Nitriding

Nitriding diffuses nitrogen into steel surfaces to form a hard, wear-resistant case with minimal distortion and improved fatigue performance.

Overview

Nitriding is a low-temperature thermochemical heat treatment that diffuses nitrogen into steel to create a hard surface layer (case) and a supportive diffusion zone. It’s typically done as gas, plasma (ion), or salt bath nitriding, and it’s most effective on alloy steels with nitride-forming elements (Cr, Mo, Al, V). The process delivers high surface hardness, strong wear and scuff resistance, and a bump in fatigue strength from compressive residual stress.

Choose nitriding when you need a hard, durable surface but can’t tolerate the distortion and size change common with high-temperature treatments. It’s a good fit for finished or near-finished parts because growth is usually small but real, and post-grind is often avoided or tightly controlled. Tradeoffs: slower cycle times than many heat treats, case depth is limited compared to carburizing, and results depend heavily on steel grade, surface condition, and masking requirements for no-nitride areas.

Common Materials

  • 4140
  • 4340
  • H13
  • Nitralloy 135M
  • 17-4 PH

Tolerances

±0.001" to ±0.003" (expect slight growth; hold critical fits by leaving grind stock or specifying post-finish)

Applications

  • Gear shafts and splines
  • Camshafts and tappets
  • Extrusion screws and barrels
  • Injection mold cores and cavities
  • Valve stems and seats
  • Hydraulic piston rods

When to Choose Nitriding

Choose nitriding for steel parts that need high surface hardness, wear resistance, and improved fatigue life while maintaining tight geometry with minimal distortion. It fits low to medium volumes for critical rotating or sliding components where consistent surface performance matters. Plan for controlled surface finish, clean condition, and clear definition of masked areas and target case depth.

vs Annealing

Choose nitriding when you need a hardened, wear-resistant surface; annealing is used to soften material for machining and to improve ductility. Nitriding is typically performed after machining/heat treat steps to add surface durability without major dimensional change.

vs Normalizing

Choose nitriding when surface wear, scuffing, or rolling contact fatigue drives failure; normalizing mainly refines grain structure and equalizes properties after forging or welding. Nitriding adds a hard case and compressive stress that normalizing does not provide.

vs Quenching and Tempering

Choose nitriding when you want a hard surface with minimal distortion and don’t want to re-machine after heat treat. Quench-and-temper sets bulk hardness and strength but can cause warp and may not provide the same surface wear resistance without additional treatment.

vs Carburizing

Choose nitriding when you need lower processing temperatures and tighter control of distortion on finished geometry. Carburizing can achieve deeper cases for heavy-duty gears, but it runs hotter and usually requires post-heat-treat finishing to recover size and roundness.

Design Considerations

  • Specify nitriding method (gas/plasma/salt), target case depth, and hardness profile requirements, not just “nitrided.”
  • Call out surfaces to be nitrided and surfaces to be masked; include sketches to avoid ambiguity and rework.
  • Control pre-nitride surface finish; rough surfaces nitride unevenly and can reduce effective contact performance.
  • Avoid heavy post-machining after nitriding; if grinding is required, leave controlled stock and state allowable stock removal.
  • Select a nitriding-suitable steel grade; plain carbon steels won’t respond like Cr/Mo/Al alloyed grades.
  • Define dimensional requirements with expected growth in mind (fits, bearing seats, threads) and state whether final inspection is pre- or post-nitride.