Use of Hardenability Tests for Selection and Specification of Automotive Steels

THE fundamental relationship between cooling rate during quenching and hardness produced in steel is pointed out in this paper. The requirements for a hardenability test bar for determining this relationship for a wide variety of section sizes and steels are given. Some of the methods that have been suggested for testing hardenability are discussed briefly, pointing out that the end-quench specimen is best suited for obtaining this fundamental relationship between hardness and cooling rate.

The use of the hardness-cooling rate curve for determining cooling rates in objects to be hardened and also for predicting hardnesses that will be obtained in such a part depending upon the steel used is described. A method of specifying hardenability of steels in terms of hardness cooling rate curves is described and the procedure outlined for determining what the hardenability limits should be. Application of this general procedure for selecting and specifying steels is illustrated in connection with a specific automobile part.

A method of interpreting hardenability information obtained from various hardenability tests in terms of hardness cooling rate curves is explained. This translation of hardenability information from any test bar into terms of a hardness-cooling rate curve permits the use of these various test bars for the purpose of predicting hardness in complicated shaped articles. The H-CR curve is the abbreviated name taken for convenience in referring to the hardness-cooling rate curve.