These are steels that are specially formulated to undergo a nitriding operation on a machined part.  Nitriding consists of heating the part in an atmosphere containing ammonia.

A thin, very hard case results from the formation of nitrides. Nitriding grades contain the strong nitride-forming elements aluminum, chromium and molybdenum.

Grade Designation:  Nit135M, Nit #3

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Chemical Composition Ranges
and Limits Charts.