Gas carburizing effects on impact toughness of low carbon steel?
Answer:
What makes materials hard is a complicated subject that has to do with internal crystal structures. These in turn are effected by heat treating or mechanical working. Most non-ferrous metals can only be hardened by "work hardening", hammering, rolling, bending. But they can be softened by "annealing" which is heating to a red heat and then cooling quickly (opposite to steel, see below).
The hardness of steel is determined by carbon content. No carbon and it can't be hardened other than by work hardening. Increasing the carbon content from 0.01% to .10% increases the hardenability and the strength. This is then modified by the addition of alloying metals as well as the alloy metals having their own properties.
To harden most steel it is heated to a medium red or slightly above the point where it becomes non-magnetic. It is then quenched in water, oil or air depending on the type of steel. The steel is now at its maximum hardness but is very brittle. To reduce the brittleness the metal is tempered by heating it to some where between 350°F and 1350°F. This reduced the hardness a little and the brittelness a lot. Most steels need to be tempered at about 450°F for maximum usable hardness but every steel is slightly different.
To soften steel so that it can be cold worked and machined is called annealing. To anneal steel is is heated to slightly above the hardening temperature and then cooled as slow as possible. Cooling is done in an insulating medium such as dry powdered lime or in vermiculite. High carbon and many alloy steels can only be cooled slow enough in a temperatue controlled furnace since the cooling rate must be only 20 degrees F per hour for several hours.
The set of processes, annealing, hardening and tempering are collectively known as "heat treating".
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