OK first of all go ahead and fast forward about a half inch on the tracking bar of the video. The first minute is pretty much just looking at a stainless socket weld in a rigid tripod pipe vise.
But the rest is worth watching, I promise.
Especially when you see the weld at the end at the end of the video.
At the beginning of the video, you see a 2” stainless steel socket weld all chucked up in a vise with the root pass already done.
You can tell the first pass is done by the slight discoloration.
The welder is using an air cooled scratch of lift arc torch with a gas lens style cup. The welder positions the tig cup on the weld and rocks the torch into the weld until the tungsten electrode just sparks and then rocks the torch back and starts walking the cup.
Walking the cup is a welding technique common amongst pipe welders. It makes a distinct weave ripple when done right and keeps the hand away from the heat.
The gas lens tig cup this welder is using is helping him maintain good shielding and that’s why the weld shows such cool colors. Too much heat on stainless and it starts to get dull and grey. But if its allowed to cool enough before the argon passes over the hot puddle area, a stainless tig welded socket weld like this will have a luster of colors like gold, purple and blue. That’s when you know you have the heat right and when you are not overheating it.
Overheating stainless usually results from either going too slow, using too much amperage, or not letting it cool enough between weld passes. ..
On a 2 pass weld like this socket weld, you need to let the first pass cool at least enough so that you can just about hold your hand on the weld without burning yourself., in order to keep it from overheating and graying up on the cover pass.
Let me tell you a secret.
You can force cool stainless without hurting it.
Some of have been taught by old timers that we could go straight to hell for fast cooling any metal.
I worked a fab shop once where every fabrication work table had a water line and a air line Y’d in together into a stainless tube nozzle with small valves to adjust the air and water into a fine mist to speed up the cooling on stainless steel.
I thought that was kind of crazy until I started studying metallurgy and the chemical composition of stainless steel.
Come to find out , the worst thing for 300 series stainless is to keep it hot for a long time during welding. Keeping it above the 900f range for extended periods can actually cause loss of corrosion resistance.
In short, the stainless might rust, or even crack from the chromium combining with carbon to cause what is called carbide precipitation...
Here is what happens in case you are wanting to be able to explain to somebody what carbide precipitation means…
1. 300 series stainless steels contain roughly 18 percent chromium. That is the main ingredient that makes it stainless.
2. Chromium and carbon seem to look for and find each other kind of like a drug user and a drug dealer.
3. When things get heated up, the carbon tries to find the chromium and latches on to it to form chromium carbides.
4. When that happens there are areas that are depleted of chromium that lose the stainless properties and rust , corrode, and even crack .
There you have it… a down and dirty explanation of carbide precipitation and why you don’t want to overheat stainless steel welds.
Speed cooling doesn’t hurt it and can actually help as long as you don’t use tap water with chlorine. So here is the thing, if you are a contractor and you are interested in getting the job done, blowing on your stainless socket welds with an air nozzle from a nearby air hose could actually be helpful and it won’t harden the weld.
A note of caution---Make sure and blow from a distance and spread the air stream evenly or you might make the weld draw from uneven cooling but from a metallurgical standpoint, you’re good to go. CHEMICAL COMPOSITION OF STAINLESS STEEL TIG WELDING RODS
Fe (iron) is not listed on these but is whatever is left to add up to 100%
308L--- 20Cr 11Ni .03C max
309--- 24Cr 13Ni
455--- 12Cr 8.5Ni 2Cu 1.1Ti
420--- 13Cr .35C
15-5--- 15Cr 4.5Ni .3Cb 3.5Cu
347--- 18.5Cr 11Ni .4Cb 1Si
321--- 18Cr 9.5Ni .4Ti
316--- 19Cr 12.5Ni 2.5Mo .08Cmax
310--- 27Cr 21.5Ni
450--- 15Cr 6.5Ni .75Mo .3Cb 1.5Cu
209--- 22Cr 13Ni 5Mn 2Mo
350--- 16.5Cr 4.5Ni 2.9Mo .1N
410--- 12.5Cr .12C
355--- 15.5Cr 4.5Ni 2.9Mo .1N
349--- 20.5Cr 9Ni 1.5W 1.2Cb .5Mo .2Ti
312--- 29Cr 9.5Ni
155--- 20Cr 20Ni 20Co 3Mo 2.5W 1Cb
909--- 38Ni 14Co 4.8Cb 1.5Ti .2Al
A286--- 15Cr 25.5Ni 1.2Mo 2.1Ti .004B .3V ELI
15-7--- 15Cr 7Ni 2.4Mo 1Al
418--- 13Cr 2Ni 3W
21-6-9--- 20Cr 6.5Ni 9Mn .27N
jethete M190--- 11.8Cr 2.8Ni 1.6Co 1.8 Mo .32V
17-7--- 17Cr 7Ni 1Al
17-4--- 16.5Cr 4.8Ni .22Cb 3.6Cu
13-8--- 13Cr 8Ni 2.3 Mo 1.1Al
more on stainless socket welds