Form: This specification covers an aircraft-quality@ low-alloy steel in the form of bars@ forgings@ mechanical tubing@ and forging stock. Application: Primarily for parts with sections 3.5 in. (89 mm) and under in nominal thickness at time of heat treatment requiring a through-hardening steel capable of developing a minimum hardness of 30 HRC when properly hardened and tempered and also for parts of greater thickness but requiring proportionately lower hardness. Certain design and processing procedures may cause these products to be susceptible to stress-corrosion cracking after heat treatment; ARP 1110 recommends practices to minimize such conditions.