Choosing Your Steel

Where to get steel for knifemaking


As many people know, you can't just use any old steel and make a good knife out of it. Different steels have different properties, and you have to make sure you get the right one.

To start off, there are two opposing properties a knife must have to be counted as a good knife. Keep these in mind, they are the basis of how good a knife is.

The first is Toughness. This is measured in how much abuse and stress the blade can take without breaking or snapping. Think of toughness as synonymous with "softness". A blade that is soft/tough can take a lot of abuse without breaking, but will not hold an edge for long. In steel terms, soft means it cannot hold an edge for much use. The atoms will move slightly in relation to each other rather than break away completely.

The second property is Hardness. This is measured in how much use a blade can go through before becoming dull. The harder the blade, the longer it will stay sharp, as well as the sharper it can be made. The downside of this however, is it is brittle. Think of a knife made of glass. Extremely sharp, and will not dull, but will shatter easily.

Now the best knife is one that is right in between the two extremes; tough enough that it can be put through a lot of abuse without breaking, but hard enough that it will hold an edge for hundreds of cuts.

One way to get the best possible physical properties out of a blade is by differential heat treating, shown here by a hamon line. The spine (center) of the blade remains soft, whereas outside the line the blade is hard. The thickest part of the blade, where all the stress goes, is the toughest, giving this blade a good cutting edge yet still very tough.

One way to get the best possible physical properties out of a blade is by differential heat treating, shown here by a hamon line. The spine (center) of the blade remains soft, whereas outside the line the blade is hard. The thickest part of the blade, where all the stress goes, is the toughest, giving this blade a good cutting edge yet still very tough.

Only with particular types of steel is it possible to change the properties of the steel to get the perfect balance. Steel is a combination of iron and carbon. Without carbon, plain iron is too soft to be an effective knife. Too much carbon, and it becomes too brittle. The optimum range is between .4% carbon and 1% carbon by weight. A steel with this amount of carbon can be manipulated to be the perfect balance of hardness and toughness. To change the properties of steel, a smith must heat treat (HT) it.

Now hold on here, this is where it starts to get scientific. The following is not essential at this point, but it will give you a good understanding of how it all works and why a smith heat treats it in certain ways.

unnamed(1).jpg


Before a smith starts to treat the steel, the iron atoms in that steel are built in a cubic pattern, with an iron atom at each corner of the "cube" and one in the center. The carbon atoms are floating between these cubes. This formation is called Body Centered Cubic (BCC).

The smith then begins to heat up the steel in a forge, until the blade is an orange-red. He knows when he is at the right temperature, called critical temperature, when a magnet will no longer stick to the blade. As the steel gets to this point, the iron atoms switch formation from BCC into Face Centered Cubic (FCC), where there no longer is an atom at the center of the cube like in BCC, but now has an iron atom at the center of the the face of every cube. Because there is more room between the atoms, and inside the cube itself, carbon atoms float from outside the cubes to the inside. Still with me?

The smith then takes the blade out of the forge, and plunges it in warm oil (some people use water but the thermal shock tends to crack the steel). Because of the cooling, the steel turns from FCC back into the original BCC. If the steel were to air cool, the change would be slow enough that the carbon atoms slip outside the cubes as it cools. However, the oil shock cools it, switching cube formations almost immediately. There is no time for the carbon atoms to slip out and so they are trapped in a box too small for them. This puts a lot of stress across the steel, keeping the atoms rigid in relation to each other, which translates to hardness, yet brittleness.

At this point, the blade is too brittle. I've snapped a piece in this stage with my hands very easily. So, the smith needs to temper the steel, toughen it just enough to where it can take a lot of stress, yet still hold an edge. The temperature needed to do this varies from steel to steel, but generally it is between the 350-450 Fahrenheit range, for 2-6 hours. Many people use the kitchen oven with a thermometer or a toaster oven. What this does is open the now BCC cubes very slightly, allowing a few carbon atoms to escape. This eases the pressure and toughens the steel. Once the tempering is completed the blade is cooled and finished.

 

A note on stainless steel

Stainless steel is steel with an added element that keeps it from rusting. Originally, this element was Chromium, which is still commonly used today, along with a number of other elements. Rust is Iron Oxide; oxygen that hooks onto the iron and forms a flaky red compound. When these flakes form, they chip off, and expose more iron to be oxidized.

When enough Chromium is added to steel, the oxygen combines with the Chromium rather than the iron. Chromium (or whatever other element is used) Oxide is harder and thinner (microscopic) than rust, forming a see-through hard coating over the steel. This coating prevents more oxygen from reaching the steel.

However, with the exception with some specialized (and expensive to make) stainless steels, the added element gets in the way of the carbon, blocking the full effect of the hardening, yet still often keeping the blade almost as brittle. Because of this, most mass produced blades do not hold an edge very well at all, yet still are brittle. This is why most people use stainless kitchen knives; it's lower performance, but it requires even less maintenance and upkeep.  Avoid stainless altogether; the stuff you would have easy access to is cheap and makes low quality blades, and good stainless is both expensive and complicated to heat treat.


Where to get knife steel

Professional (I would even say most) smiths order knife steel online, in specific types. Getting the same steel with almost no difference in between "batches" gives complete consistency, so the smith doesn't have to alter the heat treat process every time (usually by tempering temperature). The most popular supplier of carbon steel is Aldo Bruno, the New Jersey Steel Baron. Much of the advice I have been given or have seen given is to go ahead and order "official" steel from him, or other suppliers, right from the start. It's consistent, you know what you will get and how the steel will act, the steel comes annealed (as soft as you can get it, so easier working), and you can order it in dimensions it is easy to forge or cut from.
There are differing opinions on whether a newbie should order steel from a supplier, or start off with scrounging legitimate, but still uncertain, steel. Many people will say to order steel, it's not that expensive. Personally, I think it's ok not to do so (this is the broke bladesmith after all). Other steel sources make perfectly good knives, though you won't be able to HT to professional level because you won't know the exact steel type (type A might need X temperatures and type B might need Y), that doesn't matter because your first knives will not be professional anyway. My final advice would be, if none of the upcoming "scrounging" options are available easily to you, order steel online. Most smiths I've talked to recommend 5160 for starting out. You can't do fancy stuff like damascus or hamons very easily with it, but it makes some of the most physically capable blades out there.

Steel types

Types of steel are labeled by numbers, such as 5160, 1084, 1075, etc., referring to chemical makeup. The first two numbers generally refer to other elements, for example 10 means plain carbon, with no other elements (relatively) present. This is not so important to remember. What is more important are the last two numbers, which refer to "points" of carbon; one point is equal to one hundredth of a percent. For example, 5160 is .60% carbon, 1084 is .84% carbon, 1075 is .75% and so on. There are slight differences and trace elements between steel types, and some steels do not actually contain the carbon content labeled (think of it like a 2x4 is not actually 2" x 4"), but one source of 1084 is usually very similar enough to another source of 1084 (difference not big enough to matter), and online sources will tell you exactly how to treat them.

Local sources for knife steel

As mentioned before, the essential thing for a steel to make a capable blade is carbon. It is very hard to find steel over 1% carbon (around the upper limit for knives) unless if it is cast iron, so steel fit for knives is generally referred to as plain "High Carbon".
So, what applications other than knives are high carbon steels used for? Remember you can really only heat treat high carbon steel, so any application where the steel needs to be altered from plain "metal" form. Springs especially (springy is right in between hard and tough, too tough and it bends, to hard and it snaps. Carbon steel enables the manufacturer to be able to heat treat it to just the right springiness) fit the bill. Here are a few suggestions:

  • Files and rasps. The teeth on these must be hard enough to not dull for a lot of use, so they are always high carbon steel. However, rasps and files are not put under stressful situations (like prying or hitting things), so they have no need to be tough and are usually as high carbon as possible, generally the .95-1% range. The higher carbon content means tougher forging and grinding. The teeth show up nicely in the finished knife, rasps giving a scale pattern.
Demonstration of the pattern left by rasp teeth. Forged by the author.

Demonstration of the pattern left by rasp teeth. Forged by the author.

  • Springs. Particularly leaf springs from cars. They are generally 5160 or very similar to it. One leaf spring will last you a long while. Easy to forge but with an exceptional edge, however watch out for pre-existing cracks in the steel.
  • Saw blades. Generally made of L6 or 15n20 (google how to heat treat these steels when heat treating a knife made from a saw blade). Hand saws are normally too thin, ideally an old disc blade from a lumber mill would be perfect.
  • Ball Bearings. Made to be very hard and not dent or deform. They can be very tricky to forge though, and finding some big ones could be hard.
  • Railroad spikes. Use these generally for practicing forging. Use only the ones marked HC (high carbon). These are better for forging axes and such, which require less hardness. Generally they are only around .3% carbon, so they don't harden up all that well, but they are wonderful for practice.
  • Hammers. Great for axes, as they are already half formed.
  • Lawnmower blades. Common, easy to find, but very uncertain steel.
  • Chainsaw bars. I'm not certain on these but I've heard they are usually similar to O1. I got a load of these when I started out and they performed well, and I've heard good reports from a smith I know personally.
  • Chisels. Varying in steels, so it would be best to look up the brand to try to find out the steel type. However they make excellent, if small, knives.

An assortment of steels potentially fit for blades. Drill rod, circular saw blades, files, wrought iron (used for fittings, not the blade itself), tractor cable and more. The two pieces at the top were of uncertain carbon content, so the owner spark tested them (read on below); they showed up as mild steel, too low carbon for knives.

Mystery Steel Testing

So let's say you found some steel that might be fit for a knife, but it could also be mild steel (sub high-carbon). How do you know if it fits the bill? Two tests for this.
The first is spark testing. Carbon burns easier and hotter than iron (and iron does burn; burning is just oxidizing happening really fast and releasing energy)
, so sparks thrown by a grinder on high carbon tend to be brighter and have many more forks. Plain iron or low carbon steel will have less forking. Here is a video on spark testing, showing the difference between steels.
That test however is really just a quick and dirty test to tell between a definitely not option, and a maybe not option.
The second test is by hardening it. Cut off a piece of the steel, or if you have several pieces use one of those, and heat it to critical temperature; the point at which a magnet will no longer stick to it. Get it nice and even, then plunge it into a bucket of (preferably warm) water, stirring it around. There is a chance of it cracking due to shock but as it's a test piece it doesn't matter. When the steel is cool, try marking it with a file. If the file marks it or bites in/drags at all, there is not enough carbon to affect the hardness, and so it is not knife-fit. If the file just skates off of the steel instead of biting in, it means the steel is harder than the file (and files are very hard), so the steel is fit for knife use.


To recap, carbon is the deciding factor in whether a steel is fit for knife use or not. Too little carbon and it has no effect on the physical properties of the steel. Order online if you have or are willing to pay the money, but if not go to other sources, such as files or leaf springs.