304 SS Weldment Thermal Stress Relieving

I apologize for posting to my post. I failed to mention we vibratory stress relieve the weldments.

Thank you,

Ken

OK so have the parts been TIG welded or Stick welded? The 2 processes are both manual, yet very different.

MIG versus dual-shield is another problem. MIG is a very fast process, but on stainless steel creates severe localized distortion, and therefore stress. Dual-shield is a hotter process, and does a better job of fusing the material, while introducing less stress.

You need to clarify your processes. There are some very experienced members of this newsgroup. I personally have about 25 years of metals experience and I am a certified weld inspector.

I have extensive experience welding and machining stainless steel and I would want more info before rendering a judgment.

A benefit of stitch welds is that if a crack develops in a weld it can't propagate.

On 1" stainless steel I do hope you are allowing for a preheat of the weldment.

You also have not stated how or if you are dealing with post weld passivation of the surface.

Passivation is a critical step of stainless steel welding if the part is to have any atmospheric exposure.

MIG on 1" plate stainless steel is not something I would recommend. Dual-shield would be my favored process.

Thermal stress relief will also help restore corrosion resistance of the part. Also are you using 308L, 308LSI, or 309L filler?

I answer these kinds of question several times a year for manufacturers in the greater Seattle area, as a fabrication consultant.

Ernie, your reply raised a couple questions. It would seem to me the added heat of the dual shield would cause more distortion, not less. Is there a reason why doesn't it? Not doubting you, just wondering why.

Do the stitch welds have to have a break between them? If I stitch weld and then come back and fill in to form a continuous weld, does that defeat the stitch welding benefits?

Thank you, and I hope to be able to meet you one of these days, I'm in the Bremerton area.

wow.. its been a while since I've seen that. Sorry to momentarily interupt the discussion.

I've seen VSR for weldments and have never been able to get my head around how such low frequencies could have any effect whatsoever on the weldments stress levels.

I've read all the literature and I've seen the "tests".

Not being cynical here -- genuinely curious -- do you see real advantages from VSR and how do you quantify them?

Who's system are you using, if I may ask?

Thanks, Tony

I am wondering just how much welding time would be saved. Even if there are ten units to make by the time you set up for a continuous weld I can't see saving more than ten or fifteen minutes per. If the stitching is fifty percent of the length we are only looking at 90 inches if it is both sides of the channel. Sometimes the true cost of tooling up is ignored. My bet is that a lot more time than fifteen minutes has been expended discussing this alternate procedure. A continuous weld does not necessarily make things "stronger". Likely this channel only is there to stiffen the plate? If there are hundreds of these to do why not do stitch welding. One could rig the unit to stop and start the arc. As Ernie mentioned there are some very nice and easy to use dual shield flux core wires for stainless that use carbon dioxide. Randy

Gents-

Thank you for the responses tomorrow morning I will post our process and answer some of the unknowns due to my lack of welding experience. Thank you again for your time.

Respectfully,

Ken

Dual-shield is hotter than MIG, but the flux layer prevents the weld from cooling as fast as MIG, which reduces the severity of the weld contraction, and stress.

Preheating the base metal also helps to reduce the thermal shock of wire feed processes. Preheat and interpass temps are listed in reference books.

Somewhat yes. You would still get the benefit of reduced heat input, but cracks could propagate from one stitch to the next, if they touch.

Back filling between stitch welds is commonly used on long seams on boat hulls to reduce the distortion of the weld process. The full process is back-stepping skip-stitches, but most of that has been eliminated with modern pulsed MIG equipment, which allows continuous seam welds without stitching at all.

Gents-

"Our fabrication house has gone to a continuous seam weld on the reinforcement brackets in efforts to improve the appearance and long term strength of the product. We have also made some major improvements in weld quality and lowering the overall heat input and subsequent residual stress in the as welded parts. We have changed the process to a robotic application using a synergic pulsing power supply. This allows us to reduce the heat input and the amount of spatter produced. We have increased travel speeds by a factor of four combined with the pulsing parameters have lower the over all heat input by a fourth. This becomes very evident after welding as we no longer see the distortion problems we encountered using a stitch weld manual operation. We feel that to further try and eliminate stress from these parts would be non effective and to thermally reduce stress would actually damage the corrosion resistance of these parts."

The quanity of this job is 120 pieces. This is the statement from our fabrication house to present to our customer. I do not feel this is objective evidence of why we do not need a thermal stress relieve.... help!

Thank you again,

Ken

That is a good argument Ken since you are doing 120 units.... Also you might get the suits to reduce the fillet size to further economize. I worked in a yard where Tee extrusions were being stitched to shell plate and it was a nightmare. We went to a bug riding on the top of the Tee and a continuous weld. Sure enough the first time an inspector saw it he rejected it because the fillet was too small. Eventually it became the normal procedure. Randy

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