Form This specification covers an aircraft-quality@ low-alloy steel in the form of bars@ forgings@ mechanical tubing@ and forging stock. Application These products have been used primarily for parts 3.5 inches (90 mm) and under in nominal section-thickness at time of heat treatment requiring a through-hardening steel capable of developing hardness as high as 40 HRC and also for parts with thicker cross-section which require proportionately lower hardness@ but usage is not limited to such applications.