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. (9.525 mm) or less in thickness at the time of heat treatment which require a through hardening steel capable of developing hardness of approximately 40 IIRC when properly hardened and tempered and also parts with thinner sections which require proportionately higher hardness.