Grades of Stainless Steel
To make a steel "stainless" it needs to contain a minimum of 12%
Chromium (Cr). The Cr oxidises in the atmosphere forming a
passive layer on the surface. This layer, unlike coated steels,
is self repairing should it be scratched.
The problem with 12% Cr is that it is fairly brittle and only
provides the minimum corrosion resistance. Increasing the
Chromium content to 17% improves corrosion resistance but
increases brittleness. Adding 8% Nickel makes the steel ductile
again. Thus 18/8 stainless was born (304). 316 / 316L has
additional Molybdenum and higher Nickel which provides greater
corrosion resistance.
With stainless when you see two numbers they always refer to the
Chromium and Nickel content - 18/8 is 18% Cr and 8% Ni. If you
see 3 numbers like 19/12/3 they refer to the Chromium, Nickel
and Molybdenum content. 316L is 19% Cr, 12% Ni and 3% Mo.

Welding Stainless
There are 2 common grades of stainless: 304L (welded using 308L
filler), and 316L which is welded using 316L filler.
Why is 308L filler used for 304L? Basically there are a number
of grades that do similar jobs, 302L, 303L and 304L (they are
17/7, 18/8 and 19/9 respectively). 308L is 20/10 so can be used
to weld all 3 grades.
Stainless is easy to weld but very difficult to keep flat, the
coefficient of linear expansion is 1.7 times that of mild steel.
There isn’t much you can do about that except to weld it quickly
and by doing so minimise the heat input.
304 and 316 (as opposed to the L low carbon versions) suffer
from weld decay. When heated to welding temperatures the
Chromium combines with the Carbon leaving the steel short of
Chromium and therefore unable to self repair itself.
This was virtually eliminated by introducing stabilised
stainless steels 347 and 321 which contain Niobium or Titanium
which sacrifices itself to save the Chromium, however, when
lower carbon versions 304L and 316L were introduced the problem
of weld decay was eliminated. These days the higher (in fact,
normal) carbon versions are only used for applications where
heat resistance is needed.
Stainless Steel Filler Metal Choice
Select the metals to be welded from the purple bars to the top
and right. The filler metal is in yellow where the two
intersect.
304L |
316L |
310 |
347 |
321 |
410 |
430 |
Mild
steel |
|
308L |
308L |
310 |
308L |
308L |
309L |
309L |
309L |
304L |
308L |
316L |
310 |
316L |
316L |
309L |
309L |
309L |
316L |
310 |
310 |
310 |
310 |
310 |
309L |
310 |
310 |
310 |
308L |
316L |
310 |
347 |
347 |
309L |
309L |
309L |
347 |
308L |
326L |
310 |
347 |
318 |
309L |
309L |
309L |
321 |
309L |
309L |
309L |
309L |
309L |
410/309L* |
309L |
309L |
410 |
309L |
309L |
310 |
309L |
309L |
309L |
309L** |
309L |
430 |
309L |
309L |
310 |
309L |
309L |
309L |
309L |
Mild
steel |
Mild
stee |
Welding Stainless Steel to Mild Steel
The usual choice for the filler when welding stainless to mild
is 309L. 309 is over alloyed stainless steel (19/10) so when
diluted by the mild steel gives a deposit approximately like
308L / 304L.
There are other fillers that give a crack free weld, 312,
308MoL, 307 and 310 will all work but these are less widely
available than 309L.
Shielding gasses for MIG
The best gas for MIG welding stainless is 97.5% Argon +2.5% CO2.
Previously an Argon/Oxygen mix was widely used, but this doesn't
give as smooth a finish as the Argon/CO2 mix.
For mild steel welding 80% Argon plus 20% CO2 is common, with
95% Argon plus 5% CO2 often used for thin sections, but even 5%
CO2 is too oxidising for stainless and will leave the weld
looking black.

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