The Cutting Edge
March 16, 2010
What would we do without knives? Knives, from the kitchen variety to utility to hunting, are a part of our daily lives. The word knife comes from the Middle English (450-1150 A.D.) word knif, and knyf, from the Anglo-Saxon word cnif. Regardless of its intended use, a knife would not be able to do its job if not for proper heat treatment.
Speaking of Middle “Earth"...English, the Lord of the Rings trilogy seems to have given rise to a renewed interest in knives and swords, particularly in those that are hand crafted. Let’s take a look at the technology and thermal processing involved in crafting such a tool.
Two things are key in making a good quality knife blade: steel and heat treatment. Knife blades need to be made from high-carbon steel with about 0.85% C providing the best combination of final heat-treated hardness and carbide precipitation. Higher final hardness is necessary for edge retention, and carbides also help resist dulling of the edge. Adding other alloying elements, such as chrome, molybdenum, tungsten and vanadium improves the hardenability of the steel, provides refinement of the grain size, and produces alloy carbides that are more beneficial to edge retention.
Proper heat treatment of a blade involves more than hardening. It begins with annealing to remove the stresses induced during forging and to make it easier to shape the blade by cutting and grinding. Normalizing reduces the stresses induced during this shaping process while also reducing the grain size of the steel. Heat treatment is the process of hardening the blade to maximum hardness and then tempering to the optimal hardness, usually Rc 58 to 60. Depending on the alloy, a sub-zero treatment may also be used as part of the initial quenching cycle in order to fully transform the steel to attain maximum hardness.
For those who are familiar with the art of blade manufacture, Damascus steel is sure to be discussed. Blades made from Damascus steel exhibit an attractive random swirled pattern, known as damascene. Although these unique blades were forged by bladesmiths in Damascus, Syria, the steel was actually made in India beginning around the year 500 A.D. Unfortunately, this is a lost art and the date of the last blades made with the highest quality damascene patterns is probably around 1750. The keys to the success of this process were the steel, which was relatively high-purity iron with 1.5% carbon, and the unique forging process. The carbon content of these steels results in bands of Fe3C (cementite) with a characteristic spacing. These bands lie parallel to the forging plane of the blade, and the bladesmith can exploit this characteristic to produce a more exotic pattern upon polishing and etching of the blade.
If your interest is piqued and you have a desire to explore the knife-making art at a greater depth, there are a variety of resources on the web to get you started. Remember that once you have the right steel, the key to it all is the right heat treatment. You won’t be cutting much more than butter if your heat treatment doesn’t cut muster. IH