Form This specification covers an aircraft-quality@ low-alloy steel in the form of bars@ forgings@ mechanical tubing@ and forging stock Application These products have been used typically for parts@ 3.5 inch (89 mm) and under in nominal thickness at time of heat treatment@ requiring a through-hardening steel capable of developing a minimum hardness of 40 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 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