These phrases may have started out in the workshops and forges as practical admonitions, and only later became metaphors. They once meant just what they said.
Strike when the iron is hot/Act at the proper time
Too many irons in the fire/Doing too much at once
Hit the nail on the head/To do something accurately
Going at it hammer and tongs/Working hard and fast
Don’t lose your temper*/To get emotional or “hot”
Have a brittle temper/To be easily angered
Get it ironed out/Solve a problem
*A steel spring is tempered (by controlling the rate of temperature rise and fall) to achieve a balance between strength and flexibility. When a spring is overheated it "looses it's temper" and can be bent out of shape. When a person gets overly excited they too may "loose their temper" and get "bent out of shape" as well!