Form: This specification covers an aircraft-quality@ low-alloy steel in the form of bars@ forgings@ and forging stock. Application: Primarily for parts@ 0.875 inch (22.22 mm) and under in nominal section thickness at time of heat treatment@ requiring a through-hardening steel capable of developing hardness as high as 50 HRC when properly hardened and tempered and also parts of greater thickness but requiring proportionately lower hardness.