This means the weld is not complete. It doesn't penetrate fully. And is susceptible to cracking from the inside out.
I am continually confused by terminology here. For one thing the difference between penetrating %100 of the material being welded as opposed to %100 of the weld having some penetration. So, I think you are saying if the weld is not all the way thru the base metal, that will cause a stress where the seam is? The bead (MIG) is often a good amount thicker then the base metal though. When a frame is brazed, the bead is allowed to carry the load and it is considered high quality, but not if it's a weld?
I think you have additional requirements because sanitary welding must not allow cracks for bacteria etc. to resist cleaning.
Always happy to get welding advice....
Sorry. You'll have to forgive me. Without noticing. I always refer to welding in TIG welding terms.
Typically. You should be aiming for the weld to penetrate both side of the base material. However, this is usually on butt joints. On T-joints and lap joints penetrating to the other side may not be possible. On thinner materials it will be. But, on thick lap joints and T-joints. You won't be penetrating all the way through to the other side. At that point. You're aiming to have the weld penetration as deep as it is wide.
So, what I mean by "It doesn't penetrate fully", is this. On stainless steel. If the weld does not have a shielding gas on the backside, the oxide the oxygen produces prevents the weld from "burning" into the material.
A crude example.
Lets say you have to 1'' thick stainless plates. With a shielding gas the weld will penetrate the full 1''. Without it. It will only penetrate .5''. Because the oxides (commonly referred to as "sugar" or "sugaring") prevent the weld from going all the way through. And it's the lack of fusion where the sugar is, that weakens the weld. As it is not complete. So it cracks from the inside out.
That's a very crude example.
In reality. The amount of penetration into the material will always vary. Due to material, gas, wire, material thickness, joint type, etc.
In sanitary welding. The backing gas is there to complete the weld. As well as prevent the sugaring. The sugaring will harbor bacteria. So in that case it's a strength thing as well as a cleanliness thing.
Disclaimer: I AM NOT an expert stainless welder.
I too have heard the the same things about the back-purge when welding stainless tube, I have seen many failures of welds on exhaust and turbo manifolds/headers. most of the time if I am the one fixing it I look at it closely to see if I can find the cause and I have seen the funky looking growth inside when it was not back-purged. I actually went and traded an old cylinder I had for an Argon to use just for back-purge and as a back-up for the TIG. I am a firm believer of back-purging even though I do not know the exact metulurgical reasons for it but I know it seems to work
I also second the hot-dip Galvanizing option I have seen it done on Land Rover Defenders so why not a LOCOST chassis......
It's also just as important to back purge on stainless steel that goes through lots of heat cycles. Like a turbo manifold for instance. If the weld isn't 100% penetrated. When the material heats up and expands, and cools down and contracts. The portion that isn't fused (where the sugar is) will expand. Push up against each other. As well as cool down at a different rate. This puts a lot of stress on the portion of the weld that is complete...
I hope all this makes sense.. I'm kind of groggy and incoherent at the moment. So that may not make sense...
*Edit... I like that it asked me if I was sure I wanted to post in lieu of new posts being added while I was typing.. FANTASTIC FEATURE!!!
Im sort of confused really..why do people try to fix something that isnt broken? For 50 years race car builders,bicycle builders,motorcycle builders ..the list goes on..have been building frames out of mild steel..why? um well because it works well and because you dont have to do any tricky welding to make it stick together.Im sure that if collin chapman was reading this hed be asking why people are trying to complicate an easy thing,and yes i totally agree with other comments about why other people havent gone down the stainless road,look 50 years after the first 7 was dreamed of caterham are still making them out of mild steel and until recently were still brazing them together..why..because it never broke..one thing that no one has really touched on is flexing,these things do flex,in fact every race car or bike does and i know from limited experience that stainless does not like to flex..it cracks.
This is absolutely correct. Stainless doesn't mix well with two things. Excessive heat (and heat cycles) and flexing.