I saw some questions about steel recently, and I decided to write a short article on the how, why and what regarding steel as it is used in swords and knives. In this article I’ll be explaining several things, including some things that are only interesting on a theoretical level. While that is not strictly necessary, it helps you understand why some things are done the way they are. The internet being what it is, I have no doubt that for every point I make, someone else is saying the exact opposite. That being the case, why would my opinion be any better? The only assurance I can give in that area is that I am a blade smith, and I use mostly various types of carbon steel. I start off with stock, heat it, forge it to shape, grind it, heat treat it, finish, polish and sharpen it, and make a handle. I’ve been doing this for several years now, so I can say with some degree of confidence that I know what I am talking about. Master smith Mike Blue was kind enough to review this article. Any mistakes left are my own. With that said, let’s get started. Carbon steel The term steel is used to describe metal consisting of Iron, with 0.2 to 2% carbon. The actual percentage depends on the intended use and the requirements. More carbon makes the steel stronger and tougher than plain iron, but as the carbon content increases the steel becomes harder than iron or soft steel and eventually more brittle or prone to crack or break under load, instead of bend. What is important about the carbon is that it allows for the steel to be hardened. Heating and cooling cycles can be used to change the crystalline structure of the carbon dissolved in the steel, making it much harder (more on that later), so that it can take a thin sharp edge that stays sharp for a long time. The flip side of course is that with increased hardness comes increased brittleness. In terms of being ‘steel’, all that matters is the iron and carbon content. Aside from those 2, other elements can be added to change the properties of the steel. Chromium can be added for increased rust resistance. Vanadium can be added for toughness, etc Of course the reality is more complex than that, because the outcome depends on the resulting crystalline structure that is the result of this magic cake mix, and all these alloying elements have their influence on all properties. The term carbon steel is most often used to describe a type of steel that has relatively few alloying elements; few enough that the resulting properties are more or less comparable to plain steel with only iron and carbon. Another name commonly used is tool steel or ‘high carbon’ steel if it has enough carbon to be used for cutting other steel or metals. Alloying Alloying is the process of adding elements to steel, to enhance certain properties, such as resistance to rust or certain chemicals, toughness, hardness, fatigue resistance, magnetism, etc. Steel has a number of properties that are all trade-offs with each other. Toughness and hardness are 2 good examples. A very hard piece of steel will resist scratching. This means that once something is polished, it will stay polished for a very long time. But the crystalline structure is very hard. This means that if it is put under enough force to distort the structure, it will just snap without a warning. A very tough piece of steel has a structure that is much less crystalline. This means it will be less polish able, but when placed under load, the structure has enough wiggle room to deform without catastrophic failure. To give 2 practical every day examples: compare a wood chisel and a concrete chisel. The wood chisel is made of very hard steel that stays sharp so that it can cut cleanly through wood fibers. But hit a nail and the edge is destroyed. A chisel for concrete cannot hold a truly sharp edge. You could sharpen it, but it would be blunt quickly. However, you can put it to a block of concrete and hit it with a sledgehammer from all directions, and it would not budge, bend or break. This is toughness. For sharpness, you want the most pure crystalline structure you can, because that will make the sharpest edge. Once you start adding alloying elements, you will distort the structure. The benefit of adding large amounts of elements can be to make the steel stainless or impervious to certain types of chemicals. Or you want to increase toughness and impact resistance. Or you want to increase fatigue resistance. The possibilities are endless. But no matter how or what, every increase caused by alloying elements will degrade the structure and incontrovertibly make it less suited for cutting. Alloying is an important part of modern steel manufacture. Stainless steel Stainless steel is simply steel with added alloys to prevent rust. Or the alloys are treated chemically to create an oxide that does not degrade preventing further rust. The alloys commonly used for making steel stainless usually make it less suited for edged tools, because they interfere with the crystalline structure that is necessary for making a decent edge. There are stainless steels that are suitable for knives, but they are always compromises. For some applications such as simple pocket knives, this doesn’t matter, but for pushing the limit in sharpness or for sword length blades, they are generally avoided, because those alloys do not equal toughness needed in a striking blade. Super steel Super steel is a term that is used in the blade world to refer to modern steels that have very high levels of alloying. Such steels are often referred to as ‘super steel’, ‘wunderstahl’ or ‘unobtainium’. They are generally used as knife steel. Super steels are products designed to maximize some property other than cutting ability, without sacrificing too much cutting ability. The designers aim for extreme corrosion resistance for example. Or extreme toughness. In some cases, this makes sense. For example, in a salt water environment, rust resistance becomes critically important, so there is need for a steel that will not rust away before your eyes, but which still can cut pretty well. Usually, each new super steel – for blade use – is a new name for something that is almost the same as the previous type, but with slightly more of this, and slightly less of that. It gets a new name, and then knife makers use it to make new versions of the same knife. And of course, collectors and knife nuts want new knives custom made of the new stuff because the new shiny is just so much better than the old shiny, and gladly fork out hundreds of dollars to stay ahead of the game. The new steel gets used for 1 to 2 years, after which a new fad comes along. This means that some types of steel are only available for a couple of years. Tamahagane Every discourse on blade steel has to mention tamahagane, so I will discuss it here briefly as well. Tamahagane is the name for the steel which is used for making traditional Japanese swords, and has been used for hundreds of years in the same way. Literally translated, it means ‘jewel steel’ which just shows that even ancient Japanese smiths understood the power of marketing. There are a lot of myths floating around on the internet about this steel. It’s super steel. It’s magic. Tamahagane swords never break. They can cut through steel. Etc. The true wonder of tamahagane is not that it is such brilliant material. The true wonder is that a smith can take the collection of dirtballs that is raw tamahagane, and turn it into top quality sword steel. Raw tamahagane is made by building a big charcoal fire, a traditional form of bloom smelting, and then shoveling in layers of iron sands and charcoal for 3 consecutive days. This is a very sensitive process relying on experience to know when to add what and how to regulate the air. If the smelt master makes a mistake, he ends up with either soft steel or cast iron. The entire process is controlled just by the color of the flames and the roar of the air. The result is a big pile of metallic looking gunk that can be broken down into nugget sized bits of varying carbon content with a lot of crud and slag attached. That is raw tamahagane. Its only saving grace is that the metal is very pure steel, meaning there should be virtually no alloy elements in it at all. This is then sorted by carbon content, at which point the high quality stuff gets distributed to licensed sword smiths, in a pecking order determined by seniority. Only licensed smiths and people with really, really good connections can get in on that action. Everyone else using tamahagane is either using the low quality stuff that is made available to a bigger audience, or just plain lying. Because while the distribution of quality tamahagane is tightly controlled, slapping the name ‘tamahagane’ on any piece of steel is not actually illegal. Even the high quality stuff at this point is still a collection of ugly nuggets that are quite unusable for anything, with a very high carbon content. A smith will build a stack of those nuggets, heat it till it is bright yellow, and have his apprentices hit the thing with sledgehammers. This is done repeatedly for a very long time, until the stack of nuggets fuses together. This process also drives out the slag and dirt and closes up all the spaces or bubbles in the original material making an homogenous bar of metal. When the stack has become a bar, the bar is folded in on itself, and the process is repeated, driving out more dirt and homogenizing the carbon content. Eventually, the process will have produced a more or less homogenous bar of very pure steel, with a carbon content that has about halved since the beginning of the process. High quality tamahagane starts out at 1.4% carbon, and ends between 0.6-0.7%. The folding itself also does not impart any magic into the steel. Its only purpose is to homogenize and purify. The layers don’t do anything. This folding process itself is all done without modern equipment or fluxing chemicals. As a result there can be welding flaws inside the material, following the length of the bar. These inclusions are literally called folding scars, and can be anywhere in the material. That does not mean the steel is defective, as long as these are not fatal flaws exposed along the edge. A good smith will minimize these flaws as much as possible. There are modern, very pure steels that are virtually identical to properly processed tamahagane with a known carbon content. In terms of metallurgy, this steel is as good as the best tamahagane. But these still remain very simple steels compared to modern alloys. As I said: there is no magic. Real tamahagane starts our as gunk and turns into a very simple yet very pure billet of steel. The only magic is in the hands of the smith who can get from one to the other, using only methods and resources that are more than 500 years old. Authentic Japanese high quality tamahagane cannot be bought outside Japan; Nor inside Japan for that matter. Not unless you are a licensed sword smith. But people can and do make their own tamahagane. It’ll still be expensive though. Making it is a very time consuming process, and in the end you still end up with raw material that takes a long time and effort to turn into something useful. Damascus Damascus steel is a name that has existed for centuries, and has more or less changed meaning in modern times. Contrary to tamahagane, the original steel which became known as Damascus steel can objectively be called a super steel. Despite being hundreds of years old, in some applications it can still beat almost anything we can make today in a foundry. The discovery of ancient Damascus was a happy fluke, and when the specific ores that were needed for making it ran out, the craft of making it was lost because at the time, no one had a clue what really made the difference between Damascus steel and plain steel. When Damascus steel was processed correctly, it exhibited specific patterns on the surface. One of the most prized patterns was called ‘Mohammed’s ladder’, which looked a bit like a ladder running the length of a sword. Original Damasucs aka Crucible steel aka Wootz Wootz is also called several other things, because it has been independently recreated by a handful of metallurgists in the previous century. Pulat or Bulat are 2 other names. The making of wootz is a very finicky process that requires a good deal of skill and knowledge, but also a very specific mix of alloys. Wootz is made in a crucible, where iron ore is melted to a liquid state, with charcoal added to the mix for carbon. When everything has melted and dissolved, the crucible is removed from the furnace, and allowed to air cool. This is where the magic happens. As the soup is cooling down slowly, the alloys solidify first while the iron is still molten. This solidification happens in dendrite structures which permeate the mixture like a spider web. Sometime later the steel solidifies, and you have an ingot of carbon steel with carbide dendrites running throughout. These carbides give the steel a look that is not unlike wood grain or some other organic structure with branching growth. This is also what gives the steel its qualities. Just like rebar reinforces concrete, those carbides strengthen the steel. And they also have an interesting side effect. Wootz knives, when properly sharpened, out-cut any modern steel in cutting tests. The reason is that those carbides permeate the surface of the knife and the edge, and are much harder than the surrounding steel. So as the ‘softer’ steel is worn away by use, the carbides start sticking out of the surface like very fine carbide teeth. Even a relatively dull un-hardend wootz knife will cut very strongly, because of the edge exposure of those carbides. Unlike Japanese Tamahagane, modern wootz is available for knife makers around the world. But it is very expensive. It never caught on as popular knife steel because it is very expensive, not stainless, and difficult to produce. Original Damascus got lost because it depended on specific ores. Once the raw steel could no longer be obtained, the knowledge about heat treating it and working it disappeared 1 or 2 generations later. Of course, the reputation remained, so it will not come as a surprise that during the ages, various manufacturers slapped the label ‘Damascus’ on anything with a cutting edge. The original steel that was called Damascus, and their modern recreations, is today called ‘wootz’ or crucible steel. Modern Damascus aka Pattern Welded steel Eventually, smiths started making bars of steel that consisted of a sandwich of different types of steel, with different alloy elements to give different layers different corrosion resistance. When such a bar is then twisted, shaped, and flattened, these layers will form a pattern throughout the steel when it is turned into e.g. a knife. If the knife is then etched, some layers will turn dark from oxidation, and some won’t. That can create all sorts of visually interesting patterns. This type of steel is currently widely called ‘Damascus’ or ‘Pattern welded’ (PW) steel. Pattern welded is the more accurate term, but because Damascus is so widely used by almost all blade makers to refer to that kind of steel, it is not considered incorrect. Calling the original type of steel ‘Damascus’ would also be correct, but it is generally called wootz to avoid confusion. Molecular steel structures Steel consists of iron, carbon, and alloying elements. When heat is added, the mobility of those elements increases, and they will start to move around. The crystalline structure of the steel will change and settle itself in a new ground state at that temperature. In the ground state, a piece of steel that has been allowed to cool off slowly will have its carbon located in little nodules that are spread around in the iron matrix, like raisins in a cake. This state is called pearlite. It is easy enough to see that this pearlite is no harder or tougher than plain iron. It’s iron with big sprinkles. If the steel is heated, energy is infused in those carbon nodules, and they will start to diffuse into the surrounding steel structure. The carbon atoms will find spaces between iron atoms in the iron matrix, and settle there. It’s for them the most convenient place to be. This state of the steel structure is called austenite. If the steel is now allowed to cool down again very slowly, the carbon will leave those spaces and clump together again to the pearlite structure. However, if the steel is cooled down quickly (quenched), the carbon does not have the time to migrate back, and they will become locked in the steel. This structure is called martensite, and is very hard. It is also very brittle. If you’d drop it or hit it with something, it could shatter. Heat treatment – hardening Hardening is the process of cooling down a piece of steel quickly to lock the carbon in place. The exact temperatures at which point these things happen depend a lot on the exact composition, because different elements and concentrations of carbon have different effects and different crystalline matrices. Plain carbon steel has a well known heat treatment recipe, and it is pretty simple. Low alloyed steel behaves a lot like pure steel, so it can be heat treated the same. For high alloy steels, this becomes very specific, and very finicky. Not only do the temperatures vary, but also the speed with which the steel can cool off, and thus the medium in which the hot steel is quenched. Steels needing a very fast quench have to be quenched in water. Other steels need oil, which extracts heat slower than water. There are special heat treatment oils, but generally the cheapest vegetable oil will work just as well. And for some steels, even air cooling is fast enough. Now you could ask yourself: why not always dunk it in water? Logically, the faster the atoms are locked in place, the better. In theory this is true. But there is thermal expansion to consider. Hot steel and cold steel have different sizes because they have different structures. If you quench a piece of steel, one part will always cool down faster than the other. Suddenly, one part shrinks while the other is still fully expanded. As a result, the entire piece starts warping. This will correct itself as soon as the rest cools down as well, but if there is a big warp, it will deform the piece before the rest cools down. And when the rest cools down, it will lock itself in the new shape. The faster the quenching medium can cool the steel, the bigger that temperature differences and thus stresses can become, and thus the bigger the chance that the piece will warp or outright tear itself to pieces. Quenching is a very fast process, and extremely violent at a molecular level. Regardless of the amount of deformation, the hardened piece will be full of stresses. To fix that, the steel is tempered. It will be heated to a specific temperature that is lower than the point at which the crystalline structure changes, but nevertheless it allows the carbon atoms some mobility, so that the carbon in the places where the stresses are highest, can squeeze back to places where they cause less stress. Care for carbon steel Proper care for steel is not overly complex, but doing it wrong is a good way to turn a knife or sword into a piece of scrap, especially with plain carbon steel. Carbon steel will develop a gray tinge over time. This is called a patina. It is a very thin layer of oxidation, and this is normal. This is even a good thing, because it is an inert layer that offers some protection against rusting. Do not polish this away because blank steel will be exposed again. This will look shiny, but any moisture or water can now create rust. It is also a very bad idea to degrease a knife or sword using spirits or some other kind of degreasing agent. Again, this will remove all oil and protection from the steel, and the steel will rust like crazy. Don’t touch the bare blade with your fingers, and if you do. Wipe it clean as soon as possible. Finger grease is acidic, and left alone it will etch itself in the steel. To maintain your steel in good condition, clean it regularly and wipe it down. When it is clean you should oil it with a little bit of camellia oil, or a neutral mineral oil. This will put a protective layer over the steel surface which repels oxygen and water. Do not use just any oil, because it might be acidic. Also don’t use wax. The right kind of wax would not hurt the steel, but it might build up in the sheath, attract dirt and thus stain the steel. This level of care is not only valid for plain carbon steel, but also wootz, pattern welded, and tamahagane; every type of steel that is not ‘stainless’. Care for stainless and super steel This is fairly similar to care for carbon steel. Stainless does not mean that it will not be affected by chemicals or acidic materials. It just means that it will tolerate a lot more before it becomes noticeable. Oiling your Swiss army knife is not going to make a practical difference, but keeping it clean will. Steel marketing Back in the days when mass spectroscopy did not exist, companies invented names to make their knives and tools more desirable. Names like India steel, silver steel, Damascus steel were slapped on a knife for the same reason that cleaning products are regularly advertized with a ‘new and improved formula’. In the consumer market, the rule is that new -> better. Unless you have a decent working knowledge of steel, it is impossible to know anything about the metallurgy of old knives and tools. What is stamped on the blade or tang is certainly not trustworthy in any way. These days, there is such a thing as mass spectroscopy, and chemical testing. Companies can no longer make wild claims or invent new names. So they come up with new formulas that are just slightly different from the previous one, give it a name, and say that it is just so much better (for an unsubstantiated meaning of ‘better’) than the previous steel. This is why super steels exist. The real secret here, and something that many are not sharing, is this: in terms of cutting properties and edge retention vs. sharpen ability, plain carbon steel is still the golden standard. Super steels are still only trying to equal that standard. Yes, you read that correctly. Carbon steel is as good as it can possibly get, for a cutting tool. Btw, I was not entirely correct when I said ‘not making up names’. There have been cases where knife makers buy a batch of O1 tool steel, and then say they’ve had a special steel recipe mixed together just for them, and thus ‘upgrade’ plain steel to ‘special’ steel. It’s sad but it’s happened before. Conclusion With this article, I have tried to give a basic introduction to steel. An article like this is always a compromise between being comprehensible, and being accurate. The things which I have described are generalist, and have exceptions. I also didn’t include every type of steel or every process that is involved in working steel. It’s a balance. I have worked on the premise that this article should be of value for someone who knows little to nothing about steel, and would like to understand the basics. Anyone interested in details will be able to do the required research themselves.