Form This specification covers an aircraft-quality@ low-alloy steel in the form of bars@ forgings@ mechanical tubing@ and forging stock. Application These products have been used typically for parts 0.500 inch (12.70 mm) and under in nominal section thickness at time of heat treatment requiring a through-hardening steel capable of developing hardness as high as 52 HRC when properly hardened and tempered and also parts of greater thickness but requiring proportionately lower hardness@ but usage is not limited to such applications.