When TIG welding stainless steel (and mild steel, and most other metals) you have a direct-current (DC) arc running between the tungsten electrode in your torch and the workpiece. The electrode is the negative pole, hence "Direct Current Electrode Negative" or "DCEN".
The big exception I am skirting around is the TIG welding of aluminium (aluminum), which for a range of physical reasons needs alternating current (AC) and takes on a unique character which is dealt with separately.
Some points about TIG welding in DC apply to Al welding so comments here might have similarities, while in many other ways the experiences are different...
The molten stainless steel of the weld pool is so reactive to air that you need to exclude the atmosphere from the back of the weld. So as well as the supply of Argon gas from the torch which the welding machine provides, you need to have a pipeline rigged-up which supplies a blanket of Argon along the back of the weld. This picture shows the effects of backpurge and absence of backpurge.
There is some "stream of conciousness writing" from this time :-). Sift for the bits which tell you things which are helpful. Initial TIG/stainless experiences.
The standard I achieved can be see from these craft objects I made
You will often practice on mild steel because it is cheaper than stainless steel and the skills needed are just about the same. I copied this commentary from here.
stainless steel is reckoned to need about the same
skills as TIG welding mild steel. However, you have bigger problems
with distortion because of the thermal properties of austenitic
stainless steel. These include the chrome-nickel "18/8" and "18/10's"
from which kettles, pans and the like are made.
(i) thermal conductivity is about half that of ferritic steel, so heat
spreads less far and there are steeper thermal gradients near the
joint
(ii) the thermal expansion coefficient is higher than for ferritic
steel.
You really strongly see these effects, but overall, at the beginner
stage, there don't seem to be many other differences.
aluminium TIG welding looks and sounds very different, but didn't defeat me on a first try. You get a much bigger melt pool, and the AC power to the arc needed for cleaning off refractory aluminium oxides makes the arc crackle, unlike the quiet DC arc for welding steels. The large melt pool kind of reminds me of fairy-tale pictures of the moon reflecting in pools in the woods, the pool being the aluminium melt pool and the moon being the reflection of the tungsten electrode in it.
I would add now (2yrs later) that with stainless you need backpurging which is extra work compared to mild steel which needs no backpurge.