Along the lines of "chill bar" and such; when welding a barrel, it will cool quickly due to the cooler surrounding barrel. This can cause what is sometimes referred to as "self-quenching". On barrels with enough carbon content, this will cause hardening, usually right along the weld edge or even on the weld itself. Many fluid steel barrels made in the early 20th century have enough carbon to harden, I suspect, since these are often early proprietary versions of chromoly. So, a normalize or anneal step can be used to get the steel back to a softness that can be filed.
