Heat treating is the thermal process that steel must go through in order to harden. Only then will steel acquire the desired properties that make it useful. Those properties are hardness, toughness and wear resistance. The chemical composition of the steel determines the level of each of these properties.
Heat treating is NOT a black art. It is a science, a mannered chaos of the rearrangement of particles within the structure of the steel. All of the steps in the hardening process require a specific temperature and time at that given temperature to properly pass on to the next step of the process. Steel must pass through each step of the hardening process in the correct order to properly harden.
With a few exceptions, steel is generally provided in the annealed state. This annealed state is considered too soft for most tooling applications. The hardness of steel for tooling is usually measured on the Rockwell hardness “C” scale. Most tooling applications require that the steel be hardened to an RC hardness of 45 or higher in order to provide the required properties. Steel in the annealed condition will measure less than 20 on the Rockwell “C” scale.
Prior to the first step in the hardening process, most steels will need to be protected from exposure to oxygen to prevent decarburization. Decarburization is an unwanted loss of carbon during the hardening process. Carbon in the steel is what gives steel its hardness during the hardening process. Loss of carbon during the heating of the steel will cause it to stay soft or not reach its full hardness depending on the amount of carbon lost. Steps must be taken to prevent the exposure of the tool to oxygen. This can be accomplished by a number of methods including a gaseous atmosphere, a neutral or vacuum atmosphere, or stainless foil wrap.
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The first step in the hardening process is an optional step. This first step is stress relieving. In the annealed state, steel is free from built up stress. It has gone through an annealing process at the producing mill to remove the stresses acquired during the manufacturing of the bar or plate. During the subsequent manufacturing that turns a bar of steel into a tool, additional stresses build up in the material. These stresses can cause the tool to crack, chip or bend during the heat treating process or later while in use. Stress relieving is the process of heating a tool up to a medium temperature below the critical temperature. Typically the stress relieving temperature will be 1000 to 1250F. Holding the tool at this temperature range for a given time will remove or reduce the amount of stress in the tool. The tool must then be cooled prior to the subsequent heat treating steps.
Preheating is the next step in hardening process. Preheating is the act of heating the tool to a temperature at or just above the critical temperature and holding it at that temperature long enough for the temperature to equalize throughout the tool. This step provides two benefits to the tool. First, it allows the tool to heat to the austenizing temperature evenly and quickly. This in turn, minimizes the stress produced and provides for an even hardness throughout the tool.
Austenizing is the process where the actual change in hardness occurs. Austenizing is a phase change where the steel arranges the molecules into a useful order. The matrix of the steel is expanded which allows the carbon and other elements to embed into the matrix.
Quenching is the relatively rapid cooling of the steel to a point below its critical temperature. It must pass through this range at a fast enough rate to trap the carbon and other elements within the steel matrix. These trapped or frozen elements are what provide the tool steel with the desired properties such as hardness and wear resistance.
After quenching, the steel is brittle and must be tempered. Austenite is formed during the austenizing stage. As the steel cools during the quenching stage, martensite is formed. Martensite is the desired particle form, but must be tempered. During the quenching process, some retained austenite is carried beyond the critical temperature and must also be tempered.
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Tempering is the process of heating the steel to a temperature high enough to remove the stress without softening the steel. This heating and subsequent cooling of the steel transforms the retained austenite into martensite. Tempering is normally performed two to four times depending on the steel grade or alloy content. It is during this tempering process that the tool attains its final desired hardness.
Cryogenic treatment is not a necessary step in the hardening process, but may be beneficial. During a cryogenic treatment, the steel is cooled to about –300F. This deep freezing of the steel may allow some more of the retained austenite to transform into martensite, thereby reducing any retained stress.