Form: This specification covers an aircraft-quality@ low-alloy steel in the form of mechanical tubing. Application: Primarily for parts with sections 0.500 in. (12.70 mm) or less in thickness at the time of heat treatment which require a through-hardening steel capable of developing hardness as high as 50 HRC when properly hardened and tempered and also parts of greater thickness but requiring pro-portionately lower hardness.