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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 theChromium 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 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 ste el.

There isn’t much you can do about that except to weld it q uickly

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 lowe r carbon

versions 304L and 316L were introduced the problem of we ld decay

was eliminated. These days the higher (in fact, normal) carbon

versions are only used for applications where heat resistance isneeded.

Mashed potato extruder nozzle.

316 stainless with welds polished

for hygene. (Paul Holland

Fabrications)

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Welding - The DIY Guide  > Welding Different Steels  > Stainless Steel

Welding Stainless Steels

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

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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.

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.

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.   Detail of stainless steel flange

weld. (Race-Tech)

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