Hello. I am having some difficulties with weld breakage on a small engine frame made from 304L stainless square tube. The primary structure is made from 1 x 1 x 0.083 inch or 1 x 1 x 0.065 inch tube. The engine is a two cylinder 25 hp Kohler so there is definitely some vibration. While I have not had any actual primary welds break, the failure of ancillary components after only 25 hours of run time has me greatly concerned. In the past these frames were constructed using mild steel and a Miller 175 MIG welder, with no failures after hundreds of hours of use. In the interest of improved productivity I have switched to stainless steel which I can assemble in-house, without hot dip galvanizing or powder coating. Some of the frames are used in a salt water marine environment, which seemed like a great application for stainless.
The equipment I'm using is a Miller dynasty 200 DX with liquid cooled CK230 torch, 3/32" ceriated tungsten ground longitudinally to 30° included angle using a 100 grit aluminum oxide sanding belt. I have since tried 0.040 inch and 1/16 inch ceriated tungstens ground to 18° included angle which seems to improve the weld appearance, narrower etc. I have been using gaslens, either standard or the CK gas saver Pyrex set up. Pure argon flow rates have varied from 15 CFH to 8 CFH when I was using the gas saver Pyrex set up. When welding on tubes I have since increased the CFH to 15 as I was advised eight was for perfect flat situations only. The filler material is a 308L rod.
My concern is that switching to stainless steel which has higher thermal expansion and lower thermal collectivity is creating high post weld stresses that are causing failures, or could lead to more failures after hundreds of hours of use. I am already planning to do some design changes to limit welding, thereby distortion, assembly time and increased stiffness were vibration related fatigue caused one failure. In that case 1" x 1/8" flat bar was drilled in the middle to 35/64th to it except a 1/4" schedule 40. tube. The tube was welded to the bar autogenously. After 25 hours of run time to flat bar broke off adjacent to where the tube had been welded. In this case I think the flat bar should be replaced by a 3/4" by 3/4" angle.
The clearest example of the limited fatigue life of my new stainless weldments is a spacer/torque delivering part. This component is attached to a pulley driven by half of the engine's power via a long belt drive. This is accomplished by three, ¼" schedule 40 pipes welded into a triangular configuration on a 75 mm bolt circle. To handle the torque, three 1" x 0.125 inch pieces of flat bar are welded across the three tubes to connect them and form this triangular part. 5/16" bolts pass through the quarter-inch schedule 40 pipes to connect to pulley to the driven device, in this case an air impeller. Previously these parts were constructed with mild steel and have survived hundreds of hours of use. When constructed using 304L stainless the parts are breaking after about 25 hours, or about 4.6 million cycles, assuming one cycle per rev. In one case the spacer was constructed to be 4 inches long and lasted only one hour.
Any thoughts? Are your eyes glazed over yet? Do I have a hot cracking problem? Welding induced stresses that can be relieved with a heat treating of all the parts, negating my in-house solution?
Bryan
This is a tough problem to sort out without pictures. Can you post a photo of the failed weld?
When you say this: "The tube was welded to the bar autogenously." Does that mean you have not added welding rod??? That could be part of the problem. Also, consider using thicker material (1 x 1 x 0.083 inch or 1 x 1 x 0.065 inch tube) seems thin to me. I believe the strength of 304 is equal to or greater than carbon steel. You consider 304L which is low carbon stainless and thus slightly better quality. A good quality weld should not break even under the conditions mentioned . After making a weld bead using GTAW , the weld should look gold to slightly purple or even copper colored but will clean up nicely with a wire brush. As mentioned earlier, pics would be helpful.
Your problem may be due to stainless work hardening more readily than mild steel, and having a lower fatigue limit because of it.
The spacer seems a poor design to Me, My preference would be solid round stock with the holes drilled and the ends machined/ground parallel.
I've attached a few photos into the original message. Unfortunately I do not have the actual failed parts in my possession at this time. I have made some welds with the undesirable gray finish and recognized them as overheated or poorly shielded. Typically I am getting a purple or golden finishes, as my technique has improved. The spacer could be changed but the current design allows for necessary access to fasteners. The solid design would be significantly heavier, more complicated to implement and require some lathe work. I have considered replacing it with a solid plastic spacer. In any case it seems this is a symptom of what might become further problems with the other parts of the frame, if the stainless structure is more susceptible to these problems, than the previous mild steel and MIG welding.
I should have noted that this is a weight critical application, every ounce counts.
Does anyone have knowledge of what kind of beneficial stress reduction could be achieved at lower temperatures such as 800°F?
The first thing that comes to my mind when reading your post is you'd be better off with 316L rather than 304L for salt air/water environment.
Because of its higher corrosion resistance? My thought when a choose 304L was that this is not bright work, rather structure under a fiberglass cover. In any case exposure time has been short enough to where no staining or corrosion is visible.
whats visible is not always what needs to be worried about. I would suggest having a look at naces handbook and some other materials in that regards.
Bryan,
After looking at your picture.. I see brown smoke redidual at the toes of your welds... This is very uncommon for stainless steel.
Blue, pink or even grey/black on the top of the weld and it will still be quite strong... But It looks like your entire gas shielding is starved and your burning tungsten...
What color is the tungsten after each weld? If it is shiny and clean, no problem. If it is anything else you have a problem... You may want to go a bit higher on the gas flow with your GTAW.
300 series stainless... whether its 304/308/316 or 347 is super tough and sea water is not going to be a problem in a short turn around of time such as you are describing.
If your seeing smoke at the edges of your weld... This is indicative of a problem... Whatever is causing the smoke is likely the cause of the cracks. When the source of the smoke is gone, whether it is gas or surface contaminants, I suspect the cracks will be gon at that point to.
304L is marine environments can have some problems of corrosion: pitting or crevice or stress corrosion cracking. What can cause a sudden failure of a weld could be stress corrosion cracking but this usually happens at temp approx 60°C ( but this limits is a function also of the internal stress of the weld and external stress and how much chloride will consensate on the weld)
Upgrade to 316 is not the solution if the problem is SCC (the limit is the same of 304).
i think that the problem is most a design issue.
http://www.corrosionist.com/Materials_Stainless_Steel_Welding.htmV.
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