I spent 1 1/2 years looking at cubs before I got mine. Having a new fuselage on a 1951 cub was a huge selling point. Nothing wrong with a properly repaired/rebuilt cub but that "properly" part may be a bit hard to find.
DENNY
I don't actually "know" the answer to that, and I think the correct answer is "it depends" but here's my take on it.
Post heat can do two things. It can relieve residual weld shrinkage stresses, and it can re-normalize after welding induced self quenching. For re-normalizing a temp of 1600 - 1650 is required, and I don't know how long that temperature has to be maintained.
Pre heat can reduce or eliminate self quenching, eliminating the need for re-normalizing. Then post heat can be applied for stress relief if needed.
As a practical matter, I do know that 4130 is not highly susceptible to quenching, and is commonly TIG welded to aerospace criteria without either pre or post heat. A high end shop near me does that and destructive testing at a nearby metallurgy lab has shown it to be suitable, in the thin sections found in tubing. Heavy sections would be more susceptible to self quenching and pre/post heat could be required. But we don't have those heavy sections in our Cubs.
I'd say if I were in doubt about thickness and quenching risk, I'd preheat to around 400+ (amber color). That's merely opinion.
The amount of Pre and post heating a weld has more to do with the duration it took to perform the weld and the mass, in this case the thickness of material. The goal is to achieve evenness in the heating and cooling to reduce the chance of uneven stress. If you are gas welding the process is slower and the heat spreads more evenly. A slow withdraw of the torch while spreading the heat is most you need to do. This also maintains the shielded envelope of the flame till the material cools below the critical level, generally considered the color change.
With tig, starting the heating away from where the weld will begin is a good thing. This might be doing a tack near the endpoint, then move the flame to where you intend to start the weld and proceed back through the tack. Long welds or thick material one would stitch the weld and then close out with making sure the whole heat affected zone tends to cool evenly. Again you want to maintain the shield gas till the initial cooling has taken place. I find I commonly need to tap the pedal to keep the post flow going till the material is below critical level.
Larger, thicker weldments take more care during cooling such as burying the part in dry sand or using a Nomex wrap.
Question: In TIG, if the gas shield is important to protect the weld from oxygen, why don't welders pipe argon into the tube being welded? Wouldn't the shield be needed on both sides of the weld?
Question: In TIG, if the gas shield is important to protect the weld from oxygen, why don't welders pipe argon into the tube being welded? Wouldn't the shield be needed on both sides of the weld?
They do, most common with stainless and Ti. Seen with aluminum, not as often with the simpler alloys in steel. I have capped the ends of steel tubes just to reduce the air exchange and with flat sheet in all material used flux on the backside.
Question: In TIG, if the gas shield is important to protect the weld from oxygen, why don't welders pipe argon into the tube being welded? Wouldn't the shield be needed on both sides of the weld?