What is "hammer hardening"

Hammer \Ham"mer\ (h[a^]m"m[~e]r), n. [OE. hamer, AS. hamer, hamor; akin to D. hamer, G. & Dan. hammer, Sw. hammare, Icel. hamarr, hammer, crag, and perh. to Gr. 'a`kmwn anvil, Skr. a[,c]man stone.]

1. An instrument for driving nails, beating metals, and the like, consisting of a head, usually of steel or iron, fixed crosswise to a handle.

   With busy hammers closing rivets up.
   --Shak.

2. Something which in form or action resembles the common hammer; as:

   1. That part of a clock which strikes upon the bell to indicate the hour.

   2. The padded mallet of a piano, which strikes the wires, to produce the tones.

   3. (Anat.) The malleus. See under Ear.

   4. (Gun.) That part of a gunlock which strikes the percussion cap, or firing pin; the cock; formerly, however, a piece of steel covering the pan of a flintlock musket and struck by the flint of the cock to ignite the priming.

   5. Also, a person or thing that smites or shatters; as, St. Augustine was the hammer of heresies.

      He met the stern legionaries [of Rome] who had been the ``massive iron hammers'' of the whole earth.
      --J. H. Newman.

3. (Athletics) A spherical weight attached to a flexible handle and hurled from a mark or ring. The weight of head and handle is usually not less than 16 pounds.

   Atmospheric hammer, a dead-stroke hammer in which the spring is formed by confined air.

   Drop hammer, Face hammer, etc. See under Drop, Face, etc.

   Hammer fish. See Hammerhead.

   Hammer hardening, the process of hardening metal by hammering it when cold.

   Hammer shell (Zo["o]l.), any species of Malleus, a genus of marine bivalve shells, allied to the pearl oysters, having the wings narrow and elongated, so as to give them a hammer-shaped outline; -- called also hammer oyster.

   To bring to the hammer, to put up at auction.