Form: this specification covers a premium aircraft quality@ low-alloy steel in the form of bars@ forgings@ mechanical tubing@ and forging stock. Application: Primarily for parts 3.5 inches (90 mm) and under in cross-sectional thickness requiring a through-hardening steel capable of developing a minimum hardness of 52 HRC@ the parts being subject to very rigid magnetic particle inspection standards. Certain design and processing procedures may cause these products to become susceptible to stress-corrosion cracking after heat treatment; ARP1110 recommends practices to minimize such conditions.