Form This specification covers a premium aircraft-quality@ low-alloy steel in the form of bars@ forgings@ mechanical tubing@ and forging stock. Application These products have 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 and subject to very rigid inspection standards@ 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.