Form This specification covers an aircraft-quality@ low-alloy steel in the form of bars@ forgings@ and forging stock. Application These products have been used typically for parts@ 0.375 inch (9.52 mm) and under in nominal section thickness at time of heat treatment@ requiring 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@ but usage is not limited to such applications.