Form: This specification covers an aircraft-quality@ low-alloy steel in the form of bars@ forgings@ and forging stock. Application: Primarily for parts with sections 0.375 in. (9.5 mm) and under in nominal thickness at the time of heat treatment which require a through-hardening steel capable of developing hardness up to 40 HRC when properly hardened and tempered and also parts of greater thickness but requiring proportionately lower hardness.