Nitriding of Tool Steels
Nitriding puts a hard, wear resistant case on a finished tool without quenching it, so the part keeps its size and its core toughness. This page collects what the treatment produces on tool steels, the depth and hardness it is normally held to, the layer structure behind the surface, and the places in die and mold work where it earns its cost.
| Item | Value | What it means in practice |
|---|---|---|
| Typical values for tool steels | ||
| Nitriding depth | 0.1–0.2 mm | The hardened case that carries the wear. Depths of 0.5–1.0 mm belong to carburizing and are a common mistake when targets are copied from that process. |
| Surface hardness | 1000–1200 HV0.5 | Typically reached on H-series tool steels in gas nitriding. Above 1000 HV is easy for these grades, and the hardness depends on how much chromium and aluminium the steel carries. |
| Process temperature | 525–545°C | Below the tempering range of most tool steels, so the part is nitride after the final hardening and tempering rather than before it. |
| Process time | 4–10 h | The usual range in practice. Longer treatments deepen the case and thicken the compound layer rather than raising the surface hardness. |
| Core hardness | Unchanged | The interior keeps the hardness and toughness of the tempered part, which is why the treatment suits finished tooling. |
| Hardening mechanism | Alloy nitrides | Nitrogen diffuses in and precipitates fine alloy nitrides, chromium being the main former in tool steels, aluminium the strongest one where a grade is built with it. |
Source: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press), Section 4.2.3 and the measured response curves in Figure 4.11.
| Layer formed | Atmosphere | Structure | Where it is used |
|---|---|---|---|
| Diffusion layer only | 1–5% N2 | None. The hardened zone is a gradient of precipitated nitrides in the steel. | Crack sensitive tooling such as die casting dies, where a brittle skin is not acceptable. |
| Compound layer, γ′ type | 15–30% N2 | Fe4N on top of the diffusion layer | Wear surfaces that can carry a hard skin. The layer is harder than the matrix but not as brittle as the ε type. |
| Compound layer, ε type | 60–70% N2 with 1–3% methane | Fe2-3N on top of the diffusion layer | Maximum surface hardness, with the highest brittleness. Usually avoided on parts that see cracking or impact. |
Source: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press), Figure 4.12, adapted there from the ASM Handbook on heat treating. The compound layer is called the white layer in the shop, because it stays bright under the microscope and is not attacked by the usual etch.
Why the white layer gets a bad name
The compound layer is a ceramic skin sitting on a steel part, and it is very hard and very brittle at the same time. Used well, it is the highest hardness the treatment can produce. Used badly, it is the origin of the cracks that end the life of a die. The source shows both outcomes on H13, first a diffusion layer alone, then a case where a thick white layer and carbonitride precipitates along the grain boundaries turned into crack paths, with the cracks travelling through the compound line rather than around it.
Two shop rules follow. On parts that see impact or thermal cycling, keep the layer thin or suppress it, and for a nitrided forging die the source puts the practical limit at 0.25 mm of layer, below which cracking stays rare. On parts that only slide, a controlled compound layer adds the most wear resistance for the least depth of case.
Source: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press), Figure 4.11, gas nitriding response curves for H11 modified (X38CrMoV5-1), H13 (X40CrMoV5-1) and a modified H10 (X50CrMoV5-3). Values read from the published curves at the surface and at depth.
Reading the curves
The case depth in the hardness profile and the nominal nitriding depth quoted in literature are measured differently. The profile above describes a long cycle at 36 hours, and the useful hardened depth in it runs beyond the 0.1 to 0.2 mm that the source quotes as the typical tool steel target. Whichever number you quote, measure the case on the part or on a coupon that saw the same cycle, because furnace load, gas flow and surface condition all shift the result.
Gas, plasma and salt
Gas nitriding feeds nitrogen from a dissociated ammonia atmosphere and is the process the published data for tool steels mostly comes from. Plasma nitriding strikes the nitrogen in an ionised gas instead, which gives finer control over the layer and lets the shop shield areas that should stay soft. Liquid salt bath nitriding is fading for environmental reasons. The layer structure behind the three is the same, so the choice is about equipment, control and cost rather than about a different result.
Nitride after hardening and tempering, never before
The treatment runs at a lower temperature than the tempering step, so it cannot be used to harden the steel itself. It is applied to a part that is already at its final hardness, which is also why the part keeps its size through the process. A part that goes to the nitriding furnace with a soft core comes back with a hard skin on a soft core, and the first loaded contact finds out.
Nitriding is not carburizing
Carburizing runs near 900 to 1000°C, diffuses carbon for a fast and deep case, and depends on a quench afterwards to harden the surface by martensite. Its ceiling is the hardness of martensite, around 900 HV, and the case can soften again in later tempering. Nitriding runs near 540°C on the finished part, hardens by nitride precipitation rather than by a quench, and passes 1000 HV without a second heat treatment. Case depth targets do not transfer between the two processes.
Aluminium in the grade answers the nitriding furnace
The hardness of a nitrided case comes from whichever alloying elements form the nitrides, and aluminium forms the strongest ones. Grades that carry aluminium as a deliberate addition, such as some extrusion and precipitation hardening mold steels, reach a much harder case than a plain 5% Cr hot work steel under the same cycle. When a drawing specifies a nitrided hardness above the usual range for H13 or H11, the grade itself is usually the lever, not the furnace.
Where it pays for itself
Hot forging is where nitriding is applied most, because the die surface sees load and abrasion above 500°C and the surface hardness is the deciding factor. Hot extrusion follows, and the aluminium treated grades above were developed for it, along with the ESR H13 used in extrusion work. Die casting is the careful case, because the thermal cycling cracks a brittle layer quickly, so the compound layer is kept thin. In plastic molds the treatment is the normal way to protect a hardened plastic mold steel cavity against wear and to hold the polished finish, and it is one of the reasons an ESR refined H13 above 50 HRC is popular for lenses and other work that leaves the mold bright. The polishing and texturing side of that work, including the defects a nitrided surface changes, is covered in the mold steel polishing and texturing guide.
The heat treatment sequence around the furnace is in the hardening and tempering chart, and how a nitrided case behaves at temperature alongside the base hardness of the grade is in the hot hardness chart. The other surface treatments that compete with it, including PVD coating and hard chrome, are compared in the surface treatment guide.
Confirm before quoting
Case depth, hardness and layer structure are for general reference only. Actual results vary by grade, furnace, gas mixture and part geometry. Confirm the process with your heat treater and the grade data with the mill before production. Aobo Steel supplies tool steel in the annealed or prehardened condition, and nitriding remains a service performed on the finished tool.
Related reference pages
Tool steel heat treatment guide · Tool steel processing characteristics · Selection of tool steel for hot extrusion dies · Selection of tool steel for die casting cores and inserts · Hot work tool steels · Why H13 is used in die casting
Sources: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press, 2017), Section 4.2.3 with Figures 4.11 to 4.13, Section 4.3.2 and Section 6.4.3. Reference data for comparison only. Confirm the cycle and the case specification with your heat treater before production.