AR steel is often described as being through-hardened, but what does that really mean?
When grain structure changes during the initial heating stage of Q&T, the composition of the entire plate changes. This is referred to as through-hardening. Through-hardening differs from "case-hardening," also referred to as "surface-hardening," which only hardens the surface while allowing the metal deeper underneath to remain soft. In this case, the composition, or hardening, of the plate only changes at the surface level.
