MFN Handmade Knives

Malvin F. N. - a 20 year old Bladesmith

How to choose the right steel for a knife

Imagine a blacksmith asks himself “what is the best steel for a knife?”

Well, there is no such thing as one best knife steel. After four years of knifemaking, forging experience, and intense metallurgy studies, the author can explain what makes a goodknife and which steel to choose.

Introduction to knives

“The word knife in English comes from the old Old Norse word kníf. It most likely entered English vocabulary about 1100, according to mention of it in the definitive reference AngloSaxonand Old-English Vocabularies by Thomas Wright and Richard P.Wülcker, published in London in 1883.” (Brady, 2014) A knife is a one or two-edged, flat piece of steel with a handle, either made from solid materials or worked directly from the metal. There are varioustypes of knives for even more different uses. The primary categories are kitchen, outdoor, survival, hunting, diving, combat, and lastly, knives for a specific use, like horse hoof trimming, spoon carving, or skinning animals.

The definition of a good knife is quite complex. It all comes down to the perfect combination between use adapted geometry, steel choice, and processing. It is essential to know the three different cutting techniques and their purpose. Firstly there is the Pressure cut.This means that the object is cut only through the pressure of the blade. For example, Razors have to be able to perform pressure cuts. Secondly, there is the tensile cut. In other words, it is cutting an object only through the blade sliding over it, for instance, with a saw. However, this is rarely used with knives since there is always pressure involved in cutting with a knife. Thirdly there is the combination of pressure and tensile cut. This is the most commonly used technique.

Regarding geometry, the essential parameters are the size and shape of the blade, thickness, the angle and type of the bevels, and the angle of the cutting edge. The geometry has to be specially adapted to the purpose of the knife and the cutting technique used. The steel choice seems at first way more important than the geometry. Partly this might be true because, for example, mild steel will never make a good knife. However, even when the perfect steel has been chosen, it will simply be a useless knife if the geometry is not adapted.

Knife steel should contain at least 0.3% and up to 1.5% carbon. There are three types of steelto choose from carbon, stainless, and Damascus. Damascus is not really a separate category since it is a combination of at least two different alloys of one of the previously listed types.

Introduction to Steel

Steel cant be found on the periodic table. That is because steel is an artificial structure consisting primarily of Iron (Fe) and carbon (C). There are many more alloy compounds, which will be discussed later. Steel suitable for knifemaking divides into three types.

„Type T I: hypoeutectoid to hypereutectoid steel alloys with small additions of alloying elements between 0.5 and 5%.“ (Landes pag.71) Steels in this category have very few and small carbides, resulting in a very fine cutting edge with high cutting edge stability. This results in an excellent pressure-cut capability. However, some of the higher alloy steels’ qualities, such as rust resistance, must be sacrificed.

„Type T II: Hypereutectoid to partially lebeduritic steel alloys, with additions of alloying elements between 5 and 15%.“ (Landes pag.71) These steels have significantly larger carbides, which break out more quickly at the cutting edge. This results in a poorer pressure cut but increases the tensile cutting properties. Because the carbides which break away leave a saw-like structure. In addition, these steels can be stainless and, due to the increased quantity of alloying elements, very stable and resistant.

„Type T III: Lebeduritic steel alloys with alloying element additions greater than 15%.“ (Landes pag.71) These steels have many carbides, which break out very strongly at narrow cutting angles. Using coarse cutting angles, they have very high resistance to wear. In addition, these steels are very robust and rust-resistant.

Alloying elements to enhance steel

Now let us look at the influence of Alloying elements in Alloyed steel. “It should be noted that even so-called unalloyed steel always contains, in addition to Iron (Fe), the elements carbon (C), silicon (Si), manganese (Mn), phosphorus (P), and sulfur (S). Alloying elements can have a very different influence on the properties of the steel.” (Agerer) There are many more alloying elements than listed below, but those are the ones relevant for knives.

Carbon (C) Only through 0,002% to 2,06% carbon does an Iron alloy turn into a steel alloy. Nevertheless, only at a percentage of 0.3 percent steel becomes hardenable. Suppose the percentage of carbon gets too high and the other alloying elements are not adjusted accordingly. In that case, “it increases brittleness and thus lowers forgeability, weldability, elongation at fracture, and notched bar impact work.” (Agerer) Although 2,06% is the maximum, the maximum for knife steel is about 1.5% carbon.

Chrome (Cr) increases the rust resistance of steel from about 12,2% chrome. The usual stainless steel for knifemaking has between 13% to 18% chrome. Chrome adds carbidesto the steel, which are very strong and increase the Hardness, Firmness, and wear abilities. The toughness, tested with notched bar impact work, decreases. The carbides easily break outat the thin cutting edge, which decreases the pressure cut abilities but increases the tensile cut. Chrome is primarily used in Type T II and Type TIII steel but can also be found in Type

Vanadium (V) is one more element that increases carbide amounts. Vanadium carbides are very hard, increasing tensile cut abilities, wear resistance, hardenability, tensile strength, toughness, and heat resistance. Vanadium is primarily used in Type T II and Type TIII steel.

Molybdenum (Mo), in combination with chromium, improves corrosion resistance. Nevertheless, also hardenability, tensile strength, weldability, and reduces brittleness. It reduces forgeability and ductility and significantly increases the temperature required for hardening.

Manganese (Mn) is mainly added to increase the hardness and strength of the steel. It also improves forgeability and weldability. Manganese alloyed steel is often used to make beautiful Damascus patterns since if it is edged in acid (usually iron3chloride), it turns to a nice black color, giving a beautiful contrast to nickel steel.

Cobalt (Co) is added to steel to increase heat resistance, which is necessary for highspeedtools such as drills. Another good aspect of cobalt is that it refines the steel structure.

Tungsten (W) is also a carbide former, increasing the tensile cut abilities. Above all, itincreases the strength and toughness of the steel, which is needed to produce tool steels with high mechanical loads.

Copper (Cu) as an alloying element increases corrosion resistance and hardness while significantly increased susceptibility to fracture.

Silicon (Si) has many extraordinary properties. In particular, it increases tensile strength and yield strength. Therefore, it is mainly used for the production of spring steels. It also prevents the formation of carbides, which increases the pressure cut abillities. Other fantastic properties are that it makes molten steel more fluid and prevents oxidation.

Nickel (Ni) is one of the most essential elements for corrosion resistance, along with chrome (Cr) and molybdenum (Mo). At least 8% of nickel is needed. It slightly increases the flexibility of the steel. However, it dramatically reduces hardenability. A beautiful aspect is that Nickel steel in Damascus stays silver after edging it in acid (iron3chloride). This contrasts beautifully with other steels, which edge gray to black.

Nitrogen (N) is one of the two elements you do not want to see in knife steel. While it slightly improves corrosion resistance, it dramatically reduces all other aspects, such as hardness, tensile strength, stability, toughness, and more.

Sulfur (S) is the worst thing that can happen to steel. It reduces corrosion resistance and all other positive aspects of steel, such as hardness, tensile strength, stability, toughness, and more.

Finally, we answer the initial question: “What is the best steel for a knife.” Unfortunately, there is no “best steel” for a knife. The best steel has to be chosen for each individual knife, and there are always compromises. Comparing two entirely different knives will show what it is all about.

Let us imagine a knifemaker gets two orders. The first customer, a trained chef, ordersa Japanese knife. He wishes for a one-side bevel, an extra fine cutting edge, and the best pressure-cut abilities. The second customer, profession unknown, wishes for a solid bushcraftknife. He will use it mainly during his hobby bushcraft missions—carving, splitting, chopping wood, cutting food, or ropes. It must sustain in a challenging environment, with dirt, moisture, and abusive use.

At this point, the knifemaker starts to decide the fundamental geometry aspects and essential alloying elements of the steel to be chosen. He does this, considering the knife’s purpose, the user’s skill, demanded cutting technique, the working environment of the knife, and special customer wishes.

The most critical parameters for a knife regarding the geometry are for the Japanese chef’s knife length between 15 and 27 centimeters (~5.9 to 10.6 inches), and for the Bushcraftknife, 12 to 15 centimeters (~4,7 5,9 inches). The Japanese chef’s knife has a maximum widthof 4 to 6 centimeters (~1,6 to 2,35 inches), and the bushcraft knife has 3 to 4 centimeters (~1,18 to 1,6 inches). The Japanese chef’s knife has a thickness of 2 to 3.5 millimeters, and the bushcraft knife is 3.5 to 6 millimeters.

Lastly, the knifemaker chooses the angle and placement of the bevels. The main rule is that the flatter the angle, the better the cut. But also, the flatter the angle, the more fragile the knife. For the Japanese chef’s knife, the flattest angle possible is needed. This is achieved by starting the angle at the top of the knife and allowing it to stretch across the entire blade width. On the other hand, the bushcraft knife must be durable and shock resistant. For this purpose, the angle starts at the middle, leaving half of the blade thick and flat.There is one more thing to mention, the Japanese chef’s knife is beveled only on one side so that cut food falls off, while the cut is straight. The Bushcraft knife is beveled from both sides.

Let us demonstrate the difference by calculating the angles.

Japanese chefs knife

Thickness = 3mm

Width = 50mm

tan (α )=(350)=0,06=tan−1(0,06)=3,4 ̊=α

Bushcraft knife

Total Thickness = 5mm

Half Thickness = 2,5

Total Width = 40mm

Bevel with = 20mm

tan (α )=(2,520)=0,125=tan−1(0,125)=7,1 ̊=α

As can be seen from the formulas, the angle of the Japanese chef’s knife is 3,4 degrees, while the angle of the bushcraft knife is 7,1 degrees, which is more than twice as steep. Logically thinking, it is evident that the bushcraft knife will cut much less effectively through vegetables, while the Japanese knife would brake under the conditions the bushcraft knife is used.

Having determined all those parameters, the knifemaker can finally decide on the steel and then determine the final angle of the cutting edge.

For the Japanese chef’s knife, he will want a high percentage of carbon—at least 1 to 1,5 percent. It has to be a Type T I steel since pressure-cut is demanded. Since a professional chef will wield it, it can be very hard and does not need to be stainless because he knows howto care about it.

Knowing all of this, the knifemaker’s choice is Aogami / blue paper steel. It is one of the finest Japanese steel, a modernized version of traditional white paper steel. With a percentage of 1.4% carbon (C), it will be very hard, with a fantastically durable cutting edge that can be razor-sharp. The 0,3% of Molybdenum (Mo) increases the hardenability even more. In order to make the steel more durable and rugged, 2,25% of Tungsten (W) is added. Finally, small amounts of Chrome (Cr) (0,4%) and Vanadium (V) (0,5%) are added to improve wear resistance. With this steel, a hardness of 62 to 63 Hrc Rockwell can be achieved using the proper heat-treating process. Because the steel has a very fine microstructure and no large carbides, which quickly brake out of the thin cutting edge, the thinnest cutting angle can be chosen for the final sharpening. In this case, 10 to 20 degrees.

For the bushcraft knife, a totally different steel is needed. If made from Aogami/Blue paper steel, it would shatter in pieces due to its abusive use. From this, the knifemaker knows he needs steel with 0.40 to 0.60 percent carbon (C). It must also be stainless since it has to survive in rough conditions with dirt and water. Pressure-cut is unnecessary, so it must be a type T II or III steel. The final choice was a T III steel, which has 16% of alloying materials. The 1.4034/AISI420HC/N540 steel has only 0,45 % carbon (C). Due to the low percentage ofcarbon and 0,4% silicon (Si) increasing tensile and yield strength, it has an immense toughness, which is even more increased by 0,5% of Manganese (Mn). The 13,74% of Chrome (Cr) makes it highly corrosion resistant and increases the tensile cut abilities. Although it has a low percentage of carbon, it can achieve a high sharpness, thanks to the alloying elements. With this steel, a hardness of 50 to 57 Hrc Rockwell can be achieved usingthe proper heat-treating process. The highest sharpness after sharpening is lost quickly, but the basic sharpness created afterward remains for a long time. However, this is only given if an adequate cutting-edge angle is chosen. If it is too thin, the carbides of chrome would breakout and make the knife dull after no time. Even if it were pure carbon steel, it could not be a narrow-angle due to the abusive use of the blade. So the final angle is at about 35 to 50 degrees.

Locking at those two examples, it should be clear that making a knife and choosing steel is much more complex than one might think. Only by considering all aspects of the use and the associated requirements can one select the best steel for a knife.

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