SAE AMS6421F

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, 0.875 in. (22.22 mm) and under in section thickness at time of heat treatment, requiring a through-hardening steel capable of developing hardness as high as 50 HRC when properly hardened and tempered, and also for parts of greater thickness requiring proportionately lower hardness, but usage is not limited to such applications.