This specification covers an aircraft-quality, low-alloy steel in the form of bars, forgings, and forging stock. These products have been used typically for parts, 0.375 inch (9.52 mm) and under in nominal section-thickness at time of heat treatment, requiring a through hardening steel capable of developing hardness up to 40 HRC when properly hardened and tempered and also parts of greater thickness but requiring proportionately lower hardness, but usage is not limited to such applications.