For my second and final attempt to teach a little something about all things hot in honor of summer, I’d like to provide some information about heat treating—in the metalworking industry, that is.
Heat treating involves changing the properties of metals that have different crystal structures at high and low temperatures. The type of material transfiguration depends on the temperature to which the material is heated, how fast it is heated, how long it is kept heated, the temperature to which it is first cooled, and how fast it is cooled.
Quenching hardens steel by heating it to high temperatures and then quickly immersing it in room temperature oil, water or salt brine to “freeze” the new crystal structure.
The two main ways to soften a metal (to restore its ductility) are annealing, in which its temperature is slowly raised, held for a certain amount of time and then slowly cooled, and tempering, in which it is slowly heated in an oil bath and held for hours.
During heat treatment, a part’s size may change—by how much can not be accurately predicted. However, a heat treater can provide a reasonable estimate that can help a shop prepare for final machining operations. The effects are different for every material grade. But communication with the heat treater, experimentation and process control can help to provide fairly accurate, consistent size change estimates.
Variables that are related to heat treating that may affect dimensional change include the type of heat treating; high-heat/low-heat process, temperature and soak times; furnace temperature uniformity and repeatability; part and load size; load configuration; part orientation; type of furnace atmosphere; and more.
For more of these variables and to learn more about predicting size change, read “Predicting Size Change from Heat Treatment.”
As far as “heat treating” unrelated to metalworking, I would suggest a tall, ice-cold glass of the beverage of your choice and a refreshing body of water to enjoy it in!