Harden metal by heating (6)

I believe the answer is:

anneal

(Other definitions for anneal that I've seen before include "Heat and cool slowly in order to toughen" , "Destress using heat" , "Make tough" , "Treat (metal, glass) with heat" , "Heat and cool (metal/glass) to remove stresses" .)

I've seen this clue in The Guardian.

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