Question About Stainless Steel

[Megalobyte]

I know that even when a blade is stainless, it can still rust, and likely will if not given at least SOME care. Ive seen stainless guns rust, some fairly easily. And yet, i have owned many watches over the years that were probably mediocre-grade stainless at best, and i have yet to ever see a spot of rust on a watch. And, i have worn them in the ocean, every day in the shower, rain, never once did i think of wiping it down, or protecting it from rust, and again, i have never seen one rust. Why the difference between stainless steels, why should a knife be more prone to rust than a watch?

 

[tallwingedgoat]

Because knives are made of high carbon stainless steel. Though your watch has the same amount of chromium as your knife, it has 1/10th the amount of carbon. Carbon is the biggest cause of rust. Pure iron, for example, actually resist rusting to some degree.

Cheap stainless knife steels often have less carbon than premium steels like ATS-34. Thus cheap steels like 420 are ideal for diving knives.

 

[rdangerer]

Search on the phrases "free chrome" and "free chromium".

Part of the reason: Corrosion is a function of the steel's oxide layer resistance to said corrosion. Stainless steels form a chromium oxide layer that is tough and heals rapidly when abraded away. The chrome oxide prevents oxygen from reacting with the iron in the SS matrix in amounts enough to show red rust (iron oxide). The amount of free chromium left in an alloy's makeup is somewhat proportional to a steel's ability to resist rust. The "free-ness" of chromium means that the chrome isn't bound up with another element already, i.e. is not part of a molecule already. I suspect in the case of most cutlery stainless steels (those that can be hardened), some of the chrome is bound up in the form of chrome carbides, molecules of chromium + carbon that are fairly hard (Rc60 to Rc 64+). Chrome carbides resist abrasion due to their hardness, but not as well as the "big 3" carbide formers in knife steels, vanadium, molybdenum, and tungsten. If enough chrome is already bound up, either the propensity to form, or the thickness of, the chrome oxide layer is hampered.

The higher the carbon content of an alloy, in very general terms, the more chromium is bound up in the form of chromium carbides, and the less available as "free chrome" ready to bind with oxygen and form the tough, corrosion resistant outer layer. In many cases, the more chrome, the more corrosion resistant the steel (e.g. 420 and 440 series stainless steels). In most cases, more chrome means a more brittle steel at knife harness levels. Some steels violate this precept, well, better stated, have better optimized the tradeoffs... the best current example is CPM's S30V.

Stainless steels must be martensitic and hardenable to make decent knife blades... those are in the 420 and 440 series, among other high carbon chrome and chrome-moly steels. They are typically "high carbon" stainless steels (Carbon of 0.5% up to maybe as high as 2.2%). They have mediocre welding characteristics. To make the stainless category (stain resistant is a better description), steels typically need to have somewhere in the 13% chrome range. D2 is right on the border with around 11% to 12.5% depending on the maker of the particular D2 alloy.

Many other stainless things are made of non-martensitic steels, steels that fall into the ferritic (409SS, 430SS) or austenitic categories (304, 310, 316, 317). Example: the 300 series Stainless steels are austenitic. They won't harden to the knife-range of hardness, but are very corrosion resistant and relatively soft, containing say 12% to 23% chromium, less than 0.1% carbon, are often easy to form and shape, and typically are easier to weld. They may contain nickel.

Here is one random link my search engine returned that gives a quick overview and examples of what the SS might be commonly be used for:

http://pw1.netcom.com/~dwelding/stainless.htm