What is the difference between tempering and hardening metals and their end products?

I think I've heard hardening is when you dunk red-hot metal into cold water, and tempering is when you take that hardened metal, heat it slightly, and then let it cool slowly. However what is the difference between their end products?

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– A.K.
Apr 18 '19 at 19:36

2 Answers

As the names imply, hardening makes the metal more rigid but more brittle, and tempering (from "temperate", moderate), forgoes some hardness for increased toughness. Iron alloys are hardened by rapid quenching and toughened by annealing, but some copper alloys are made more ductile by quenching and are hardened by working.

In brief, annealing allows atoms to migrate to a less-strained position, whereas quenching lock atoms in place; this is used to make tempered glass, as well as to harden metals.

Extremely rapid quenching can produce a metallic glass, where precipitation of constituents is prevented.

Edged-tools may need the body to be ductile, so that it does not shatter, but require for the edge to be hardened so that it retains sharpness. See http://www.smt.sandvik.com/en/products/strip-steel/strip-products/knife-steel/hardening-guide/purpose-of-hardening-and-tempering/ or http://www.stormthecastle.com/blacksmithing/blacksmithing-a-knife/stock-removal-method-of-knifemaking-part-4.htm.

Though the other answer provides a good insight, I think annealing and tempering could have been confused.

I will talk with respect to iron-carbon alloys or steel here. Tempering is usually a post-quenching or post hardening treatment. It is done to relieve internal stresses, decrease brittleness, improve ductility and toughness. The usual heating range for tempering in steel is from $$150\ \mathrm{^\circ C}$$ to $$600\ \mathrm{^\circ C}$$ and it is below the upper critical temperature or the eutectoid line. The cooling is done in still air.

Annealing can induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous (though grain coarsening can also happen), and improve cold working properties. Cooling in case of annealing is usually furnace cooling which gives the slowest of cooling rates.

The products for annealing will depend on the carbon % and the temperature ranges from which it is cooled. But usually it is pearlite + ferrite/cementite.

The product of tempering is usually just tempered martensite or bainite.

Hope this was helpful.