Normalizing
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 the 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 that 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, Any 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 out the carbide/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 down and refine the sizes of those constituents.
Annealing
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
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 iron 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. 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.
Tempering
Tempering is very often, and very incorrectly, confused with hardening. Fully hardened steel has its normal bcc atomic configuration distorted into a tetragonal state due to the carbon atoms that were trapped in the interstitial spaces by the rapid cooling of the quench. This “unnatural” state, known as alpha martensite, is under much stress from the strain induced hardeness, also making it brittle. While this results in high abrasion resistance and strength, it renders the steel useless for operation requiring impact strength or toughness. To correct this, a “compromise” must be made through tempering. In the tempering process the martensitic steel is heated just enough to release some of the trapped carbon atoms to a desired degree, reducing stress and increasing toughness as it is transformed to beta martensite. Not only does this increase toughness and ductility but it reduces the chance of distortion or cracking from hardening. This is why tempering should be done as soon as possible after the martensitic transformation has completed. So, very contrary to the popular misuse of the word, tempering can actually be called the opposite of hardening. Different steels, due to varying alloy compositions, have different temperature ranges to achieve the desired results. In tempering the end result is affected by two factors, time and temperature. Of these, temperature has the most immediate and profound affect. Time at temperature has a more subtle effect, relieving stresses and increasing toughness with less loss of hardness.
