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Nitriding and QPQ

Anodizing & conversion coatings · Thermal

Nitriding diffuses nitrogen into steel to create a hard wear-resistant case without depositing a conventional coating.

CNC machiningSheet metalHigh cost $$$$Unchanged
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Reference page — Nitriding is not in the instant-quote configurator. Upload the part, name the finish in the notes, and an engineer quotes it with the part.

Nitriding and QPQ on a machined part

What is Nitriding?

Gas, plasma, and salt-bath nitrocarburizing or QPQ are selected for gears, shafts, dies, moulds, and firearm components.

How it works

Gas or plasma nitriding holds alloy steel in a nitrogen-active atmosphere, commonly around 480–580 °C, so nitrogen diffuses inward and forms alloy nitrides. Salt-bath ferritic nitrocarburizing runs near 570–590 °C and forms a thin compound layer over a diffusion zone; QPQ adds polish and post-oxidation. The case is supported by the core, so steel chemistry and prior heat treatment control the result.

Why specify it

  • Surface hardness commonly 700–1200 HV
  • Case depth about 0.1–0.7 mm
  • Low distortion compared with carburizing
  • QPQ adds black appearance and corrosion resistance

Where it falls short

  • Works mainly on suitable ferrous alloys
  • Compound layer can be brittle if excessive
  • Process heat can alter prior temper
  • Case depth requires long cycles

Nitriding at a glance

Nitriding is strongest on surface hardness (73 of 100), at a high cost.

Corrosion protectionGood55 / 100
Surface hardnessVery good73 / 100
Cosmetic qualityGood55 / 100
LubricityGood58 / 100
Relative costHigh72 / 100
CorrosionHardnessCosmeticLubricityEconomy

Each score is 0–100 against every finish in this catalog. Corrosion comes from salt-spray life (ASTM B117, log scale) and hardness from the finish layer's Vickers or pencil hardness; cosmetic, lubricity and cost are FabDigit's engineering assessment. Economy on the radar is the inverse of cost.

Nitriding key numbers

Typical values from the governing specifications and shop practice; bars are this finish against the highest value among anodizing & conversion coatings finishes.

Thickness5–20 µm0.2–0.79 mil
Hardness700–1,200 HV
Max service temp.500 °C932 °F
Thickness
Compound (white) layer about 5–20 µm over a 0.1–0.7 mm diffusion case; salt-bath QPQ can sit at the high end.
Hardness
700–1,200 HV — Grade- and process-dependent surface hardness.
Electrical
Unchanged
Colours
  • silver-grey
  • black QPQ
Appearance
Grey after gas or plasma nitriding; uniform black after polished post-oxidized QPQ.

Where Nitriding sits among all finishes

Surface hardness (Vickers, log scale) against corrosion protection (salt-spray hours, log scale) for every coating FabDigit offers, one cloud per finish group. Nitriding is highlighted; hover a dot for its name or open the full map.

02550751000255075100Corrosion protection (0–100)Surface hardness (HV, log scale, 0–100)Plating — 19 finishesHard coatings — 17 finishesAnodize & conversion — 8 finishesPaint & powder — 7 finisheshard, low corrosion protectionhard and protectiveprotective, softsoft, little protectionType II anodize — corrosion 71, hardness 54BSA anodize — corrosion 71, hardness 52Type I anodize — corrosion 71, hardness 49Hard anodize — corrosion 71, hardness 61PTFE hard anodize — corrosion 71, hardness 61PEO — corrosion 70, hardness 74Sherardizing — corrosion 83, hardness 56Brass plate — corrosion 35, hardness 44Bronze plate — corrosion 40, hardness 49Cadmium plate — corrosion 70, hardness 26Copper plate — corrosion 32, hardness 39Decorative chrome — corrosion 64, hardness 71Electroless copper — corrosion 20, hardness 41Electroless nickel — corrosion 70, hardness 68Nickel plate — corrosion 51, hardness 55Gold plate — corrosion 68, hardness 39Hard chrome — corrosion 40, hardness 73Hot-dip zinc — corrosion 90, hardness 42Nickel-PTFE — corrosion 72, hardness 57Silver plate — corrosion 55, hardness 39Solder plate — corrosion 42, hardness 9Tin plate — corrosion 34, hardness 0Zinc plate — corrosion 60, hardness 30Zinc-nickel — corrosion 86, hardness 60Cerakote — corrosion 83, hardness 27E-coat — corrosion 83, hardness 15Epoxy coat — corrosion 83, hardness 15Powder coat — corrosion 83, hardness 14Wet paint — corrosion 59, hardness 10AlCrN PVD — corrosion 58, hardness 93AlTiN PVD — corrosion 52, hardness 94CrN PVD — corrosion 62, hardness 83DLC — corrosion 55, hardness 93MoS2 PVD — corrosion 20, hardness 26TiAlN PVD — corrosion 50, hardness 93TiCN PVD — corrosion 40, hardness 92TiB2 PVD — corrosion 40, hardness 94TiN PVD — corrosion 45, hardness 87WC/C PVD — corrosion 48, hardness 77ZrN PVD — corrosion 52, hardness 90CVD alumina — corrosion 70, hardness 85CVD TiCN — corrosion 38, hardness 92CVD TiN — corrosion 45, hardness 87Multilayer CVD — corrosion 62, hardness 88HVOF coating — corrosion 78, hardness 77Plasma spray — corrosion 60, hardness 68Plastic chrome — corrosion 72, hardness 71Electroless plastic — corrosion 58, hardness 39Clear coat — corrosion 20, hardness 12Soft-touch — corrosion 20, hardness 5PLATINGHARD COATINGSANODIZE & CONVERSIONPAINT & POWDERDLCTiN PVDType II anodizePowder coatZinc plateZinc-nickelHard chromeTin plateCadmium plateE-coatCerakoteWet paintHot-dip zincHVOF coatingNitriding — corrosion 55, hardness 73Nitriding
02550751000255075100Corrosion protection (0–100)Surface hardness (HV, log scale, 0–100)Plating — 19 finishesHard coatings — 17 finishesAnodize & conversion — 8 finishesPaint & powder — 7 finishesType II anodize — corrosion 71, hardness 54BSA anodize — corrosion 71, hardness 52Type I anodize — corrosion 71, hardness 49Hard anodize — corrosion 71, hardness 61PTFE hard anodize — corrosion 71, hardness 61PEO — corrosion 70, hardness 74Sherardizing — corrosion 83, hardness 56Brass plate — corrosion 35, hardness 44Bronze plate — corrosion 40, hardness 49Cadmium plate — corrosion 70, hardness 26Copper plate — corrosion 32, hardness 39Decorative chrome — corrosion 64, hardness 71Electroless copper — corrosion 20, hardness 41Electroless nickel — corrosion 70, hardness 68Nickel plate — corrosion 51, hardness 55Gold plate — corrosion 68, hardness 39Hard chrome — corrosion 40, hardness 73Hot-dip zinc — corrosion 90, hardness 42Nickel-PTFE — corrosion 72, hardness 57Silver plate — corrosion 55, hardness 39Solder plate — corrosion 42, hardness 9Tin plate — corrosion 34, hardness 0Zinc plate — corrosion 60, hardness 30Zinc-nickel — corrosion 86, hardness 60Cerakote — corrosion 83, hardness 27E-coat — corrosion 83, hardness 15Epoxy coat — corrosion 83, hardness 15Powder coat — corrosion 83, hardness 14Wet paint — corrosion 59, hardness 10AlCrN PVD — corrosion 58, hardness 93AlTiN PVD — corrosion 52, hardness 94CrN PVD — corrosion 62, hardness 83DLC — corrosion 55, hardness 93MoS2 PVD — corrosion 20, hardness 26TiAlN PVD — corrosion 50, hardness 93TiCN PVD — corrosion 40, hardness 92TiB2 PVD — corrosion 40, hardness 94TiN PVD — corrosion 45, hardness 87WC/C PVD — corrosion 48, hardness 77ZrN PVD — corrosion 52, hardness 90CVD alumina — corrosion 70, hardness 85CVD TiCN — corrosion 38, hardness 92CVD TiN — corrosion 45, hardness 87Multilayer CVD — corrosion 62, hardness 88HVOF coating — corrosion 78, hardness 77Plasma spray — corrosion 60, hardness 68Plastic chrome — corrosion 72, hardness 71Electroless plastic — corrosion 58, hardness 39Clear coat — corrosion 20, hardness 12Soft-touch — corrosion 20, hardness 5PLATINGHARD COATINGSANODIZE & CONVERSIONPAINT & POWDERDLCTiN PVDPowder coatZinc plateNitriding — corrosion 55, hardness 73Nitriding
Anodize & conversionPlatingPaint & powderHard coatings

Specifications for Nitriding

The documents a drawing cites to define this finish, with the Type / Class rows that apply. Each links to a page explaining the standard.

Drawing call-out

PLASMA NITRIDE DESIGNATED SURFACES, EFFECTIVE CASE DEPTH 0.30–0.50 mm, SURFACE HARDNESS 900 HV0.5 MIN, COMPOUND LAYER 5–15 µm; MASK THREADS; DISTORTION 0.02 mm MAX

Which materials take Nitriding?

Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library.

Material familyVerdictNotes
Steel
YesAlloy and tool steels designed for nitriding give the best response.
  • Confirm tempering temperature exceeds nitriding temperature
DMLS / SLM
With careSteel DMLS/SLM parts can be nitrided after density, residual-stress, and prior-temper review.
Stainless Steel
With careSpecial low-temperature plasma processes may preserve corrosion resistance; conventional treatment can reduce it.
Titanium
LimitedTitanium nitriding is a different specialized diffusion process.
Aluminum
NoThis record covers ferrous nitriding, not aluminum nitride deposition.
Brass
NoBrass is incompatible with nitriding.
Copper Alloy
NoCopper alloys do not form a useful nitrided case in this process.
FDM
NoPolymers cannot be nitrided.
Magnesium Alloy
NoProcess temperature and chemistry are incompatible.
MJF
NoPolymers cannot be nitrided.
Polymer
NoProcess temperature and mechanism are incompatible.
SLA
NoPolymers cannot be nitrided.
SLS
NoPolymers cannot be nitrided.
Zinc Alloy
NoProcess temperature exceeds zinc-alloy limits.

How Nitriding is applied, and what it does to the part

  1. 1Select and heat-treat steel
  2. 2Clean
  3. 3Mask or stop off
  4. 4Gas, plasma, or salt-bath treat
  5. 5Diffuse or quench
  6. 6Polish and oxidize for QPQ
  7. 7Inspect hardness and case depth
DimensionsGrowth is far below a plated case depth but is not zero; leave finish-grind allowance and qualify distortion on precision parts.
Build-up per side0–10 µm — Diffusion causes little buildup; QPQ compound-layer growth and swelling are process-specific.
MaskingUsually needed. Stop off threads, sharp edges, weld zones, and surfaces that must remain machinable or soft.
ElectricalUnchanged
Temperature limit500 °C (932 °F) continuous
AppearanceGrey after gas or plasma nitriding; uniform black after polished post-oxidized QPQ.
Added lead time7–15 working days on top of the bare part

Design rules for Nitriding

What to decide on the drawing before the part goes to the finisher.

  1. Name gas, plasma, or QPQ.The processes do not produce interchangeable cases.
  2. Specify effective case depth and hardness load.Surface hardness alone does not define load support.
  3. Choose a nitriding steel.Chromium, molybdenum, aluminum, and vanadium improve nitride response.
  4. Set compound-layer limits.An excessive white layer can be brittle.

Nitriding versus the alternatives

Finishes engineers weigh against Nitriding, and when each wins.

FinishCorrosionHardnessCosmeticCostThicknessChoose it when
Nitriding
this page
557355$$$$5–20 µm—
Diamond-Like Carbon Coating
Nitriding vs Diamond-Like Carbon Coating →
559372$$$$1–3 µmlower friction with only micrometre buildup is required
Titanium Nitride PVD
Nitriding vs Titanium Nitride PVD →
458788$$$$1–5 µma deposited hard film is preferred
CorrosionHardnessCosmeticLubricityEconomy
NitridingDiamond-Like Carbon Coating

The radar overlays Nitriding with Diamond-Like Carbon Coating, its most common alternative — chosen lower friction with only micrometre buildup is required. Scores in the table are 0–100 across every finish in the catalog.

Cost, lead time and compliance

Relative cost

Nitriding and QPQ is a high-cost finish ($$$$): more expensive than about 77 % of the finishes FabDigit quotes.

Finishing is priced per part with a lot minimum, from FabDigit's own China-side rates. The exact figure appears on the instant quote once the part is uploaded.

Lead time and compliance

Added lead time7–15 working days
RoHSCompliant
NotesFinal nitrided steel is generally RoHS-compatible; salt-bath chemistry and gas handling require controlled industrial processing.

What Nitriding is used for

  • Gears
  • Pump shafts
  • Extrusion dies
  • Injection moulds

Nitriding — frequently asked

How deep is a nitrided case?

Typical engineering case depth is about 0.1–0.7 mm. Compound layer at the surface is much thinner, commonly about 5–20 µm.

How hard is nitrided steel?

Surface hardness is commonly 700–1200 HV depending on steel and process. Case depth and core hardness are equally important.

Gas nitriding vs plasma nitriding: what is the difference?

Both diffuse nitrogen, usually around 480–580 °C. Plasma offers selective control and reduced gas use, while gas nitriding is established for larger batch loads.

What is QPQ nitriding?

QPQ is salt-bath ferritic nitrocarburizing followed by polishing and post-oxidation. It produces a black surface, a thin compound layer, and a diffusion case.

Does nitriding change dimensions?

It has much less growth than a 50–300 µm sprayed coating, but growth and distortion are not zero. Precision parts should include a qualified grind or size allowance.