Form This specification covers an aircraft-quality@ low-alloy steel in the form of heat treated bars and forgings@ and of forging stock. Application This material has been used typically for nitrided parts requiring high surface hardness@ resistance to heat@ and less distortion than parts fabricated from steel requiring quenching to case harden@ but usage is not limited to such applications. This steel may be case hardened in dissociated ammonia gas to provide a minimum case hardness of 900 HV.