AISI 9840 steel is a nickel-chromium-molybdenum (Cr-Ni-Mo) triple-alloy, medium-carbon, quench-and-temper steel with carbon 0.38–0.43%, nickel 0.85–1.15%, chromium 0.70–0.90%, and molybdenum 0.20–0.30%. Also designated DIN 39NiCrMo3 / EN 1.6510, it achieves tensile strength 930–1,180 MPa and hardness 27–34 HRC after quenching and tempering. It is widely used for forged crankshafts, gear shafts, connecting rods, and transmission components in automotive, energy, and heavy machinery sectors. Jiangsu Liangyi Co., Limited manufactures custom AISI 9840 open die forgings and seamless rolled rings with full EN 10204 MTC documentation.
AISI 9840 is a nickel-chromium-molybdenum alloy steel — officially classified as a "Triple Alloy Steel" — engineered to deliver an exceptional combination of high strength, wear resistance, fatigue life, and impact toughness after heat treatment. It is the material of choice for engineers designing forged components that must perform reliably under extreme mechanical loads across millions of operating cycles.
01 What Exactly Is AISI 9840 Steel?
AISI 9840 is a low-alloy, medium-carbon quench-and-temper steel standardized under the SAE/AISI classification system. Every digit carries precise technical meaning defined by the American Iron and Steel Institute.
The prefix "98" identifies the alloy family: a nickel-chromium-molybdenum triple-alloy series. The trailing "40" directly states the average carbon content of 0.40% (exact range: 0.38–0.43%) — the primary driver of hardenability and achievable strength. The presence of Cr, Ni, and Mo together earns this steel the "Triple Alloy" designation.
AISI 9840 belongs to the same broad family as AISI 4340, but with lower nickel content and slightly elevated manganese. This balance delivers excellent through-hardening at lower material cost — a decisive advantage for high-volume production of AISI 9840 forged components including open die forgings, seamless rolled rings, gear shafts, and crankshafts.
International Equivalent Grades
| Standard | Designation | Region | Notes |
|---|---|---|---|
| AISI / SAE | 9840 | United States | Primary designation |
| DIN / EN | 39NiCrMo3 / 1.6510 | Europe (Germany) | Most common EU equivalent |
| EN 10083-3 | 36CrNiMo4 | European Union | Structural applications |
| JIS G4103 | SNCM439 | Japan | Nearest JIS equivalent |
| GB/T 3077 | 40CrNiMoA | China | Chinese standard |
| BS 970 | EN 24 | United Kingdom | Legacy British standard |
| ASTM A519 | 9840 | United States | Seamless mechanical tubing |
02 Chemical Composition of AISI 9840 Steel
The alloying balance of AISI 9840 defines its performance envelope. The three principal elements — chromium, nickel, and molybdenum — contribute distinctly and synergistically to the steel's final mechanical properties.
| Element | Symbol | Min % | Max % | Primary Engineering Function |
|---|---|---|---|---|
| Carbon | C | 0.38 | 0.43 | Core strength and hardness driver |
| Manganese | Mn | 0.70 | 0.90 | Hardenability, deoxidation, hot workability |
| Silicon | Si | 0.15 | 0.35 | Deoxidation, ferrite strengthening |
| Nickel | Ni | 0.85 | 1.15 | Toughness, low-temperature impact resistance |
| Chromium | Cr | 0.70 | 0.90 | Hardenability, wear and oxidation resistance |
| Molybdenum | Mo | 0.20 | 0.30 | Prevents temper embrittlement, elevated-temp strength |
| Phosphorus | P | — | 0.035 max | Controlled impurity — reduces ductility if excessive |
| Sulfur | S | — | 0.040 max | Controlled impurity — affects machinability |
Role of Each Alloying Element
Chromium (Cr), 0.70–0.90%: Increases hardenability by shifting the TTT curve, allowing effective oil quenching in larger sections. Promotes fine carbide formation for improved wear resistance and good response to nitriding surface treatments.
Nickel (Ni), 0.85–1.15%: The toughness provider. Nickel stabilizes austenite during heat treatment and significantly improves Charpy impact resistance at room and sub-zero temperatures. It refines the martensitic microstructure and is the key differentiator from lower-toughness grades such as 4140 or 5140.
Molybdenum (Mo), 0.20–0.30%: Suppresses temper embrittlement and adds solid-solution strengthening. Promotes fine Mo-carbide precipitation during tempering, maintaining strength at elevated service temperatures. Even in small quantities, molybdenum has a disproportionately large positive effect on long-term mechanical stability.
03 Mechanical Properties of AISI 9840 Steel
The table below presents typical properties of AISI 9840 in the quenched-and-tempered (Q&T) condition at approximately 28–32 HRC — the most common specification for forged industrial components.
| Property | Typical Value | Unit | Test Basis |
|---|---|---|---|
| Tensile Strength (UTS) | 930 – 1,180 | MPa | EN ISO 6892-1 |
| Yield Strength (Rp0.2) | 750 – 1,000 | MPa | 0.2% proof stress |
| Elongation at Break | ≥ 12 | % | Gauge length 5d |
| Reduction of Area | ≥ 50 | % | Cross-section reduction |
| Charpy Impact Energy | ≥ 60 | J | Room temperature, V-notch |
| Brinell Hardness | 277 – 352 | HB | EN ISO 6506-1 |
| Rockwell Hardness | 27 – 34 | HRC | After Q&T at 550–650°C |
| Elastic Modulus (E) | ~205 | GPa | Room temperature |
| Density | 7.85 | g/cm³ | — |
| Poisson's Ratio | ~0.29 | — | — |
| Thermal Conductivity | ~38 | W/m·K | At 20°C |
Actual properties are influenced by cross-section size and quench rate. Sections larger than 75 mm diameter may show reduced core hardness after standard oil quenching. A Jominy end-quench hardenability test (EN ISO 642) is recommended for critical large-section forgings to verify through-hardening consistency.
Comparative Performance — AISI 9840 vs 4340 vs 4140
04 Heat Treatment Protocol for AISI 9840
A properly executed quench-and-temper cycle transforms an AISI 9840 forged blank into a component with exceptional through-hardening — consistent from surface to core even in sections approaching 100 mm diameter. Every step in the sequence is critical and must not be skipped or combined.
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01Hot Forging — 1,050 to 1,200°C
Work within the recommended temperature window. Finish forging at or above 850°C. AISI 9840 is classified as a white-spot sensitive steel — hydrogen dissolved during steelmaking can cause internal cracking if the post-forge cooling is too rapid.
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02Post-Forge Annealing or Normalizing — Mandatory, Immediate
Transfer to furnace immediately after forging. Full annealing: 830–860°C, furnace cool ≤20°C/hour. Normalizing: 870–900°C, air cool. Either process relieves residual stresses, eliminates white-spot risk, and produces a uniform microstructure ready for machining and heat treatment.
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03Austenitizing — 820 to 860°C
Heat uniformly to austenitizing temperature. Soak for approximately 1 hour per 25 mm of effective section thickness to ensure complete, homogeneous austenitizing through the core. Insufficient soak time results in incomplete carbide dissolution and reduced hardness after quenching.
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04Oil Quenching — Immediate from Austenitizing Temperature
Transfer to oil quench bath immediately upon completing the austenitizing soak. Do not water-quench — quench cracking risk is too high for this alloy. Oil quenching achieves full martensitic transformation in sections up to approximately 75–100 mm diameter. Agitated oil improves uniformity.
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05Tempering — 550 to 650°C (Do Not Skip or Delay)
Begin tempering while the part is still warm from quenching (above 80°C). Soak at 550–650°C for minimum 1 hour per 25 mm section thickness. Target hardness: 27–34 HRC. Double tempering is recommended for complex or large sections to fully stabilize the microstructure.
Never temper AISI 9840 between 250–400°C. This range causes tempered martensite embrittlement (TME), where impact energy can fall by 60–80%. Any structural forged component tempered in this range must be considered rejected and re-treated. Always temper above 500°C for all load-bearing applications.
05 Forging Characteristics and Parameters
AISI 9840's alloying balance provides good hot workability for both open-die and closed-die forging. The triple-alloy system ensures the forged microstructure responds predictably to subsequent heat treatment, producing consistent final properties across production batches.
AISI 9840 occupies a uniquely practical position in the forge shop — economical enough for volume industrial production, yet technically capable enough for safety-critical power transmission components where dimensional consistency and material reliability are non-negotiable.
— Jiangsu Liangyi Engineering Team, Material Selection Reference 2025| Parameter | Value / Range | Notes |
|---|---|---|
| Forging Temperature | 1,050 – 1,200 °C | Stay below 1,220°C to avoid grain coarsening |
| Minimum Finishing Temperature | ≥ 850 °C | Prevent deformation in two-phase region |
| Post-Forge Cooling | Slow controlled or immediate anneal | Mandatory — white-spot prevention |
| Applicable Processes | Open die, closed die, ring rolling, press forging | — |
| Preheating (large sections) | 400 – 500 °C | Reduces thermal shock on billets >75 mm |
| Available Product Forms | Round bar, flat bar, forged billet, rings, custom shape | 30 kg to 30,000 kg per piece |
06 Industrial Applications of AISI 9840 Forgings
AISI 9840 forged components are specified wherever engineers require a reliable combination of high strength, fatigue resistance, and impact toughness in moderate to heavy cross-sections across multiple demanding industries.
High-cycle torsional and bending fatigue — 9840 delivers consistent through-hardened performance across full pin diameters.
Excellent wear surface after gas nitriding or induction hardening applied post-forging.
Cyclic bending and high compressive loads handled reliably across hundreds of millions of cycles.
Shafts, hubs, and synchronizer rings in manual and automated transmissions.
Turbine rotor shafts, pump shafts, and compressor shafts requiring superior fatigue life.
Hydraulic cylinder rods, excavator pivot pins, and press tooling requiring reliable toughness.
Drill collars, wellhead housings, and downhole tool components requiring extreme toughness.
Rotor spindles, turbine discs, and coupling flanges in power generation equipment.
07 AISI 9840 vs AISI 4340 — Which Should You Choose?
Both grades are Cr-Ni-Mo triple-alloy steels with excellent hardenability, but they differ in ways that matter for application suitability and project economics.
| Characteristic | AISI 9840 | AISI 4340 |
|---|---|---|
| Nickel Content | 0.85 – 1.15% | 1.65 – 2.00% |
| Manganese Content | 0.70 – 0.90% | 0.60 – 0.80% |
| Maximum Tensile Strength | ~1,180 MPa | ~1,310 MPa |
| Impact Toughness | Very High (≥60 J) | Exceptional (≥80 J) |
| Through-Hardening Depth | Very Good (to ~100 mm) | Excellent (to ~150 mm) |
| Material Cost (relative) | Lower | Higher |
| Fatigue Resistance | High | Very High |
| Best Application Fit | Volume industrial forgings | Aerospace, ultra-critical parts |
Select AISI 4340 when absolute maximum strength and toughness are required regardless of cost — primarily aerospace structures and ultra-critical drivetrain components. Select AISI 9840 when approximately 90% of 4340's capability is needed at meaningfully lower material cost — which describes the vast majority of automotive, industrial machinery, and energy sector forging applications. For volume production, AISI 9840 is almost always the economically correct choice.
08 Machinability and Weldability
Machinability
In the annealed or normalized condition, AISI 9840 machines with a relative machinability rating of approximately 55–65% compared to AISI B1112 (free-machining steel baseline = 100%). The alloy produces long, somewhat gummy chips in the soft condition. Chip-breaking inserts and adequate coolant are recommended for CNC turning.
Recommended sequence: normalize or anneal to 170–220 HB → rough machine → quench and temper to final hardness → finish machine with light cuts (0.1–0.3 mm) using carbide inserts → grind to final tolerance.
Surface Hardening Options After Q&T
- Gas / Plasma Nitriding: Hard case 600–750 HV at 0.1–0.5 mm depth. Maintains tough core. Ideal for gear flanks and bearing journals.
- Induction Hardening: Selective surface hardening to 55–60 HRC on specific zones (journals, teeth) without affecting the full component.
- Nitrocarburizing: Improves wear resistance and corrosion resistance simultaneously for components in wet or corrosive service environments.
Weldability
AISI 9840 has a carbon equivalent of approximately 0.80–0.90%, classifying it as difficult to weld. Structural welding in production is not recommended. Where unavoidable: preheat to 200–300°C, use low-hydrogen electrodes (E9018-G or equivalent), maintain interpass temperature, and perform post-weld stress relief at 500–550°C immediately upon completion.
09 Frequently Asked Questions — AISI 9840 Steel
AISI 9840 chemical composition per SAE/AISI standard: Carbon (C) 0.38–0.43%, Manganese (Mn) 0.70–0.90%, Silicon (Si) 0.15–0.35%, Nickel (Ni) 0.85–1.15%, Chromium (Cr) 0.70–0.90%, Molybdenum (Mo) 0.20–0.30%, Phosphorus max 0.035%, Sulfur max 0.040%. The three principal alloying elements Cr, Ni, and Mo give it the "Triple Alloy Steel" designation.
Yes. AISI 9840 and DIN 39NiCrMo3 (EN 1.6510) are equivalent steel grades under different national standards with nearly identical chemical composition and mechanical properties. Mill test certificates (MTC) can reference both designations simultaneously. Jiangsu Liangyi can supply material with EN 10204 3.1 or 3.2 MTC referencing both AISI and DIN/EN designations on the same document.
After quenching and tempering at 550–650°C, AISI 9840 typically achieves: Tensile Strength 930–1,180 MPa, Yield Strength 750–1,000 MPa, Elongation ≥12%, Reduction of Area ≥50%, Charpy Impact Energy ≥60 J at room temperature, hardness 27–34 HRC (277–352 HB). Exact values depend on section size, tempering temperature, and quench rate.
AISI 9840 is called a Triple Alloy Steel because it contains three principal alloying elements: Chromium (Cr), which improves hardenability and wear resistance; Nickel (Ni), which enhances toughness and impact resistance; and Molybdenum (Mo), which prevents temper embrittlement and maintains strength at elevated temperatures. The synergy of all three elements produces superior all-round performance compared to single or dual-alloyed steels.
White spots are hydrogen-induced internal cracks formed during rapid post-forge cooling. The nickel and chromium content retards hydrogen diffusion from the steel, making 9840 sensitive to this defect. Prevention: (1) Slow controlled post-forge cooling at ≤20°C/hour; (2) Immediate transfer to annealing furnace after forging — do not allow rapid air cooling; (3) Low-hydrogen melting practices during steelmaking (vacuum degassing/VD). Detection: ultrasonic testing (UT) per EN 10228-3 after heat treatment.
Jiangsu Liangyi provides the following standard documents with AISI 9840 forged parts: (1) Mill Test Certificate (MTC) per EN 10204 Type 3.1 or 3.2; (2) Full chemical composition analysis (heat and product analysis); (3) Mechanical property test results including tensile, impact, and hardness; (4) Ultrasonic testing (UT) report where specified; (5) Dimensional inspection report. Additional third-party inspection can be arranged upon request at additional cost. Our ISO 9001:2015 quality system governs all production and documentation processes.
AISI 9840 offers approximately 90% of AISI 4340's mechanical performance at lower material cost. Key differences: 9840 has lower nickel (0.85–1.15% vs 1.65–2.00%), lower maximum tensile strength (~1,180 MPa vs ~1,310 MPa), and lower cost. 4340 is specified for aerospace and ultra-high-strength applications. For automotive, industrial machinery, and energy sector forgings — crankshafts, gear shafts, connecting rods, transmission shafts — AISI 9840 is typically the most economical and technically sound material selection.
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