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@ 3.5 inches (89 mm) and under in nominal thickness at time of heat treatment@ requiring a through-hardening steel capable of developing minimum hardness of 30 HRC when properly hardened and tempered and also for parts of greater thickness but requiring proportionately lower hardness@ but usage is not limited to such applications.