Form This specification covers an aircraft-quality@ low-alloy steel in the form of mechanical tubing. Application This tubing has been used typically for parts@ 0.375 inch (9.52 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 hardened and tempered@ and also for parts of greater thickness but requiring proportionately lower hardness@ but usage is not limited to such applications.