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The very name of "normalizing" best describes what this operation does; it brings everything inside the steel back to a normal or equalized state. By everything I mean grain size, carbide size and distribution, dislocation densities and stresses resulting from the strain of working or thermal effects. The idea is to heat the steel above the recrystallization temperature in order to reset that austenite grains, evenly soak at a temperature sufficient to dissolve large carbide concentrations and in the process wipe out any strain energy that could be in the structure. The most important consideration in normalizing is the heat is as even as possible and that the cooling is as even as possible, but not too slow and not too fast. If one were to cool from normalizing heats too slowly the carbide would diffuse out in rather coarse structures and in places that you may not want them, and thus you would not be normalizing but annealing instead. A slightly faster cooling rate will also promote finer structures, so air cooling is the method used for normalizing, Andy faster than this and you are hardening. Normalizing is used after forging the blade to even out all the chaos inflicted by the hammer. Industry specifies much higher heats for normalizing than many bladesmiths, 1600oF.-1700oF., and I always start out with a higher temperature to be certain that I put things into solution. At this first stage I am not so concerned about how fine the grain is but that they are all the same size, uneven grain size can be worse than larger grains, so using the high heat levels the carbide and grain size and actually "normalizes" the inside of the steel. I then follow this heat with two or three more normalizings at subsequently lower heats to step refine the sizes of those constituents.
Annealing is the operation by which the qualities of softness, malleability and machineability are achieved. It produces the most stress free state by allowing as much carbon (cementite) to diffuse from the ferrite as possible. It is accomplished by heating a steel to an austenitic condition and then cooling slow enough for thorough diffusion, resulting in the microstructure pearlite. Another process known as spheroidizing is used to create a very soft and machineable state in steel by producing spheroidal cementite microstructures instead of the lamellar structure of pearlite. For more information on annealing common steels follow the links below.
Hardening is the operation in which steel is heated to an austenitic condition and then quenched or rapidly cooled in order to obtain the properties desired in a hardened steel. A microstructure of martensite is the most common goal of hardening. To accomplish this the steel is heated to a temperature above 1335oF where the irons atomic stacking will shift to fcc (gamma iron) which has many more spaces for carbon atoms to occupy than at room temperature. The next step, often referred to as "the soak", the steel is held at the predetermined temperature long enough for the carbon atoms to diffuse through the iron and into those newly created spaces. When this is accomplished the steel is force cooled rapidly enough to trap the carbon in this position and create a supersaturated solution at room temperature. This new condition heavily distorts the atomic stacking resulting in a very hard phase of steel known as martensite. Being the most critical aspect of a quality blade, I leave nothing to chance for this operation. To heat the steel I use an electronically controlled molten bath of NaCl based salts capable of holding exact temperatures very evenly. Since each steel will have it own particular cooling requirements I utilize several quench mediums formulated by industry specifically for the task, ranging from state of the art heat treating oils to low temperature molten salts.