Can 4140 be nitrided?
Yes. 4140 gas- or ion-nitrides to a 50-55 HRC surface (roughly 550-650 HV) with a case of 0.2-0.6 mm (0.008-0.024") and almost no distortion, provided it is quenched and tempered above the nitriding temperature first.
4140 is one of the most commonly nitrided steels. Its 1% chromium and 0.2% molybdenum form fine alloy nitrides when nitrogen diffuses into the surface at 500-565 °C (930-1050 °F), producing a hard diffusion zone under a thin compound (white) layer. Because the process runs below the tempering temperature and needs no quench, the part hardly moves: growth of about 0.01-0.03 mm (0.0005-0.001") per surface and negligible warp, so finished, ground parts can be nitrided as the last operation. The surface reaches about 550-650 HV (roughly 50-55 HRC on 4140; higher-aluminum nitriding steels such as Nitralloy 135M reach 900-1100 HV, and H13 about 1000 HV), with good wear resistance, a large gain in fatigue strength from the compressive residual stress, and some corrosion resistance from the compound layer.
The core must be prepared first. 4140 for nitriding is quenched and tempered, and the tempering temperature has to be at least 30 °C (50 °F) above the nitriding temperature, so pre-hard 4140 at 28-32 HRC (tempered around 620-650 °C) is ideal, and a part tempered to 40 HRC at 540 °C would soften during a 565 °C nitride. Annealed 4140 can be nitrided but the soft core lets the thin case crack under point loads. Gas nitriding in dissociated ammonia takes 24-72 hours for a 0.3-0.6 mm (0.012-0.024") case; ion (plasma) nitriding runs at 400-520 °C, controls the white layer better, masks by simple mechanical cover and is preferred for parts with sharp edges and holes; ferritic nitrocarburizing (FNC, gas or salt bath at 565-580 °C for 1-4 hours) gives a thin case of 0.05-0.2 mm with a corrosion-resistant compound layer for less demanding wear. AMS 2759/6 covers gas nitriding of low-alloy steel parts, AMS 2759/8 ion nitriding, and AMS 2759/10 nitriding controlled by nitriding potential.
The compound layer is the design detail most often missed. Conventional gas nitriding leaves 5-25 µm of epsilon and gamma-prime iron nitride on the surface: hard, brittle, and prone to chipping at sharp edges. For gears, bearing journals and anything that mates under load, specify a white-layer limit or its removal by light grinding or lapping, and put radii on edges. Surfaces that must stay soft for later machining or welding are masked with copper plating, a stop-off paint, or in ion nitriding by a fixture cover. Nitrided surfaces cannot be machined afterwards except by grinding or EDM, so the sequence is: rough, quench and temper, finish machine and grind, stress-relieve if heavy stock was removed, nitride, then lap or lightly finish.
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What to specify
- Core condition before nitriding: "quench and temper to 28-32 HRC, temper at or above 595 °C (1100 °F)" so the core survives the nitriding temperature
- Process: gas nitride per AMS 2759/6 (or AMS 2759/10), ion nitride per AMS 2759/8, or ferritic nitrocarburize, and the case depth as effective depth to a stated hardness (for example 0.3-0.5 mm to 400 HV)
- Surface hardness, typically 550-650 HV (about 50-55 HRC) for 4140, and the measurement method (microhardness traverse per ASTM E384 on a coupon)
- Compound (white) layer: maximum thickness, or "remove white layer by lapping" on mating surfaces; and edge radii of at least 0.4 mm (0.015")
- Masking: surfaces to remain unnitrided (later machining, welds, threads that must not be brittle), by copper plate, stop-off or fixture
- Growth allowance of about 0.01-0.03 mm (0.0005-0.001") per surface on tight fits, and whether the part is finish-lapped after nitriding
- Stress relief before nitriding if the finish machining removed heavy stock, to stop the part moving during the long cycle
Pitfalls
- Core tempered below the nitriding temperature: the part comes back with a hard skin over a softened core, and the drawing hardness is gone
- Sharp edges and unbroken corners: the compound layer chips off in flakes and takes the case with it
- Expecting Nitralloy or H13 surface numbers: 4140 tops out around 55 HRC; choose Nitralloy 135M or a 3% chromium steel for 65 HRC surfaces
- Machining after nitriding: only grinding, lapping or EDM touch a nitrided surface; a drill or tap is ruined immediately
- Threads nitrided by accident: they become brittle and gall; mask them or cut them after by EDM
- Decarburized or contaminated surfaces (scale, oil, phosphate) nitride unevenly; the part must be clean and bright going in
Frequently asked questions
- How hard does nitrided 4140 get?
- About 550-650 HV at the surface, roughly 50-55 HRC, over a diffusion case of 0.2-0.6 mm (0.008-0.024"). That is lower than aluminum-bearing nitriding steels or H13 (900-1100 HV), but the case is tough and the process barely distorts the part.
- Does 4140 need to be hardened before nitriding?
- Yes. Quench and temper it first, with the tempering temperature at least 30 °C (50 °F) above the nitriding temperature; pre-hard 28-32 HRC 4140 tempered around 620-650 °C is the usual starting condition. An annealed core is too soft to support the case.
- How long does nitriding 4140 take?
- Gas nitriding runs 24-72 hours at 500-565 °C (930-1050 °F) for a 0.3-0.6 mm case; ion nitriding is similar or a little faster; ferritic nitrocarburizing takes 1-4 hours for a shallow 0.05-0.2 mm case with a corrosion-resistant compound layer.
- Does nitriding change the dimensions of 4140 parts?
- Very little: about 0.01-0.03 mm (0.0005-0.001") of growth per surface and negligible warp, which is why finished, ground parts are nitrided last. Allow for the growth on press fits and lap it off where a tight bearing fit is needed.
