Tall Dark and Swarfy wrote:You have the rare ability of combining technical writing with proper English. Much more satisfying to read than the banal, data-driven rubbish that is considered scholarly these days. I'm looking forward to your completion of this effort.
Thanks! I spend much of my workday mediating between corporate suits (who usually don't understand, don't want to understand, and don't believe they have to understand what they're buying) and mechanical engineers (who think anyone who can't communicate entirely in Reverse Polish Notation must be moral degenerates).
Anyway, tired of reading my own writing so I'll leave this here for tonight. I'd appreciate it if someone with lots of experience with 52100 could review the last but two paragraphs in 3.1.2; I'm not too confident on it.
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.
1- Steel- What is it?
First, let us understand what steel is. 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.
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- Importance and function of carbon and other alloying elements
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.
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
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) 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.
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 (by mass). 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."
A great man once said, "Science is more art than science."
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, 5120 and 5160:
Carbon 0.17 - 0.22
Chromium 0.7 - 0.9
Manganese 0.7 - 0.9
Phosphorus 0.035 max
Silicon 0.15 - 0.35
Sulphur 0.04 max
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
What immediately leaps to the eye is that, except for the carbon content, these two alloys are identical. 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 all 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 being much less prone to corrosion 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, however.
3.1.4- SAE Tool Steel Nomenclature
3.2- The JIS system
3.3- 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)