Nitriding
Also: gas nitriding, ion nitriding, plasma nitriding, ferritic nitrocarburizing, FNC, nitrocarburizing, salt bath nitriding, Tufftride, Melonite, QPQ
Nitriding diffuses nitrogen into steel below its transformation range: a hard, wear- and fatigue-resistant skin with no quench and very little distortion.
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Nitriding holds steel at about 495-565 °C (925-1050 °F) in a nitrogen-rich environment so nitrogen diffuses into the surface and forms hard nitrides with the chromium, molybdenum, vanadium and aluminum in the alloy. There is no quench: the part never passes through the transformation range, so it changes size only slightly and distorts far less than a carburized or through-hardened part. The case is thin, typically 0.1-0.6 mm (0.004-0.025"), but very hard, and the compressive stress it adds improves fatigue strength.
Three process families are sold. Gas nitriding uses ammonia in a sealed retort, with long cycles of roughly 10-80 hours; modern furnaces control the nitriding potential automatically (AMS 2759/10) to set the compound layer. Ion or plasma nitriding (AMS 2759/8) uses a glow discharge in a vacuum vessel, masks easily with steel covers, and can leave little or no compound layer. Ferritic nitrocarburizing (FNC) adds carbon, in gas or in a salt bath (sold as Tufftride and Melonite, and as QPQ with a post-oxidize and polish), runs at about 540-590 °C (1000-1090 °F) for 1-4 hours, and puts a hard, corrosion-resistant compound layer on inexpensive plain-carbon parts.
The outer compound layer, called the white layer for how it etches, is iron nitride about 5-25 µm (0.0002-0.001") thick. It resists wear, galling and corrosion, which is why FNC is used on hydraulic rods and firearm parts, but it is brittle and can spall under high contact stress, so gears and heavily loaded parts specify a thin white layer or none, or have it ground or lapped off. Below it, the diffusion zone carries the hardness gradient into the core. Because the core is not re-hardened, steel is quenched and tempered before nitriding, at a tempering temperature at least 30 °C (50 °F) above the nitriding temperature so the core does not soften.
At a glance
| Typical tolerances | Growth is small and predictable, typically about 2-13 µm (0.0001-0.0005") per surface for gas and ion nitriding and similar for FNC, so parts are finish machined or ground to size first with the growth allowed for. What distortion there is comes mainly from machining stresses released at temperature, so stress relieve after rough machining at a temperature above the nitriding temperature. Case depth is specified as a range, for example 0.25-0.40 mm (0.010-0.016"), usually measured to a hardness 50 HV above the core or to a stated hardness; the compound layer is specified as a maximum or a range such as 5-12 µm (0.0002-0.0005"). Surface hardness is a minimum in HV or HR15N (for example 500 HV min on 4140), since the case is too thin for a standard HRC test. |
|---|---|
| Size limits | Gas nitriding retorts and pit furnaces commonly take loads roughly 600-1,200 mm (24-48") in diameter and 1-3 m (3-10 ft) deep, with larger units at shops that serve extrusion and forging dies. Ion nitriding vessels run from bench size to several meters, with load weight limited by the vessel base and power supply. Salt-bath FNC lines take racks and baskets of small parts, typically up to about 1 m (40") long. Long, slender parts are hung vertically to stay straight. |
| Surface finish | Gas- and ion-nitrided parts come out a uniform matte grey with no scale; roughness increases slightly, and seal surfaces with a compound layer may be lapped or polished afterwards. FNC leaves a grey to black surface; QPQ and post-oxidized FNC are a glossy to satin black that often replaces chrome plating or black oxide on rods and firearm parts. Surfaces must be free of oil and machining residue, and passive stainless and heavily ground surfaces are activated or blasted first so the case forms evenly. |
| Lead time | FNC and salt-bath nitriding in 3-7 business days. Gas nitriding in 1-2 weeks, because cycles of 20-80 hours run as full loads. Ion nitriding in 1-2 weeks. Add time if the parts must first be quenched and tempered or stress relieved. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Nitriding steels contain nitride formers. Nitralloy 135 (with about 1 percent aluminum) reaches the highest hardness, about 900-1,100 HV at the surface. Chromium-molybdenum steels such as 4130, 4140 and 4340 reach roughly 500-650 HV (about 50-58 HRC), and hot-work and high-alloy tool steels (H13, D2) exceed 1,000 HV, which is why H13 die-casting and extrusion dies are routinely nitrided. Plain carbon steels (1018, 1045) and cast irons gain little from gas nitriding but take FNC well for wear and corrosion. Stainless steels, including 17-4 PH, can be nitrided once the passive film is broken, but conventional nitriding costs much of their corrosion resistance. Aluminum and copper alloys cannot be nitrided; titanium can only at much higher temperatures in specialist shops.
What drives the cost
- Case depth and cycle time: gas nitriding cases grow slowly, so a 0.5 mm (0.020") case can take several days of furnace time
- Process: FNC is the cheapest per part for small parts in bulk; ion nitriding costs more per load but masks without paint or plating
- Masking areas that must stay soft: tin-based paint or copper plate for gas nitriding, steel caps and shields for ion nitriding
- Pre-treatment: quench and temper, and stress relief after roughing, if the part is not bought prehardened
- Fixturing to hang or space parts, especially long shafts
- Testing and documentation: case-depth sections, compound-layer measurement and aerospace certs
When to use it
- Wear, galling and fatigue resistance on parts that are already finish machined and cannot tolerate quench distortion
- Hot-work dies, extrusion dies and mold components in H13 and similar tool steels
- Crankshafts, gear shafts, spindles and lead screws in 4140 and 4340 that need a hard surface over a tough, prehardened core
- Low-cost wear and corrosion protection of plain carbon steel parts (FNC or QPQ) in place of chrome plating or black oxide
- Precision parts where grinding after hardening would be difficult or impossible
When not to
- Heavy contact loads that need a deep case: carburizing or induction hardening goes 1-3 mm (0.040-0.120") deep, nitriding rarely beyond 0.6 mm (0.025")
- Parts that depend on stainless corrosion resistance: conventional nitriding lowers it
- Steel tempered below the nitriding temperature, such as tool steel given a low temper for maximum hardness: the core softens in the nitriding furnace
- Aluminum, copper alloys and plastics
Design tips
- Finish machine, grind or hone to final size before nitriding and allow for a few microns of growth per surface; tell the nitrider which surfaces are critical.
- Stress relieve after rough machining at a temperature above the nitriding temperature, then finish machine, so the part does not move in the nitriding furnace.
- Specify prehardened steel, for example 4140 at 28-32 HRC, or a heat-treat condition tempered at least 30 °C (50 °F) above the nitriding temperature.
- Call out case depth as a range with its definition, surface hardness as a minimum in HV or HR15N, and the compound layer, for example "white layer 12 µm max", or "no white layer" on gear teeth and parts under high contact stress.
- Break sharp edges and corners: the case forms from two sides at an edge and becomes brittle there.
- Mark areas to stay soft; ion nitriding masks threaded holes and bores with simple steel caps and plugs, which is cheaper than paint or plating.
- Name the process: gas nitriding (AMS 2759/6 or AMS 2759/10), ion nitriding (AMS 2759/8) or FNC. They give different case structures and are not interchangeable.
Nitriding by material
Frequently asked questions
- Does nitriding cause distortion?
- Very little. Nitriding runs below the transformation temperature of steel and has no quench, so most parts grow only a few microns per surface and stay within finish-machined tolerances. What movement there is usually comes from machining stresses relaxing at temperature, which a stress relief before finish machining prevents.
- How hard is a nitrided surface?
- It depends on the steel. 4140 reaches roughly 500-650 HV (about 50-58 HRC), H13 and other high-alloy tool steels 1,000 HV and above, and Nitralloy 135 about 900-1,100 HV. Because the case is thin, hardness is tested with a superficial Rockwell (HR15N) or microhardness test, not a standard HRC test.
- What is the white layer in nitriding?
- The compound layer of iron nitrides at the surface, about 5-25 µm (0.0002-0.001") thick, named for how it looks after etching. It resists wear, galling and corrosion but is brittle, so it is kept thin or removed on gears and parts under high contact stress, and kept on FNC-treated rods and wear parts where it does the work.
- Can stainless steel be nitrided?
- Yes, once the passive film is removed, and 17-4 PH and 410 are nitrided for wear. Conventional nitriding ties up chromium as nitrides, so corrosion resistance drops noticeably. Specialist low-temperature processes harden austenitic stainless while keeping most of its corrosion resistance.
- What is the difference between nitriding and nitrocarburizing (Melonite, QPQ)?
- Nitrocarburizing (FNC) adds carbon along with nitrogen, runs a little hotter and much shorter (hours rather than days), and builds a thicker compound layer over a shallow diffusion zone; it is a low-cost wear and corrosion finish for plain carbon steel. Gas and ion nitriding build a deeper diffusion case on alloy and tool steels for load-bearing wear and fatigue resistance.
