This specification covers an aircraft-quality, low-alloy steel in the form of mechanical tubing. 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 parts of greater thickness but requiring proportionately lower hardness, but usage is not limited to such applications.