Form This specification covers an aircraft-quality@ low-alloy steel in the form of mechanical tubing. Application This product has been used typically for parts@ 0.875 inch (22.22 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@ and also parts of greater thickness but requiring proportionately lower hardness@ but usage is not limited to such applications.