Restricted hardenability steels have been in use for some time but the specific restrictions for a particular grade depend upon customer needs and vary from mill to mill. Such steels are desirable to provide more controlled heat treatment response and dimensional control for critical parts. Because of increasing interest in steels with restricted hardenability, the SAE Iron and Steel Technical Committee directed Division 8 to prepare a set of standard steels with restricted hardenability.
In 1993, the American Society for Testing and Materials (ASTM) adopted the twelve SAE restricted hardenability steels and added ten more. SAE decided to include in SAE J1868 the additional 10 steels.
In general, steels with restricted hardenability (RH steels) will exhibit a hardness range not greater than 5 HRC at the initial position on the end-quench hardenability bar and not greater than 65% of the hardness range for standard H-band steels (see SAE J1268) in the "inflection" region. Generally the restricted hardenability band follows the middle of the corresponding standard H-band. An example of the RH band compared with the standard H-band is given for SAE 4140 in Figure 1.