The Steel
Steel is iron with carbon dissolved into it. This technically makes steel a solution, somewhat like salt dissolved into water. The large iron atoms are arranged uniformly, with the small carbon atoms filling in the spaces. As the carbon atoms fill in the spaces between the iron atoms the iron becomes stretched apart increasing the energy between the iron atoms and making the structure stronger. Other alloying elements add different qualities to steel in various proportions.
The atoms in steel form into crystal structures referred to as grains. Each grain is a group of atoms that are arranged geometrically. The atoms in these grains can be pictured as cubes stacked next to and on top of each other. This is known as the lattice structure. In each separate grain the atoms are oriented at different angles to its neighboring grains.
When steel is heated past the critical temperature ( about 1600 degrees) the grains reform and begin to grow (as long as left above this teperature). The grains can be pictured as soap bubbles in a jar. The shapes are irregular and 3-dimensional. As the grains grow, they absorb the smaller ones next to it. The size of the grains influences how strong the steel will be. If the grains are large the steel will be weak and brittle; if they are small the steel will be stronger and tougher.
When the metal is heated past a certain point known as the critical temperature (about 1600˚F ) a few changes occur in the steel. The steel becomes non-magnetic, the atoms rearrange, and new grains begin to form on the boundaries of the old grains. The grains start small but continue to grow as long as left above the critical temperature. Grain growth can be pictured like bubbles growing if one were to put a straw in the jar and blow. The knowledge and control of this behavior of steel is critical to the quality of the final product.
When the steel is cooled slowly, the iron atoms once again rearrange back to the original structure, and the carbon atoms have time to rearrange themselves comfortably in the gaps between iron atom. However, when the steel is cooled rapidly from above the critical temperature the carbon atoms do not have to time to become rearranged and become trapped. This results in a different structure than encountered in the forging process. It is this structure that is responsible for the incredible hardness of steel.
Forging
Forging is the process of deforming metal using heat and pressure. There are a number of benefits to forging knives as opposed to stock removal. One is that there are almost limitless possibilities for design. The maker is not limited to work within the dimensions of the piece of stock. This can result in very creative and unique pieces. The other benefits are more practical and technical.
The micro-structure of steel has a natural flow to it. Forging a knife to its near finished shape preserves this natural structure giving it more strength.
The act of deforming the steel during forging also changes the structure of steel, imparting strength. The impact of the hammer fractures the crystalline structure of the steel and inhibits grain growth. The smaller grain structure results in a stronger blade.
Our blades are forged to a high degree, using very little cutting or grinding to make the shape of the blade.
Heat Treating
Thermal Cycling/ Normalizing
Thermal cycling, or normalizing, is the process of heating the steel just past its critical temperature (non- magnetic) and then air cooling. We repeat this process three times after forging, before annealing, and again before the quench. The effect this has on the metal is eliminating any stresses in the steel, and a smaller grain structure. As mentioned before, when the steel is heated past its critical temperature many small grains form on the boundaries of the old grains. Repeating this process results in a refined grain structure, and a stronger piece of steel.
Knife being thermal cycled before the quench. The holes in the tang are drilled and the profile of the blade refined before it gets hardened.
Annealing
Annealing is the process of heating the steel up to the critical temperature, and then cooling at a slow rate, usually in some sort of insulating media. This is done to eliminate internal stresses in the steel. It also leaves the steel in its softest state which makes grinding much easier. Annealing is performed after normalizing.
The Quench
After the second thermal cycling process, the metal is hardened in the quench and the blade is truly born. For the quench, the blade is brought up to the critical temperature and cooled in oil. This rapid cooling results in an entirely different atomic structure than encountered in the forging process, and a piece of steel that is very hard, but also very brittle.
A knife being quenched and the resulting flare up.
For most of our smaller knives the entire blade is quenched, but for many larger ones we perform a differential heat treat. In this process, the edge is hardened while the spine is left “soft”. One way to do this is to apply clay or mortar to the spine of the blade before the quench. When heated, the clay will prevent the spine from reaching the critical temperature. Another way is to only immerse only the edge of the blade in the oil. A oxy-acetylene torch can also be used to heat only the edge up for the quench. Whichever method is used the result is the same. A blade with a soft spine will better absorb the shock of high impacts. It will also allow the blade to bend without breaking, should such forces be applied.
Tempering
In order to relieve the steel of its brittleness, it is heated to a specific temperature between the range of 300-450˚F, depending on the blade’s intended use. This allows the atoms to slightly readjust and relax. The result is a piece of steel the is much tougher. This means the blade will be able to absorb shock without breaking. It will resist bending, returning to straight when bent. It will also be somewhat flexible and tolerate a moderate degree of flex. Tempering allows all this while still retaining a hard piece of steel; a piece of steel that will stay sharp. A lower temperature leaves more hardness in the blade but sacrifices toughness. The result is an edge that will stay sharp longer but will be more prone to chipping. This is usually done for kitchen knives. A higher temperature results in a tougher blade that will be able to take more abuse. This is done with blades that are designed for chopping.
We temper our blades three times each. The rule of three seems to produce the best results.