tig welding low ferrite 316L S.S.(correction of heat stress fractures) 5

Can you add a filler wire? If so pick one with a good potential ferrite level.

Is your base metal or weld metal low ferrite? Wrought (forged, rolled, etc.) 316L usually doesn't have much ferrite, if any.

What do you mean by "heat stress" fractures-base metal cracking? What filler metal and ferrite number?

The cracking comes from low ferrite and sulfur in the 316. If you can't increase the delta ferrite to 5% or more, you can lower the sulfur to under 0.002%.
What is happening physically is that the 316 is solidifying in a austenitic mode and the sulfur is being rejected from the austenite dendrites to grain boundaries where it is a weak film. Ferrite doesn't reject sulfur, so a little delta ferrite eliminates the problem. So, does not having the sulfur. That's the easy way to solve the problem. Laser welding would avoid it as would using consumable electrodes with low sulfur or higher ferrite.

the stress cracks are in the base material and mate material, base material is a german cast 1/2" globe valve with a .065 wall thickness joined with standard 1/2"x.065 tube. no filler wire is used as it is being orbitaly welded.the ferrite is so low that it does not show a measurement on our fisher ferrite meter.the sulfer level is within .005/.017

Bad news. Tubing almost always is high sulfur. I don't know why anyone would make tubing with a low ferrite number and high sulfur. It's like they want their customers to suffer a lot.
I don't want to be comercial on line, so I won't. You know what you need, either higher ferrite, lower sulfur, or something clever. Inquire at MFM304@AOL.COM if you want advice eng-tips.com doesn't want posted.
If you can increase ferrite or reduce sulfur, you'll be O.K.
Why doesn't the German casting have higher ferrite? That would be normal. Is this an international design?

MCGUIRE,THE VALVE IS AN INTERNATIONAL DESIGN, BUT THE VALVE CAN BE PRODUCED IN ANY PART OF THE WORLD.SIGNICAST IN THE U.S. PRODUCES A VALVE WITH A COMPOSITION MORE WELDER FRIENDLY.

Are you sure you are getting heat stress fractures and not also incomplete or asymetric weld penetration? Sulfur has the effect of increasing the weld penetration substantially. Therefore when you are welding two pieces with very different sulfur levels, you can get asymetric penetration, i.e. the weld will be at the joint on the external surface, but be away from the joint at the root of the weld. This can be overcome by careful weld set-up.

This is why most tubing has .005 to .017 sulfur, to increase penetration. I have had people tell me that 95 percent of their welders CANNOT weld tubing with lower sulfur levels. We did it every day, but it takes some experience! Valex makes tubing with lower sulfur levels.

The other comments about using a filler wire with ferrite or laser welding will definitely help reduce or eliminate weld cracking, or microfissuring.

tev,thank you for your information. Low sulfer material is not as difficult to weld as most welders say that it is, I have been welding low sulfer german made valves for about five years.the problem most welders have is trying to weld the material without using a machine with a pulsing capabilaty.

You can weld low ferrite material, even with some sulfer in it without hot cracking. The Suutala diagram shows that when the Crew/Nieq is over about 1.5, that solidification cracking is greatly reduced. Below 1.5, you do need very low sulfer levels. Read more at the links below.

That should have been Creq/Nieq, not Crew/Nieq.

Creq = Cr +1.37Mo +1.5Si + 2Nb + 3Ti
Nieq = Ni + .31Mn + 22C + 14.2N