I am having problems with heat stress fractures in low ferrite(0%-.2%) 316L sainless!Just wondering if there is something that i can do to correct the problem when making butt weld fussion welds.

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.

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.

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