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Looks like most of the important stuff was covered. Stainless is a great choice really. race cars don't use it because it isn't strong enough to be worth the cost, and the purging requirements make it a real PITA to test for sound welds visually.
As mentioned, 409 works fine for med-temp stuff, but protects itself with a coarse oxide that appears similar to rust. 304 and most of the Austenitic stainless alloys have a homogenous, smooth chromium oxide on the surface that looks much nicer. 304 for a frame would be great as it has excellent fatigue properties, near as makes no difference double the elongation elongation(ductility) of mild steel, and it's stronger than 1018 by a good margin. All these things sound like good design pluses is a structural member under stress and dynamic loads.
Now, here are some drawbacks...
Fit-up must be perfect. As mentioned, it will distort more for a given heat input, so any gaps that require more filler and heat, will shrink more and cause issues. It also welds pretty terrible when fit-up is not precise. Add in that your shielding gas will be escaping at an alarming rate through said gaps, and it's less than perfect welds based on contamination. The leaking will cause a sharp jump in cost also.
TIG welding is really the only thing for a frame like this, so add about 4-5 times the hours if you were planning on a MIG job for your mild steel. Filler and gas costs go up about 5 fold, and so does the required skill level to get a quality result.
Cleanliness:
Stainless has to be near-surgical in terms of surface prep to be done right. This requires more equipment, more chemicals, careful selection of PPE, etc, etc.
Weight: Stainless weighs about 2.5% more than 1018 depending on the alloy. This is not insurmountable, but does add up. You can go thinner on the wall, but then you lose stiffness, and bending ability during fabrication. On the average frame this would only amount to about 3-5 lbs. Except for what's next....
You must, as mentioned, drill holes in every tube juncture in every tube to ensure good purge gas flow and fill. These should not be 1/8" holes like you need to reduce the pressure effect, but about 1/4-1/2" holes to allow the gas to move around efficiently. This will add in a very long wait time for a proper purge versus a typical exhaust job. I would guess a regular bottle of argon may only last one shot of purging and welding. This would mean a very long day to avoid doing it twice, if it's even possible to weld in a day.
Amperage: Stainless requires about 50% less heat to melt because of it's poor thermal conductivity. This means less amperage, and smaller filler wire. All this translates into much slower welding speeds. Whatever your buddy the chassis builder tells you about welding time on his 4130 stuff, add in 50%, plus the purge wait time.
In conclusion, this is an excellent choice for a frame given the properties. The extra cost is substantial, but it is the extra time to do it right that will cause most to pass. The extra skill is also a factor, although welding is not brain surgery. It merely requires 100% of the hand skill, and 10% of the theory.
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