By Harris, Technical Team, SHUNFU METAL
One Grade, Four Standard Systems
AISI 4140 is the world’s most widely specified chromium-molybdenum alloy steel, but the same steel travels under different names in different markets: EN 42CrMo4 (1.7225) in Europe, JIS SCM440 in Japan, BS 708M40 in Britain, GOST 40KhM in Russia, and 42CrMo under the Chinese standard GB/T 3077. In demanding service, the steel is usually delivered in quenched and tempered (QT) condition and then given a surface treatment, because the hardened core alone cannot win the wear and corrosion battle. This article summarizes a 2021 master’s study from Anhui University of Technology comparing two surface routes, plasma nitriding and plasma oxynitriding, and cross-checks the findings against published international work on the same grade.
| Standard system | Designation |
|---|---|
| AISI / SAE, USA | 4140 (UNS G41400) |
| EN 10083-3, Europe | 42CrMo4 (1.7225) |
| DIN, Germany | 42CrMo4 / 1.7225 |
| JIS G4053, Japan | SCM440 |
| BS, United Kingdom | 708M40 |
| GOST, Russia | 40KhM |
| GB/T 3077, China | 42CrMo |
Why Surface Engineering Comes After the Heat Treatment
Quenching and tempering give 4140 its strength and toughness, but the as-QT surface hardness of about 360 HV0.1 is not enough for components that must also slide, seal and resist chloride attack. Plasma nitriding (glow discharge plasma nitriding) solves this by ionizing a nitrogen-hydrogen gas mixture in a glow discharge, driving energetic nitrogen into the surface to build a compound layer, the so-called white layer, over a deeper diffusion zone. International literature is unanimous on the architecture: the compound layer consists of ε-Fe2-3N (epsilon iron nitride) and γ′-Fe4N (gamma-prime iron nitride), and it is the epsilon phase that carries the hardness and corrosion resistance.
Two Routes, Two Layer Architectures
The study compares two routes on the same grade. Plain nitriding builds a nitrogen-rich layer; oxynitriding adds plain air as the oxygen source in the same furnace, a one-step process that deposits an oxide layer on top of the nitride layer in a single cycle. The phase difference decides the performance difference. Notably, oxynitriding produced a 3-4 μm oxide layer with nano-scale Fe2-3N particles uniformly distributed inside it, a composite architecture that plain nitriding cannot deliver.
| Layer | Phases identified | Key notes |
|---|---|---|
| Nitrided layer | αN, Fe2-3N, Fe4N | PN520-4 surface mainly Fe2-3N; PN560-4 becomes Fe2-3N + Fe4N |
| Oxynitrided layer | αN, Fe2-3N, Fe2O3, Fe3O4 | PON0.175 richest in Fe3O4; oxide layer 3-4 μm with nano Fe2-3N |
| Oxide layer quality | Fe3O4 vs Fe2O3 | Low oxygen gives loose oxide; too much oxygen thins the layer |
Hardness: From 360 to Beyond 1000 HV
Both routes push the surface hardness from about 360 HV0.1 to roughly 1000 HV0.1, close to a threefold increase. The trends are what matter for process control. Raising the nitriding temperature from 400 to 520°C increases surface hardness and flattens the hardness gradient, but at 560°C the hardness drops, because the surface nitrogen concentration falls and the phase shifts from Fe2-3N toward an Fe2-3N plus Fe4N mixture. The nitrogen-to-hydrogen ratio of 1:1 gave the best hardness distribution, and longer time thickens the layer and softens the gradient. Oxynitrided samples match the nitrided ones at the surface, around 1000 HV0.1, but run slightly softer in the subsurface, simply because the nitrided route carries a higher proportion of hard Fe2-3N there.
Harder Is Not Always More Wear-Resistant
Here the study delivers its sharpest point. Plasma nitriding made the surface three times harder, yet the friction coefficient rose and fluctuated strongly, and the dominant mechanism stayed adhesive wear with mild oxidation. Nitriding did not cure adhesion; it merely cut the wear rate by an order of magnitude, with the best nitrided sample, PN520-4, reducing wear by 96.3 percent versus the untreated steel. Oxynitriding behaved differently: the friction coefficient was lower and stable, because the dense oxide layer interrupts metal-to-metal adhesion, and the wear mechanism shifted to abrasive and oxidative wear. The best oxynitrided sample, PON0.175, beat even the best nitrided sample by another 47.3 percent. Hardness wins the wear battle by armor; Fe3O4 wins it by lubrication.
Corrosion: The Magnetite Key
Polarization tests in 3.5 percent NaCl solution quantified the corrosion gains. Plain nitriding improved corrosion resistance, and the best nitrided condition, PN520-4, showed a clear passivation region. The reason is phase-specific: Fe2-3N, with its crystal structure and higher nitrogen content, resists attack better than Fe4N, so the fraction of epsilon phase on the surface is the controlling variable. Oxynitriding went further. The dense Fe3O4 (magnetite) layer, with its inverse spinel structure and high electrochemical stability, widened the passivation window dramatically: the best oxynitrided sample, PON0.175, reached a ΔE of 770.786 mV, more than double the 314.016 mV of the best nitrided sample. This mirrors international post-oxidation literature on 4140, which states plainly that only magnetite, never Fe2O3 (hematite), improves wear and corrosion, and that oxidation control is everything.
| Specimen | Ecorr (mV) | Icorr (A/cm²) | ΔE (mV) |
|---|---|---|---|
| Untreated | -641.527 | 2.051×10-6 | No passivation |
| PN520-4 (nitrided, 1:1) | -201.268 | 7.540×10-7 | 314.016 |
| PON0.175 (oxynitrided) | -323.640 | 5.895×10-7 | 770.786 |
| PON0.25 (oxygen overload) | -399.666 | 1.409×10-6 | 190.448 |
The Process Windows That Matter
The data collapse into a small set of windows. Temperature is a sweet spot, not a monotonic curve: 520°C outperforms 560°C on hardness, wear and corrosion alike, because higher temperature lowers surface nitrogen and creates the less protective Fe4N mixture. The gas ratio behaves the same way: N2:H2 of 1:1 beats 1:3 and 3:1. Time helps steadily: 8 h beats 4 h on corrosion, with PN520-8 reaching a ΔE of 533.717 mV. For oxynitriding, the air flow is the sharpest knife, and the optimum N2:H2:air ratio is 0.3:1:0.875; too little air gives a loose oxide, too much thins the layer and collapses the passivation window.
| Route | Optimum | Surface hardness | Wear vs untreated | ΔE |
|---|---|---|---|---|
| Nitriding | 520°C, N2:H2 = 1:1, 4-8 h | ~1000 HV0.1 | -96.3% | 314 mV (533.7 mV at 8 h) |
| Oxynitriding | 520°C, N2:H2:air = 0.3:1:0.875 | ~1000 HV0.1 | -96.3% then -47.3% vs nitrided | 770.786 mV |
What Buyers Should Take Away
Five conclusions are worth keeping. First, if the component must slide or seal in a corrosive environment, oxynitriding with a dense Fe3O4 surface is the stronger choice; it beats nitriding on friction, wear and corrosion at nearly the same hardness. Second, the epsilon phase is the load-bearing phase of the whole system, so a supplier who cannot state the Fe2-3N fraction of the compound layer is flying blind. Third, temperature is a sweet spot, not a race: 520°C beats 560°C on every measured property. Fourth, surface treatment upgrades the surface, but the QT core is still the foundation; order the steel, its heat treatment and the nitriding as one specification, not three separate purchases. Fifth, demand evidence: a hardness gradient curve and a polarization ΔE value tell more about process control than any certificate word.
