AISI 9310 (SAE 9310, UNS G93100) is a low-alloy nickel-chromium-molybdenum carburizing steel containing 3.00–3.50% Ni, 1.00–1.40% Cr, 0.08–0.15% Mo, and 0.08–0.13% C per ASTM A29/A29M. After gas carburizing at 900–950°C and full heat treatment per AMS 6260/6265, it achieves 58–62 HRC surface hardness with a tough 35–42 HRC core — the standard material for aerospace helicopter transmission gears. Its European equivalent is EN 1.6657 (14NiCrMo13-4); its Japanese equivalent is JIS SNCM815. Jiangsu Liangyi Co., Limited manufactures custom AISI 9310 open die forgings from 30 kg to 30,000 kg, ISO 9001:2015 certified, EN 10204 3.1 MTC standard.
Grade Designations Covered in This Article
AISI 9310 is a nickel-chromium-molybdenum low-alloy steel engineered for one specific outcome: a hard, wear-resistant carburized surface over a tough, impact-resistant core. It is the primary material specification for helicopter transmission gears, aircraft gearbox components, and heavy-duty industrial drivetrain parts where fatigue failure under cyclic contact loading is the defining design constraint. This guide gives engineers the complete technical picture — and gives procurement teams the right language to source correctly the first time.
What Is AISI 9310 Steel?
AISI 9310 (also designated SAE 9310, UNS G93100) is a low-alloy carburizing steel standardised under ASTM A29/A29M, with aerospace-grade variants covered by AMS 6260 (bar stock) and AMS 6265 (forging stock). It belongs to the SAE 9000-series family, where "93" indicates a nickel-chromium alloy base and "10" denotes approximately 0.10% nominal carbon content.
The alloy was designed for components that simultaneously require an extremely hard, wear-resistant surface (achieved by carburizing to 58–62 HRC) and a tough, ductile core that resists fracture under shock and fatigue loading. Its relatively low base carbon of 0.08–0.13% keeps the core ductile after quench and temper, while the 3.0–3.5% nickel content drives the deep hardenability needed to achieve ≥35 HRC core hardness in sections up to 150 mm diameter — far beyond what simpler carburizing grades such as AISI 8620 can deliver in the same section size.
AISI 9310 is chosen when engineers need a case-to-core hardness gradient that minimises sub-surface shear stress concentration at the case/core interface — the typical spalling initiation point in heavily loaded gear contacts. Its nickel content raises core hardenability enough to produce ≥30 HRC core hardness in sections up to 150 mm without through-hardening the entire part, preserving ductility where gear tooth roots experience tensile bending stress reversals.
In aerospace production, AISI 9310 is routinely processed as VIM+VAR (Vacuum Induction Melted + Vacuum Arc Remelted) double-melt per AMS 2300 and AMS 2304 to achieve maximum inclusion cleanliness. For industrial applications without AMS certification requirements, EAF+VD (Electric Arc Furnace + Vacuum Degassing) quality is generally sufficient. The melt route must be agreed with the forging supplier before order placement — it directly affects raw material cost and lead time.
Chemical Composition
The composition of AISI 9310 is tightly controlled across all alloying elements. The table below shows heat analysis limits per ASTM A29/A29M alongside the tighter AMS 6260/6265 aerospace requirements. Nickel (highlighted) is the grade-defining element that distinguishes 9310 from all other common carburizing steels.
| Element | ASTM A29 (%) | AMS 6260/6265 (%) | Function in Steel |
|---|---|---|---|
| Carbon (C) | 0.08 – 0.13 | 0.07 – 0.13 | Base hardness potential; kept low to preserve core toughness after carburizing |
| Nickel (Ni) ★ | 3.00 – 3.50 | 3.00 – 3.50 | Primary hardenability and toughness driver — the defining element of this grade |
| Chromium (Cr) | 1.00 – 1.40 | 1.00 – 1.45 | Surface hardness, wear resistance, and oxidation resistance in carburized case |
| Molybdenum (Mo) | 0.08 – 0.15 | 0.08 – 0.15 | Suppresses temper embrittlement; improves through-hardening in heavy sections |
| Manganese (Mn) | 0.45 – 0.65 | 0.40 – 0.70 | Hardenability contribution; binds sulfur to prevent hot-shortness during forging |
| Silicon (Si) | 0.15 – 0.35 | 0.15 – 0.35 | Deoxidiser; moderate effect on strength and high-temperature oxidation resistance |
| Phosphorus (P) | 0.025 max | 0.015 max | Grain-boundary embrittlement risk; AMS tightens limit for fatigue-critical parts |
| Sulfur (S) | 0.025 max | 0.010 max | Inclusion control; lower AMS limit reduces MnS inclusion density and fatigue crack initiation sites |
For aerospace applications, always specify AMS 6260 (bar) or AMS 6265 (forging stock) — not simply "AISI 9310". The tighter phosphorus and sulfur limits in AMS specifications directly reduce non-metallic inclusion density. This distinction is mandatory for rotating gear components in aerospace programs that reference MIL or DoD standards — always confirm the specific melt route requirement with your program office.
Why 3.0–3.5% Nickel Is the Key Design Choice
This nickel range was deliberately chosen to maximise hardenability in large cross-sections without pushing the alloy into fully through-hardening territory. At 3.0–3.5% Ni, quenching a carburized AISI 9310 forging up to 200 mm diameter produces a controlled case-to-core gradient: the surface achieves 58–62 HRC while the core reaches 35–42 HRC — hard enough to resist sub-surface shear under high Hertzian contact loading, yet ductile enough to absorb impact without brittle fracture.
Mechanical Properties
All values below represent typical quench-and-tempered core properties after carburizing and final hardening, tested on standard coupons per AMS 2370. Properties are strongly section-size and heat-treatment dependent.
| Property | Typical Value | Condition / Notes |
|---|---|---|
| Modulus of Elasticity (E) | 207 GPa (30 × 10⁶ psi) | Room temperature |
| Poisson's Ratio | 0.28 | Room temperature |
| Density | 7.83 g/cm³ (0.283 lb/in³) | — |
| Thermal Conductivity | ~36 W/m·K | At 20°C |
| Coefficient of Thermal Expansion | 12.3 × 10⁻⁶ /°C | 20 to 200°C |
| Charpy V-Notch Impact | ≥ 68 J (50 ft-lbf) | Core, Q&T condition |
| Rotating Bending Fatigue Limit | ~550 MPa | Carburized + shot-peened |
| Machinability Rating | ~55% of AISI 1212 | Best in normalised condition before carburizing |
Heat Treatment Guide
AISI 9310 requires a multi-stage heat treatment sequence. Temperature uniformity (±5°C across furnace load) and transfer time from furnace to quench tank are the two most critical process controls. The 7-step sequence below is the standard aerospace production protocol per AMS 2759.
Relieves residual forging stresses and produces a uniform ferritic-pearlitic microstructure before carburizing, ensuring consistent case depth across all section thicknesses in the same furnace load.
900 – 930 °C → Air cool to ambientGear blanks are rough-machined to leave adequate carburizing stock and remove decarburized forging surface. Non-carburize surfaces must be copper-plated or stop-off lacquered before furnace loading.
Parts held in endothermic or nitrogen-methanol atmosphere at elevated temperature. Carbon diffuses into the surface, raising surface carbon to 0.70–0.90%. Cycle time determines total case depth; effective case depth (ECD to 50 HRC) is the measured output specification.
900 – 950 °C · Carbon potential 0.80–0.95% · Endothermic / N₂-methanolA separate core hardening cycle refines the core grain structure that coarsens during extended high-temperature carburizing, restoring core impact toughness. Commonly required in aerospace OEM specifications — confirm with your applicable heat treatment drawing.
830 – 880 °C → Agitated oil quenchImmediately after quenching, the part is cooled cryogenically to transform retained austenite in the carburized case to martensite. Omitting this step allows retained austenite to transform in service under cyclic contact stress, causing dimensional change and accelerated surface fatigue (spalling). This is the most commonly omitted and most critical step for aerospace gear applications.
−70 to −80 °C · Hold 1–2 hours · Transfer within 1 hour of quenchThe case is re-austenitized at a lower temperature to minimise distortion, produce fine case grain size, and reduce residual retained austenite content before final tempering.
780 – 820 °C → Oil quench (agitated)Temper within 1–2 hours of final quench. Low temperature preserves case hardness above 58 HRC while relieving quench stresses that would otherwise cause cracking under service loading.
150 – 175 °C · 2 hours minimum · Air coolAlways specify the sub-zero treatment step explicitly on the heat treatment drawing or purchase order. Many commercial heat treaters omit it unless specifically called out. The incremental cost is negligible compared to the cost of a premature gear failure from retained austenite transformation in service.
Carburizing & Case Hardening In Depth
Carburizing is the defining process for AISI 9310. Understanding the key variables allows engineers to specify case depth accurately and enables procurement teams to evaluate supplier capability with the right technical questions.
ECD vs. TCD — What to Specify on the Drawing
Effective Case Depth (ECD) is the depth at which hardness falls to 50 HRC on a microhardness traverse from the surface. This is the only engineering-relevant case depth parameter and is what must be specified on the component drawing. Total Case Depth (TCD) is the depth at which carbon returns to the nominal base carbon (~0.13% for AISI 9310) — always deeper than ECD, used only as a manufacturing process control check, never as a design specification.
| Application | Typical ECD Range | Reference / Notes |
|---|---|---|
| Helicopter transmission gears | 0.90 – 1.40 mm | AMS 2301 / OEM drawings — standard specification range |
| Fixed-wing aircraft gearboxes | 0.75 – 1.25 mm | Confirm per applicable OEM or MIL specification |
| Heavy industrial gears (mining) | 1.50 – 3.00 mm | Deeper case for higher Hertzian contact stress; longer cycle time |
| Automotive / motorsport gears | 0.40 – 0.80 mm | Lower contact stress; cycle time optimised for cost |
| Piston pins / small shafts | 0.25 – 0.60 mm | Thin sections; through-hardening risk if ECD too deep |
Surface Carbon Content Control
For AISI 9310, optimal surface carbon after carburizing is 0.70–0.90%. Carbon above 0.90% creates carbide netting at prior austenite grain boundaries — a rejection criterion in AMS 2301 and AMS 2759/7. Carbon below 0.70% produces insufficient case hardness and reduced wear resistance.
Engineers sometimes substitute AISI 8620 for AISI 9310 in large-section gear forgings (>80 mm) to save material cost. In sections above 80 mm, AISI 8620 does not develop sufficient core hardness to support the carburized case under high Hertzian contact loads — the soft core yields beneath the case and spalling initiates sub-surface. AISI 9310 is the correct specification for heavy-section applications. The material cost premium is recovered many times over in service life extension.
Forging Process & Key Parameters
Open die forging is the standard manufacturing route for custom AISI 9310 components above approximately 30 kg. Forging closes internal voids in the as-cast ingot and creates controlled fibrous grain flow aligned with the stress-carrying direction of the finished component — benefits casting and rolled bar cannot replicate.
| Parameter | Recommended Range | Critical Control Note |
|---|---|---|
| Ingot Soak Temperature | 1,180 – 1,220 °C | Soak 1 hr per 100 mm of section; uniform temperature throughout |
| Start Forging Temperature | 1,150 – 1,200 °C | Above this range: incipient melting risk at grain boundaries |
| Finish Forging Temperature ★ | ≥ 925 °C (≥ 1,700 °F) | Never work below this limit — surface cracking and tearing will occur |
| Minimum Forging Reduction Ratio | ≥ 4:1 (≥ 6:1 preferred) | 4:1 for void closure; 6:1 for UT Class 4 acceptance |
| Post-Forge Cooling | Controlled slow cool to ≤ 300 °C | Rapid cooling risks hydrogen-induced flaking in heavy Ni-steel forgings |
| Inter-Pass Reheat | Return to 1,100 – 1,150 °C | Reheat if temperature approaches 925 °C; never force reduction when temperature is uncertain |
Forging Shapes Available in AISI 9310
At Jiangsu Liangyi Co., Limited, we produce the complete range of AISI 9310 forging shapes in a wide range of sizes and weights:
- Gear blanks (discs with bore): Most common 9310 product — round discs for gear tooth cutting. Max OD 2,000 mm.
- Gear shafts and pinion shafts: Stepped-diameter shafts with integral flanges. Max length 15 m, max diameter 1,800 mm.
- Seamless rolled rings: For ring gears, race rings, and bearing rings. OD up to 6,000 mm.
- Hollow forgings and sleeves: For ring gear bores and planetary carrier housings.
- Forged bars and blocks: For multi-part cutting from a single forging.
Specify the grain flow direction relative to the principal stress axis on your drawing. For a 9310 gear shaft, longitudinal grain flow (along the shaft axis) places the strongest microstructure orientation parallel to the bending and torsional stresses the part carries in service. A supplier who cannot provide a grain flow sketch on request is a capability red flag.
Global Grade Cross-Reference
AISI 9310 is specified under different designations in different national standards systems. The table below provides the most complete international cross-reference available for procurement and dual-standard compliance documentation. Always verify specific chemistry ranges against your project standard before finalising any grade substitution.
| Standard System | Grade Designation | Key Reference | Notes |
|---|---|---|---|
| USA — AISI / SAE ★ | AISI 9310 / SAE 9310 | ASTM A29/A29M | Reference grade — this article |
| USA — Aerospace | AMS 6260 / AMS 6265 | SAE Aerospace Material Specs | Tighter P, S; VIM+VAR typically required |
| Germany / EU (DIN/EN) | 1.6657 · 14NiCrMo13-4 | EN 10084, DIN 17210 | Very close equivalent; minor Cr range difference |
| Japan (JIS) | SNCM815 | JIS G4053 | Close; slightly higher Ni upper limit |
| United Kingdom (BS) | 835M15 / 835A15 | BS 970 Part 3 | Approximate — confirm full chemistry before substitution |
| France (AFNOR) | 14NCD13 | NF A35-551 | Near equivalent; used in French aerospace programs |
| China (GB) | 12Cr2Ni4A | GB/T 3077 | Similar; Cr slightly higher — verify with supplier MTC |
| Sweden (SS) | SS 2523 | Swedish Standard | Approximate; verify nickel range |
When a project specifies AISI 9310 but material is supplied to EN 1.6657, Jiangsu Liangyi prepares a dual-standard compliance statement on the EN 10204 3.1 MTC — confirming the supplied material satisfies both EN 10084 requirements for 1.6657 and AISI 9310 chemistry limits where they overlap. This service is provided at no additional cost.
AISI 9310 vs. Other Case-Hardening Grades
Engineers frequently evaluate several Ni-Cr-Mo alloy steels for carburizing applications. The comparison below clarifies where AISI 9310 is the unambiguous correct choice versus its most common alternatives.
- Aerospace / helicopter gears requiring AMS 6260 or 6265 compliance
- Heavy sections (>80 mm) needing ≥35 HRC core after carburizing
- Maximum fatigue life in contact-loaded rotating components
- Applications requiring high inclusion cleanliness per AMS 2300 (confirm melt route with supplier)
- Parts with strict retained austenite and sub-zero treatment specifications
- Light-to-medium section gears (≤ 80 mm diameter)
- Cost-sensitive industrial gearboxes without AMS certification
- High-volume automotive drivetrain parts at scale
- Applications where core hardness ≤ 30 HRC is acceptable by design
- Spline shafts, camshafts, lightly loaded carburized components
- Through-hardened structural parts — not typically carburized
- High tensile strength (1,200–1,500 MPa) without case requirement
- Aerospace landing gear, drive shafts, structural forgings
- Bending/torsion fatigue dominant — not Hertzian contact-stress driven
- European-standard projects requiring EN 10084 / EN 10228 MTC
- Wind turbine gearbox ring gears designed to ISO 6336
- Projects where DIN / EN mill certificates are contractually mandatory
- Functionally equivalent to 9310 — specify dual-standard MTC for both
Typical Applications of AISI 9310 Forgings
AISI 9310 has established a dominant position in applications where fatigue life, surface hardness, and core impact toughness under dynamic loading must coexist. Its use spans three major industry groups.
Aerospace & Defense
- Helicopter main transmission gears: The primary application for AISI 9310 worldwide. Spiral bevel and planetary gear sets in main rotor transmissions are nearly universally specified as 9310 or equivalent, driven by AMS 6260/6265 requirements for tighter chemical composition and inclusion cleanliness — confirm specific melt route requirements with your program documentation.
- Turbofan accessory gearbox components: Spur and helical gears transmitting power from the engine core to accessories — hydraulic pumps, generators, fuel control units, and environmental control compressors.
- Flight control actuator gearing: Where backlash control, dimensional stability, and long wear life under cyclic loading are critical to flight safety.
- Piston engine gudgeon (wrist) pins: In piston aircraft engines, combining high surface hardness with impact toughness under oscillating contact load.
Industrial Power Transmission
- Mining mill drive gears: Bull gears, pinion shafts, and ring gears for SAG, ball, and rod mills — often above 5,000 kg per piece — with ECD of 1.5–3.0 mm to handle very high Hertzian contact stresses.
- Wind turbine gearbox internals: Planet gears, ring gears, and sun shafts in multi-MW drivetrains, where AGMA 9309 case depth requirements align well with AISI 9310 carburizing capability.
- Marine main propulsion reduction gears: For naval vessels and large commercial ships requiring certified melt traceability and extended service intervals.
- Rolling mill drive shafts: For aluminium and steel rolling mill main drives subjected to high torque reversals and cyclic bending loads.
Automotive & Motorsport
- High-performance racing gearboxes: Sequential transmissions in FIA-class motorsport where 9310 is preferred over 8620 in large-section gear packs for its superior fatigue life per unit weight.
- Heavy-duty commercial vehicle differentials: Ring-and-pinion sets in Class 8 truck rear axles and multi-axle drive systems for high-payload applications.
Sourcing & Procurement Checklist
When issuing an RFQ for custom AISI 9310 open die forgings, the checklist below ensures a technically complete quotation and avoids the most common misalignments that cause specification disputes or delayed delivery after order placement.
| # | RFQ Data Point | What to Specify | Why It Matters |
|---|---|---|---|
| 1 | Material Standard | AISI 9310 / AMS 6260 / AMS 6265 / EN 1.6657 | Determines chemistry limits, melt route, and MTC type |
| 2 | Melt Route | EAF+VD (industrial) or VIM+VAR (aerospace) | Directly affects inclusion cleanliness, cost, and lead time |
| 3 | Effective Case Depth ★ | ECD range in mm to 50 HRC (e.g., 0.90–1.20 mm) | The most critical carburizing parameter — must be on the drawing |
| 4 | Sub-Zero Treatment | Yes/No; specify temperature (°C) and hold time (hours) | Often omitted by default; critical for retained austenite elimination |
| 5 | Delivery Condition | Normalised; or carburized + Q&T (specify hardness targets) | Must align with your downstream processing plan |
| 6 | Heat Treatment Drawing | Provide your HT drawing or request supplier's for approval | Confirms carburize cycle, case depth, and temper match design intent |
| 7 | Forging Reduction Ratio | Minimum 4:1 or 6:1 per UT class requirement | Determines internal soundness level and UT pass rate |
| 8 | NDT Requirements | UT class (ASTM A388 / EN 10228-3), MT/PT acceptance criteria | Inspection costs and manufacturing yield depend on acceptance level |
| 9 | Mill Test Certificate | EN 10204 3.1 standard | Confirm MTC type with supplier before order; specify early |
| 10 | Forging Dimensions & Weight | Rough-forged envelope + machining allowance + estimated weight | Enables press sizing, billet weight calculation, and accurate pricing |
Supplier Qualification — Questions That Reveal Real Capability
- Can you provide a microhardness traverse (surface-to-core profile) from a recent 9310 batch at the same ECD we require?
- What atmosphere control method do you use for carburizing — probe, infrared carbon sensor, or shim stock?
- Is sub-zero (cryogenic) treatment performed in-house or subcontracted? What chamber temperature and calibration interval?
- What melt routes are available (EAF, EAF+VD, or others)? Confirm in writing before order placement.
- What is your standard minimum forging reduction ratio, and can you provide a forging flow sketch showing grain direction?
- Which third-party inspection bodies can you coordinate with for your project's requirement?
For custom AISI 9310 open die forged gear blanks, shafts, and rings, seamless rolled rings, and gear blanks from 30 kg to 30,000 kg, Jiangsu Liangyi provides EN 10204 3.1 Mill Test Certificates as standard. ISO 9001:2015 certified since 1997. 80,000 m² facility. 120,000 ton/year capacity. 50+ countries served. 24-hour quote turnaround. Submit your drawings and RFQ via our inquiry form.
Frequently Asked Questions About AISI 9310
Concise, authoritative answers to the questions engineers and procurement teams ask most often about AISI 9310 steel and open die forgings.
AISI 9310 (SAE 9310, UNS G93100) is a low-alloy nickel-chromium-molybdenum case-hardening steel per ASTM A29/A29M. It contains 3.00–3.50% Ni, 1.00–1.40% Cr, 0.08–0.15% Mo, and 0.08–0.13% C. After gas carburizing at 900–950°C and heat treatment per AMS 6260/6265, it achieves 58–62 HRC surface hardness with a tough 35–42 HRC core. It is the standard aerospace helicopter transmission gear material and is used in heavy industrial drivetrain components requiring deep-section hardenability.
The closest European equivalent to AISI 9310 is EN 1.6657, also designated 14NiCrMo13-4, per EN 10084 and DIN 17210. The Japanese JIS equivalent is SNCM815 per JIS G4053. The UK equivalent is BS 835M15 per BS 970 Part 3. The French equivalent is 14NCD13 per NF A35-551. The Chinese GB equivalent is 12Cr2Ni4A per GB/T 3077. These are near-equivalents — verify the specific chemistry against your project standard before finalising a substitution. Jiangsu Liangyi can supply dual-standard compliance documentation for AISI 9310 / EN 1.6657 on the same MTC at no additional cost.
AISI 9310 requires a 6–7 stage heat treatment: (1) Normalize at 900–930°C; (2) Gas carburize at 900–950°C to 0.70–0.90% surface carbon; (3) Core quench at 830–880°C + oil quench; (4) Sub-zero treatment at −70 to −80°C for 1–2 hours — mandatory for aerospace applications; (5) Case re-austenitize at 780–820°C + oil quench; (6) Low-temperature temper at 150–175°C for 2 hours minimum. The sub-zero step is the most commonly omitted and most critical step for aerospace gear applications.
The key difference is nickel content: AISI 9310 contains 3.00–3.50% Ni versus AISI 8620's 0.40–0.70% Ni. This higher nickel gives 9310 significantly greater hardenability — achieving ≥35 HRC core hardness in sections up to 150 mm after carburizing. AISI 8620 is limited to sections below 80 mm where it can develop adequate core hardness. For aerospace transmission gears, helicopter drivetrain components, and large industrial gear forgings above 80 mm diameter, AISI 9310 is the correct specification. AISI 8620 is suitable for lighter-section, cost-sensitive automotive and industrial applications.
For helicopter transmission gears, the typical effective case depth (ECD) of AISI 9310 is 0.90–1.40 mm measured to 50 HRC, per AMS 2301 and typical OEM drawings. For fixed-wing aircraft gearboxes, ECD is typically 0.75–1.25 mm. For heavy industrial gears such as mining mill bull gears, ECD ranges from 1.50–3.00 mm. Always specify ECD (not Total Case Depth) on engineering drawings and purchase orders — ECD is the engineering-relevant parameter.
Jiangsu Liangyi supplies AISI 9310 forgings with EN 10204 3.1 Mill Test Certificates as standard, confirming chemical composition, mechanical properties, and heat treatment results. We hold ISO 9001:2015 certification. For projects requiring specific melt route documentation or NDT reports, please specify all requirements in your RFQ so we can confirm feasibility and lead time. Contact us via our inquiry form for a 24-hour quotation.
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