A local fabricator has made some machine frames for me and the welds on these structures are failing after being transported across-country on a flat bed trailer.
http://photobucket.com/albums/b355/TARS14/?action=view¤t=Image19.jpg
I really don't know any details of the process / filler etc. used and have little expertise with diagnosing weld failures.
The strutures are quite large box-type frames ( ~10' H x 8" W x 20' L ) made from formed 7 Ga. "C" channels which are tabbed together for location and then fillet welded.
I have not seen these in person yet but it looks to me that the weld beads in most all of the cases have cracked roughly down the center of the bead and that the bead sort of bridges the joint with poor penetration into the base materials.
I would like to get opinions on these failures in general as there seems to be some very knowledgable individuals who post here.
I realize that more specific knowledge of the process used would be helpful but at this time I am just looking for general commentary on the welding job.
Thanks,
Tom Rice
Tom,
Generally, a center line crack is an indication of the solidification process. In other words, the weld process (heat input, travel speed, weld process used, etc.) allowed the weld to cool down too rapidly and did not allow the weld bead to fully solidify. When a low heat welding process is used and the heat dissipates too rapidly into the base metal, this can put a strain on the molten weld pool and cause a center line crack, commonly called a solidification crack. When you get more details, we can have more information as to the possible causes, but based upon what you have told us, this is the likely cause.
Chuck
Thanks for the input Chuck.
That does make a lot of sense due to the scale of the parts being joined I guess that they will tend to sink the heat away from the joint in a hurry. What is generally done to avoid this?
This also makes good sense from the standpoint of strain buildup in the weld joints regarding the mode of failure; i.e. the welds looked OK when the structure was put on the truck and failed presumably due to fatigue during over the road shipment.
Also what specific details would be most helpful?
Thanks again!
Tom
By -
Date 06-20-2005 19:44
Tom, you need to retard the cooling down rate, most likely by slightly preheating the general area you are going to be welding. Also use a higher heat input. Stay away from autogenous welding, too. I have a feeling that the solidification crackw were there when you loaded them on the truck, but you might not have seen them to the extent of the pictures. The loading and bouncing around on the truck could have certainly added to the problem. It appeared from the pictures that this was a single pass weld bead, too, and rather small bead. The transfer of the heat to the heavier base metal caused the heat to rapidly leave the weld bead before it could fully solidify. I am confident this was the problem. Preheat the areas (around 100-150F) you will be welding, use a highe heat input, and put a heavier bead on there. That should help.
Chuck
Tom,
the topic title indicates that the material is 304 stainless steel, but this cracking does not look typical of 304 base metal/308 filler metal. First of all, cracking of 304/308 is not common, it is a very ductile, forgiving material. "Hot cracking" or 300 series may occur during solidification, due to very high sulfur content, as in type 303 or from severe contamination, but I don't think it would look quite like this. It appears that the weld metal itself may be something hardenable, something other than 304/308 mix. Could one or both of the base metal components, and/or the filler metal be incorrect? This really looks like a brittle fracture.
Stan
By -
Date 06-20-2005 22:52
Hi Tom,
If I may add to your response without sounding disrespectful.... I see solidification cracking frequently in a 308 filler when shallow beads are used to join thick materials. The 308 contains only around approximately 3-6 ferrite potential, which doesn't give much protection from hot cracking when used on thick sections. Even at higher ferrite levels, hot cracking (solidification cracking) is common when joining thick materials with shallow single beads. The center of the weld bead is the last to solidify, so that is normally when center line, hot cracking, solidification cracking (it's called many things) occurs. Given the information given to us, and looking at the pictures, I still believe this is a cooling down problem. I agree with you about 308 cracking being unusual, but it is not unusual if not welded properly, or if it is cooled down too rapidly as when welding thick members using shallow, single beads.
Chuck
The stitch welds do apear to be rather small, generally concave in shape and have underfilled craters. I would be interested to know the welding process, electrode classification and what fillet weld size/spacing was specified. It's tough to tell without all the details. You may want to contract a local testing lab to perform chemical analysis of the frame material and weld deposit. A ferrite measurement of the weld deposit would be good, but it could also be estimated from the WRC-1992 diagram with the deposit chemistry results. There should be at least 5FN present, but as little as 3FN would prevent hot cracking provided sulfur content of the base metal is within spec. We look for a combined phosphorous + sulfur content of 0.025 wt% or less.
By -
Date 06-21-2005 11:50
Tom,
It is quite apparent that the welds that cracked will need to be removed and re-welded. Also apparent is that you state that the base metal is 304 S.S. Before spending a lot of money on various lab tests, and they are not cheap, try heating up the area to be welded to approx. 150F, use a higher heat input regardless of the weld process you choose to use, and deposit a heavier weld bead. I have seen 2205 Duplex stainless steel crack in the root pass, and this is around 40-45 FN, due to a shallow bead deposited on 3/4" open butt root passes due to the lack of proper cooling properly. Just because a certain filler has a certain FN, does not guarantee there will be no cracking. Tom, your weld has to have sufficient time to solidify. Try using the above mentioned suggestions and see if that helps your problem. That is the quick and much cheaper way to see if that will solve your problem. Then if you want to spend money on various metallurgical tests, that is up to you, but try the obvious first.
Chuck
Thanks for the information.
The fabricator tells us that the filler used was T316. And the fillet radius was 3/16" to 1/4".
By -
Date 06-21-2005 14:01
Hi again Tom,
Knowing that the filler was 316 and the base metal is 304, that tells me, based on the pictures, past experience, and knowledge, that the welds cracked due to the rapid dissipation of the heat from the molten weld puddle to the larger volume of the base metal. This is not uncommon based on the joint configuration and the way this was welded. Suggestions have been made to you, now it's up to you how you proceed. Good luck...
Chuck
Tom Rice,
A few metalurgical / failure analysis tests, can positively determine the cause of the cracking. This can be extremely valuable information when it comes time to sort out who pays for the mistake. If your subcontractor supplied you with something other than you specified and you can prove it, you have some recourse. If you alter it before knowing for sure what you have, you may no longer have any recourse.
Good luck and keep us posted on the results.
If you need the name of some reputable failure analysis labs, I can provide a few.
Koz
Koz,
The welds were rejectable from a visual inspection, not to mention other inspection methods. If there is any metallurgical testing done, it should be at the expense of the fabricator, not the customer. It's quite obvious in this case who is at fault for the cracks in the welds. I seriously doubt that Tom Rice specified cracks in all those welds, so the bad welds were done by the fabricator who is at fault. One doesn't need an expensive metallurgical test to prove there are cracks in those welds, we can see them. That right there is proof that Tom was given something other than what he specified. He has all the recourse he needs by the poor quality of the cracked welds. A few metallurgical/failure analysis tests can run into a lot of money when it is pretty obvious what the problem was. I would not recommend doing a bunch of destructive testing on his parts. Tom should send the parts back to the fabricator and have it welded properly. If the fabricator doesn't know that welding shallow beads on a thick piece can be a major reason for solidification cracking, maybe the fabricator should ask for advice of eliminating the problem. Maybe Tom Rice should turn these suggestions over to his fabricator and let him decide how best to give Tom a satisfactory product.
Chuck
Well, this posting has certainly had some good feedback! Chuck, having an enormous respect for your knowledge and experience working with stainless steels, I'm not sure I completely agree with your assessment in this case.
Tom states that the materials are formed channels and only 7 ga. in thickness. I am not qualified to argue the metallurgical points with you but to me, all pictures seem to initiate from the craters; we have welded tons of ss in my shop and if my guy's leave a "fisheye" they will almost always get cracks. I also don't know if I could recommend preheating due to potential distortion concerns. Okay, just had to toss in my two cents! ;-)
Hi my Friend,
Thank you for your response. The good part about these forum's is one's ability to disagree or agree without any bad feelings. As you said, a "fisheye" is certainly a reason for a potential crack. In my experience, this does not run the full length of the weld right in the center. A "fisheye" can cause a transverse crack as easily as a longitudinal crack. The great majority of cracks that run the length of the weld right in the center is due to solidification, or lack of. Also, I'm not recommending a formal preheat, just enough to retard the cooling rate of the weld, not enough to distort the piece. Even on 7 ga. the volume can be great enough to suck the heat away from the weld and not allow complete solidification. Actually, probably using a higher heat input with a heavier bead will serve the purpose without heating the channel. A shallow, single pass bead is just asking for trouble anyway.
Chuck
I agree with you on the "usually doesn't run the full length" and the "shallow, single pass bead is just asking for trouble" ~ there are probably several factors involved in these failures. Obviously, Tom needs to go back to his fabricator and have them sort out the "why's and what to do's." A bad position to be in on both sides of the fence!
By -
Date 06-21-2005 17:46
I certainly agree with that.....You're right about Tom needing to get back to the fabricator and find out exactly how they welded it, what process was used, and all the facts. It's pretty obvious that they don't want to do what they did the first time. Like you, I think that more than one variable contributed to this problem. Personally, I think that the weld process, the shallow bead, the accelerated cooling, and any other number of things are contributiing causes of this cracking problem. As serious as it is, I believe it is pretty easily corrected. Have a good day like we're having here in Dallas. :)
Chuck
Evident cracking mechanism is most likely fatigue failure.
Cause is likely that the welds are too small (too cold, high residual stress) and poor bead geometry, especially at starts and finishes. Heavy structure, small beads, large cyclic energy input (them slab joints) during shipment.
Tend to to think not due to any solidification cracking condition, but ANY contaminant on the material surface can predispose an austenitic weld metal to crack. Cracks may easilly be subsurface, and very convenient for fatigue propagation.
Fab shop has apparent little understanding of how to work with the material.
Disagree about fatigue cracking on welds very recently made. Other points are viable. Contamination on the surface will more than likely cause porosity than center line cracking the full length of the weld. Very unlikely these cracks were subsurface that propogated to the full length of the weld right on the surface. Maybe Tom Rice will inform us when the piece is re-worked exactly what the problem was.
CM
I think the truth lies somewhere inbetween Bonniweldor's comments and Cmeadows reply. I don't know if fatigue is involved, but the welds certianly look very similar to other failures I have seen due to overloading of the welds due to shrikage stresses imposed by the welds. The welds are basically just too small to carry the load imposed on them, whether by shrinkage stress, fatigue, or a combination of the two. However, pre-existing defects such as solidification cracks due to poor weld profile or other causes would have contributed if present. I don't think this is a metallurgical problem though. It looks to me like it would have been as likely to happen on carbon steel given the same weld sizes and stresses as on the stainless steel that was used.
Based on lab analysis tests regarding fatigue of welds, the failure usually, not always, but by the vast majority, happens at the weakest point of the weldment which is at the toe of the weld where it meets the HAZ, not at the center of the weld. The necessity of a stainless steel to cool down properly to achieve maximum phase balance and properties is definitely a metallurgical concern. When ignored, it becomes a metallurgical problem.
As I mentioned, I wasn't sure if fatigue was involved, but your study would be another good reason it may have not been fatigue, but simple overloading of the weld. As you know, and overstressed fillet weld usually breaks through the shortest path, which is the root (with the built in notch) straight throught the face, just at shown in the photos. Does the study you reference study fillet welds, groove, or fillet combined with groove? It would be interesting if fatigue caused fillets to break at the toe. I'm not familiar with that aspect.
I'm also not disagreeing that stainless metallurgy is important, and the posts regarding the metallurgical aspects seem to be accurate. However, the similar failures (based on a small amount of information and a few photos) I have seen have been from simple overstressing of the weld, so that is my guess. As always this kind of discussion would be easier with all the information and the part in front of you.
I don't disagree with your facts of the failure of fillet welds, but I really don't think that the photos are indicitive of the failure of fillet welds. The weld profile, depth, and single shallow beads (as in the photos) are usually not associated with fillet welds. In our studies, a fillet weld will react differently to fatigue than an open-butt or autogenous weld. Of course so many variables have to be taken into consideration to determine any failure. In our investigations it is easier and probably more accurate to test butt welds rather than fillets. If a butt weld and a fillet weld were subjected to the exact medium and conditions, I'm not sure they will both fail at the same rate due to the a number of factors. But, I guess anything is possible.
Overloading would be indicated by plastically strained material, and these welds do not exhibit that characteristic.
Assuming the welds were not fractured as indicated in the pictures when put on the truck (would expect that to be observed), fatigue is the likely mode for crack propagation, if not initiation.
The significant stress raiser in this context would be the root, rather than the toe.
By -
Date 06-29-2005 16:03
"Assuming the welds were not fractured..." Too much assumption is being done here. Who knows who loaded the truck? Did anyone inspect the welds after they were loaded? Bottom line.... welds cracked causing an unsatisfactory condition. Fact....Welds were improperly constructed due to a number of possible circumstances. Who knows for sure? I'm sure Tom Rice will give the fabricator the "possible" reasons for the failures and go from there.
As a welder the comentary here is interesting to one who is always looking for new info. On the other hand whenever one takes on a job a thing called implied warrent of merchantability comes into play. This is very old law and exists in some form in common law upon which our system is built. Unless the welder was given a detailed procedure which he followed to the letter this is his responsibility (welds that crack before the parts are even put into service are not merchantable). If the welder claims that the damage was caused by the trucker then that is between them.
Bill
Tom,
It looks to me that perhaps the welds were made with the wrong welding wire. If the welds were fusion (no wire added) than I would say that weld failure was due to a high sulfer content as 304 as opposed to 304L could (I have seen this occur early in my career) cause this to happen.
When I first looked at these photo's I was baffled as to why they would fail. The weld beads seemed to have enough re-enforcement and the fillet welds have equal legs so technique is not the issue.
To have a weld (here, a series of welds) fail from loading the truck doesn't make much sense to me. Your pictures are very clear. It looks to me like the welding wire used has a high nickel content such as inconel or maybe 317.
I am only speculating, but I was doing a job welding super-duplex ss steel while others were doing a 304 stainless job and one of my co-workers accidently grab a wire off my table and used it to weld his frame. Shortly before someone noticed this exact condition and stub wire (with the flag still on it indicating it was 317) was near the couple of welds this happend to. Just my opinion though.
Medicinehawk,
What wire were you using to weld the Super Duplex? 317L?
Chuck
That job was quite awhile ago .....it might have been 2209, but it was like 7 or 8 years ago and I just cannot remember Chuck.
Hawk
By -
Date 07-01-2005 02:08
Hey Hawk,
How's it going? I was just wondering.....I probably misread yur post and thought you were using 317 to weld the Super Duplex. Anyway, the 2209, as you already know, is used to weld regular Duplex (UNS31803 and UNS32250 primarily) Since there is no AWS classification for the "Super Duplex" (UNS32750), the filler metal is propriatary. Did I spell that right?? Anyway, I was just wondering. Have a good one !!!
Chuck
