Quick Answer: Key Differences at a Glance
Molybdenum Content
1.5918: ≤0.10% Mo
1.6587: 0.25–0.35% Mo — the defining difference
Max Section (oil quench)
17CrNi6-6: ~80 mm dia.
18CrNiMo7-6: ~150 mm dia.
Core Tensile Strength
1.5918: 900–1,200 MPa
1.6587: 1,100–1,350 MPa
Design Fatigue Life
1.5918: 10&sup6;–10&sup7; cycles
1.6587: 10&sup7;–10&sup8; cycles
Material Cost Premium
18CrNiMo7-6: +8–15%
Over 17CrNi6-6 on raw billet cost
Governing Standard
EN 10084:2008
Both grades: case hardening steels
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Written by Jiangsu Liangyi Co., Limited Engineering Team
This guide draws on direct production experience forging both 17CrNi6-6 and 18CrNiMo7-6 across hundreds of gearbox applications. JNMT (Jiangsu Liangyi) has manufactured custom alloy steel forgings since 1997.
28+ years forging experience
clients in 50+ countries
50+ countries served
ISO 9001:2015 certified
Every gear tooth failure costs far more than the gear itself — downtime, warranty claims, and reputational damage follow. When your engineering team specifies a case hardening steel for a gearbox forging, the choice between 17CrNi6-6 (1.5918) and 18CrNiMo7-6 (1.6587) is rarely obvious from a datasheet alone. This guide provides the decision framework that goes beyond raw numbers — written by engineers who forge both grades every week.
Scope of This ArticleThis comparison focuses on forged components for power transmission gearboxes. All mechanical property values follow EN 10084:2008 unless otherwise stated. Properties of cast or bar-rolled products may differ. Always verify specifications against the applicable standard for your jurisdiction and application.
The Engineering Problem: Why Two Grades Exist
Gearbox designers face a fundamental metallurgical contradiction: a gear tooth must be simultaneously hard at the surface — resisting contact fatigue, pitting, wear, and scuffing — and tough at the core — absorbing shock loads without brittle fracture. No through-hardened steel composition resolves this cleanly across all section sizes, which is why case hardening (carburizing followed by quenching) became the dominant processing route for high-performance gear forgings.
Within the EN 10084 case hardening steel family, two grades dominate commercial procurement across Europe, Asia, North America, and Australia:
- 17CrNi6-6 (EN 1.5918) — A balanced chromium-nickel grade containing no deliberate molybdenum addition. Predictable processing behaviour, well-characterised global supply, and cost-effective for sections up to approximately 80 mm effective diameter.
- 18CrNiMo7-6 (EN 1.6587) — The same Cr–Ni base composition, enhanced with 0.25–0.35% molybdenum. The Mo addition fundamentally transforms hardenability depth, temper embrittlement resistance, and fatigue performance.
The molybdenum delta between these two grades appears minor in a composition table. In engineering practice, it determines whether a gear achieves 107 load cycles or only 106 — an order-of-magnitude difference in service life.
Chemical Composition: EN 10084:2008 Specification
The composition ranges below are taken directly from EN 10084:2008. Both grades share the same chromium and nickel ranges; the molybdenum content is the sole but consequential distinguishing factor.
Chemical composition (% mass) of 17CrNi6-6 and 18CrNiMo7-6 per EN 10084:2008
| Element | 17CrNi6-6 · EN 1.5918 | 18CrNiMo7-6 · EN 1.6587 |
|---|
| Carbon (C) | 0.14 – 0.20% | 0.15 – 0.21% |
| Silicon (Si) | ≤ 0.40% | ≤ 0.40% |
| Manganese (Mn) | 0.50 – 0.90% | 0.50 – 0.90% |
| Chromium (Cr) | 1.40 – 1.70% | 1.50 – 1.80% |
| Nickel (Ni) | 1.40 – 1.70% | 1.40 – 1.70% |
| Molybdenum (Mo) ★ | ≤ 0.10% — trace only | 0.25 – 0.35% — deliberate addition |
| Phosphorus (P) | ≤ 0.025% max | ≤ 0.025% max |
| Sulfur (S) | ≤ 0.035% max | ≤ 0.035% max |
The Three Metallurgical Roles of Molybdenum
Molybdenum in 18CrNiMo7-6 serves three distinct metallurgical functions that 17CrNi6-6 cannot replicate through its chromium and nickel content alone:
Hardenability enhancement. Mo suppresses the bainite transformation nose on the CCT diagram, allowing thicker sections — up to ~150 mm — to achieve a martensitic core on oil quench without requiring water quenching, which risks distortion in precision gear blanks.
Temper embrittlement resistance. Mo segregates to prior austenite grain boundaries and blocks the Ni–P and Ni–Sb embrittlement mechanisms. Forgings tempered at 250–400 °C retain significantly higher impact toughness in 18CrNiMo7-6 than in 17CrNi6-6 at the same tempering temperature.
Carbide stability during carburizing. Mo forms stable Mo&sub2;C and mixed (Fe,Mo)&sub3;C carbides that resist overcarburizing, giving the heat treater a wider, more forgiving process window and reducing the risk of surface carbide networking.
Mechanical Properties After Carburizing and Hardening
EN 10084:2008 specifies properties for heat-treated test specimens in the carburized + hardened + low-tempered condition (150–200 °C). This is the engineering-relevant state for gearbox performance prediction.
17CrNi6-6
EN material number 1.5918
Core Tensile Strength (Rm)900–1,200 MPa
Core Yield Strength (Rp0.2)≥ 700 MPa
Elongation (A)≥ 9%
Impact Energy KV (−40 °C)≥ 40 J
Surface Hardness (CHD 0.3 mm)58–63 HRC
Effective Case Depth0.3–1.5 mm
Max Section (core hardening)~80 mm dia.
Forging Temperature1,100–800 °C
18CrNiMo7-6
EN material number 1.6587
Core Tensile Strength (Rm)1,100–1,350 MPa
Core Yield Strength (Rp0.2)≥ 850 MPa
Elongation (A)≥ 8%
Impact Energy KV (−40 °C)≥ 50 J
Surface Hardness (CHD 0.3 mm)58–63 HRC
Effective Case Depth0.3–2.5 mm
Max Section (core hardening)~150 mm dia.
Forging Temperature1,150–850 °C
Critical Engineering InsightSurface hardness is essentially identical in both grades after correct carburizing (58–63 HRC). The performance difference is entirely in the core: how strong it is, how tough it remains at sub-zero temperatures, and how reliably that core condition is achieved in sections above 80 mm. Specifying the wrong grade does not affect surface appearance — it affects core structural integrity.
Heat Treatment Protocols: Process Window Comparison
Both grades follow the carburizing route defined in EN 10084, but the processing windows differ in ways that matter for manufacturing control and quality assurance.
Heat treatment process stages for 17CrNi6-6 vs 18CrNiMo7-6
| Process Stage | 17CrNi6-6 (1.5918) | 18CrNiMo7-6 (1.6587) |
|---|
| Soft Annealing | 680–720 °C, furnace cool | 660–710 °C, furnace cool |
| Normalising (optional) | 880–900 °C, air cool | 870–890 °C, air cool |
| Carburizing Temperature | 900–950 °C, atmosphere | 900–960 °C, atmosphere or vacuum |
| Diffusion Soak Window | Narrower — time-critical control | Wider — Mo carbides buffer activity |
| Maximum Case Depth | ~1.5 mm effective | ~2.5 mm effective |
| Quench Medium | Oil (reliable ≤80 mm dia.) | Oil (reliable up to ~150 mm dia.) |
| Low Tempering | 150–200 °C, ≥1 hr | 150–200 °C, ≥1 hr |
| Sub-zero / Deep Freeze | −60 °C optional | −60 to −80 °C recommended for large gears |
| Vacuum Carburizing (LPC) | Compatible — standard grades | Preferred — Mo retards grain growth at 950–1,050 °C |
Application Mapping: Grade Selection by Gearbox Type
The correct grade depends on section size, operating temperature, fatigue life requirement, end-market certifications, and total cost of ownership. The matrix below is based on experience across more than 2,000 gearbox engineering programmes.
Gearbox / Application
17CrNi6-6 (1.5918)
18CrNiMo7-6 (1.6587)
Automotive manual gearbox (passenger car)
✓ Primary recommended choice
Overkill for most car gearbox designs
Truck / heavy commercial vehicle transmission
✓ Suitable for shafts ≤80 mm
✓ Preferred for large axle shafts
Agricultural / construction equipment gearbox
✓ Cost-efficient standard choice
Consider for extreme shock loads
Industrial reducer (mining, paper, cement)
✓ Suitable for pinions ≤60 mm
✓ Preferred for large rings and shafts
Wind turbine main gearbox (planet carrier, ring gear)
⚠ Not recommended — section size exceeds hardenability limit
✓ Industry standard — required by most OEM specs
Aerospace gearbox
⚠ Rarely specified in aerospace
✓ Meets AMS and military specifications
Marine propulsion gearbox
✓ Suitable for smaller units ≤70 mm
✓ Preferred — Mo improves corrosion resistance
Precision robot joint gearbox
✓ Excellent for compact precision forgings
Optional for critical fatigue life
Railway traction gearbox
✓ Possible for smaller section components
✓ Standard for large axle gearboxes
Fatigue Life: ISO 6336 Allowable Stress Numbers
For gear engineers, fatigue is the governing failure mode, not static overload. ISO 6336 rates gear steel quality through the permissible contact stress number σH lim and tooth root stress number σF lim. These values determine the maximum tooth surface pressure and root bending stress the gear can sustain for the target number of load cycles.
ISO 6336 fatigue stress numbers for 17CrNi6-6 and 18CrNiMo7-6 case hardened gears
| Fatigue Parameter (ISO 6336) | 17CrNi6-6 (typical) | 18CrNiMo7-6 (typical) |
|---|
| σH lim — contact (pitting) fatigue | 1,380–1,450 MPa | 1,450–1,550 MPa |
| σF lim — bending (root) fatigue | 400–430 MPa | 430–470 MPa |
| Design fatigue life target (gear tooth) | 10&sup6;–10&sup7; cycles | 10&sup7;–10&sup8; cycles |
| Residual compressive stress in case | −400 to −600 MPa | −500 to −700 MPa |
| Shot-peening response | Good | Excellent — higher pre-peening compressive stress baseline |
Design ImplicationSwitching from 17CrNi6-6 to 18CrNiMo7-6 for an identical gear geometry can extend calculated bending fatigue life by 30–60%. Alternatively, the higher allowable stresses allow the designer to reduce face width or use a smaller centre distance without sacrificing rated life — partially or fully offsetting the 8–15% material cost premium in heavy-duty designs.
Forging Process Considerations
Open die forging and seamless ring rolling are the two primary forming routes for gearbox blanks in both grades. Process differences between the grades affect manufacturing cost, yield, and the documented quality that customers receive.
Minimum Forging Ratio
Both grades require a minimum forging ratio of 3:1 to break down the as-cast dendrite structure and achieve grain refinement necessary for gear-quality properties. For 18CrNiMo7-6, we target a minimum of 4:1 on large shaft forgings to ensure homogeneous Mo redistribution — molybdenum exhibits moderate micro-segregation at dendrite cores in the original ingot, and insufficient forging reduction can leave residual banding that causes scattered hardness in the heat-treated part.
Seamless Ring Rolling
17CrNi6-6 ring blanks up to approximately 1,200 mm OD can be rolled in a single heat without microstructural issues. 18CrNiMo7-6 requires tighter surface temperature control in the final sizing passes because molybdenum creates a narrower hot working window. On our ring rolling mill, surface temperature is monitored continuously by calibrated pyrometer, with pass schedules adjusted in real time.
Lead Time NoteRaw billet availability for 17CrNi6-6 is typically 2–4 weeks shorter than 18CrNiMo7-6, because 1.5918 is a higher-volume grade held in more mill stock positions globally. For time-critical projects where both grades would technically satisfy the design requirements, 17CrNi6-6 may offer a meaningful procurement advantage beyond material cost.
Cost Analysis: Total Cost of Ownership
Material price is only one element of cost. For complete economic comparison, teams should evaluate all cost drivers over the product life cycle.
Total cost of ownership comparison for 17CrNi6-6 vs 18CrNiMo7-6 gearbox forgings
| Cost Factor | 17CrNi6-6 (1.5918) | 18CrNiMo7-6 (1.6587) |
|---|
| Raw Billet Cost Premium | Baseline | +8–15% above 17CrNi6-6 |
| Heat Treatment Complexity | Standard atmosphere carburizing | Slightly higher (vacuum LPC option) |
| Forging Yield / Scrap Risk | Low | Low with correct procedure |
| Design Optimisation Potential | Moderate — standard allowable stresses | High — higher σ lim enables lighter, smaller gear geometry |
| Field Warranty / Failure Risk | Moderate for sections >60 mm | Lower across all standard section sizes |
| Service Life Multiplier | 1.0× (baseline) | 1.3–1.6× typical in bending fatigue |
| Billet Lead Time | Shorter — widely stocked globally | 2–4 weeks longer in most markets |
For automotive gearboxes in high volumes (100,000+ units/year), the raw material cost differential is significant and 17CrNi6-6 is typically the correct engineering and commercial choice, provided design analysis confirms adequate fatigue margin. For industrial, wind turbine, or aerospace gearboxes in low volumes where a field failure triggers disproportionately large downstream costs, the 18CrNiMo7-6 premium is almost universally justified.
The Grade Selection Decision Framework
Specify this grade when →
17CrNi6-6 · EN 1.5918
Section Size
Effective diameter ≤ 80 mm
Core hardening to specification is reliably achieved on oil quench within this range without Mo.
Typical Applications
Automotive, agricultural, light industrial
Tens of millions of operating hours globally confirm adequate life in these duty cycles.
Production Volume
Medium to high volume (>10,000 units/yr)
The 8–15% material premium becomes very significant at scale.
ISO 6336 Fatigue Margin
Analysis shows adequate margin with 1.5918
If σF and σH limits for 17CrNi6-6 provide ≥15% design margin, use this grade.
Specify this grade when →
18CrNiMo7-6 · EN 1.6587
Section Size
Effective diameter 80–200 mm
Mo-enhanced hardenability ensures martensitic core on oil quench — 17CrNi6-6 cannot reliably achieve this.
Typical Applications
Wind turbine, aerospace, heavy marine, rail
Life targets of ≥10&sup7;–10&sup8; cycles and high field replacement costs demand this grade.
Operating Conditions
Sub-zero temperatures or high shock loads
Impact toughness KV at −40 °C is 25% higher (≥50 J vs ≥40 J). Mo resists temper embrittlement.
OEM Specification
Customer or standard mandates 1.6587
Wind OEMs (Vestas, Siemens-Gamesa, GE Vernova) and aerospace certifications typically specify 18CrNiMo7-6 explicitly.
How to Apply This FrameworkRun the ISO 6336 fatigue analysis with allowable stress numbers for both grades. If 17CrNi6-6 provides adequate margin at the target life and section, specify it. If the design requires the higher σ lim values of 18CrNiMo7-6, or if the effective section exceeds 80 mm, specify 18CrNiMo7-6. Never compensate for an under-specified steel grade by tightening heat treatment tolerances — hardenability is a composition property, not a process variable.
Frequently Asked Questions
What is the main difference between 17CrNi6-6 and 18CrNiMo7-6?
The primary difference is molybdenum content. 17CrNi6-6 (EN 1.5918) contains only trace Mo (≤0.10%), while 18CrNiMo7-6 (EN 1.6587) contains a deliberate 0.25–0.35% Mo addition. This significantly increases hardenability (reliable core hardening up to ~150 mm vs. ~80 mm on oil quench), improves impact toughness at sub-zero temperatures, resists temper embrittlement, and extends gear fatigue life by 30–60% in bending. Core tensile strength is also higher: 1,100–1,350 MPa vs. 900–1,200 MPa. The raw material cost premium for 18CrNiMo7-6 is typically 8–15% over 17CrNi6-6.
Which steel is better for wind turbine gearboxes — 17CrNi6-6 or 18CrNiMo7-6?
18CrNiMo7-6 (EN 1.6587) is the industry standard for wind turbine main gearboxes. Planet carriers and ring gears in multi-megawatt turbines typically have effective section sizes of 100–200 mm, exceeding the reliable hardenability limit of 17CrNi6-6. Additionally, wind turbine gearboxes must achieve fatigue lives of 10&sup7;–10&sup8; cycles under variable load spectra, and high field crane replacement costs make warranty failures extremely expensive. Most major wind turbine OEMs (Vestas, Siemens-Gamesa, GE Vernova) specify 18CrNiMo7-6 explicitly in their material standards.
Can 17CrNi6-6 substitute for 18CrNiMo7-6 if 1.6587 is out of stock?
Not without a formal engineering deviation review. For components with effective hardening diameters ≤80 mm, substitution may be technically permissible after the design engineer confirms adequate fatigue margin using 17CrNi6-6 allowable stress numbers. For sections above 80 mm, 17CrNi6-6 cannot achieve the required core hardness on oil quench and substitution must be rejected. All substitutions must be documented with updated EN 10204 3.1 material certifications and a formal deviation record in the quality management system.
What are the international equivalents of 17CrNi6-6 and 18CrNiMo7-6?
For 17CrNi6-6 (EN 1.5918): Russian 17HN3A (GOST 4543), Polish 17HN3 (PN), Chinese 17CrNi6 (GB/T 3077). For 18CrNiMo7-6 (EN 1.6587): American AISI 4820 (closest U.S. equivalent), SAE 9310 (aerospace USA), Japanese SCM822H (JIS G4053). Always verify cross-references by full chemical composition and Jominy hardenability band (HH curve), not by trade designation alone.
What documentation should accompany 17CrNi6-6 or 18CrNiMo7-6 forgings?
Standard documentation for gear-quality case hardening steel forgings should include: (1) EN 10204 Type 3.1 or 3.2 material test certificate covering chemical analysis, mechanical properties (Rm, Rp0.2, A, KV at −40 °C), surface and core hardness after heat treatment, and forging ratio declaration; (2) NDE test report (UT to ASTM A388 or EN 10228-3); (3) grain size certificate per ASTM E112; (4) heat treatment records including furnace charts and carbon potential log for carburizing. For wind turbine and aerospace applications, EN 10204 Type 3.2 (third-party witnessed) is required.
Does JNMT supply custom forgings in both 17CrNi6-6 and 18CrNiMo7-6?
Yes. JNMT Forged Parts (Jiangsu Liangyi) manufactures custom open die forgings and seamless rolled rings in both 17CrNi6-6 (1.5918) and 18CrNiMo7-6 (1.6587), with full in-house heat treatment capability including computer-controlled atmosphere carburizing, oil quench, and low-temperature tempering. EN 10204 3.1 certification is standard; 3.2 third-party witness inspection can be arranged on request. Visit our
request stock sizes and lead times for EN 1.5918 forgings for standard size ranges and a request-for-quote form. We have been serving European, American, and Asian OEMs since 1997.
Conclusion: The Right Steel for the Right Gearbox
The choice between 17CrNi6-6 and 18CrNiMo7-6 is not a question of which steel is generically superior — it is a question of which steel is correctly matched to the engineering constraints of your specific gearbox application.
For passenger car transmissions, light commercial vehicle gearboxes, agricultural equipment drives, and compact industrial reducers with sections below 80 mm, 17CrNi6-6 (EN 1.5918) is the rational first choice: technically well-proven, globally available, cost-effective, and fully adequate when design analysis confirms sufficient fatigue margin. You can review our full range of available product forms and ordering details.
For large-section ring gears, planet carriers, wind turbine gearboxes, heavy marine propulsion drives, railway traction gears, and any application where field failure triggers a disproportionately large downstream cost, 18CrNiMo7-6 (EN 1.6587) earns its premium by providing hardenability and fatigue performance that 17CrNi6-6 cannot physically match at equivalent cross-sections.
The engineering decision rule is straightforward: run the ISO 6336 fatigue analysis with allowable stress numbers for both grades. If 17CrNi6-6 provides adequate margin at the design life and section, specify it. If it does not, or if the effective section exceeds 80 mm, specify 18CrNiMo7-6. Do not attempt to compensate with process tightening alone — hardenability is a composition property.
Free Engineering ReviewOur engineering team reviews customer gearbox forging drawings at no charge as part of our quotation process. We have evaluated both grades across hundreds of gearbox families and can identify potential hardenability risks, forging ratio requirements, and heat treatment challenges before they affect production.
Contact us to begin a technical discussion.
17CrNi6-618CrNiMo7-6EN 1.5918EN 1.6587Case Hardening SteelGearbox ForgingEN 10084:2008ISO 6336Carburizing SteelOpen Die ForgingSeamless Ring RollingChina Forging Manufacturer1.5918 vs 1.6587ASTM A388Wind Turbine Gearbox
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