Steel Used in Knives

When choosing the best knife, part of the job is making sure it's design is appropriate for your needs. However, one thing you'll need to take careful look at is the composition of the stell used in the knife.
In this article we'll discuss both the components that make up the various types of knife steel, as well as which steels are better for which situations.

Photo courtesy of flickr user Frédéric Bisson

Components of Steel

Before going into the various types of steel, we should first discuss the various components that go into making a good quality steel and the properties they add to the steel.

Iron: Iron is the main component of steel. Pure iron is actually quite soft, and industrial iron can reach a purity of roughly 99%. Iron at its core is a relatively soft metal that will hold it's shape when worked and is capable of being brought to a very fine edge (the only thing that can be sharper is glass). Iron is easily worked and maleable, and is quite durable.

Carbon: The basic formula for steel is iron with carbon added in. Most wrought or pig irons have a large amount of carbon in it, as well as other compounds, and this must be reduced industrially to about 1% for a good knife. Knife steels are generally categorized into groups: low carbon (between .1% and .5%), medium carbon (between .59% and .89%) and high carbon (between .9% and 1.2%). Adding carbon to the steel significantly increases both the iron's toughness and tensile strength (the amount that it can be pulled before breaking). Carbon's main property, however, is that it will crystalize when cooled rapidly. This is called tempering and it is how we achieve maximum blade hardness. A tempered knife will become more brittle, yet hold an edge for much longer.

Chromium: Chromium is added for two reasons: first it increases wear resistance and hardenability  and most importantly it strongly resists corrosion (i.e. rust). A steel that is at least 13% Chromium is considered "stainless", however this is a misnomer, as even stainless steel will form rust if not cared for properly. Chromium itself has a tendency to form carbides, which increases wear resistance, but there is some debate between the composition of "free chromium" and chromium "trapped in carbides" regarding its efficacy as a stain resister.

Manganese: Mostly present in cutlery steel (and the KABAR knife brand) Manganese mostly increases strength and wear resistant, and has a slight increase in the knife's ability to be worked.

Molybdenum: Molybdenum is another carbide former. Molybdenum is added to help prevent brittleness, and also a slight heat resistance. This is typically present in steels that "air harden" rather than being quenched.

Nickel: Colloquially believed to make the steel stainless, nickel is actually added to increase toughness and prevent brittleness, especially in low temperature situations.

Phosphorus: A useless element that is almost always in trace amounts of steel. These amounts were not completely removed during the iron's purification process and actually make the steel softer.

Silicon: Helps to contribute to strength, similar to Manganese.

Sulfur: Sulfer makes a steel capable of being worked by machines in moderate temperatures, but decreases toughness.

Tungsten: Tungsten is another carbide former, and as with all carbide formers, it increases wear resistance.

Vanadium: Another carbide former, vanadium increases wear resistance and hardenability  It's main feature is that it increases edge retention and is often found in high amounts in certain stainless steels.

Properties of Knife Steel

Before we go into the specific steels used in knife manufacturer, we should first speak about the properties of a good steel.

Wear Resistance: Increased wear resistance allows the steel to withstand abrasion (scratching) without deforming in some way. Carbides in general increase this property.

Strength: Although I've used the term "strength" quite loosely in other guides, simply speaking strength refers to a blade's ability to take a load (force) without deforming (bending). This is critically important for your thick, chopping type blades (think machete, bowie or axe).

Toughness: Often conflated with strength, toughness is the ability of a steel to take an impact without cracking or chipping.

Strength vs Toughness: Its important to clarify strength vs toughness before going on to specific knife steels. As a general rule of thumb, as the strength of a steel increases, the toughness decreases, and vice-versa. This isn't always the case, depending on composition, but the rule typically holds out. The property that increases strength (the ability not to be deformed or bent when stressed) is the knife's level of hardness. This is done by tempering the knife. As hardness increases, strength (or resistance to deformation) is increased, but toughness (resistance to chipping) decreases. Usually there is a trade off, as the hardness of a knife will allow the edge to be taken to a finer point, but increases likelihood of chipping. For a bowie, strength is the key element and toughness is compromised to facilitate this. For a fillet knife, toughness vitally important as a knife scraping against a bone may chip if not tough enough.

Stain Resistance: Self explanatory, stain resistance is the property of the steel to resist corrosion, usually in the form of rust. Increasing a steel's chromium content increases stain resistance, but this added chromium reduces the room of the steel for other important elements, making them weaker.

Edge Retention: Edge retention is not a simple matter. The ability of the knife to hold an edge will depend on three other properties of a steel: strength, toughness and hardness. This is because the ability to hold an edge is dependent on the type of work being done by the knife. If a blade is deformed (because it was not strong enough) it will not be as sharp. If the blade chips, sharpness too will decrease. Toughness is the main component in edge retention when cutting through objects that may contain hard impurities, such as drywall or card board. Strength is the key factor in edge retention when a blade will be carving wood or other hard materials. Additionally, wear resistance is important in preventing edge degradation, as is stain resistance.

Tempering

Tempering is the process of making the steel hard. A harder steel increases edge retention, but it is more brittle. A strong steel will need to be hardened quite a bit, but too much hardening will significantly reduce a blade's toughness, increasing the likelihood of chipping.
A blade is tempered when it is heated in a forge so that it is "cherry red", or about 1900 degrees fahrenheit. It is then quickly placed in a tempering bath (quenched) composed of either water, oil, or salt brine. As the heat is absorbed from the steel, the carbon crystalizes, making it very hard and rigid  The quicker the steel is cooled, the harder it will be.

Oil Tempering allows for a hardened steel that is more flexible, at the cost of edge retention. This is because oil absorbs heat rather slowly. This makes for a less hard, and therefore less brittle steel. If making a knife at home, this is the type of quenching solution that would be used for a sword or chopping type knife, as it will need to be able to withhold stress without breaking.

Water Tempering allows for a harder steel, as water absorbs the heat more quickly

Brine Tempering: Brine is simply salt added to water. This saltwater solution allows for a slightly faster quenching (although probably not noticeably so) but most importantly allows for an even temper due to the sodium content. This is mostly used in industrial manufacturing, or in freezing temperatures (an ice water bath will make for a VERY hard steel that may not be desirable in all situations).

Now that we've spoken about the various components of steel, lets get into the basic steels themselves. I'll only be discussing a few steels, those which are most suitably used for knives, and there are a myriad of other steel formulas that can be used in various situations.

Steel Grades

There are several organizations that grade steel depending on their composition. Being familiar with these grading schemes can help you decide which knife is best for you.

SAE: The Society of Automotive Engineers grades steels of a wide range of compositions, including steel that can be used for knives. The table below lists the SAE designations and their properties.

SAE designation	Type
1xxx	Carbon steels
2xxx	Nickel steels
3xxx	Nickel-chromium steels
4xxx	Molybdenum steels
5xxx	Chromium steels
6xxx	Chromium-vanadium steels
7xxx	Tungsten steels
8xxx	Nickel-chromium-vanadium steels
9xxx	Silicon-manganese steels

The SAE also grades stainless steel. As a general rule, the last two digits of the SAE system roughly corresponds to the carbon content. The SAE also grades steel allows using a letter system to define the grades and numbers to designate steels within each grade.

Stainless Steels

I personally dislike stainless steels. The chromium content itself makes the steel less able to be razor sharpened, which is why straight razors are not made of stainless. However, it is very useful in reducing rust and is the most common type of steel used in manufactured blades.

400 Series Stainless: The two most common types of stainless steel used for knives are 420 and 440. As a general rule, as the number increases, do does the carbon content. 420 stainless has a carbon content of less than .5%, and is generally only used in the cheapest of stainless steel knives (Buck Knives uses 425 stainless steel whose carbon content is set right at .5%). 440 steel is separated into 3 different categories based on their carbon content: 440AA (.75%), 440B (.9%) and 440C (1.2%). 440B would best be used for a good chopping type knife, and 440C would best be used in a fillet or slicing type knife.

ATS-34: ATS-34 became quite popular in the 1990's. Manufactured by Hitachi, it holds an edge very well though is somewhat less rust resistant than the 400 series. ATS-34 has a carbon content of 1.5%.

There are other stainless steels used in knife manufacturing, and I'll attempt to add in information as soon as I verify and compile it.

Non-Stainless Steels

1095: This steel, as can be seen by the chart above, is regular carbon steel. The "95" designates that the steel contains ~.95% carbon. This carbon content allows for a very good edge yet allows the blade to remain very strong. KABAR uses a variation of this (with added chromium and manganese) to make their trademark knife. Most non-stainless survival or bowie type knives that are commercially manufactured utilize 1095.

5160: 5160 steel has at least 1% chromium and .6% carbon. It is very popular with forgers, and most of the knives I've made are of 5160, because many leaf springs are composed of it. It has excellent wear resistance and toughness, and the chromium increases its hardenability.

D2: D-grade tool steels contain between 10% and 18% chromium and are often called semi-stainless. D2 tends to be tougher than most stainless steels, yet not as tough as many non-stainless steels.

W1 and W2: Used in the manufacturing of files, W1 steel is composed of 1.5% carbon. Depending on the temper they can be very brittle, and are best used for slicing knives. W2 is the same as W1 except W2 has .2% vanadium which helps it hold an edge very well.