📍 Key takeaways
What Are These Two Steels?
18CrNiMo7-6
20MnCr5
18CrNiMo7-6 (DIN 1.6587) is the correct choice for forged gear applications with section diameters above 80 mm, operating under high cyclic loading, shock loading, or sub-zero temperatures. It achieves tensile strength ≥ 1,080 MPa, ≥ 78 J Charpy impact, and maintains 28–36 HRC core hardness at 200 mm section. 20MnCr5 (DIN 1.7147) is the preferred choice for gear sections below 80 mm where cost efficiency is important — it delivers equivalent 58–62 HRC surface hardness at 15–30% lower material cost, but cannot match 18CrNiMo7-6's hardenability in larger sections.
Both grades are covered under EN 10084 — Europe's governing standard for case hardening steels — and both are designed to be carburized, hardened, and tempered to achieve a hard, wear-resistant surface over a tough, fatigue-resistant core. That is where the similarity ends.
18CrNiMo7-6 (material number 1.6587, previously catalogued as 17CrNiMo6 in older DIN editions) is a high-alloy Cr-Ni-Mo steel. The deliberate addition of molybdenum — absent in 20MnCr5 — dramatically increases hardenability in thick sections. This makes it the industry standard for large, heavily-loaded forged gears, planet carriers, and shafts in wind energy, mining, and heavy marine gearboxes. Jiangsu Liangyi supplies custom 18CrNiMo7-6 forged parts from ring OD Ø250 mm up to Ø6,000 mm, all EN 10084 compliant.
20MnCr5 (1.7147) relies on manganese and chromium alone for its case hardening response. Its hardenability is excellent up to about 80 mm section diameter — perfectly suited for automotive transmission gears, machine tool spindles, and smaller industrial gearboxes. Its lower alloy content translates directly into lower material and processing costs, which is why it dominates high-volume gear production worldwide.
"Case hardening steel" does not describe a finished surface hardness — it describes a family of low-carbon steels (typically 0.15–0.25% C) engineered to be carburized. Carbon is diffused into the surface layer, raising surface carbon to ~0.7–0.9%, allowing hardening to 58–62 HRC while the low-carbon core remains tough. Both grades follow this philosophy; their alloy differences determine how deeply and uniformly this hardening reaches.
Chemical Composition Compared
The table below shows standard chemical composition ranges per EN 10084. On your mill test certificate (EN 10204 3.1 MTC), verify that the actual ladle analysis falls within these limits.
| Element | 18CrNiMo7-6 (1.6587) | 20MnCr5 (1.7147) | Role in Steel |
|---|---|---|---|
| C (Carbon) | 0.15–0.21% | 0.17–0.22% | Core carbon; must stay low for toughness |
| Mn (Manganese) | 0.50–0.90% | 1.10–1.40% | Hardenability; primary strengthener in 20MnCr5 |
| Cr (Chromium) | 1.50–1.80% | 1.00–1.30% | Carbide formation; wear resistance; hardenability |
| Ni (Nickel) | 1.40–1.70% | — none — | Core toughness; sub-zero ductility; fatigue strength |
| Mo (Molybdenum) | 0.25–0.35% | — none — | Deep hardenability; temper embrittlement resistance |
| Si (Silicon) | ≤ 0.40% | ≤ 0.40% | Deoxidation during melting |
| P / S | ≤ 0.025 / ≤ 0.035% | ≤ 0.025 / ≤ 0.035% | Tramp elements — lower is better |
| Carbon Equiv. (IIW) | CE ≈ 0.62–0.72 | CE ≈ 0.40–0.52 | Hardenability proxy; preheat indicator for welding |
Three elements set 18CrNiMo7-6 apart from 20MnCr5:
- Higher Cr (1.5–1.8%): Forms stable alloy carbides during carburizing, improving wear resistance and slowing austenite transformation on cooling — better through-hardening.
- Nickel (1.4–1.7%): The most important toughness additive. Ni dissolves in the iron matrix, lowering the ductile-to-brittle transition temperature significantly. Critical for shock-loaded gears and cold-environment service.
- Molybdenum (0.25–0.35%): The key to deep hardenability. Mo inhibits diffusion-controlled transformation of austenite, allowing the steel to harden in thick cross-sections without distorting fast quench rates. Mo also suppresses temper embrittlement during stress-relief.
Mechanical Properties After Full Heat Treatment
Values below are typical after carburizing + core hardening + low-temperature temper (160–200°C), reported for a 30 mm reference diameter per EN 10084.
| Property | 18CrNiMo7-6 | 20MnCr5 | Notes |
|---|---|---|---|
| Tensile strength Rm | ≥ 1,080–1,320 MPa | 980–1,280 MPa | At 30 mm dia. |
| Yield strength Rp0.2 | ≥ 785 MPa | ≥ 735 MPa | — |
| Elongation A | ≥ 11% | ≥ 10% | Similar |
| Impact energy KV (+20°C) | ≥ 78 J | ≥ 55 J | Charpy V-notch; Ni drives this gap |
| Case hardness (carburized) | 58–62 HRC | 58–62 HRC | Equivalent at surface |
| Core hardness at 100 mm Ø | 35–42 HRC | 22–30 HRC | Critical gap in large forgings |
| Core hardness at 200 mm Ø | 28–36 HRC | 18–22 HRC | 20MnCr5 drops sharply — inadequate |
| Bending fatigue strength | ~650–700 MPa | ~550–600 MPa | Per ISO 6336 gear root bending fatigue |
| Machinability | Good | Better (lower alloy) | 20MnCr5 is easier to machine |
Core hardness at large section sizes is where the grades diverge most dramatically. A 200 mm diameter 20MnCr5 forging may achieve only 18–22 HRC at its center — insufficient for gears under heavy cyclic loads. The same size in 18CrNiMo7-6 maintains 28–36 HRC. This difference cannot be corrected by process adjustment — it is inherent to the alloy composition.
Hardenability and Critical Section Size — The Decisive Difference
Hardenability is the ability of a steel to achieve sufficient hardness at depth when quenched. It is the single most important differentiator between these two grades, and determines which grade is technically viable for your specific gear geometry.
| Section Diameter | 18CrNiMo7-6 Verdict | 20MnCr5 Verdict |
|---|---|---|
| Up to 40 mm | ✓ Excellent | ✓ Excellent — preferred for cost |
| 40–80 mm | ✓ Excellent | ✓ Good — adequate for moderate loads |
| 80–150 mm | ✓ Excellent — recommended | ⚠ Marginal — core hardness drops |
| 150–300 mm | ✓ Good — grade of choice | ✗ Insufficient — core will not harden adequately |
| Over 300 mm | ✓ Acceptable with proper process | ✗ Not recommended |
If your gear's critical section exceeds 80 mm and the component operates under high cyclic loading, specify 18CrNiMo7-6 as the default. The alloy cost premium is negligible compared to the cost of a field failure or unplanned downtime.
Heat Treatment Protocols Side by Side
Both grades require a multi-stage heat treatment sequence. Parameters differ due to varying alloy content — notably, 18CrNiMo7-6 demands specific post-forge precautions against white spot (hydrogen-induced cracking).
| Stage | 18CrNiMo7-6 (1.6587) | 20MnCr5 (1.7147) |
|---|---|---|
| Forging temp. | 900–1,200°C | 900–1,200°C |
| Post-forge cooling | Slow furnace cool + dehydrogenation anneal required (white spot risk) | Sand or furnace cool; air permissible for smaller sections |
| Soft annealing | 650–700°C, furnace cool; max 229 HBW | 680–720°C, furnace cool; max 207 HBW |
| Normalizing | 850–880°C, air | 860–900°C, air |
| Carburizing | 880–980°C; surface C target 0.70–0.85% | 850–950°C; surface C target 0.70–0.85% |
| Core hardening | 820–860°C, oil quench | 840–880°C, oil quench |
| Tempering | 150–200°C (avoid 250–400°C temper embrittlement zone) | 150–180°C; less sensitive to embrittlement |
| Final case hardness | 58–62 HRC | 58–62 HRC |
18CrNiMo7-6 is significantly more sensitive to hydrogen-induced white spots than 20MnCr5, because higher alloy content slows hydrogen diffusion out of the steel. Immediate dehydrogenation annealing after forging is mandatory — typically at 600–650°C for several hours before slow furnace cooling. Skipping this step is the number-one cause of internal cracking defects in this grade. Verify that your forge supplier's procedure explicitly addresses this requirement before placing an order.
Typical Applications by Industry
Real-world application mapping based on component size, loading profile, and industry-standard material specifications.
Wind Turbine Planet Gears
Section 150–400 mm, extreme fatigue cycling. Per IEC 61400 gearbox design guidelines.
Mining Mill Ring Gears
Large ring gear OD up to 6,000 mm; heavy shock loading from ball/SAG mills.
Marine Propulsion Gearboxes
Offshore environments; impact toughness at sub-zero temperature required.
Steel Mill Drives
Rolling mill pinion shafts; high torque, continuous duty, large section.
Automotive Transmissions
Manual gearbox gears, differential bevel gears; section < 60 mm, high volume.
Machine Tool Spindles
CNC machining centers, milling spindles; moderate loads, 30–80 mm section.
Agricultural Gearboxes
PTO shafts, tractor final drives; cost-sensitive, light-to-medium duty.
Conveyor and Packaging Drives
Light industrial gears; section < 50 mm; cost efficiency is the primary driver.
Head-to-Head Scorecard
Choose Your Grade by Application
Use this matrix to identify the correct grade for your application. When in doubt, specify 18CrNiMo7-6 — the cost premium is modest relative to the performance margin it delivers.
Cost and Supply Chain Considerations
Material cost
18CrNiMo7-6 commands a material premium over 20MnCr5, driven primarily by its nickel and molybdenum content. As a rough benchmark, 18CrNiMo7-6 steel billets typically cost 15–30% more than equivalent 20MnCr5 billets, though this varies by market conditions and specification (VD vacuum degassing, ESR remelting, etc.).
For forged components — where steel material often represents only 20–35% of total part cost, with forging labor, heat treatment, machining, and certification making up the rest — this premium rarely exceeds 8–12% of finished part cost. In lifecycle cost analysis that accounts for replacement costs and downtime, 18CrNiMo7-6 almost always wins for critical applications.
Supply and lead times
20MnCr5 is one of the most widely stocked case hardening steels globally, with short lead times from steel service centers virtually everywhere. 18CrNiMo7-6 in large forging sizes (Ø 300 mm+) requires specialist forge capability — vacuum degassing, sufficient press capacity, and controlled heat treatment. Lead times from China for large forgings typically run 30–70 days after deposit, longer when EN 10204 3.2 third-party inspection is required.
When sourcing 18CrNiMo7-6 forged parts from China, verify that the forge performs vacuum degassing (VD) during steelmaking. This is non-negotiable for large forged parts — it reduces hydrogen content (preventing white spots), lowers non-metallic inclusions, and ensures cleanliness levels required by EN 10084. Always request the full ladle analysis on the MTC, not just a certificate of conformity.
Engineer FAQ
Can I substitute 20MnCr5 for 18CrNiMo7-6 to reduce cost on a large gear?
Only if the component's critical section is below approximately 80 mm and the load profile is within 20MnCr5's performance envelope. For components exceeding 100 mm section diameter under significant cyclic or shock loading, substitution is not recommended. The hardenability gap is a physical property of the alloy — not correctable by heat treatment optimization. Substitution in this range can result in inadequate core hardness, reduced fatigue life, and potential field failure.
What is the US equivalent of 18CrNiMo7-6?
The closest AISI equivalent is AISI 4320H. The Japanese equivalent is SNCM420, and the Chinese standard equivalent is 17Cr2Ni2Mo. Note that "equivalent" does not mean "identical" — minor differences in allowable composition ranges can affect specific mechanical properties. Always verify the actual heat analysis against your specification requirements.
Does 18CrNiMo7-6 require special precautions for welding?
Yes. With a high Carbon Equivalent (CE ≈ 0.62–0.72 by IIW formula), preheat to 200–300°C is typically required. Post-weld heat treatment (PWHT) is recommended for structural welds. Welding is generally avoided on carburized and hardened surfaces. Consult a qualified welding engineer and follow EN ISO 15614 procedures.
How do I specify carburizing depth for 18CrNiMo7-6 gear forgings?
Carburizing depth (CHD to 550 HV) should be based on gear module and application loading. As a guide: module 4–8 gears typically require CHD 0.6–1.2 mm; module 10–20 gears require 1.2–2.5 mm; module 25+ can require up to 4.0 mm for heavy-duty gears. Specify on your drawing as "CHD = X.X to Y.Y mm per EN ISO 2639."
What sizes does Jiangsu Liangyi supply in 18CrNiMo7-6?
Jiangsu Liangyi manufactures large 18CrNiMo7-6 (1.6587) open-die forgings and seamless rolled rings, with press capacities from 2,000 to 6,300 tons. We supply rings up to Ø 6,000 mm OD and parts up to 30,000 kg, all EN 10084 compliant with EN 10204 3.1 MTCs. For full technical specifications, size capability, and a free quote, visit the 18CrNiMo7-6 (1.6587) forged parts page on our website.
Referenced Standards and Sources
- [1] EN 10084:2008 — Case hardening steels. Technical delivery conditions. European Committee for Standardization (CEN).
- [2] EN 10204:2004 — Metallic products — Types of inspection documents. CEN.
- [3] ISO 6336:2019 — Calculation of load capacity of spur and helical gears. ISO.
- [4] IEC 61400-4:2012 — Wind turbines — Part 4: Design requirements for wind turbine gearboxes. IEC.
- [5] EN ISO 2639:2002 — Steel — Determination and verification of the depth of carburized and hardened cases. ISO/CEN.
- [6] SEP 1921 — Steels for highly stressed components — Ultrasonic testing. Verein Deutscher Eisenhüttenleute (VDE/VDEh).
All technical data in this article is based on EN 10084:2008 standard ranges and Jiangsu Liangyi Co., Limited' 25+ years of production experience with both grades. Values cited for mechanical properties are minimum guaranteed values per standard unless otherwise noted.
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