Nitriding

Nitriding diffuses nitrogen into steel at low temperature to create a hard, wear‑ and fatigue‑resistant case with minimal distortion and no quench.

Overview

Nitriding is a low-temperature heat treatment that diffuses nitrogen into the surface of alloy steels to form a hard, wear-resistant case while keeping the core tough and ductile. It runs below the tempering temperature of most steels and does not require quenching, so dimensional change and distortion are very low compared to through-hardening processes.

Use nitriding when you need high surface hardness, excellent fatigue strength, and good seizure resistance on precision parts: gears, shafts, molds, dies, and sliding components. It works best on nitriding-grade and alloy steels containing aluminum, chromium, and molybdenum, which form hard nitrides. Tradeoffs: case depth is usually limited (often 0.1–0.8 mm), cycle times can be long, and the process is mostly limited to steels and some cast irons. You must design and specify case depth, hardness, and masking requirements clearly to control cost and performance.

Common Materials

  • 4140 steel
  • 4340 steel
  • H13 tool steel
  • Nitralloy
  • 17-4 PH stainless

Tolerances

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Applications

  • Transmission gears
  • Crankshafts and camshafts
  • Injection mold cavities and cores
  • Extrusion and injection screws
  • Hydraulic piston rods
  • Valve stems and spools

When to Choose Nitriding

Choose nitriding when you need a very hard, wear-resistant surface on a precision steel part but must keep distortion and dimensional change to a minimum. It’s ideal once the part is fully machined and ground, and you want to improve fatigue life, pitting resistance, and sliding wear without through-hardening the entire section. Best for medium to high-value parts where long life justifies the added heat treat cost.

vs Annealing

Pick nitriding instead of annealing when the goal is surface hardness, wear resistance, and fatigue strength, not softening or stress relief. Annealing prepares material for machining and forming; nitriding is a final or near-final hardening step on already machined parts.

vs Normalizing

Choose nitriding over normalizing when you need a hard case on a tough core with precise dimensions. Normalizing refines grain and homogenizes structure but does not give the very high surface hardness and wear resistance that nitriding provides.

vs Quenching and Tempering

Use nitriding instead of quench-and-temper when distortion and cracking risk must be low and only the surface needs to be very hard. Quenching and tempering through-harden the section and can move parts out of tolerance; nitriding gives a hard shell on a stable, tempered core with minimal size change.

vs Carburizing

Select nitriding over carburizing when low distortion, controlled shallow case depth, and no oil or gas quench are critical. Carburizing can achieve deeper cases in low-carbon steels but runs hotter and usually requires quenching, increasing distortion and post-grind requirements, while nitriding is more dimensionally stable for precision components.

Design Considerations

  • Select nitriding-grade or alloy steels (Al, Cr, Mo bearing) and confirm core hardness before nitriding to achieve the desired case properties
  • Specify required case depth, surface hardness, and inspection method on the drawing to avoid over- or under-processing and surprise costs
  • Finalize machining and most grinding before nitriding; avoid heavy stock removal afterward, which will cut through the hardened case
  • Call out masked or non-nitrided areas clearly with dimensions and surfaces; masking complex features increases cost and lead time
  • Avoid sharp corners, notches, and abrupt section changes in highly loaded zones to reduce the risk of case cracking and stress concentration
  • Consider surface finish before and after nitriding; specify whether post-nitriding polish or grind is allowed and how much material (if any) is available to remove