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