Form: This specification covers an aircraft-quality@ low-alloy steel in the form of bars@ forgings@ mechanical tubing@ and forging stock. Application: Primarily for parts having thickness 0.750 in. (18.75 mm) and under in nominal section thickness at time of heat treatment requiring a through-hardening steel capable of developing a minimum tensile strength of 180@000 psi (1241 MPa) at the section center when properly hardened and tempered and for parts of greater section thickness but requiring proportionately lower strength.