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316L steel - research on what Panerai are made of

March 31 2007 at 2:34 PM

bo  (Login expat-oz)
High Heid Yin
from IP address

At the Firenze exhibition, Panerai had a display of the various steps required to create a case for both Luminors and Radiomirs. Pictures would be redundant, I think everyone's seen them. I've always been fascinated with machining/milling/metallurgy, so I decided to do a bit of research. I'm going share what I've come up with - and I'm not a chemist, metallurgist, whatever, so any mistakes are my own. Something of a more cerebral nature with no pics might surprise some - a post like this from me may be perceived much like the village idiot spouting poetry - so feel free to correct me if this is WAY off! happy.gifhappy.gifhappy.gif

Panerai makes their watches from a material called 316L steel. Well, what the heck is that? 316L is steel made from a blend of iron, carbon, chromium, nickel, and molybdenum (with some other stuff thrown in). It's the second most common type of steel, after 304 (304 is also called A2 steel, commonly used in something else I'm passionate about - fine woodworking tools. A2 tool steel is about as good as you can get when making a blade for a plane...especially when it's cryogenically treated, but that's a story for another day.)

So, back to this 316L stuff - what the heck is it? Technically, it's an austenitic alloy with some other good stuff. Okay, what the heck is an austentic alloy, you ask? Good question! Let me give it a stab...

Didja know that steel has a crystalline structure? That structure is important. If you care, read and do the following, but if not, feel free to skip ahead...

Sit down, and draw a simple cube. That is simple cubic structure. No room to "hang" anything - the cube is self contained, and you've got no other points to connect.

Now, draw another cube. Draw lines from the corners of the cube through the center of the cube - this is body centric structure. There is ONE junction for all your connect-the-dots lines - and it's in the middle of the cube. That ONE point is where you can "hang" something...again, not germane, but illustrates the next point...

Now, draw another cube. Instead of connecting lines though the CENTER of the cube, draw lines that connect the corners of the cube on the SIDES of the cube - THIS is face-centric cubic structure. There is a place to "hang" something on EACH of the cube's 6 faces...

What the heck is all this crap about "cubes" and "hang points", Bo? Too easy - in this structure, the coners of the cube are iron, and the stuff we're "hanging" is carbon. Iron and carbon in such a solution (and it IS a solution, not a molecule, with some other stuff thrown in) makes for a stable, non-magnetic and ductile material we call austenitic stainless steel. Austenite is the pure carbon/iron crystalline structure - but I said that 316L steel has some "other stuff" in it as well - here it is:

Carbon .030%
Manganese 2%
Silicon 0.75%
Chromium 18% - 20%
Nickel 10% - 14%
Molybdenum 2% - 3%
Phosphorus .045%
Sulfur .030%
Nitrogen .10%
Iron the rest of it...

These elements, when alloyed, are "316L", or the same stuff our watches are made out of. Lots of other stuff is 316 steel too - nuclear containment vessels, ships hulls, chemical and petroleum processing and storage tanks, all sorts of things - but this is about watches, right? I just thought that "nuclear containment vessels" was too cool for school - not that I'd want a nuke in my watch, but still...

Back to it - what's all this stuff DO??? Well, the chromium and nickel contribute to a process called passivation - the chromium in the steel interacts with oxygen to form a layer off chromium oxide on the the surface of the steel. WAY too thin to see or feel, it's this protective oxidation of the chromium that protects your watch from water and air - and the best part, aside from not interfering with the lustrous appearance, is that if scratched, it reforms almost immediately. Cool! No elbow grease and a self-regenerating protective layer? What else do you want???

316L steel is also extremely resistant to corrosion, due to the addition of molybdenum to the "mix". It's nearly impervious to atmospheric and other mild corrosive agents. It also displays remarkable resistance to acidic corrosion, even in high concentrations and at elevated temperatures. A notable exception is nitric acid, but if you're dipping your watch into boiling, concentrated nitric acid, you've got more worries than a bit of discoloration to your Panny's case! 316L steel has survived 1000+ hours in boiling salt (sodium chloride), without cracking.

All this is great, but most sources state that 316L is NOT designed for continuous immersion in seawater!!! That was kind of shocking, until I read more about it - it IS considered adequate for some things, like boat hardware and rails, as well as building facades - 316L steel, exposed to seawater spray for 100 hours at room temperature will show NO signs of corrosion. Additionally, coastal conditions have no effect on the steel.

Well, THAT'S good news! If you can stay down longer than 100 hours, or survive 1000 hours in boiling salt, you need to tell the rest of us how! That said, it's a good idea (as any diver will tell you) to rinse your equipment with fresh water after a dive - and your watch is no exception.

Most notably, though, and why this steel is commonly used in stuff smaller than nuclear containment vessels like cookware, surgical steel implants, flatware, food processing and handling, water delivery, and most importantly to us, watches - it's especially resistant to organic and fatty acids. The kind of stuff that your body makes. So, as caustic as I might be, I'm still not narsty enough to corrode my caseback! w00t!

Interestingly, 316L steel cannot be "hardened" by heat treating it - while easy to form and relatively easy to machine, you can't "harden" it as you can with other types of steel. It's Rockwell hardness (max) of 95 is about as good as you can get. It's some TOUGH stuff too - and it's appropriate that the world's coolest "tool" watch has a maximum TENSILE strength of 70,000PSI - wow!

It also takes quite a lot to MELT this stuff - here's some figures (with other elements for comparison)

316L: 1390 to 1440ºC or 2450 to 2630ºF
silver: 960ºC or 1760ºF
gold: 1063ºC or 1945ºF
platinum: 1769ºC or 3216ºF
the earth: really hot. think nuclear hot.

As far as density goes, it's not terribly heavy or dense. I've dug up some MORE figures, and here there are, with other elements for comparison:

316L: 8.027 grams per cubic centimeter (g/cm3)
silver: 10.49 g/cm3
gold: 19.32 g/cm3
platinum: 21.45 g/cm3
the earth ~5.00 g/cm3 - it's heavy 'cause it's big.

A few last miscellaneous bits - 316 is materially tough and stable over a very wide range of temperatures - it won't deform, stretch, bend or creep from cryogenically low to extremely high temperatures. It's pretty easy to machine. It's not the best steel to weld, but it ain't bad either - care must be exercised when selecting material (even types of steel) to mate mating with it, so that you don't interfere with it's anti-corrosive properties.

That's a WHOLE lotta info, so forgive me if I went too far into it - what we end up with though, is a material that is extremely resistant to corrosion, fairly ductile, easy to machine, and tougher (literally) than nails. Good stuff.

All that's left to ask is "how are those blanks we saw in Firenze made? cut from sheet, stamped, or cast???" Anybody? I have no idea! And what are vintage pieces made from? Same stuff???

sources were mostly wikipedia,, and the results of various google searches.


H T H!!!

This message has been edited by expat-oz from IP address on Apr 30, 2010 5:01 PM

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