godan verb (-su), transitive verb
anneal; temper
Technical term in metallurgy: heating metal and then cooling it slowly to remove internal stress and increase toughness.
鋼を鈍してから加工する。
Anneal the steel before working it.
Rarely used kanji form.
焼きなます is a compound meaning 'to anneal by heating', while 鈍す alone is the basic verb.
The reading なます is likely a native Japanese verb. The kanji 鈍 is used for its meaning 'dull' or 'soften', which fits the annealing process. The exact derivation is uncertain.