51-

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

51-

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

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.