Peen

From SwordWiki

Peening is the process of hammering a protrusion flat, like a rivet, to hold two metal objects together. A properly performed peen is extremely strong.

Many sword makers use a method of "hot peening", in which the portion of the tang being peened is heated during the hammering. This expands and softens the metal, making it easier to hammer, and then in cooling it will tighten the peen's grip even further. Hot peening is not necessary by any means. Cold peens can be just as resilient if executed well. And depending on the sword construction, the peen may not be entirely structural, with each of the other components holding themselves in place individually.

Usually the peen is ground and polished to make it invisible, or at least flush with the pommel or peen block. Occasionally they are left as rough aesthetic details.

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