Form: This specification covers an aircraft-quality@ low-alloy steel in the form of bars@ forgings@ flash welded rings@ and stock for forging or flash welded rings. Application: Primarily for parts 0.75 in. (19.0 mm) and under in normal section thickness at the time of heat treatment requiring a through-hardening steel capable of developing hardness as high as 40 HRC when properly hardened and tempered and for parts of greater thickness but requiring proportionally lower hardness.