Fri Mar 07, 2014 9:44 pm
Latest version. Suggestions and constructive criticism are always appreciated.
NOTES TOWARD AN ESSAY ON KNIFE STEELS
For the prospective buyer of a high-quality knife, there are few topics as potentially confusing as the myriad varieties and opinions on knife steels. In this essay I attempt to clarify the issues and perhaps explode some myths, from the perspective of an engaged amateur in knifemaking and collecting with experience and training in the metalworking industry.
Before we start, allow me to clarify that the differences between knife steels can sometimes be exaggerated. Steels of excellent quality for knifemaking are inexpensive and easily available, and some steels that are quite good if used in a properly designed and made knife, such as 420HC, 440B and AUS-8, have gotten reputations as dogs because they are associated with cheap, mass production knives that are badly made. Others, like the Hitachi White series (aka "Shirogami" or "White Paper") have become nearly legendary because they are often selected by some of the best knifemakers for their products. In truth, the difference between knives made of steels in the same broad category, such as carbon steels Hitachi White and its far more prosaic cousin 1095, will be minimal so long as the knives are equally well designed and made.
In summary, unless there's a complete deal killer (such as stainless when you want carbon, or vice-versa), selection of the steel can take a back seat to other considerations when choosing your new knife.
1- Steel- What is it?
Steel is an alloy of iron and carbon, sometimes with other elements required or merely allowed in a particular grade.
There is an enormous variety of steel grades; for this essay, I will focus exclusively on those used in the overwhelming majority of knives, that is to say, martensitic steels with a high carbon content. Thanks to their chemical composition, these steels have the property that they can be effectively hardened through heat treatment, a process which I will explain below.
Although many people divide steels into "carbon" and "stainless," there is an extremely important category of alloys that contain elements besides iron and carbon and yet cannot be termed "stainless." Although all steels are alloys, the term "alloy steel" is generally understood to mean a steel that has a significant amount of an element besides iron and carbon. In this essay, I will follow that definition, but will only use the term for those alloys that do not have enough chrome to be considered stainless. In short, "alloy steel" will be used only for those steels that cannot be classified as "carbon" or "stainless."
Stainless steel is an alloy that contains at least 10.5% chrome (by mass). "Stainless" is a bit of a misnomber; given time and the right (or rather, *wrong*) conditions, any steel will corrode. Within the stainless steel family, martensitic stainless with a high carbon content (that is to say, the type of stainless steel that is used in high-quality cutlery) is the least corrosion resistant.
1.1- Defining the physical properties of steel
In speaking of steels, we'll borrow terms from materials engineering that are sometimes counterintuitive to the layman. We'll also use certain common terms in highly specific ways, and other terms that are largely specific to knives and other cutlery. Here, I seek to define these terms in a way that will be easily understood and memorable.
HARDNESS: The ability of a material to withstand force without plastic deformation (bending). STRENGTH is the amount of force required to induce material failure (bending or breaking). Because steel is a ductile material, its strength is highly correlated to its hardness. When talking about steel and similar materials, "strong" may be taken as simply another way of saying "hard." Hardness is an important property to consider in knife steels because a harder material will allow a steeper, keener edge.
TOUGHNESS: The ability of a material to withstand force without fracture (breaking). Also an important property, as materials lacking in toughness will tend to chip in use.
These two properties are related but also opposite. A useful example is that a ceramic plate is very hard but not very tough, while a rubber ball is very tough but not very hard.
CORROSION RESISTANCE: The ability of a material to resist chemical conversion. Its importance will be decided by the user's willingness to perform routine maintenance, its exposure to acidic foods and salt, etc. Carbon steel has the lowest corrosion resistance and stainless steel the highest, but keep in mind that any steel can rust and good cutlery steels will rust quicker than some marginal ones.
ABRASION RESISTANCE and WEAR RESISTANCE: Although abrasion is technically only one type of wear, in the context of knives the two terms may be taken as synonymous. The terms are more or less self-explanatory. Wear resistance will increase with a steel's hardness, but also with the presence of carbides in the steel matrix. Materials with higher wear resistance will have better edge retention (see below), but will also be harder to sharpen.
EDGE RETENTION: The ability of a knife to keep its edge in normal use. A given material's edge retention is directly correlated to its hardness and wear resistance.
GRAIN SIZE: The average size of the crystalline grain in the steel matrix. Has important effects in properties like strength and ductility, but don't worry about this too much; the relevant aspect here is that a knife made of a finer-grained material will take a more polished edge. Carbon steel has the finest grain size and therefore can take the most polished edge, while alloys with large carbides, such as D2, will resist polishing.
SHARPNESS: A subjective and often contentious property. I regard it as a combination of the keenness of the edge, its polish, thickness behind the edge, overall thickness of the blade, etc. Steel selection is only one of many factors influencing a knife's potential sharpness.
1.2- Importance and function of carbon and other alloying elements
One of the most common things you'll see in discussions of knife steels is carbon content. High carbon steels can be hardened more than low carbon steels, so it must follow that steelmakers can just pour more carbon into the mix until they obtain the hardest, best steel, right?
Things are slightly more complicated that that. In a simple carbon steel, maximum hardness can be achieved with relatively low carbon content (compared to the amount of carbon we find in the "best" knife steels, anyway). Extra carbon is added to make the hardening process (see heat tretment, below) simpler and more predictable, or to make up for the carbon that ends up forming carbides with other alloying elements. Adding too much carbon to the alloy will simply turn it into in cast iron, which is useless for knifemaking.
Other alloying elements can be added to the mix. Common ones include chromium (for corrosion resistance and wear resistance), vanadium (for grain refining in alloy steels), manganese (helps remove impurities in the steelmaking process) nickel (toughness) and many others.
Generally speaking, carbon steels will sharpen easier and take the finest edge (for a given hardness). In knifemaking, other alloying materials are only considered when other qualities are desired. These qualities can be important to the end-user, for example, corrosion resistance or edge-holding. On the other hand, alloying elements can also result in undesirable properties, for example by forming large carbides that make obtaining a highly refined edge difficult.
2- Heat Treatment
To achieve the keenness desired in an edged tool, hardness is required. Martensitic steel has the quality that, when heated to a certain temperature and rapidly cooled, it becomes much harder than it was in its original state. There are several methods and scales to measure steel hardness; the most commonly used in the knife world is the Rockwell C scale, usually abbreviated as HRC. The dominance of the Rockwell C system is such that even if the measurements are taken with another method (such as Vickers, Brinell, or Knoop) they will commonly be converted to HRC in the documentation. In the Rockwell system, the harder a material is, the higher number it will have. Thus, a piece of steel measured at 62 HRC will be harder than one hardened to 55 HRC. One artifact of the way the HRC scale was calculated is that the "leap" between one number and the next is not even; the numbers go further apart the higher you go on the scale. Thus, the difference in hardness between 61 HRC and 62 HRC is greater than between 54 HRC and 55 HRC.
The process of heating and then rapidly cooling (quenching) steel is simply known as hardening. It usually happens that the hardening process will induce stresses into the steel, as well as leave it somewhat harder than desired. The result is a workpiece that is too fragile for use and may warp over time. Thus, the practice of tempering -which consists of heating the workpiece to a certain temperature, allowing it to soak in the heat, and then cooling slowly- is used to relieve the internal stresses and bring the part's hardness down to its desired level.
Together, the hardening and tempering processes are known as heat treating. The physical characteristics of the finished product will depend very heavily on the heat treatment, making this arguably the most important part of knife manufacturing.
It is very important to note that heat treatment will allow the knifemaker to vary the hardness (and therefore the other characteristics related to hardness) by varying the time and temperature of the tempering process, within the limits imposed by the alloy's chemical composition. While one steel might have a higher maximum hardness than another, an individual knife made of the first may have a lower hardness than one made from the second.
There are other processes such as stress relief, normalizing, etc. that are also part of the heat treatment family but these are not typically part of the knifemaking process and are therefore outside the interest of this essay.
2.1 Hardenability and differential hardening
Despite the name, hardenability does not refer to the maximum hardness that can be obtained from a given alloy. Rather, it is the depth of the hardened material after the hardening process is completed. In the case of knives, with their thin cross-section, through-hardening can easily be achieved even with alloys with low hardenability. However, with alloys that have low hardenability, the craftsman can differentially harden a knife, typically making the material hard at the edge and leaving the spine soft but tough. This technique is common in Japanese swordmaking and other traditions. While the benefits of this process are marginal in a kitchen knife, many conoisseurs prize differentially hardened knives for their craftsmanship and the often beautiful temper lines, usually called by their Japanese name, "hamon."
3- Steel Grade Nomenclature
Steels are made in enormous variety, for products ranging from rebar through auto parts to razor blades. This is thanks to the great flexibility of steel chemistry, which allows metallurgists to engineer steels that maximise the desirable qualities in the material (such as hardness, toughness, corrosion resistance, economy of production or of processing into a finished product, etc). Naturally this means there is a need to classify steels into grades, so that purchasers of raw steel can easily specify the steels suitable for their needs.
There are different systems that classify steels in use throughout the world. Generally, each major industrial power has developed their own system, which the countries in their economic sphere have adopted. In this essay I will focus on the AISI/SAE system (USA), the EN system (European, appears to be generally based on DIN [Germany] and is rather slowly superceding various national systems) and the JIS system (Japan), as these are the ones most typically encontered by the knife enthusiast. Also of interest are proprietary names sometimes used by steel and knife manufacturers, either of standard steels (perhaps to obscure the plebian origin of the materials used) or of their own unique alloys.
Keep in mind that these systems merely classify concrete objects; they are not the objects themselves. Pretty much any alloy you could name in one system will have a twin or a close-enough analogue in all the other systems.
3.1- The SAE system
Although the chemical composition of the alloy is less immediately obvious than some other systems (notably the German DIN), the AISI/SAE system is very easy to understand once you know what to look for, and perhaps the easiest to use casually. A four or five digit number indicates a carbon or alloy steel, a three digit number (usually followed by one or more letters) indicates stainless steel, and a letter followed by one (occasionally two) digits indicates a tool steel.
For carbon and alloy steels, the first two digits indicate the alloying materials, while the last two (or three, in the case of five-digit alloys) indicate the carbon content.
3.1.1- SAE Carbon Steel
Carbon steel composition is the easiest to understand. The first two digits will be 10, while the last two indicate the (nominal) carbon content. Thus, 1095 tells us it is a carbon steel that contains (approximately) 0.95% carbon. Let's look at the actual composition of 1095:
Carbon 0.9 to 1.03%
Manganese 0.3 to 0.5%
Phosphorus 0.04 maximum
Sulphur 0.05 maximum
As you can see, some variation in the actual amount of carbon is allowed, as are trace amounts of sulphur and phosphorus, two contaminants that, in significant quantities, can render steel brittle and unworkable. The only surprise here is the small amount of manganese required; manganese is used in steelmaking as an agent to remove those pesky contaminants, makes the steel easier to work with, and generally improves the desirable characteristics (such as strength, toughness, resistance to cracking) of the steel, yet it does not change the qualities of the alloy so much that it is no longer considered "carbon steel."
1095 is probably the most important of the conventional SAE carbon steels used in kitchen cutlery. With competent heat treatment, it can take a keen, fine edge, sharpens quite easily and resists chipping well. The "Old Hickory" brand uses this alloy for their amazingly inexpensive knives; unfortunately these are very cheaply made, but they make fine "project knives" for people with the basic tools and skills necessary to modify or replace the handle and thin the edge.
Although the exact alloys used are a mystery, it is a safe bet that most European carbon steel knives such as Opinel and Sabatier are made from something very similar to 1095.
3.1.2- SAE Alloy Steels
Now that we can read a SAE carbon steel grade, let's look at the SAE alloy steels.
Since we know that "10" as the first two digits will indicate a carbon steel, it becomes obvious that the first two digits in the grade will indicate the elements present in the alloy. For example, 5160:
Carbon 0.56 - 0.64
Chromium 0.7 - 0.9
Manganese 0.75 - 0.9
Phosphorus 0.035 max
Silicon 0.15 - 0.35
Sulphur 0.04 max
We can see this has an important chromium component, but not enough to call it "stainless." In fact, we can tell a lot about a steel just by looking at the first digit. Compare 5160 above to 52100 below. We already know that 52100 is going to contain more carbon (by reading the digits after the second), but the other alloying elements may surprise us:
Carbon 0.98 - 1.1
Chromium 1.3 - 1.6
Manganese 0.25 - 0.45
Phosphorus 0.025 max
Silicon 0.15 - 0.35
Sulphur 0.025 max
The presence of a generous amount of chromium in the above steels should be the giveaway - SAE grades that start with a "5" are all chromium-carbon steels.
5160 and 52100 are by far the most important SAE alloy steel grades in knifemaking (excepting tool steels, on which more below). While 5160 is common in camping knives where toughness is paramount, 52100 is prized in kitchen cutlery for its excellent sharpening qualities, which approach carbon steels, while having better edgeholding and corrosion resistance than the carbon alloys. For many users, it may be in a "sweet spot" between carbon and stainless.
It should be noted that 52100 is available in a special high grade version called "52100 Vac Melt." This is not an official SAE grade but rather an indication that it is made in special equipment to produce the highest quality steels - see more below on methods of steel production.
It is left as an exercise to the reader to investigate the alloying elements in other SAE alloy steel grades such as 4140, 4360, 6150, etc., as well as the effect these materials have on the physical characteristics of the alloy if they wish to learn more about this topic; they may do so by exploring the links provided in "Suggested Reading," below. These alloys are much less important in knifemaking.
3.1.3- SAE Stainless
SAE stainless steel grades consist of a three-digit number that indicates their alloy composition, followed by one or two letters indicating carbon content. 100, 200 and 300-series steels cannot be hardened by heat treatment and are therefore useless in knifemaking, outside of perhaps cladding or separate bolsters. 500-series is used for special grades of heat-resistant stainless steels for industrial use. The SAE grades of interest to us fall in the 400-series.
By far the most important types of SAE stainless steel grades in knifemaking are the 420 and 440 grades.
420HC, the high carbon version of 420, is widely used for pocketknives. Competently heat treated, it may be called "not terrible" for this application. Sharpens easily, although relatively low hardness limits its ability to take a keen edge. Edgeholding is also rather low. Buck Knives makes a line of kitchen knives in this material.
440A is the lowest carbon (and least desirable) of the 440 line of steels. It is widely used in low-end kitchen cutlery, will not take a keen edge. Knives made of 440A are often serrated. It has the best corrosion resistance of the common stainless grades used in cutlery.
440B has somewhat higher carbon content than 440A. Also widely used in (somewhat) better grades of kitchen cutlery. In my opinion, it is roughly equivalent to 420HC.
440C is the original "super steel." Highest carbon content of the 440 series, can be hardened to 57 HRC and takes a better edge than its siblings. In the premium cutlery market, it has been largely overshadowed by newer, better alloys, usually offered under proprietary trade names, which I will discuss below.
Because of their high alloy content, all stainless steels suffer from sharpening problems compared to carbon steels. No stainless steel will take an edge as polished as its carbon counterpart, and "burr chasing" and wire edges are facts of life for people sharpening knives made of it.
3.1.4- SAE Tool Steel
We sometimes hear the claim that this or that knife has a special quality because it is made of "tool steel," or that "tool steel" is not as good as whatever the speaker is touting. This sort of generalization is a good sign the speaker doesn't really understand what they're talking about. In fact, there is a huge variety of tool steels, and there is nothing truly magical about them.
Tool steel nomenclature was developed for alloys used in industrial environments, where extremely predictable behavior is desirable even at a higher price, because the cost of the material is a tiny fraction of the value of the part. Examples of these products include machine tool tooling, molds, dies, etc. In each of these, even slight variations in their characteristics could result in defective products, broken machinery, unscheduled downtime and other expensive outcomes.
18.104.22.168 What is Tool Steel?
Simply put, tool steels are steels that are made to a tighter specification than conventional ones. Many of the SAE tool steel specifications overlap with conventional SAE steel specifications, the only differences being less contaminants allowed and more control over the exact amounts of alloying elements.
22.214.171.124 Tool Steel Nomenclature
Befitting the rather special purpose of these materials, tool steels have a unique nomenclature that hasn't so much organized as accrued by users and manufacturers over decades. Some of them (W, O and A series) are categorized by their quenching media, others (S and H series) are categorized by their most notable quality, others (D and P series) by the use they were developed for, and still others (L, T, M and F series) are categorized by an alloying element.
126.96.36.199.1 We'll start with tool steels by their quenching media.
W series is quenched in water. These are very high quality, almost pure carbon steels, with low hardenability. As such, it has all the characteristics of a carbon steel, meaning it will take a very fine, keen edge easily but is also prone to rusting, toughness is relatively low and its edge-holding ability could be better. W-2 is widely used in knifemaking, and is the least expensive tool grade steel.
O series is quenched in oil. These have some alloying elements, but may be regarded as carbon steels in practice. They have higher hardenability and are easier to work than the W series. O-1 is the most commonly used in knifemaking, while O-6 is tough and retains its edge well.
A series is quenched in air. Definitely alloy steels, these have quite high hardenability. A-2 is widely used in outdoors knives, where its toughness is prized.
188.8.131.52.2 Tool steels by their notable quality:
S series: Designed to resist shock, the only cutlery use that I'm aware of is in reproduction katana swords.
H series: Designed to retain their hardness at high temperatures, with no cutlery applications.
184.108.40.206.3 Tool steels by their use:
D series: Originally used in steel cutting dies. D2 is used by makers such as Bob Dozier and Queen, generally for pocketknives. Due to its chromium carbides, this steel is notoriously hard to sharpen, but has excellent edge retention (Editor's comment: D2 will take a mediocre edge and hold it forever). Also has some corrosion resistance.
P series: Used in plastic molds. No cutlery applications.
220.127.116.11.4 Tool steels by alloying material:
L series: Low alloy. L6 is appreciated in outdoors knives for its excellent toughness and takes a good edge. Generally overlooked in kitchen cutlery, where toughness is less critical and carbon steels like the W series overhadow it for their sharpening qualities.
T series: High-speed steel with tungsten content. No significant cutlery use that I'm aware of.
M series: High-speed steel with molybdenum content. No commercial cutlery use that I'm aware of, although some machine shop mechanics like to make "neck knives" out of M2. In this application, it's quite hard to sharpen, somewhat brittle and has excellent edge retention.
F series: Another tungsten steel, this one with significantly less alloy. Might be called a cross between L and M series. Seemingly ignored in the knifemaking world, which is a mystery to me, since it should make excellent knives (at least on paper).
Some conventional steels (such as 52100) and proprietary alloys (such as CPM 10V) are sometimes referred to as tool steels, particularly when they are manufactured in a process that lends itself to high purity and tight tolerances in alloying, such as VacMelt or powder metallurgy (see below). This is technically incorrect, but such a steel can be have characteristics as good as any "real" tool steel.
3.2 The EN system
3.3- The JIS system
3.4- Proprietary Steels and Nomenclature
4- Steel by Manufacturing Process
4.1- Conventional Steel Manufacturing Overview
4.2- Tool Steel Manufacturing Overview
4.3- Powder Metallurgy Overview
5- Steel by its Characteristics in the Product
5.1- Carbon and quasi-carbon Steels
6- Suggested Further Reading
NBS Monograph 88 (link)
cashen blades (links)