This page was last modified on 8 December 2014, at 20:24.
This page has been accessed 3,656 times.

» Create a book
» On Steel
Page Discussion View source History

On Steel

This article talks about steel, and the various types of steel that are commonly encountered in the knife or razor world.

Contents


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.


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 too brittle or prone to cracking or breaking under load, instead of bending.


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 as well as the heat treatment, 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. It is an important part of modern steel manufacture.


Steel has a number of properties that are all trade-offs with each other. Toughness and hardness are Two 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 two 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 perfect crystalline structure you can, because that will make the sharpest edge. Once you start adding alloying elements, you begin to 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 to increase toughness and impact resistance, 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.


Stainless steel


Stainless steel is simply steel with other chemical elements added to prevent rust. Or the alloy is treated chemically to create an oxide that does not degrade preventing further rust. The alloying elements used for making steel stainless also make it usually 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 very little other 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 likely 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 Damascus 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. 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. 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 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.


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.


Powered by MediaWiki
 
.. .