This specification covers an aircraft-quality, low-alloy steel in the form of mechanical tubing. This tubing has been used typically for parts, 0.75 inch (19 mm) and under in section 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 thickness but requiring proportionately lower hardness, but usage is not limited to such applications.