AWS Process to minimize distortion

[dsgn2mfg]

I was wondering if anyone knows of an AWS (or similar) process specification to minimize distortion when welding stainless steel plate. We are assembling large vacuum chambers and need to control heat distortion that occurs during welding. We don't do the work in house but want to be able to provide as much assistance as possible to our vendors and to accurately specify what is reasonable to expect as an outcome from the assembly process.

Thanks
dsgn2mfg

 

[metengr]

AWS has published a paper in the past on controlling or minimizing weld distortion. This is not a specification, standard or other enforceable item, it is only general guidance.

What is the specific grade of stainless steel and wall thickness?

 

[dsgn2mfg]

Mostly we are working with 304, 304L to 304, 304L or 316L. We are making vacuum chambers so the walls, flanges and passages are all SS. We often weld mild low carbon steel stiffeners and gussets on to the larger external areas for structure. Depending on size/spans, function, and number of penetration plate range from 1/2" to 1-1/2" finished. They are often Blanchard ground for finish and flatness prior to pre-machining. Once assembled into the weldment, we often do secondary machining and sometimes follow up secondary welding. To add more heat generated distortion we often welding cooling tubing/channels to carry water all over these chambers. The channels require continuous welds on both sides. Tubbing is only slightly less problematic as we still do 4" x 4" stitch weld to assure good conduction.

Usually we do structural GMAW stitch welds on the exterior and GTAW full length all around on the interior for vacuum seal. We then leak test the chambers with helium to requiring at least 2 x 10-8 atm-cc/sec.

A typical chamber will be a box 66" long, 60" wide and 12" tall. Construction will be to weld materials ground to 1.25" for top plates, 1.13" for end plates, .5" for side and bottom plates. Tops and bottoms will almost always have external carbons steel stiffeners. We almost always do post weld secondary machining on the end plates that requires flatnesses and parallelisms from end to end of .005".

 

[metengr]

dsgn2mfg;
I was not sure if I completely understood your fabrication sequence - it appeared to me that you are using external fillet, stitch welds to join the end pieces, and are using a continuous, seal weld (GTAW process) on the inside surface. Correct?

 

[dsgn2mfg]

Yes, that is correct. Almost all structural welds occur on the outside of the vessel. To accomplish this the boxes are constructed with top, bottom and ends overhanging the sides about 1-1/2". Structural members are most common on the tops and bottoms and not required on the sides because of the short distance from top to bottom plate.

 

[dsgn2mfg]

More,

When possible the chambers are structurally welded first and this helps a lot, but so much heat is put into the chamber it seems to distort a fair amount. Between that and actively cooling them we rarely have a problem. What I'm trying to do is write a welding specification and was hoping I could find some AWS standard the clearly outlines good practices. Similarly I am looking for the same thing for weld preparation, precleaning the parts prior to welding and post cleaning after.[u/]

 

[metengr]

dsgn2mfg;
Have you looked at specifying a continuous fillet weld on the outside of the T-joint versus a 4" stitch weld? Having a continue fillet on the outside surface along with the GTAW seal weld on the inside surface of the T-joints may do the trick to reduce distortion.

 

[Tomtation]

TIG welding produces a lot of heat. Use pulsed-MIG, it is very quick, gives very nice welds, is easy to learn and minimizes heat and distortion.

 

[dsgn2mfg]

Thanks for the tip. We usually use pulsed Mig on the exterior for structural welds and TIG for the interior Vacuum tight welds which are usually just fusion welds. What I'm looking for is not how to do it (we have that down) but for a specification that helps me quantify it and that will have meaning for potential vendors.