Case-hardening alloy steel · Open-die forgings

17NiCrMo6-4 Forgings (W.Nr. 1.6566, 18NiCrMo5, 18NCD6, 815M17)

Published 18 May 2016 · Last updated 8 September 2026 · Reviewed by the Jiangyin Jiangnan Metal metallurgical engineering team

Short answer: what is 17NiCrMo6-4?

Material number

1.6566EN 10084:2008

Steel type

Case hardeningNi-Cr-Mo carburising grade

Carbon

0.14–0.20%Cr 0.80–1.10, Ni 1.20–1.50

Case hardness

58–62 HRCafter carburise + temper

Core tensile

1000–1300 MPasection dependent

Supply hardness

≤ 229 HBsoft annealed, +A

Carburise at

880–980 °Cthen quench 780–870 °C

Forge at

1100–900 °Creduction 4:1 minimum

What 17NiCrMo6-4 forged products can you buy?

Jiangyin Jiangnan Metal produces 17NiCrMo6-4 by three routes, chosen by geometry and order quantity. Open-die forging covers shafts, blocks, discs and near-net blanks. Seamless ring rolling produces gear-ring and bearing-race blanks, which is the dominant route for anything annular, because a rolled ring has circumferential grain flow that a machined-from-solid disc cannot reproduce. Upset forging handles short, large-section hubs, flanges and pinion heads.

Grain flow matters more on this grade than on a through-hardening steel. A carburised gear tooth fails by pitting or by root bending fatigue, and both are surface phenomena governed by how the fibre runs under the tooth root. Ring rolling puts that fibre where the load is. It is also the cheaper route: on a typical ring gear blank it removes half the purchased weight compared with machining from a solid disc.

What is 17NiCrMo6-4, and why does it exist?

A gear tooth is asked to do two incompatible things. The flank has to be hard enough to resist contact pressure and pitting, which means high carbon and high hardness. The root has to be tough enough to survive shock and bending fatigue, which means low carbon and low hardness. No single uniform hardness satisfies both, and a through-hardened tooth that is hard enough at the flank will snap at the root.

Case-hardening steels resolve that by putting the two properties in different places. 17NiCrMo6-4 is melted with only 0.14–0.20% carbon, so the whole part is soft and tough as delivered. It is machined in that condition, then carburised: held at 880–980 °C in a carbon-rich atmosphere so that carbon diffuses into the surface layer and raises it to roughly 0.8% carbon over a controlled depth. Quenching then transforms that high-carbon skin into hard martensite while the low-carbon core stays comparatively soft and ductile.

The three alloying elements each do a specific job:

• Nickel at 1.20–1.50% supplies core toughness. Nickel is the element that keeps the core ductile at high strength and holds impact values up at low temperature. It is why this grade is preferred over plain chromium-manganese carburising steels for shock-loaded drivetrains.
• Chromium at 0.80–1.10% carries hardenability and case hardness. Chromium raises the depth to which the quench takes effect and forms carbides in the carburised layer that resist abrasion.
• Molybdenum at 0.15–0.25% suppresses temper embrittlement and adds hardenability. It also reduces the grain-boundary weakness that Ni-Cr steels are otherwise prone to after slow cooling through the 450–550 °C range.

The result sits in the middle of the case-hardening family: more hardenable and tougher than 16MnCr5 or 20MnCr5, less hardenable and considerably cheaper than 18CrNiMo7-6. For ruling sections up to roughly 60–80 mm it usually does everything the heavier grade would do, at lower cost and with less distortion.

Which standard applies: EN 10084, not EN 10083

Two further points on standards. The phosphorus limit in EN 10084:2008 is 0.025% maximum, not 0.035%; several published tables, again including our own earlier one, carried the higher figure. And the grade also appears in EN ISO 683-3, the international standard for case-hardening steels, which is the right citation where an ISO callout is required instead of a European one.

What are the equivalents of 17NiCrMo6-4?

Buyers meet this one steel under half a dozen national names, because each European standards body issued its own designation before EN harmonisation. All of the designations in Table 1 describe substantially the same Ni-Cr-Mo carburising chemistry, and we accept purchase orders under any of them, issuing a certificate that lists every specification the heat satisfies.

What is the chemical composition of 17NiCrMo6-4?

The composition below is the EN 10084:2008 requirement, and it is what we buy raw material against unless a drawing calls for a tighter band. Melting is by EAF + LF + VD: electric arc furnace, ladle furnace refining and vacuum degassing. The vacuum degassing step is not optional on a gear steel. It removes hydrogen, which causes flaking in heavy Ni-Cr-Mo sections, and it lowers oxygen, which controls the oxide inclusions that start subsurface fatigue under a carburised case.

What are the mechanical properties of 17NiCrMo6-4?

A case-hardened part has no single set of mechanical properties, and asking for “the tensile strength of 17NiCrMo6-4” without saying where in the part is the commonest source of confusion on an enquiry. There are three separate answers, and a purchase order should state which one it is specifying.

Case depth: CHD, and how to specify it

Case depth is the number that most often goes wrong on a drawing. The convention in EN ISO 2639 is CHD, case hardening depth, defined as the perpendicular distance from the surface to the point where hardness falls to 550 HV1. Writing “case depth 1.5 mm” without naming the limit hardness leaves it open to interpretation, and a supplier measuring to 513 HV or to a visual etch boundary will hand you a different part.

What are the physical properties of 17NiCrMo6-4?

Hardenability: what the +H suffix means

Hardenability is not hardness. It is the depth to which a quench takes effect, and on a case-hardening steel it governs two things at once: how hard the case gets, and how strong the core is at the centre of a heavy section. EN 10084 lets you buy it as a guaranteed property using the +H suffix, which places the heat inside a specified Jominy band.

• 17NiCrMo6-4+H — supplied within the standard Jominy hardenability band. Specify this whenever gear performance depends on repeatable heat treatment across batches.
• +HH / +HL — restricted to the upper or lower half of the band. Worth the premium when distortion is tightly controlled or when several suppliers must produce interchangeable parts.
• No suffix — chemistry only. The heat may sit anywhere in the band, and two conforming batches can need different quench severities to reach the same core hardness.

For a gear programme running across more than one heat, +H is the cheapest insurance on the order. It costs far less than re-qualifying a heat-treatment cycle for every delivery. The Jominy test itself is run to EN ISO 642: a standard bar austenitised, water-quenched from one end, and hardness measured at fixed distances along it.

How is 17NiCrMo6-4 heat treated?

Every temperature in Table 7 is a working range, not a set point. The figure you use inside that range depends on section size, furnace loading and whether the part is direct quenched from the carburising temperature or cooled and reheated.

Direct quench, single quench or double quench?

Three routes are in common use, and the choice is a trade between cost, distortion and core properties.

• Direct quench from the carburising temperature, after equalising down to about 840 °C. Cheapest and fastest, with the least oxidation. Grain is coarser and retained austenite higher, so it suits parts that are ground afterwards and are not fatigue-critical.
• Single quench and temper: cool from carburising, reheat to 780–820 °C, quench, temper at 150–200 °C. The normal industrial route, and the best general balance of case hardness, grain size and distortion.
• Double quench: harden first from 830–870 °C for core properties, then again from 780–820 °C for the case. Highest core strength and finest grain, but two quenches mean more distortion and more cost. Reserve it for heavily shock-loaded parts.

How is 17NiCrMo6-4 forged?

The forging window is wide and the steel is not difficult to work hot. The two things that catch people out are hydrogen and grain flow.

1. Raw materialEAF + LF + VD melted billet to EN 10084, fine-grain treated with 0.020–0.050% aluminium. Heat number traced, chemistry verified by optical emission spectrometry before cutting.
2. HeatSoak to about 1150 °C, held long enough to bring the whole section to temperature rather than just the skin.
3. ForgeWork between 1100 °C and 900 °C. Stop deformation above 900 °C and reheat rather than finishing cold. Total reduction ratio of at least 4:1 to close porosity and orient the fibre.
4. Ring roll, where the part is annularRadial-axial rolling to give circumferential grain flow under the tooth root or race, which is the whole reason to buy a rolled ring instead of a machined disc.
5. Controlled cooling and dehydrogenationCool slowly in furnace or sand. On heavy sections, dehydrogenation annealing goes in immediately after forging, before the piece is allowed to reach room temperature.
6. Normalise or soft annealNormalise at 830–880 °C to refine the forged grain, or soft anneal at 650–700 °C to ≤ 229 HB, depending on what the machinist needs.
7. Rough machineTo near-net form, leaving carburising and grinding stock where the drawing calls for it.
8. Test, examine and certifyTensile, impact and hardness, grain size to ISO 643, ultrasonic examination to EN 10228-3 or SEP 1921, then EN 10204 3.1 or 3.2.

17NiCrMo6-4 compared with 16MnCr5, 20MnCr5, 20NiCrMo2-2 and 18CrNiMo7-6

The case-hardening steels form a ladder of increasing hardenability and cost. 17NiCrMo6-4 sits in the upper middle: it is chosen over the manganese-chromium grades when the section is too heavy for them to harden through, and over 18CrNiMo7-6 when that grade's hardenability is not needed and its price and distortion are not wanted. Selecting by ruling section is the reliable method; selecting by grade name is how parts end up over-specified.

Three questions that settle the choice

1. What is the ruling section at the tooth root or the race? Not the overall part size. If it is under 40 mm, a manganese-chromium grade will probably harden through and costs less. Above 80 mm, look at 18CrNiMo7-6.
2. Is the load shock or steady? Nickel buys core toughness, and core toughness is what stops a root crack propagating under impact. Mining, marine and off-highway drives justify it; a steady-load industrial reducer often does not.
3. How tight is the distortion budget? Higher hardenability means a more severe transformation gradient and more movement in the quench. If the tooth flanks are ground afterwards, distortion matters less and the higher grade is affordable. If they are not, the lower grade may give a better finished part.

Machining, grinding and distortion control

Machining

17NiCrMo6-4 machines like a medium-alloy steel of similar hardness, which is to say without drama, provided it arrives in the right condition. Soft annealed at 229 HB maximum is the usual delivery state and is what most shops want. The one trap is that a fully spheroidised structure can be too soft: it smears rather than cutting cleanly and gives poor surface finish and long stringy chips. Where finish matters, order +FP, treated to a ferrite–pearlite structure and hardness range, rather than a plain soft anneal.

• Carbide tooling, moderate speeds, generous feed. Do not let the tool rub.
• Where machinability governs the cost, specify 17NiCrMoS6-4 (1.6569). The 0.020–0.040% sulphur addition forms manganese sulphides that break the chip, at some cost in transverse toughness. Do not use the resulphurised variant on parts loaded transversely or in fatigue-critical directions.
• Leave grinding stock on any surface where case depth and tolerance both matter. The hardest metal is at the surface, and grinding removes the best of the case.

Distortion, and how to design against it

Distortion is the dominant cost driver on carburised parts, and most of it is designed in rather than caused in the furnace. The part changes shape for three reasons that add together: stress released from machining, thermal gradient during the quench, and the volume increase when austenite becomes martensite, which is roughly 4%.

The practical route is to accept a distortion allowance and grind after hardening on any surface with a tight tolerance, rather than trying to hold the tolerance through the quench. Budget the grinding stock at the design stage, and make sure it is not so generous that grinding cuts through the effective case.

How do 17NiCrMo6-4 parts fail, and how do you prevent it?

What can Jiangyin Jiangnan Metal forge in 17NiCrMo6-4?

17NiCrMo6-4 is a made-to-order grade for us: we buy the heat against your specification rather than pulling from stock, which is why the drawing, the delivery condition and the required certificate all matter at enquiry stage rather than after the order.

Rolled ring OD

200–2,500 mmplant envelope

Ring wall, min

30 mmradial-axial rolled

Disc diameter

≤ 1,800 mmdiscs, hubs, carriers

Shaft length

≤ 8,000 mmpinion and gearbox shafts

Bar diameter

25–500 mmforged round bar

Single piece

≤ 8,000 kgplant envelope

Condition

+A / +N / +FPor case hardened to drawing

Lead time

8–14 weekstypical, heat dependent

Jiangyin Jiangnan Metal Co., Ltd. employs approximately 460 people, including 9 senior engineers and 32 intermediate engineers. Send the drawing and we will confirm the size, weight, delivery condition and lead time for your specific part before quoting.

Which standards and certificates apply to 17NiCrMo6-4 forgings?

Quality management is certified to ISO 9001:2015. Third-party witness certificates are issued through the inspection body you nominate: Lloyd's Register, DNV, Bureau Veritas, ABS, TÜV or SGS. Customers keep an unrestricted right to witness any production stage, including chemistry, forging, heat treatment and mechanical testing.

How do you specify a 17NiCrMo6-4 forging order?

1. Name the grade and the standard correctly. Write 17NiCrMo6-4 / W.Nr. 1.6566 to EN 10084. Do not write EN 10083; that standard does not cover this steel.
2. State the delivery condition. +A soft annealed to 229 HB max, +N normalised, or +FP for a controlled ferrite–pearlite structure. If we are also case hardening, say so explicitly, because it changes the whole route.
3. Add +H if repeatability matters. A guaranteed Jominy band is the cheapest insurance available on a multi-batch gear programme.
4. Require the fine-grain treatment. Al 0.020–0.050% reported on the certificate, and grain size 5 or finer to ISO 643.
5. Specify case depth properly. CHD to EN ISO 2639 at 550 HV1, stated after grinding, and say where on the part it is measured. On a gear, that means at the root fillet, not at mid-flank.
6. Give surface and core hardness separately. For example surface 58–62 HRC, core 30–40 HRC at the tooth root, measured at a stated depth.
7. Define NDE and the acceptance class. “UT per EN 10228-3, quality class 3” is a specification. “Ultrasonic test” on its own is not.
8. State test direction and position. Above 50 mm section, say whether tensile tests are longitudinal or transverse, and where the test material is taken from the forging.
9. Give quantity, date, Incoterm and destination. Quantity drives the melt: small orders are consolidated onto a larger heat, which affects both price and schedule.

Drawing callout you can copy

MATERIAL:      17NiCrMo6-4 / W.Nr. 1.6566 to EN 10084
               (equivalents accepted: 18NiCrMo5, 18NCD6, 815M17, SS 2523)
MELT ROUTE:    EAF + LF + VD, vacuum degassed
CONDITION:     +A soft annealed, 229 HB max, for machining
               (state +N or +FP if required instead)
HARDENABILITY: 17NiCrMo6-4+H, Jominy band per EN 10084, curve on certificate
GRAIN:         Al 0.020-0.050% reported. Grain size 5 or finer per ISO 643
CASE HARDEN:   Carburise 900-930 deg C, single quench 780-820 deg C in oil,
               temper 160-180 deg C. Sub-zero -70 deg C + re-temper if stated
CASE DEPTH:    CHD ______ mm at 550 HV1 per EN ISO 2639, measured AFTER
               grinding, at the tooth root fillet
HARDNESS:      Surface 58-62 HRC. Core 30-40 HRC at root, at ______ mm depth
RET. AUSTENITE: 20% max at surface by XRD (10% max for precision gearing)
FORGING:       Reduction ratio 4:1 minimum. Circumferential grain flow on
               rolled rings. Dehydrogenation anneal after forging
NDE:           UT per EN 10228-3 quality class 3
               MT per EN 10228-1 on machined surfaces
CERTIFICATE:   EN 10204 3.1 (3.2 with third-party witness if stated on the PO)
MARKING:       Heat number, grade, standard, condition and drawing number,
               low-stress stamped or vibro-etched

Seven mistakes buyers make with 17NiCrMo6-4

1. Citing EN 10083 instead of EN 10084. The commonest error on this grade, copied from datasheets that have carried it for years. EN 10083 is for quenched and tempered steels and does not apply.
2. Treating SAE 4317 as a drop-in equal. It carries more nickel and less chromium. It is a candidate substitute, not an equivalent, and the heat-treatment cycle has to be re-qualified.
3. Specifying case depth without a limit hardness or a measuring position. CHD at 550 HV1 to EN ISO 2639, measured at the root fillet after grinding. Anything less precise is not enforceable.
4. Forgetting that grinding removes case. The depth on the drawing must be the depth on the finished part. If 0.3 mm comes off in grinding, the carburising cycle has to put it there in the first place.
5. Accepting a certificate with no aluminium and no grain size. On a carburising steel, fine-grain treatment is a functional requirement, not a formality.
6. Over-specifying hardenability. Buying 18CrNiMo7-6 for a 40 mm section adds cost and distortion for no gain. Select by ruling section.
7. Ordering a machined-from-solid disc when a rolled ring would do. On an annular part the ring route gives better grain flow and typically halves the purchased weight. It is usually the largest single saving available on the order.

Where is 17NiCrMo6-4 used?

Two worked examples

Example 1: choosing between 17NiCrMo6-4 and 18CrNiMo7-6 for a ring gear

Given. An internal ring gear, 1,150 mm outside diameter, module 12, rim thickness below the root of 55 mm, in an industrial mill drive. Steady load with occasional shock on start-up. Tooth flanks will be ground after hardening.

Assessment. The ruling section that governs is the rim below the root, 55 mm, not the 1,150 mm outside diameter. That sits comfortably inside the 80 mm working limit for 17NiCrMo6-4, so the higher-alloy grade would not add usable core strength. Module 12 implies a CHD of roughly 1.8–2.4 mm, which is a long carburising cycle either way, so grain-size control and the +H hardenability band matter more than the choice of grade. Because the flanks are ground, a moderate distortion allowance is acceptable, which removes the main argument for the lower-hardenability alternative as well.

Decision. Specify 17NiCrMo6-4+H to EN 10084, rolled ring with circumferential grain flow, CHD 2.0 mm at 550 HV1 measured at the root fillet after grinding, surface 58–62 HRC, core 32 HRC minimum at the root, grain size 5 or finer, UT to EN 10228-3 class 3. If the rim thickness later grows past about 80 mm in a design revision, revisit 18CrNiMo7-6 at that point.

Example 2: why a rolled ring beats a machined disc

Given. A finished ring gear blank, 620 mm OD × 480 mm ID × 210 mm high, in 17NiCrMo6-4.

Method. Net volume = π/4 × (0.620² − 0.480²) × 0.210 = 0.02539 m³. At 7,850 kg/m³ that is about 199 kg finished. Rolled as a ring with 25% stock, the input forging is roughly 249 kg. Machined from a solid forged disc of the same outside diameter and height, the input billet is 620 mm × 210 mm solid, about 498 kg. You would buy, forge, heat and then cut away some 299 kg of alloy steel.

Result. Ring rolling halves the purchased weight on this geometry, and it removes roughly 250 kg of machining time as well. The metallurgical argument points the same way: the rolled ring has continuous circumferential fibre under the tooth root, while the machined disc has fibre cut through at every tooth. On annular parts in this grade, the forging route is usually worth more than anything else you can negotiate on the order.

Glossary

17NiCrMo6-4 frequently asked questions

References

1. EN 10084:2008, Case hardening steels — Technical delivery conditions. CEN. The governing standard for 17NiCrMo6-4 / 1.6566, including chemical composition, permissible deviations, delivery conditions and Jominy hardenability bands.
2. EN ISO 683-3, Heat-treatable steels, alloy steels and free-cutting steels — Part 3: Case-hardening steels. The international equivalent standard.
3. EN ISO 2639, Steels — Determination and verification of the depth of carburized and hardened cases. Defines CHD at 550 HV1.
4. EN ISO 642, Steel — Hardenability test by end quenching (Jominy test).
5. ISO 643, Steels — Micrographic determination of the apparent grain size.
6. Saarstahl, material specification sheet for 1.6566 / 1.6569 / 1.6571, 17NiCrMo6-4, 17NiCrMoS6-4 and 20NiCrMoS6-4. Composition and hot-forming and heat-treatment temperature ranges.
7. Lucefin Group, technical card 17NiCrMo6-4 to EN 10084:2008. Composition, permissible product deviations and heat-treatment temperatures.
8. Rodacciai, datasheet RC2 / RC2PB, 17NiCrMo6-4 Nr. 1.6566 and 17NiCrMoS6-4 Nr. 1.6569. Equivalent designations and Jominy hardness limits.
9. Harris, F. E., Case depth — an attempt at a practical definition, Metal Progress, 1943. The diffusion relationship used by the case-depth calculator on this page.
10. ASM Handbook, Volume 4, Heat Treating, ASM International. Carburising practice, retained austenite and distortion control.
11. ASM Handbook, Volume 14A, Metalworking: Bulk Forming, ASM International. Open-die forging and ring rolling of alloy steels.
12. ISO 6336, Calculation of load capacity of spur and helical gears, and ANSI/AGMA 2001, for gear rating and material quality grades.
13. EN 10228-3 and SEP 1921, ultrasonic examination of steel forgings; EN 10204, types of inspection documents; EN ISO 6892-1, tensile testing.

Standards are cited by number; always work to the revision in force at your contract date. Property values on this page are published typical figures for screening and enquiry purposes and are not design allowables. Test results on our certificates are independent and traceable to calibrated equipment.

About the manufacturer, and how to cite this page

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, with approximately 460 employees including 9 senior and 32 intermediate engineers. The plant runs 1 t to 9 t open-die hammers, 4,500 t and 5,000 t hydraulic presses and radial-axial ring mills up to 2,500 mm outside diameter, with in-house heat treatment, machining, mechanical testing and non-destructive examination. Alongside 17NiCrMo6-4 / 1.6566 we forge the carbon, alloy and tool steels, the precipitation-hardening and duplex stainless families, and the nickel and cobalt high-temperature alloys. Quality management is certified to ISO 9001:2015; material is supplied with EN 10204 3.1 certification as standard and 3.2 with third-party witness on request.

Contact for technical questions or a quotation: Jiangyin Jiangnan Metal Co., Ltd., No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China · 0086-189-2135-9659 · sales@steelforgepieces.com · WhatsApp

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