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Tempering at a slightly elevated temperature for a shorter time may produce the same effect as tempering at a lower temperature for a longer time. Tempering times vary, depending on the carbon content, size, and desired application of the steel, but typically range from a few minutes to a few hours. Tempering quenched steel at very low ...
Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching.
The high temperature of annealing may result in oxidation of the metal's surface, resulting in scale. If scale must be avoided, annealing is carried out in a special atmosphere, such as with endothermic gas (a mixture of carbon monoxide, hydrogen gas, and nitrogen gas). Annealing is also done in forming gas, a mixture of hydrogen and nitrogen.
Differential tempering (also called graded tempering, selective tempering or local tempering) is the inverse of differential hardening, to ultimately produce similar results. Differential tempering begins by taking steel that has been uniformly quenched and hardened, and then heating it in localized areas to reduce the hardness.
The exact boundaries of the austenite phase region depend on the chemistry of the alloy being heat treated. However, austenitizing temperatures are typically between 790 and 915 °C (1,454 and 1,679 °F). [5] The amount of time spent at this temperature will vary with the alloy and process specifics for a through-hardened part.
However, precipitates of chromium, copper, or other elements can strengthen the steel by similar amounts in comparison to hardening and tempering. The strength can be tailored by adjusting the annealing process, with lower initial temperatures resulting in higher strengths. The lower initial temperatures increase the driving force of nucleation.
Depending on the temperature and composition of the steel, it can be hardened or softened. To make steel harder, it must be heated to very high temperatures. The final result of exactly how hard the steel becomes depends on the amount of carbon present in the metal. Only steel that is high in carbon can be hardened and tempered.
Holloman and Jaffe determined the value of C experimentally by plotting hardness versus tempering time for a series of tempering temperatures of interest and interpolating the data to obtain the time necessary to yield a number of different hardness values. This work was based on six different heats of plain carbon steels with carbon contents ...