Steel selection by application — hot-work, warm-work, and the special cases

Nitralloy 135M (UNS K24065, also called Nitralloy N) is a medium-carbon low-alloy steel with about 1.0% Cr, 0.2% Mo, and 1.0% Al, designed specifically to be nitrided. The aluminum is the active ingredient. AlN precipitates form during nitriding and produce a surface microhardness of 1100-1200 HV (higher than H13 nitrides to), while the lower base alloy content keeps the core at 30-40 HRC and tough.

Where Nitralloy 135M earns its place on a forge build: large warm-forging dies running below about 400°C die surface, and drop-forge or gear-blanking dies where toughness and case life matter more than red hardness. The aluminum-bearing chemistry buys a harder nitride case than H13 can make, at the cost of hot-hardness retention you do not need below that temperature threshold. Above about 450°C die surface, Nitralloy fails. The core over-tempers in service because the alloy was never designed for hot-hardness retention. H13 stays the default for hot-die work above that line.