Heat is a powerful energy that has positive and negative effects on metal. While heat treating to restore a metal’s ductility, dimensional or shape distortion can also occur. An article written by Miles Free from the PMPA, titled, “Craftsman’s Cribsheet: Distortion in Heat Treatment.”
“The rule of thumb that I have used for medium carbon alloy steels is to expect a change in linear dimensions of about 0.125 percent maximum,” says Mr. Free. “That is, one eighth of a percent of the linear dimensions could be the change encountered from heat treatment and quench. It generally is less, but 0.125 percent gives me a rule of thumb to evaluate capability to hold dimensions after heat treat.”
Shape distortion, or warpage, is a result of processing or design problems, not phase changes of the material. For eight reasons why steel parts can warp upon quench and tampering, click here.
Choosing steels with high hardenability and fine grain size and paying attention to the details of loading, time and temperature, and quenchant delivery are steps that can minimize shape distortion.
For more information on variables related to heat treating that may affect dimentional change and to learn more about predicting size change, read “Predicting Size Change from Heat Treatment.”