This specification covers an aircraft-quality, low-alloy steel in the form of bars, forgings, mechanical tubing, and forging stock. These products have been used primarily for parts 3.5 in. (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 parts with thicker cross-section which require proportionately lower hardness, but usage is not limited to such applications.