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All About Stainless Steel

"The case of the sloppy barrel"

Mrs P. Frotteringwart, whose husband Alf made knives and forks in Sheffield was endlessly grumbling (with a name like that she would wouldn't she?) that the cutlery was always going rusty. “Alf, the bleedin' knives are rusty again, can't you do something about it you useless pile of bowel products.”

Alf was polishing the the nice new cup he had just won (he was, as you all know, the world tiddlywink champion of 1742) and, as he stared disconsolately into the shiny silverware, he had an idea. Off he went to find a silversmith, he had the cutlery silver plated to stop it rusting. Mrs. P was happy, well for a couple of hours at least, and showed off her silverware to all her friends and so it became the standard practice to silver and nickel plate cutlery to stop it rusting.

Time passed, it does doesn’t it?

Between 1820 and 1909 there were blokes all over Europe messing about in their garden sheds mixing up brews of hot iron. Englishmen Stoddard, Farraday, Woods and Clark, Frenchmen Berthier, Brustlein and Guillet, Germans Goldschmidt, Monnartz, Borchers and the Krupps, Daniel Widdon, Harry Hawk, Old Uncle Tom Cobley and all. They tried mixing all sorts of things in with iron to try and stop it rusting: chromium, nickel, molybdenum, old socks, dead frogs, bat's wings and newt’s testicles. It was all a bit shambolic really with people urinating around all over the place like a pack of dogs without a tree.

But then one day someone finally got it right.

“Knock, knock.”
“Who’s theer?”
“I say, is that Harry, Harry Brearley?”
“Aye tis, wot d’ yer want?”
“I say old chap, my barrel’s a little sloppy in the middle.”
“Now don’t tha go bringin’ thi pers’nal problems ‘ere.”
“Oh no old chap don’t get me wrong, it’s my gun, the barrel’s worn out you see.”
“Wot’s that t’do wi’ me?”
“Well I understand from your dear old mater that you’re a pretty smart sort of a chappie, been studying steel what ho. I wonder, old bean, could you be so good as to find a strong steel for my gun that won’t wear out after downing a mere three pheasants?”
“Maybe I’ll si wot I can do.”
“Oh I say, spiffing old chap.”

The invention of 400 series stainless steel

Harry was a clever sort of a chap who came from that famous steel town of Sheffield and while developing a steel for gun barrels that didn’t wear out he inadvertently found a steel that didn’t rust. He called it “rustless” steel. His bosses (obviously very shrewd Yorkshire chappies) weren’t interested so instead he went to see a bloke called Ernest Stuart at RF Mosley who made cutlery. It was Ernest that changed the name to “stainless steel” and together they started the widespread use of stainless steel for making cutlery.

It is generally regarded that it was Harry Brearley who, in 1911, finally got it right. He realised that to make fully stainless steel you needed to mix at least 10.5% chromium and, very importantly, have less than 0.15% carbon in your steel. His early development produced a stainless steel that was what we now know as the 400 series specifically stainless steel 430.

Austenitic stainless steel

It was a team effort though and as order came from chaos others made their contributions. Guillet and a chap called Guissen did a lot of work on the addition of nickel which produced the famous 300 series of “Austenitic” stainless steels. Once again the Germans Monnach and Borscher also made their mark.

“I zay Klaus, vot iff vee add zee molybdneum?”
“Molybdenum? Vos isch das?”
“Ich veis nicht Klaus, but isch zounds veery good.”
“Donner undt blitzen Hans, vee haff zee veery, veery schtainlees schteel!”
“Off korse Klaus, vee can make zee vunderful schtainless schteel koz vee haff zee mussels in zee schpittal.

It is the addition of molybdenum which gives us the really good stuff - stainless steel 316 which resists attack by chlorides.

The secret of stainless steel is hidden in the crystals of which it is made. We melt steel, mix things with it and then cool it and it forms different crystal structures depending on what’s in it. If we add lots of chromium to steel the molecules of chromium fill up the gaps in the crystal structure of the iron such that oxygen molecules can’t get in. In this way we can protect the iron from oxygen and it won’t rust. On the surface where the stainless steel is exposed to oxygen a very thin layer of corrosion forms which forms a barrier helping to protect the metal underneath.

If we don’t add enough chromium there are still gaps left and the steel will rust, this is why stainless steel must have a minimum 10.5% chromium to be a genuine stainless steel.

The crystal structure of stainless steel

Sorry but I’ll get a bit metallurgical I’m afraid. As we’ve said before pure iron is “ferrite” which is formed of crystals that are combinations of atoms in a body centred cubic crystal structure. If we heat iron above around 1,000 degrees C the atoms all jiggle around and change their form to a face centred cubic crystal structure (I know, I know, it is all very technical but you never know, somebody might be interested). This face centred form of iron is not called “ferrite” but is now “austenite”, named after one Sir William Chandler Roberts Austen, the only man stupid enough to get out his microscope and start looking at lumps of iron while it is still red hot.

This face centred cubic structure is more resistant to rusting and incidentally is not magnetic. As the iron cools it will once again revert back to “ferrite” but by adding nickel to our steel we are able to stop it changing to “ferrite” and we can keep this “austenite” form of iron even at room temperature.

Different types of stainless steel

These days there are many different types of stainless steel with a wide range of alloying elements added in differing amounts to give different qualities but generally speaking the commonly used stainless steels fall into 2 broad categories we call the 300 series and the 400 series.

Of these the Austenitic 300 series are the most commonly used and these are steels with around 18% chromium and 8% nickel in them. As we have said the nickel makes these “Austenitic”.

Within the 300 series two particular grades form the majority (70%) of the world’s stainless steel production. These are grades 304 and 316.

304 is the most commonly used stainless steel used for probably 50% of all stainless steel applications. It is widely used in kitchens, food processing equipment, breweries, bakeries, slaughter houses, coffee urns, cooking stoves, refrigerators, pots, pans and cutlery.

316 (also known as marine grade stainless steel) is similar to 304 but with a little more nickel and with 2 to 3% molybdenum added. It is used for more severe environments and withstands attack from many industrial chemicals but, most importantly from our point of view, it also withstands pitting by chlorides (steels hate chlorides).

This is all very well but unfortunately a lot of “stainless steel” isn’t, well not very anyway. Why? Well two reasons really. The first is to save money, Nickel isn’t cheap and the poverty alleviation gene (that infamous PAG) cuts in and the nickel gets left out.

Austentic stainless steel is very difficult to machine

The second reason is more practical. Austenitic steel has a problem in that it tends to be very stressed. If you work it by hitting or cutting it the stress is relieved, the crystals change and the steel becomes very hard. To cut a long boring metallurgical story short (even more boring than watching the Pope playing chess) good quality stainless steel is technically difficult to work in order to manufacture things. Try drilling a hole in it and you’ll see what I mean.

The poorer quality stainless steels, the “Ferritic” 400 series, are far easier to machine or to deep draw into things like deep bowl kitchen sinks than the higher quality 300 series. In certain foreign countries noted for low cost manufacture of many of the world’s goods quality will often be sacrificed if an extra dollar can be made.

All over Bali there are beautiful new villas with wonderful looking “stainless steel” fittings like locks, hinges and handles that look great for the first week but mysteriously after a short time start to look like over cooked puff pastry. These fittings are NOT made from 316 or even 304 stainless steel in spite of many of these products carrying trade names that sound like respectable European manufacturers.

So we now know that much of what is sold as stainless steel isn’t, so how do we tell?

How to tell if steel is genuine stainless steel

Well there is in fact a very useful easy way of quickly assessing the stainlessness (if there is such a word) of steel magnetism.

As we said the only part of steel that is magnetic are the crystals of ferrite - pure iron in a body centred cubic structure. The better stainless steel is austenitic, it doesn’t have any crystals of pure ferrite in it and so is not magnetic at all.

So go and buy a small magnet. Try it out on something made of ordinary steel so you know how strong it is. Now you can use it to test your stainless steel, the more it sticks the less stainless is the steel.

You will find many of the supposedly stainless steel hinges, locks, handles and other fittings you can buy in Bali are magnetic to varying degrees. A good friend of mine gave up trying to find genuine stainless steel fittings in Bali and bought all her fittings in Singapore.

I’ll repeat - genuine 304 and 316 stainless steels are not magnetic at all, not even a little bit.

A word of warning - be careful with that magnet. Don’t let it get near your computer, your mobile phone, your bank cards or your husband’s brain. Magnets have a disturbing tendency to wipe magnetic strips and memories.

So will stainless steel rust?

Contrary to popular belief stainless steel will rust. Steels hate chlorides. It’s one of those sort of hate - hate things.

I am sitting here looking at a typical stainless steel water tank, the sort that many of us use. It is full of tiny holes holes and has rust stains down the sides. Checking it shows that it is in fact genuine 304 stainless steel though probably poor quality.

Here in Sanur, and in many other parts of Bali, particularly near the sea, the ground water and salt water spray has a lot of chloride in it. The water in your tank, if drawn from your bore, may attack the steel (yes even stainless 304 steel). It “pits” the steel forming little holes in the surface. Once “pitted” the corrosion tends to become more concentrated at the “pit” and quickly blows a hole through the steel.

That wonderful marine grade 316 will not rust, with its extra molybdenum it is designed to resist chlorides, sadly it is very difficult to find here in Bali.

Copyright © Phil Wilson 2011
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