18NCD6 forgings

Metallurgy

What 18NCD6 steel is and why it is specified

18NCD6 is a low-carbon alloy steel designed to be carburised. The bulk carbon content of 0.15 to 0.21 % is deliberately low so that the core stays tough; carbon is then diffused into the surface during carburising and locked in as hard martensite by quenching. A finished part therefore carries a hard, wear-resistant case in compression over a ductile core that absorbs impact.

18NCD6 differs from cheaper carburising steels such as 16MnCr5 and 20MnCr5 mainly in hardenability. With 1.50 to 1.80 % chromium, 1.40 to 1.70 % nickel and 0.25 to 0.35 % molybdenum, the core still transforms to a strong low-carbon martensite in sections far heavier than manganese-chromium grades can handle. The grade therefore turns up on drawings for wind turbine ring gears, mining mill pinions and marine reduction gearing, where a tooth can be 40 mm thick and the blank several tonnes.

Nickel also keeps core toughness high at low temperature and lowers the risk of brittle failure at the case-core transition, where fatigue cracks in a carburised gear normally start.

18NCD6 is not the same steel as 18NCD5

The two AFNOR designations are one digit apart and are regularly confused on purchase orders. 18NCD5 is the leaner grade equivalent to 18NiCrMo5 and BS 822M17, with roughly half the chromium of 18NCD6 and lower hardenability. Ordering 18NCD5 for a section sized on 18NCD6 hardenability data will leave a soft core. If the drawing shows either designation, state it exactly as written and we will confirm the grade before cutting billet.

Chemistry

Chemical composition of 18NCD6

The older French specification NF A 35-551 quotes silicon as 0.15 to 0.40 %. A typical Saarstahl heat analysis runs C 0.17, Si 0.25, Mn 0.50, Cr 1.65, Ni 1.55, Mo 0.30. Tighter internal limits such as controlled sulphur for machinability, restricted phosphorus or a narrowed hardenability band to 18CrNiMo7-6H, can be agreed at order and recorded on the mill certificate.

Carbon equivalent

Calculated from the specification limits with the IIW formula, CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15, 18NCD6 gives a carbon equivalent of approximately 0.68 to 0.90. Any value above 0.45 means preheating is required for welding. The procedure is set out in the welding section below.

Cross reference

18NCD6 equivalent grades and designations

Table compiled by Jiangyin Jiangnan Metal Co., Ltd. from EN 10084, the Saarstahl material specification sheet for 1.6587 and published national cross-reference tables. Equivalence tables indicate the closest known grades only; the governing document is always your purchase order specification.

Mechanical data

Mechanical properties of 18NCD6 forgings

Case-hardening steels are specified by their core properties, because the case is defined separately by hardness and case depth. The values below are the EN 10084 minimums for the core of a test piece that has been carburised, hardened and tempered.

These are specification minimums and ranges, not the results of any particular heat. Actual values are reported on the EN 10204 certificate issued with the goods. On a heavy forging the test location materially affects the result, so state on the order whether test pieces are to be taken from a prolongation, at mid-radius, or at a defined depth below the surface.

Hardenability

18NCD6 is supplied against a Jominy end-quench band under EN 10084. Where the design depends on core strength at a defined depth, order the H variant, 18CrNiMo7-6H, and nominate the Jominy positions and hardness limits on the purchase order so that the hardenability curve is reported on the certificate.

Physical data

Physical properties of 18NCD6

Physical constants are typical values for low-alloy Cr-Ni-Mo case-hardening steel and are given for design guidance. They are not certified values and are not measured on individual heats.

Process route

Forging and heat treatment of 18NCD6

18NCD6 billet is melted in an electric arc furnace, refined in the ladle and vacuum degassed. Degassing keeps hydrogen low enough to avoid flaking in heavy sections and cuts the oxide inclusions that fail ultrasonic inspection and start fatigue cracks under a gear tooth. ESR remelting is available where a customer specification calls for it.

Cycle data follows the Saarstahl material specification sheet for 1.6587. Water quenching is standard practice on bar; on heavy open-die forgings with changing section we normally recommend oil or polymer to control distortion and the risk of quench cracking, and we will say so on the quotation.

How 18NCD6 forgings are normally supplied

Unless a drawing says otherwise, we forge, normalise and soft anneal 18NCD6 and ship it at 229 HB maximum, so that the customer or the customer's gear cutter can rough machine, cut teeth and then carburise. Carburising a finished forging before machining traps the hard case where it has to be cut away, which is expensive and can crack the part. Where the buyer prefers a single supply chain we can carry out carburising, hardening and tempering to a nominated case hardening depth and deliver in the finished heat-treated condition.

Fabrication

Can 18NCD6 be welded?

18NCD6 can be welded in the annealed or normalised condition with a qualified procedure, but its carbon equivalent of roughly 0.68 to 0.90 puts it well into the range that requires preheat and controlled cooling. Welding after carburising or through hardening is not recommended, because the weld thermal cycle destroys the case and reintroduces the untempered martensite the tempering operation was there to remove.

• Preheat the weld area to 250–450 °C and maintain that temperature throughout welding.
• Use low-hydrogen consumables, correctly baked and held in a heated quiver.
• Cool at a maximum of 100 °C per hour, in ashes, vermiculite or a furnace.
• Follow with a stress-relief cycle wherever section thickness or restraint allows.
• Weld before carburising, never after.

Product forms

18NCD6 forged shapes we produce

Every item is forged to the customer's drawing. Near-net shaping matters more on a case-hardening grade than on most steels. Forged grain flow that follows the tooth or journal contour raises bending fatigue strength at the tooth root, which is where a carburised gear usually fails.

• Seamless rolled ringsGear rings, bearing races, ring-rolled blanks
• Gear blanks and gear ringsSolid or bored, with web to drawing
• Pinion and gear shaftsStraight, stepped, eccentric
• Forged discs and disksSolid and bored
• Round and step barsRough turned or black
• Forged flangesWeld neck, blind, custom bore
• Sleeves and bushingsBored and honed
• Forged blocksRectangular and profiled
• Hollow forged pipes and tubesTrepanned or expanded
• Forged rolls and spindlesMill and drive service
• Sprockets and cam followersWear-critical drive parts
• Near-net shapes to printProfiled to reduce machining loss

See also our forged tube sheets and forging sleeves, or the full product range.

Capability

18NCD6 forging size range and equipment

Send the drawing and we will confirm the achievable envelope, the forging reduction ratio, the machining allowance and the heat treatment route before quoting. Where the reduction ratio available from an economical billet size is too low for the section, we will say so rather than forge a part that will not pass ultrasonic inspection.

Applications

What 18NCD6 forgings are used for

Wind, marine and industrial gearboxes

Ring gears, planet carriers, planet and sun pinions, pinion shafts and gear couplings for wind turbine gearboxes, marine propulsion and reduction gearing, and industrial gear units. Gearboxes are the largest single market for the grade.

Mining, cement and heavy plant

Girth gear segments, mill pinions, crusher and mixer drive gears for cement mills, sugar mills, concrete mixers, earth-moving equipment and mobile cranes, where shock loading rules out a through-hardened grade.

Rolling mills, paper and pulp

Mill pinions, drive spindles, forged rolls, gear rings and heavy bushings for rolling mills, paper machines and pulp plant drives.

Rail and traction

Traction gears, pinion shafts and axle drive components for railway and locomotive systems.

Bearings, bushings and wear parts

Large-diameter bearing races and rings, heavy-duty bushings, sprockets, cam followers, wear pins, tappets, clutch dogs, extractors and wire guides.

Oil, gas and power

Compressor and turbine gearing, gearbox components on offshore platforms, fan and pump shafts, and drive components on process and power generation equipment.

Quality assurance

Testing, inspection and certification of 18NCD6 forgings

• Ultrasonic testing to EN 10228-3, SEP 1921 or ASTM A388, at the acceptance class you nominate. On gear blanks we recommend inspection after normalising and before tooth cutting.
• Chemical analysis by optical emission spectrometry on the heat, with positive material identification on the finished piece on request.
• Mechanical testing covering tensile, hardness and Charpy impact, including low-temperature impact testing where the design calls for it.
• Case verification by hardness traverse to the 550 HV limit under ISO 2639 and EN 10328, reported as case hardening depth, where we perform the carburising.
• Grain size to ASTM E112 or ISO 643, normally required within grades 5 to 8 for gear steel.
• Jominy end-quench test to EN 10084 or ASTM A255 where hardenability is specified.
• Surface NDT by magnetic particle inspection or liquid penetrant examination to ASTM E1444 or ASTM E165.
• Dimensional inspection against the approved drawing, with a report issued per piece.
• Material certificate to EN 10204 3.1, or EN 10204 3.2 witnessed by a third party such as TÜV, BV, SGS, Lloyd's Register or DNV.
• Traceability by heat number, hard stamped or vibro-etched on every piece.

In-house equipment includes a universal testing machine, impact testing machine, hardness testing machine, metallographic microscope and magnetic particle flaw detector. The quality system is certified to ISO 9001:2015.

Standards

Standards and specifications for 18NCD6

Not every specification applies to every product form. Name the governing specification on your enquiry and we will confirm compliance in writing before accepting the order.

Manufacturer

About Jiangyin Jiangnan Metal Co., Ltd.

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory in Jiangyin, Jiangsu Province, China. It produces forged rings, seamless rolled rings, gear blanks, shafts, flanges, discs, bushings, sleeves, tube sheets, blocks and bars in carbon steel, alloy steel, tool steel, stainless steel and nickel alloys, including 18NCD6, 18CrNiMo7-6, 1.6587, 17CrNiMo6, AISI 4140, 42CrMo4, 17-4PH and Alloy 625. Every part is forged to the customer's drawing and supplied with EN 10204 3.1 or 3.2 certification.

The company employs approximately 460 people, including 9 senior engineers and 32 intermediate engineers. Production equipment includes 1, 3, 5 and 9 tonne open-die forging hammers and a 5000 tonne hydraulic press, with in-house heat treatment, machining, and mechanical and non-destructive testing.

Published 18 May 2016. Last reviewed and updated 8 September 2026 by the technical sales department, Jiangyin Jiangnan Metal Co., Ltd. Property data is compiled from EN 10084, the Saarstahl material specification sheet for 1.6587 and published national standards, and is given for guidance only. The governing documents are the purchase order specification and the mill certificate supplied with the goods.