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          Welding Stainless Steels and steel grades

 
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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