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 nominal wall thickness at time of heat treatment requiring a through-hardening steel capable of developing hardness as high as 40 HRC when properly hardened and tempered and for parts of greater wall thickness but requiring proportionately lower hardness@ but usage is not limited to such applications.