Form This specification covers a premium aircraft-quality@ low-alloy steel in the form of bars@ forgings@ mechanical tubing@ and forging stock. Application These products have been used typically for parts requiring a through-hardening steel@ usually with hardness of approximately 60 HRC in section thicknesses 0.50 inch (12.7 mm) and under@ and subject to very rigid inspection standards@ but usage is not limited to such applications.