Heat treating is the thermal process that improves steel properties such as hardness, toughness and wears resistance. The chemical composition of the steel determines the level of each of these properties and type of heat treating process applied.
Each heat treating process requires a specific temperature and time to process the steel. Steel must be passes through each step of the heat treating process in the correct order to properly change in properties.
In general, steel is provided in the annealed state. This is to facilitate the subsequent machining process. Steel is machined to the required shape and size before sending to a hardening process. 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 58 or higher in order to provide the required properties. Steel in the annealed condition will measure less than 20 on the Rockwell ?C? scale.
Steel has gone through an annealing process at the producing mill to remove the stresses introduced during the manufacturing of the bar or plate. After the subsequent machining process that machined the steel into a tool, additional stresses will 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 in the field. As such, stress relieve is introduced to remove the stress during the machining state.
Stress relieving is the process of heating a tool up to the 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.
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.
Preheating is the next step in hardening process.
Preheating is the act of heating the tool die 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 rates 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.
More informatin on tool die making and heat treatment of tool steel, please vist our metal stamping blog.
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