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 typically for parts 0.250 inch (6.35 mm) and under in nominal section thickness at time of heat treatment requiring a through-hardening steel capable of developing hardness as high as 50 HRC and also parts of greater thickness requiring proportionately lower hardness, but usage is not limited to such applications.