CH. 3 UNDERSTANDING STEEL
Steelmaking is one of the largest industries in the world today. Steel has countless uses in our daily lives, and there are many different kinds produced to meet these various needs. For the bladesmith, choosing an appropriate steel for a blade is just the first step. By manipulating its composition through a process of regulated heating and cooling, a skilled knifemaker works to bring out specific characteristics from the steel. The combination of these factors will determine the overall quality of the final knife.
Ultimately, knifemaking is a conversation between the bladesmith and a piece of metal. As a knifemaker, picking an appropriate steel for your knife will give you a good start. To bring out the qualities you want, you have to be able to tell the steel what you need from it. Understanding what’s happening inside the chunk of metal you choose will help.
A VARIETY OF DIFFERENT KINDS OF STEEL STOCK READY TO BE MADE INTO BLADES
WHAT IS STEEL?
The base element of steel is iron. Iron is not only the most common element on earth but also one of the most common elements in the entire universe. Compromising much of our earth’s core, it is generally found in the form of iron ore. This ore is heated through a process called smelting to create a nearly pure form of iron, known as wrought iron.
THIS WROUGHT IRON FIGHTING AX, MADE BY BLACKSMITH STEVE ASH, HAS A 1075 STEEL BIT WELDED INTO THE EDGE TO MAKE IT A FUNCTIONAL WEAPON. WITHOUT THIS PIECE OF STEEL, THE EDGE WOULD BE MUCH SOFTER. MANY OLDER TOOLS USED STEEL SPARINGLY DUE TO ITS LIMITED AVAILABILITY.
Like all metals, iron is crystalline in its solid state. These crystals are formed when metallic atoms bond. The crystalline structure of iron at room temperature, ferrite, allows the molecules to slip over one another easily. This makes it a soft and incredibly malleable metal. To make a lasting weapon or tool, a harder metal is required. Adding carbon to iron increases the hardness dramatically, creating an alloy we know as steel.
The first time steel appears in the historical record is about 4,000 years ago on the land mass that makes up modern-day Turkey. Ancient metalworkers would create usable chunks of iron by heating iron ore in charcoal fires. One of these smiths would have realized that by leaving the iron in the fire longer, the soft and ductile metal became much harder and stronger; the carbon from the charcoal was mixing with the iron to form a new alloy. While they might not have known the chemistry involved, they would have recognized the value of these properties and intentionally replicated the process. In this way, steel was discovered.
AN IRON ATOM IS COMPRISED OF TWENTY-SIX PROTONS, TWENTY-SIX ELECTRONS, AND THIRTY NEUTRONS, MAKING IT ELEMENT NUMBER 26.
Carbon atoms are much smaller than iron atoms. When added together, carbon arranges itself between the iron. The carbon atoms prevent the iron atoms from easily sliding over one another. In this way, the content and distribution of carbon helps to determine the hardness and strength of a steel.
In addition to carbon, most steel has a number of other elements mixed in. Each different element causes specific changes in that steel’s molecular structure. By knowing the mixture of elements used to create a specific steel, you can predict how it will perform.
AN ATOM OF CARBON ONLY HAS SIX PROTONS, SIX ELECTRONS, AND SIX NEUTRONS AND IS MUCH SMALLER THAN AN IRON ATOM.
The structure of the crystals ultimately gives each metal its specific properties. In addition to the changes caused by different elements, iron can exist in different crystal shapes with different temperatures. At room temperature, steel is a mixture of ferrite and cementite. By adding heat, this structure changes as the spacing between the iron atoms in the crystal changes.
This ability to assume different structures makes steel an incredibly versatile and useful metal. By purposefully manipulating the molecular structure through heating and cooling, you can bring out a number of different properties in the steel.
A RUSTY KNIFE IS SOMETHING NOBODY WANTS TO SEE. WHEN IRON ATOMS COME INTO CONTACT WITH OXYGEN IN THE PRESENCE OF WATER, THEY GIVE AWAY SOME OF THEIR ELECTRONS IN AN OXIDATION REACTION. WE CAN TELL THAT THIS REACTION HAS HAPPENED BY THE PRESENCE OF IRON OXIDE, OR RUST. GIVEN ENOUGH TIME, AN ENTIRE PIECE OF IRON CAN TURN INTO IRON OXIDE.
HEAT AND STEEL
There’s an old adage in knifemaking, “Don’t talk about religion, politics, or heat treating.” You can talk to ten different knifemakers and hear twenty different processes they use to treat their metal.
Heat treating is the controlled heating and cooling of steel to cause specific changes in its properties. While “heat treating” is a blanket term for all of these processes, it is often used specifically in reference to hardening.
In any heat treating process, ensuring uniform heating and cooling is important to avoid only affecting part of the steel. The exception to this is if you want different parts of the steel to have different properties, a process commonly used in making swords or other large blades.
Every metal undergoes specific changes at certain temperatures, known as critical temperatures. Variations in critical temperatures are what make different steels require different temperatures for heat treating. Do your research to find out the particular critical temperature for the steel you are using, as well as the recommended heat treating processes.
The following is a brief overview of the different heat treatments you will use in knifemaking. By familiarizing yourself with how each treatment changes the molecular structure of the steel, you can understand the importance of each step. This will allow you to create the best possible heat treatment routine.
HEAT-TREATING A BLADE IN THE FORGE
NORMALIZING
When steel is heated above 1350°F (732°C), the carbon and other alloys start to dissolve in the iron. The crystal structure of iron begins to change into a new structure, austenite. The level of austenite increases as the temperature rises. When the steel reaches about 1450°F (788°C), all the crystal structures in the steel have become austenite. In this structure, the grain formation is broken up and formations are redistributed evenly.
When you cool this austenite steel slowly, a new structure called pearlite is formed. During the process of normalization, the steel is allowed to cool slowly in still air. This causes the metal to form a fine pearlite structure with relatively small, evenly distributed carbides. Large, unevenly distributed carbon can create a non-uniform structure in steel. This causes stress points in the metal, which can make for problems in the knifemaking process.
Normalizing is often used after forging, which can disrupt grain structure and cause internal stress. The uniform structure created during normalization evens out your steel and sets you up for success.
ANNEALING
The process of annealing is very similar to normalizing. The steel is brought to a temperature slightly above critical temperature but then undergoes a much slower and more controlled cooling. The slower austenitic steel is cooled, the more the carbon will be able to diffuse and pool away from the iron. This prevents the carbon from supporting the iron quite as much, resulting in a much softer metal. Utilizing this process before trying to drill or file your steel makes it much easier to work with.
HARDENING
When austenitic steel is cooled very quickly, carbon doesn’t have much time to diffuse and is trapped in its position. A new structure called martensite is formed. In martensitic steel, the carbon is distributed in a way that it holds the iron very rigidly in place. This process causes a lot of stress, and the resulting metal is extremely hard but very brittle. Using a hardened blade would result in a near certain breakage, so it’s important to handle the steel gently until you can put it through the process of tempering to relieve some of this stress.
WHEN STEEL IS ELEVATED TO ITS CRITICAL TEMPERATURE, IT LOSES ITS MAGNETIC PROPERTIES. METALWORKERS CAN GAUGE WHETHER STEEL HAS REACHED ITS CRITICAL TEMPERATURE WITHOUT USING A THERMOMETER BY TESTING IT WITH A MAGNET. THIS POINT, AROUND 1400°F (760°C), IS KNOWN AS THE CURIE POINT.
QUENCHING
Quenching is a process used to cool metal rapidly, usually using water or oil. Water produces an extremely quick quench, while oil cools the metal relatively slower. If cooled too quickly, certain kinds of steel can fracture. Preheating the liquid can also be used to create less of a shock and makes the oil less viscous so that it transfers heat more efficiently.
QUENCHING HEATED STEEL. IT’S IMPORTANT TO HOLD THE BLADE FIRMLY AND AGITATE IT AFTER IT IS IMMERSED TO AVOID STEAM BUBBLES THAT COULD CAUSE UNEVEN COOLING.
TEMPERING
Gently heating steel causes the atoms in the metal to vibrate. In martensitic steel, this vibration allows some of the trapped carbon to escape and recombine. This process is called tempering and is used to take away some of the hardness caused by the hardening process. This brings the steel halfway back between annealing softness and being fully hardened. The steel will now be hard enough to be useful but soft enough to be sharpened and not easily break.
SELECTING STEEL
According to the World Steel Association, there are over 3,500 grades of steel. Trying to choose the best kind of steel seems like an impossible task. Many beginners start out using old files, tools, or whatever piece of steel they come across. While it’s possible to make a knife out of some of these materials, it can be tricky to know exactly what you’re working with. Most stock steel isn’t expensive. It’s worth spending a few dollars to know exactly what you’re using so you can save yourself a lot of trouble. By picking a specific kind of steel to work, you can find out the exact steps you need to take to bring out the qualities in your steel that will make the best knife possible.
COMMON STEELS FOR KNIFEMAKING
The steels listed below are a starting point to get you familiar with some of the more common steels you’ll find in the knifemaking world. This is by no means an all-inclusive list. Every type of steel has a separate system that gives it a specific number. It can be difficult to remember what the numbers of each system mean, but a quick Internet search can usually bring up everything you need to know. Certain additional elements, such as chromium and vanadium, have a big impact on the characteristics of the steel. One of the most important things to look for is the carbon content. The optimal range of carbon is between 0.6 percent and 1.6 percent. A steel near the higher end of this range is able to obtain a higher level of Rockwell hardness, and one toward the lower end of this range will be more resilient.
As a knifemaker, the main properties you are looking for in your steel are hardness and toughness. Your knife needs to be hard enough to hold an edge but tough enough to not break easily. These two properties can work against each other, so finding the appropriate balance of hardness and toughness is an important consideration.
In addition to hardness and toughness, consider the qualities of forgeability, hardenability, edge retention, ductility, workability, corrosion resistance, availability, and finally, your own skill level. It’s important to do your research on whatever steel you choose, as every steel performs differently. Each steel also has a slightly different heat treating process to maximize its potential. Some steels are much easier to treat, and these are ideal to use as you’re first starting out. The more you use a specific steel, the more you’ll learn about how to treat it to bring out the qualities you want. However, don’t let this stop you from experimenting with different kinds; the best steel for you might not be the first one you try.
High Carbon Steels
High carbon steels are a group of steels that are primarily an alloy of steel and carbon, with very little additional elements added. The high carbon content gives these steels hardness and strength, and they tend to hold a great edge. These steels generally don’t contain enough added elements to make them corrosion resistant, so rust can be an issue.
THIS BUSHCRAFT KNIFE, MADE OUT OF 1095 STEEL, HAS SERVED ME WELL ON MANY EXPEDITIONS IN THE FIELD. THIS IS A SAKER BUSHCRAFT KNIFE MADE BY ABE ELIAS.
The group of steels known as “the 10 series” is extremely common for knifemaking. Perhaps the best known of these steels is 1095, which has a 0.95 percent carbon content. This steel is inexpensive, tough, and easy to find and holds a great edge. It is often seen with a coating on it to prevent rust. It is not necessarily a beginner’s steel, as it can be a bit difficult to heat-treat properly; but overall it makes a great blade. In the 10 series, any steels between 1095 and 1045 would be suitable for knifemaking.
Both 1084 and 1080 are great steels to learn on. They are nearly identical, with only a very small difference in carbon content. These steels won’t hold an edge forever, but they’re easy to sharpen, easy to treat, and inexpensive. While they don’t have the alloying elements that make other steels better performers, those elements also make them difficult to treat. These two are very forgiving steels and are a pleasure to forge. A 1084 steel is a great stepping stone in your knifemaking journey and what I recommend for anyone starting out.
Alloy Steels
Alloy steels used in knifemaking have a similar range of carbon content and are technically considered high carbon steels. However, this group of steels typically contains added elements to cause certain characteristics in the steel. These added elements can make certain kinds of alloy steels a bit trickier to heat-treat.
For example, 5160 is essentially 1060 plain carbon steel with added chromium. It contains between 0.56 percent and 0.64 percent carbon. While the added chromium isn’t enough for rust-proofing, it does serve to strengthen the steel. This is a very forgiving steel that is great for forging. It also is fairly easy to heat-treat. It’s tough, holds an edge, and does great if you’re going to be hard on your knife.
Another type of alloy steel, 6150 was originally used in coil springs, and has 0.48 percent to 0.53 percent carbon content and a small amount of added vanadium. This steel is nice to work with because it performs well even with less-than-ideal temperature control during the forging and heat treating processes. It doesn’t hold an edge quite as well as other steels but is very tough and easy to sharpen.
The high carbon content of 52100 steel (0.98 percent to 1.10 percent) makes it very hard, so it holds a great edge. It is very strong but can be a bit difficult to find in useful sizes. This is a good choice for hunting knives or other knives that you need to hold an edge.
A2 is a tough and relatively hard steel with 0.95 percent to 1.05 percent carbon content. It is very flexible but can be difficult to grind. It has less wear resistance than other steels but is often used in combat knives because of its toughness.
D2 steel has a 1.5 percent to 1.6 percent carbon content, so it is very hard. Its high 12 percent chromium content puts it just below the level of being considered a stainless steel, so it is resistant to rust. It is tougher than stainless steel, but not as tough as other tool steels. D2 has good edge retention, but it can be hard to sharpen. This steel can be difficult to work with and is considered a good steel for experts.
O-1 steel has a carbon content of 0.85 percent to 1.0 percent. It is a hard material with good edge retention, although it has a tendency to rust quickly. While it is relatively expensive, this is because it is usually sold as a precision ground bar, which makes for easy grinding. O-1 is a great beginner steel for using the stock removal method.
Stainless Steel
Stainless steel is characterized by its ability to resist corrosion. The presence of chromium in the steel creates a thin oxide layer on the surface of the steel, which helps to prevent the iron from oxidizing and creating rust. This makes it an ideal choice for knives that will be used in a wet or saltwater environment, or for a knife that won’t be used a lot and is at risk of rusting in its sheath.
While stainless steel has a reputation for having poor edge retention and less strength than other kinds of steel, newer grades available have proved these theories wrong. When making a blade out of stainless steel, the stock removal method is preferred, as it can be unforgiving and difficult to work in the forge. The heat treatment process can also be a bit challenging to do at home, so many knifemakers send their stainless steel blades out to be professionally done.
STAINLESS STEEL HAS THE ABILITY TO HOLD AN UNRIVALED MIRROR POLISH AND CAN MAKE AN ABSOLUTELY BEAUTIFUL BLADE, AS SEEN IN THIS KNIFE MADE BY PAUL BRACH.
One of the most commonly known stainless steels is 440c. It has 0.95 percent to 1.20 percent carbon, is wear resistant, and is a hard steel. It is nearly impossible to forge, but it is easier to grind than most other kinds of stainless. A knife made from 440c takes a nice edge that is easy to sharpen. While it takes a good deal of skill and patience, it does take a very nice polish. It is still used in some production knives but has been largely replaced by newer, higher-performing alloys.
CPM154CM is a stainless alloy that has 1.05 percent carbon. This steel was developed as a powdered metallurgy version of a different type of stainless called 154CM. The process of making CPM154CM involves using a metal powder and results in the uniform distribution of tiny carbides. This makes it a much better to steel to grind and polish. It is also considered an improvement over 440c in edge holding, toughness, and corrosion resistance.
S35VN is a fantastic steel that was created specifically for the knife industry by Crucible steel in collaboration with renowned knifemaker Chris Reeve. While expensive and a bit hard to work with for a beginner, it has an excellent balance of toughness and edge retention. It is considered by many to be one of the best available knife steels.
Damascus Steel
I’ve long had a soft spot in my heart for the legendary beauty, strength, and flexibility of Damascus steel. Damascus has its origins in the wootz steel of Sri Lanka, where early steelmakers were able to reach high temperatures required for production in their furnaces by harnessing the power of monsoon winds. The steel became famous after it was introduced into the Arab world, where it became the favorite steel for making weapons. It was here that was named after the capital city of Syria and developed its reputation as the best steel in the world. With its distinctive mottled pattern, superior edge retention, and excellent resistance to shattering, it reportedly had the ability to cut through other blades in battle.
More recently the name “Damascus steel” has also been used to describe steel that is made by welding together layers of different kinds of steel into a billet. The billet is then forged, drawn out, and folded repeatedly. Though different than the original Damascus steel, the result is an incredibly beautiful and functional metal. The amount of time and effort required to make Damascus makes it much more expensive than most steels, and it can be difficult to heat-treat, depending on the combination of steels used in its creation. While it’s probably best to invest in Damascus after you have some experience under your belt, it makes an incredibly beautiful blade and is my favorite type of steel.
Blade Thickness
Once you’ve chosen your steel, you’ll have to plan for the thickness of your blade. As the thickness of the steel increases, the angle of the grind on your future knife changes. When you have more acute angles and less mass, your knife will have less drag and will cut easier. When choosing your blade’s thickness, you want to have steel that is thick enough to be strong but not so thick that it adds unnecessary weight and makes the knife cumbersome. I recommend starting with a piece of steel between 1/8 inch (3 mm) and 5/32 inch (4 mm) thick for a good bushcraft knife.
DAMASCUS STEEL BILLETS IN A VARIETY OF DIFFERENT PATTERNS MADE BY VEGAS FORGE. THE PATTERN IN MODERN DAMASCUS IS DETERMINED BY THE WAY THE STEEL IS LAYERED AND PRESSED, AND SOMETIMES INCLUDES HUNDREDS OF LAYERS.
CUTTING OUT A BLANK
If you’re forgoing blacksmithing and heading straight into stock removal, start with a piece of stock steel roughly the length that you would like your knife to be. You’ll then need to cut out the rough profile of your knife in the stock steel using a band saw, hacksaw, or angle grinder.
If you plan on taking your knife to the forge, start with a piece of steel a bit longer than your knife template. This will allow room for error as you work. A lot of the basic profiling and shaping can be done with a hammer, but the learning curve of blacksmithing can make it difficult and time-consuming. If you’re having a hard time with getting the right basic shape at the forge and you want to move on in the process, you can switch to stock removal at any time. After any forge work, the knife should be normalized and annealed as outlined at the end of chapter 4.
To cut out the profile, use a permanent marker or a soapstone pencil to trace your design on the steel blank. Use your angle grinder and a metal cutting wheel to cut off the excess metal. While the knife profile will end up a little bit bigger than you want, you can finish up the profiling using a sanding attachment on your grinder or a belt sander.
IF YOU PLAN TO FORGO BLACKSMITHING AND HEAD RIGHT TO THE GRINDER, YOU’LL NEED TO CUT OUT THE PROFILE OF THE BLADE. TAKE CARE NOT TO CUT INTO ANY OF THE MARKED LINES, AS ANYTHING INSIDE THESE LINES WILL BE A PART OF YOUR BLADE.