51-

Etymology
From AISI and SAE standards nomenclature for iron and steel grades for chromium steels with roughly 1% chromium.

Prefix

 * 1)  Identifying a chromium steel grade with roughly 1% chromium, and a varying amount of carbon.

Usage notes
It is used in forming a four or five digit term.
 * In the four digit form, the remaining two digits attached to the prefix signifying the sub-1% fractional percentage value of the carbon content of the steel. For example, a steel with 0.95% carbon would be graded 5160. (51+60, from 0."60"%)
 * In the five digit form, the remaining three digits attached represent the carbon content in hundredths of a percentage, with at least 1% carbon content.