Autogenous weld cracks in 316L 1

[Capntom]

We fuse 1.25" Diameter 316L to 316L tubing; .083 wall, without filler and we have recently been experiencing microcracks that fail Pressure test (200 psig). We used to use 304 and occasionally 316, problems were very infrequent. The joint is a square groove without backing or purge gas. 50% Penetration. The surface must be flush w/o post weld finishing. Hence, no filler.
Capt'n Tom

 

[LAMAN75]

If your penetration is 50% your only getting about a .043 filler and of this .043 filler how much of this filler is good metal, your not using any backing gas? The problem seems to be thermal expansion causing the cracking. You simply do not have enough good metal to hold the joint together. Even if you have no root spacing your still experiencing sugaring of the root pass on the ID of the joint. A good backing purge and post flow of purge gas on the OD of the tube upon termination of the arc on the welders pull out is also necessary to prevent crater cracks. Are you welding in compliance with any particular code and or client specification? I have never seen a welding procedure qualified to weld ss without the requirement of a backing gas enless it was a fillet weld.

 

[Capntom]

We are an OEM and one of my battles is "We have always welded it like this". I have been with the company for 5 yrs. We have just recently become certified to Weld to MIL-278. All of our new designs have joints that are in compliance to MIL-22. The issue is "older" models that have been "approved" with letters. I am looking for specific justification to add "filler, backing gas, etc.." This will obviously add to the unit cost

 

[LAMAN75]

What process are you using, I assumed it was gtaw, I almost forgot but there is a procedure for welding ss without a back purge using the gmaw process short curcuiting tranfer with a special gas mixture. This application may or may not suit thin small bore tubing, but it may be an option. You would be adding filler and you may have to requalify your procedure. Either way I can`t really see much added cost.

 

[unclesyd]

We have welded thousands of tubes from 5/8"-1 1/2" 10-16 gauge. We used an orbital tube welder and all autogenous welds. We were safe ending S/S tubes mostly, but we regularly we welded 1" and 1 1/4" 10 and 12 gauge tubes. We welded 304/304, 304/304L, 304/316, 304L/316. We used an Argon purge system. At times we experienced similar problems to yours, but were able to lower the reject rate to under 0.1% simply by the addition of a square end prep machine and take the emery paper and files away from the welders. We pneumatic tested all tubes and hydrotested 10% or more.

Check and see if anyone is putting anything on during the prep.

Check and see if anyone is touching up the prep, Si-Carbide paper or a file