits to the size of fillet welds that can be done in one pass. After that, you just use more passes, and this limits the heat input all other factors remaining the same. But you are right on the lamellar tearing, the bigger the weld, the more the chance of a tear. If you have critical welding with large welds and are worried about tearing, it is advisable to UT the material before welding to reject &/or repair it first.
I think fillet welds in shear would be required to be larger but I cannot recall why I think this and I don't have my books with me on this trip. Someone straighten me out:)
Here is a link that may shed some light for you and I. http://hcgl.eng.ohio-state.edu/~ceg532/chap8/ch8_3.htm
G Austin
Sanders:
Good responses on your post by these guys. Many schools of thought here.
Fillet welds in shear (SMRF shear plate) are now being specified with "oversize" welds by some local designers. These (in shear) are where oversized fillets are most common, but I still find many compressive/tensile loading on many oversized fillet-welded structures or equipment.
Distortion is USUALLY taken into account by the designer specifying large weld sizes. This is common in aerospace weldments on thick to thin titanium (wing spars), I asked the same question Tim asked about distortion years ago. The part was prestressed in fixture to allow for expansion/contraction for these large fillet welds. No PWHT.
A few ocassions including a recent one where the drawing noted welding a # 8 rebar to the sheet steel framing (?). (I SAID USUALLY).
As a general rule of thumb, I believe that you are right that the largest weld size does not need to be larger than the thinnest base material being joined. Many of the examples given by some of the other posts look at examples of "larger" welds that might have a larger leg length, but do not have a larger throat. The throat is the critical dimension.
As stated in another post, when you are looking at a weld, it is just one of the links in the chain that is transmitting the forces through the weldment. You gain nothing by making the one link much stronger than the others. It will just break right next to the weld.
The only exceptions that I can think of is where the weld metal being deposited is playing another role besides "joining" the different base metals. An example is where it is used as a re-inforcement to strengthen the weldment locally, rather than to "join" the materials. This is often done when welding nozzles into pressure vessels where a larger weld may obviate the need to use compensation plates.
Obviously, if your filler metal strength is lower than that of the weaker of the base metals, you could also use a greater size of weld. This is however an unusual situation.
Regards
Niekie Jooste
Niekie Jooste,
To add to your post, another use would be transitions, to prevent stress risers.(welds playing other roles)
John Wright
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