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It's important to bear in mind is that the choice of blade steel is only one component in how a knife performs.  One must consider the blade's intended use and match the characteristics to that use.  Hence, factors such as blade profile are important; a tanto isn't the best choice to skin a deer, for example. But perhaps most important characteristic is the heat treatment. A good solid heat treatment on a lesser steel will often result in a blade that outperforms a better steel with inferior heat treatment.  Bad heat treatment can cause a stainless steel to lose some of its stainless properties, or cause a tough steel to become brittle. Unfortunately, of the three most important properties (blade profile, steel type, heat treatment), heat treatment is the one that is impossible to assess by eye, and as a result excessive attention is sometimes paid to the other two.

Again, the blade's intended use is of paramount importance.  Many of the cognoscenti deride 440A stainless steel, but there are few better materials for a marine or wet weather blade.  Properly heat treated 5160 is wonderfully tough, but if the application is skinning deer, the edge holding qualities of 52100 are probably more important.

STEEL ALLOYS
At its most basic, steel is iron with carbon in it.  Other alloying metals are added to bring out different characteristics in the steel. Here are the important steel alloy materials in alphabetical order, and some sample steels that contain those alloys:

Carbon: Present in all steels, it is the most important hardening element. Also increases the strength of the steel. Knife grade steel should usually  have greater than 0.5% carbon, which makes it "high carbon" steel.

Chromium: Added for wear resistance, hardenability, and (most importantly) for corrosion resistance. A steel with at least 13% chromium is classified as a "stainless" steel.  Despite the name, all steel can rust if not maintained properly.

Manganese: An important element, manganese aids the grain structure, and contributes to hardenability. Also strength & wear resistance. Improves the steel (e.g., deoxidizes) during the steel's manufacturing (hot working and rolling). Present in most cutlery steel except for A-2, L-6, and CPM 420V.

Molybdenum: A carbide former, molybdenum prevents brittleness and maintains the steel's strength at high temperatures.  It is present in many steels, and air hardening steels (e.g., A-2, ATS-34) always have 1% or more molybdenum -- molybdenum is what gives those steels the ability to harden in air.

Nickel: Used for strength, corrosion resistance, and toughness. Present in L-6 and AUS-6 and AUS-8.

Silicon: Contributes to strength. Like manganese, it makes the steel more sound while it's being manufactured.

Tungsten: Increases wear resistance. When combined properly with chromium or molybdenum, tungsten will make the steel into a high speed steel. The high speed steel M-2 has a high amount of tungsten.

Vanadium: Contributes to wear resistance and hardenability. A carbide former that helps produce fine-grained steel. A number of steels have vanadium, but M-2, Vascowear, and CPM T440V and 420V (in order of increasing amounts) have high amounts of vanadium. BG-42's biggest difference with ATS-34 is the addition of vanadium.
 

CARBON AND ALLOY (NON-STAINLESS)
These steels are the steels most often forged. Stainless steels can be forged but it is very difficult. In addition, carbon steels can be differentially tempered, to give a hard edge holding cutting edge and a tough springy back.  Stainless steels are not differentially tempered. Of course, carbon steels will rust faster than stainless steels, to varying degrees. Carbon steels are also often a little bit less of a crap shoot than stainless steels -- all of the steels named below are fine performers when heat treated properly.

In the American Iron and Steel Institute (AISI) steel designation system, 10xx is carbon steel, any other steels are alloy steels. For example, the 50xx series are chromium steels.

In the Society of Automotive Engineers (SAE) designation system, steels with letter designations (e.g., W-2, A-2) are tool steels.

There is an American Society for Metals (ASM) classification system as well, but it isn't seen often in the discussion of cutlery steels, so I'll ignore it for now.

Often, the last numbers in the name of a steel are fairly close to the steel's carbon content. So 1095 is ~.95% carbon. 52100 is ~1.0% carbon. 5160 is ~.60% carbon.

O-1
(Note:  the accompanying tables show standard alloy compositions by percentages of alloyed elements - the primary (and  assumed) component in all steels is iron)

W-2

W-1

The 10-series -- 1095 (and 1084, 1070, 1060, 1050, etc.) 1095 as an example

Carbon V (data for 50100B listed)

0170-6 - 50100-B

A-2

L-6

M-2

5160

52100

D-2

Vascowear

STAINLESS STEELS
Remember that all steels can rust. But the following steels, by virtue of their greater than 13% chromium, have much more rust resistance than the above steels. It should be pointed out that there doesn't appear to be consensus on what percent of chromium is needed for a steel to be considered stainless. In the cutlery industry, the de-facto standard is 13%, but the ASM Metals Handbooks says "greater than 10%", and other books cite other numbers. In addition, the alloying elements have a strong influence on the amount of chromium needed; lower chromium with the right alloying elements can still have "stainless" performance.

420

440 A - 440 B - 440C (data for 440C listed)

425M - 12C27

AUS-6 - AUS-8 - AUS-10 (aka 6A 8A 10A) (data for AUS-8 listed)

GIN-1 aka G-2

ATS-34 - 154-CM

ATS-55

BG-42

CPM T440V - CPM T420V (data for CPM440V listed)

400 Series Stainless
Before Cold Steel switched to AUS-8, many of their stainless products were marketed as being of "400 Series Stainless".  Other knife companies are beginning to use the same term. What exactly *is* 400 Series Stainless? It might be 440-A, but there's nothing to keep a company from using any 4xx steel, like 420 or 425M, and calling it 400 Series Stainless.

NON-STEELS USED BY KNIFEMAKERS

Cobalt - Stellite 6K

Titanium
Newer titanium alloys can be hardened near 50 Rc, and at that hardness seem to take something approaching a useful edge. It is extremely rust resistant, and is non-magnetic. Popular as expensive dive knives these days, because the SEALs use it as their knife when working around magnetically detonated mines. Mission knives uses titanium. Tygrys makes a knife with a steel edge sandwiched by titanium.

Ceramics
Numerous knives have been offered with ceramic blades. Usually, those blades are very very brittle, and cannot be sharpened by the user;
however, they hold an edge well. Boker and Kyocera make knives from  this type of ceramic. Kevin McClung recently came out with a ceramic composite knife blade that much tougher than the previous ceramics, tough enough to actually be useful as a knife blade for most jobs. It is also user-sharpenable, and holds an edge incredibly well.
 
 
 

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