Welding Stainless Steels

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.

Stainless steel seat
Stainless seat used in a changing room. (Paul Holland Fabrications)

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.

316 Stainless extruder nozzle
Mashed potato extruder nozzle. 316 stainless with welds polished for hygene. (Paul Holland Fabrications)

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 Steel

* depends on environment - if Sulphurous it must be 410
** preheat of 150°C required

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.

Stainless to mild steel weld
Dragonfly sculpture in stainless and mild steel. (Alan Ross)

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.

Stainless steel ductwork for an incinerator
304 ductwork for an incinerator

The (Unofficial) History of Stainless Steel

Harry Brearley of Brown-Bayley Steels, Sheffield is often recognised as the inventor of stainless steel. My father worked for him and told me this story.

They were making a cast of 14% Manganese Steel and someone added the wrong alloy FeCr instead of FeMn. When they realised their mistake they scrapped the melt. It was stored outside awaiting use but no-one could decide what they could do with it. It stood there for months. One day Brearley noticed it hadn’t rusted and the rest is history. If you read the official version it is very different, more scientific.

Stainless TIG weld
Detail of stainless steel flange weld. (Race-Tech)
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