I wish to build my exhaust using gas welding.The tubes and bents will be 304 or 321 stainless with a thickness of .035" or .049".
Any advice will be very usefull as the fiiler rod and flux choice.
PittsS1S, The melting point of stainless is higher than the flame temperature of an oxy/acetylene torch. The few fuel gasses that can burn hotter are not suitable for welding due to embritalment and other issues.
The real time in fabricating exhaust headers is fitting all the pipes slip connectors, and flanges into a package that clears things like the motor mount and stays clear of the cowl. You might tack weld all the pieces together
with a portable MIG welder like Harbor Freight sells and then have someone with a TIG machine weld it up. Finish weld it up in sections and trial fit it to see if adjustments for warpage are needed. Alternatively carry
the engine along and have the final welding done with the flanges bolted to the engine. If the welder doesn't know what filler rod to use or that back side flux is needed he isn't your welder.
wizzardworks, most 300 series stainless is fully weldable with a gas torch. not even close to an issue with heat. I can't offer any guides for filler rod but a good welding forum will point you in the right direction.
I have TIG'd them. I think the problem you will find in gas welding is controlling heat. Even with tig and a foot pedal, it's easy to burn a hole in thin metal. WeldingFAQs "gas welding stainless not for the faint of heart"
get a Tig rig. You will find it very useful for aircraft parts.
Mild steel rod can be used for stainless and flows better than stainless rod ( much easier to weld, no flux needed).The weld won't be quite as anti- corrosive with mild steel rod but is more crack resistant, I think. Stainless isn't really corrosion proof anyway since the exhaust tends to corrode it from exhaust acids. It also gets brittle and cracks.
The cracks in old corroded stainless are hard for me to weld with stainless rod but can be welded with mild steel rod much easier and better. These old, dark and brittle stainless pipes are hard to weld with gas because it just foams up.
I use MIG with mild steel wire for almost everything.
When I TIG stainless headers, I always backgas the pipe. No sugar on the backside, cleaner weld on the OD and the inside looks like you silver soldered it. Make sure you wash the stainless bends before you weld them, the grease left over from the bending process will mess up a weld when it heats up and starts running out the joints. Make sure you use a very tight fit up (watertight), dont bevel the edge and use a very, very small diameter filler rod (wall thickness of material or less). doing it this way, it takes very little heat (30-40A topside setting)and makes a pretty, clean weld. I dont bother with flux when tig welding.
As a AWS CWI and CWE I can tell you that you can weld SS with your Oxyfuel torch. you just need to use a mild steel filler rod with a flux. it will weld perfectly that way. The electrodes used for SMAW(stick) welding of SS are a mild steel rod because SS is a terrible conductor of electricity, the flux coating on the rod contains the elements that are added to the molten puddle to make it a corrosion proof metal.
Go down to your local welding supply store and have them get you the right flux, dip your hot filler rod in it as you weld. Heat control is touchy but no more than welding 4130 of the same thickness.