Welding Stainless Steels

Grades of Stainless Steel

What is stainless steel? It is a kind of steel that also contains at least 12% chromium. Chromium forms a protective layer for the steel due to its oxidizing properties. It is a self-repairable coat.

However chromium is brittle and not too resistant to corrosive influences. When you increase chromium to 17%, that helps with corrosion resistance but it also makes it more brittle. To make the steel ductile, try adding 8% nickel. Now you've got 18/8 stainless steel (304). 316 / 316L has additional Molybdenum and higher Nickel which provides greater corrosion resistance.

The 18/8 stands for chromium and nickel content - 18/8 is 18%Cr and 8%Ni. For 3 numbers like 19/12/3 it is 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.

Its difficult to keep stainless steel flat since they possess a coefficient of linear expansion 1.7 times that of mild steel. Therefore you have no choice but to keep heat input minimal and weld it quick.

304 and 316 are prone to weld decay. When heated, the Chromium sticks with the Carbon and the chromium layer loses its protective properties as a result.

 To take care of this problem, we use stabilized stainless steels 347 and 321 which contain Niobium or Titanium. They sacrifice themselves to save the Chromium.

Welding Stainless Steel to Mild Steel

309L is the most common choice for the filler when welding stainless to mild. Take note that 309 is over alloyed stainless steel (19/10) and in its diluted form it gives us 308L / 304L.

312, 308MoL, 307 and 310 are other fillers that give a crack free weld, but are difficult to find than 309L.

Shielding gasses for MIG

The recommended gas for MIG welding stainless is 97.5% Argon +2.5% CO2. Argon/Oxygen mix was widely used in the old days, but it doesn't give as smooth a finish as the Argon/CO2 mix.

For mild steel welding it is recommended to use 80% Argon plus 20% CO2. For thin sections with 95% Argon plus 5% CO2 often used for thin sections, but even 5% CO2 is too oxidizing for stainless and will leave the weld looking black.

The (Unofficial) History of Stainless Steel

Harry Brearley of Brown-Bayley Steels, Sheffield is credited with inventing stainless steel.

He stumbled across his discovery by accident when someone added the wrong alloy of FeCr rather than FeMn to Manganese steel. It didn't rust for months.