What makes stainless steel stainless

[Joel]

:wink:  Since this is a forum, which by definition means discussion and debate, let's try this as a topic.                              As anybody who as spent some time studying some about knife steels knows there's really no such thing as truly stainless steels in knives, at least with the currently available steels out there.  "Stain(or rust) Resistant" is actually the proper term.  Fine, but here you are about to buy or maybe make or have made a knife, and you're going to be using it in either a hot, humid climate or at least where it's gonna get wet or damp.  So you do a little research and find out that if a knife steel has at least 12.5% chromium in it, it's considered "stainless".  So you check out a bunch of steels an find out they have 14,15,16 percent chromium so any will work.  Right?  Not really quite.  What the knife manufacturers or the folks who print those knife steel charts forget to tell you is that amount refers to 12.5% of "free chromium", an I can't think of a knife steel offhand that has that; well maybe one, but its a crappy steel in other ways.  I suppose it's too much troube to explain to folks that carbon(the element that makes steel, steel) has a great affinity for chromium.  When a steel is heattreated to it's critical temperature, the carbon combines with the chromium at the ratio of 1% carbon to 10% chromium by mass weight to form chromium CARBIDES.  When chromium combines with carbon in this fashion, it is no longer "free" to provide rust or stain resistance.
If you take a knife steel that has 1% carbon and 14% chromium and heattreat it, you're left with 4% free chromium. In this case the math is simple; since there's 1% carbon 14 -10 = 4. The actual number that metallurgists look at for a steel to be REASONABLY stain/rust resistant is 4% free chromium.  My example just happened to have that number.  That's why great steels such as D-2 are considered "semi-stainless".  While D-2 does have around 12.5 percent chromium prior to heatreat, when it combines with the carbon in the steel matrix, there's usually only around 2.5% of free chromium available.  Not enough to meet the 4% criteria.  What happens to that free chromium when the steel is heattreated?.  It floats to the surface of the steel to form an extremely tight layer of Chromium OXIDE which coats the entire surface of the steel.  The more chromium oxide, the thicker the layer(we're talking molecules here).  It not only binds tightly to the surface, it's essentially invisible and it's self repairing.  If you scratch or sharpen your blade and remove a bit of it, it flows to replace the lost chromium.  Scary.  Best way to make stainless steels with only around 4 or so percent free chromium rust and stain?  Beadblast it.  Not only does beadblasting greatly increase the surface area of the knife blade, in many cases those little "Holes" are below the layer of the Chromium Oxide, allowing moisture to penetrate into the vulnerable interior of the steel.  Know this from personal experience.  Had two blades beadblasted during heattreat and will NEVER do that again.  Defeats the whole purpose of stainless if stain and rust resistance is an important criteria.
Just a topic for discussin.

[Graybeard]

I didn't know that. Well I did know it was chromium that made it stainless but didn't know about the 1 to 10 stuff and it needing to be free chromium. thanks.

GB

[Naphtali]

Chromium oxide layer constantly renews itself.

In the absence of other elements that are carbide formers, such as Vanadium, the general rule is Chromium: Carbon ratio of 12:1 renders a martensitic steel "stainless." Some brands of D-2 are "stainless" because other alloying elements make up the difference as carbide formers.

1. Carbon makes steel hard. Hardness gives knife steels their ability to hold an edge.

2. Chromium is a strong carbide former when used in amounts over 5 percent, improving steel's ability to be hardened. Carbides promote wear resistance. When chromium content above 13 percent (I think 12 times carbon content is more accurate.), steel is classified as stainless.

3. Vanadium is also a strong carbide former. When found with chromium, it frees chromium to render steel stain resistant. It also retards grain size during hardening.

4. Nearly all tool steels contain small amounts of silicon, for much the same reason as manganese. It is nearly always used with manganese, molybdenum, and chromium.

5. Nickel improves toughness, wear resistance, and extends hardenability in amounts up to 2.75 percent.

6. Tungsten, when 1.50 percent or higher, increases wear resistance in high-carbon steels. Four percent tungsten in steel containing 1.30 percent carbon renders the steel so abrasive resistant when hardened that it is difficult to grind. It also give steel "hot hardness" — that is, the ability to be used at high temperatures without losing carbon content. The downside is that knives tend to be more brittle with high amounts of tungsten.

7. Molybdenum has similar properties to chromium and tungsten. It gives air-hardening steels (A-2) ability to harden in air. In large amounts in conjunction with chromium (154-CM, ATS-34), it enhances hardenability.

8. Manganese expands the range of critical quenching, the "hardening window," making it easier to treat all areas of an object. This is important for large objects where a plain carbon steel would cool too slowly on the inside to be hardened.

[Joel]

:-) The chromium/carbide ratio of
10:1 I used was obtained from the Chief Metallurgist from CPM steels in an article he co-authored for Blade Magazine a couple of years ago, as was the 4% "free chromium" criteria..  The Key Words for stainless are still "free Chromium".  Not Chromium carbides.  A number of articles I've read over the years have referred to both ATS-34 and 154CM, both of which can have 14% chromium and 1.05% carbon(more or less) as "Marginally stainless" due to the minimal 4% free chromium remaining after austenizing.

There is a "new" D-2 steel out called
D-2(modified) that IS stainless.  Haven't seen the analysis on that, so I don't know why.  I'm assuming they increased the level of chromium.  Never even had the "normal" D-2 rust
on me, but I take care of my knives.

In order to be considered an "air hardening" steel, it must have at least 5% chromium.  Steels with either less than that, or none, require either a water/brine or oil quench. Molydenum, nickle and manganese enchance the air hardening process.  Can't think of an air hardening steel that doesn't have it(Moly), but the amounts vary from .75-5.0 percent.  ALL air hardeners have at least 5.0% chromium; that is the prime prerequisite.

I've read a lot about Vanadium functioning as a hardener and grain refiner.  Never have found mention that it frees chromium.  I'd appreciate your source on that.

Everything else you posted seems to be what I've learned over the years.

[Naphtali]

Joel:

The ratio 12:1 I obtained from Arvend Meedah, chief metalurgist at Cartech circa 1995. Apparently, like nearly everything else associated with metalurgy, how "stainless steel" is defined is a moving target.

[Joel]

:? Naphtali, you're absolutely right.  I've run into instances where AISI and SAE have different criteria for the same thing.  I imagine part of the differnces lay in how the steel is forumlted, i.e. batch smelting vs the more precise use of powdered elements(as in CPM steels), and whether they're open air smelted or vacuum(or double vacuum) smelted. Oxygen, or the lack of it, appears to play a big part in what the final product is.

[Joel]

Yeah, the smaller the surface pores the better. I've still a small game knife that I made out of a file that I polished to about 1500 grit that has NEVER rusted.  Course, when I got home, it got washed, dried and waxed.  I polish everything to a mirror polish, including the stainless(I use mostly 154CM) an even though it "orange peels" a little on me, neither my customers or I much care.   :grin: