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What is stainless steel, and why is it stainless?
Stainless steel is and alloy of iron containing at least 10.5% chromium. The chromium atoms at the surface react with the atmosphere’s oxygen to form a thin layer (just a molecule or two thick) of passive chromium-oxide, this layer forms a barrier to prevent oxygen from reaching and reacting with the iron atoms. If the barrier layer is disturbed, it quickly reforms in a self-healing manner in an oxygen environment. It has had several trade names over the years such as Stay-Brite, Corr-Bloc, Durbar, Hi-Proof and Steelex, as well as various non-English designations, the most common being French - acier inoxydable, or simply inox, and German - Edelstahl or Rostfrei
There are five different families of stainless steel: Duplex, Precipitation hardening, Martensitic, Ferritic and Austenitic. Duplex and Precipitation hardening are very uncommon, so we will not bother with them. The remaining three families are classed by their crystalline structure, Martensite, Ferrite, or Austenite. I won’t go into the weeds about how the crystals form, or how or why they impart the properties they do, but I will leave that as extra-curricular study on the part of the reader if he/she wishes. In general, I will state that Martensitic and Ferritic stainless grades show lower resistance to corrosion due to lower chromium content, are magnetic due to the iron orientation in their crystalline structure and are generally harder than Austenitic steels due to their crystalline structure. Martensitic stainless steel can be hardened by thermal treatment; the only way to achieve improved hardness in Ferritic and Austensitic steels is through work hardening.
What grade of stainless steel is my watch made from?
Depends on the age of the watch and the manufacturer. Older watches are probably made from 18/8 stainless steel which is comparable to modern 302 or 304 grade. Modern watches are most likely 316 or 316L. There are a few companies out there that use more exotic/expensive alloys, as there are probably some that still use the cheaper 302/304.
Why does (almost) everyone use 316 stainless, isn’t there something better?
Like all engineering material selection, there is no such thing as “the best”, there is only “which limitations can I live with”. Watch cases which are in close proximity to the skin and its corrosive environment, tend to place corrosion resistance at the head of the table, then trying to shoe-horn it the other desirable traits as best possible (hardness, mainly). 316 is a good choice, it is relatively hard, and very corrosion resistant, as well as being very plentiful (it's one of the most produced stainless steel alloys) and relatively inexpensive.
What is the difference between 316 and 316L?
316L is just 316 with slightly lower carbon content, intended to improve weldability. It is slightly weaker and slightly softer, although not enough to matter, at least from a watch case point of view. In application where high strength and high strength at elevate temperatures id required, low carbon “L” grades should be avoided.
What about 904L, is it not better?
904L does have a higher chromium content, so it does resist corrosion better, but this only is seen in high chloride environments, such as seawater, it also has a higher nickel content that helps resist stress cracking and crevice corrosion. However, there is always a trade-off. The higher nickel content, coupled with the manganese content, makes it easier to form the Austenite crystal structure and the alloy is softer and weaker than 316. It also may aggravate those sensitive to nickel. Further, 904L, due to the higher use of chromium and nickel is a more expensive alloy and is much harder to machine.
I have heard that Chinese use 200 series stainless steel, isn’t that bad?
Due the increase price of nickel of the last decade, or so, there have been many who would wish to reduce the nickel content of their stainless steel, and so reduce the cost of the product. The 200 series stainless steels replace the nickel content with manganese and nitrogen, both of which perform the same function as nickel, namely they are all Austenite formers, but are cheaper than nickel. This can be good, or bad, depending on how you go about it. The chromium atoms are trying to form Ferrite with the iron atoms, the manganese, nickel and (if present) nitrogen atoms, aka Austenite formers, are trying to form Austenite, the more chromium present, the more Austenite formers are needed to counter-act this. Some alloys reduce the chromium content, some increase the manganese and nitrogen content.
Grade 205 stainless is just as good as 316 as far as corrosion goes, is slightly harder, and is better for those with nickel sensitivity, but this comes at the expense of machinability. Some other grades of 200 series stainless are just as good as 304 stainless (which use to be the most common watch case grade prior to 316) and are similar to 205 in terms of hardness and difficultly of fabrication, some have decreased corrosion resistance due to reduced chromium. So, whether it is an improvement or a downgrade really depends on which specific alloy is being used, in some cases, such as 205, there would be no change in the corrosion or strength properties.
Why can’t my case be made scratch proof?
First, let’s establish what is “scratch-proof” or “scratch-resistant”.
Typical values of hardness:
Sapphire - 9 on the Mohs scale
Steel file - 70 Rockwell C scale, or about 6.5 Mohs
Full hard 440C Martensitic stainless steel knife blade - 56 Rockwell C
General grade axe head - 40 Rockwell C
Full hard 316 stainless - 24 Rockwell C or 101 Rockwell B
Annealed 316 stainless - 95 Rockwell B
Annealed 904L stainless 79 Rockwell B
Work hardened Cartridge brass - 93 Rockwell B
Aluminum 2024-T4 - 75 Rockwell B
Annealed brass - 55 Rockwell B
Next, let’s discuss through hardness and surface hardness. Through hardness is, as the name suggests, when the material is the same hardness throughout the entire billet, this will increase the overall strength of the material. Surface hardness is when the just a layer on the surface is the specified hardness. This improves wear, but does not increase the strength.
In order to resist scratching, the material needs high strength. A scratch is caused by small sharp object (i.e., sand or dirt) pressing on a small area of the surface, and depending on the force and the actual contact area, the surface may yield, once the surface yields, movement of the object will cut a groove in the surface, your scratch. Wear is similar, but the contact area is much larger, and instead of digging a groove in the surface, continued relative motion will slowly abrade off layers of the surface molecules.
Around 50 Rockwell C is about the minimum hardness for “scratch-resistance”, the average knife blade is this hard or harder and you would have to actively use abrasives to scratch it.
Surface hardening techniques.
Work hardening occurs when the material is plastically deformed. Forging, stamping , cutting, abrading, polishing, peening and burnishing all involve plastic deformation of the surface layer of the steel, this will work harden the top crust of the steel. Polishing by burnishing followed by glass bead blasting will probably result in about as hard a surface you can get without resorting to something more exotic that just plain austenitic steel. Some of these “Exotic” surface hardening techniques are:
- Aluminum-titanium nitride (TiAlN) is probably what everyone means when they say “PVD,” it is a black hard surface coat that is around 92 Rockwell C, and about 3 to 6 microns thick.
- Diamond Like Carbon (DLC) which is similar in hardness (around 90 HRc) but generally not as thick a coating, 2 to 3 microns.
- Klosterizing is not a coating but it is a process that will produce a hard shell (70 to 74 Rockwell C) over the substrate, or base material. Klosterizing involves diffusing carbon into the steel surface without forming chromium carbides that would inhibit the formation of the chromium oxide layer. Think of it as case hardening stainless steel. The depth of the hard layer is 20 to 30 microns.
All of these surface hardening treatments will resist wear, but not necessarily scratches. As the strength of the inner layers is still the same as an unhardened piece, under concentrated loads, the thin high strength crust may fail. Obviously, the thicker the crust, the better it will resist scratches. Surface treatments, such as described above, are best suited to resist wear as the pressure are generally lower and distributed over a larger area.
Similarly, the hard shell over a soft core will not be as resistant to impact damage (dents).
What is “ice-hardening”?
“Ice hardening” more correctly, cryogenic tempering is a process using liquid nitrogen that can increase the hardness of thermally harden alloys (Martensitic stainless steels, and plain steel alloys) in which the reduced temperature converts any Austenite that did not convert to Martensite during the heating-quenching cycle. It also will remove internal stresses in the parts caused by uneven cooling during quenching.
Cryogenic cooling of Austenitic stainless steels also yields improved hardness, but through a different mechanism. The rapid cooling will cold work the outer layer, and when removed from the cryogenic bath, the warming of the outer layer will cold work the core. With Austenitic stainless you should be able to achieve near full hardness throughout the piece (10% to 20% above full annealed hardness).
Why not use a Martensitic stainless steel, these can be hardened to scratch-resistant levels?
At least one company does. But, it does have its limitations, as stated above, Martensitic steels do not have as much chromium in the alloy and are not as corrosion resistant. Also, there is increased cost, as after the fabrication and polishing operations, where you get a finished part with Austenitic steels, the part must be further processed by thermal hardening, which takes time and effort.
Will my Martensitic steel case corrode?
Knife blades have been made from 440C, a hardenable grade of Martensitic stainless for years, many are subjected to abuse and neglect in high salt water environments and show reasonable performance in that area. So, it all depends on the grade of steel used, its chromium content will largely govern what level of corrosion resistance you can expect. But, in the case of Martensitic stainless steel, rinsing with fresh water after exposure to a corrosive environment is much more important in preventing corrosion.
Titanium, why not just use that?
Titanium is quite a bit stronger that stainless steels (but not stronger than some regular alloy steels), able to be thermally hardened, corrosion resistant and light in weight. Sounds like a perfect material for watch cases, doesn’t it? It would be perfect if it did not have the bad qualities of being difficult and expensive to machine, and just plain expensive to purchase. There are less than 20 facilities in the world that produce titanium metal.
For a long time in the US, before Russia started to allow export of titanium, it was so scarce and important to certain industries, it was rationed by the government. That’s why there aren’t many titanium watches (or golf clubs or engine valves, or any titanium consumer goods) prior to 1989....
Plastics, Gold and other stuff...
The materials called “plastics” covers quite a bit of ground from exotic carbon fiber impregnated polymers to cheap nylon and polystyrene. Generally, these materials are non-corrosive and have reasonable strength properties, but are often softer than metals and more easily deformed. This usually requires special or non-standard strap fitting to deal with the ease in which conventional spring bar can pull out of plastics. Also, the softer surface allows cosmetic damage.
Gold is another material that will not corrode. It is much softer and heavier than steels. It is very easy to machine and polish. Due to its expense, it is generally reserve for “jewelry” watches.
Platinum does not oxidize at any temperature, although it is corroded by halogens, cyanides, sulfur, and caustic alkalis; it is insoluble in hydrochloric and nitric acid, but dissolves in aqua regia to form chloroplatinic acid, in short, in is pretty corrosion resistant. Again, due to the cost of the material, it, like gold is for “jewelry” watches.
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Originally Posted by fasthandssam
