Form: This specification covers an aircraft-quality@ low-alloy steel in the form of bars@ forgings@ mechanical tubing@ and forging stock. Application: Primarily for parts@ 0.750 inch (19.05 mm) and under in 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.