Hardened steel nozzle

A hardened steel nozzle is made from a tougher alloy than the brass used in most stock nozzles, specifically so it can survive filaments packed with abrasive particles. Standard brass wears down quickly when abrasive material passes through it repeatedly, gradually widening the nozzle opening in a way that hurts dimensional accuracy and surface quality.

Filaments like carbon fiber or metal filled blends, and even some glow-in-the-dark filaments that use abrasive phosphorescent powder, contain particles that act like sandpaper on a soft brass nozzle. A hardened steel nozzle holds its shape and diameter far longer under that kind of wear, though it usually conducts heat slightly less efficiently than brass, which can mean small temperature adjustments.

If a printer is only ever going to run PLA or PETG, a stock brass nozzle is perfectly fine and cheaper to replace. The mistake worth avoiding is running carbon fiber or metal filled filament through a stock brass nozzle expecting it to last, since the opening can visibly enlarge within a single spool and quietly ruin print accuracy afterward.