What is AISI 9840 steel?
AISI 9840 is also written SAE 9840 and carries the number UNS G98400. It is a medium-carbon nickel-chromium-molybdenum alloy steel containing nominally 0.40% C, 1.0% Ni, 0.80% Cr and 0.25% Mo. It is a through-hardening grade with high hardenability, specified for highly stressed components of large cross section in aircraft, automotive and general engineering work: forged propeller shafts, connecting rods, gear shafts, crankshafts and landing-gear parts.
AISI 9840 is chemically close to AISI 4340 but carries about half the nickel. That makes it a lower-cost substitute in section sizes where the extra hardenability of 4340 is not needed. In the quenched and tempered condition (25 mm round, 540 C temper) it reaches roughly 1240 MPa tensile strength, 1105 MPa yield strength and 361 HB.
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory in Jiangyin City, Jiangsu Province, China that forges AISI 9840 to customer drawings in single-piece weights from 50 kg to 30 tonnes. Contact: sales@steelforgepieces.com, +86 189 2135 9659.
AISI 9840 key facts
| Grade designation | AISI 9840 / SAE 9840 |
|---|---|
| UNS number | G98400 |
| Steel type | Medium-carbon nickel-chromium-molybdenum (Ni-Cr-Mo) low-alloy steel, through hardening |
| Nominal composition | 0.40% C, 1.0% Ni, 0.80% Cr, 0.25% Mo |
| Covering standards | SAE J404, ASTM A322, ASTM A519, ASTM A274, AMS 6342 |
| Nearest equivalents | DIN/EN 36CrNiMo4 (1.6511), BS 816M40, UNI 38NiCrMo4 |
| Typical supply condition | Normalised, or quenched and tempered (Q+T) |
| Tensile strength (Q+T, 540 C temper) | 1240 MPa (180 ksi) |
| Yield strength (Q+T, 540 C temper) | 1105 MPa (160 ksi) |
| Hardness (Q+T, 540 C temper) | 361 HB, about 39 HRC |
| Forging temperature | 1175 C down to 925 C (2150-1700 F) |
| Density | 7.85 g/cm3 (0.284 lb/in3) |
| Forged forms supplied | Round, flat and square bars; hollow bars and sleeves; seamless rolled rings; forged rings; discs, blocks and plates; gear shafts, pinion shafts and spindles |
| Manufacturer | Jiangyin Jiangnan Metal Co., Ltd., Jiangyin, Jiangsu, China |
Chemical composition of AISI 9840
AISI 9840 is specified in SAE J404 as a 98xx-series steel. The "98" identifies the nickel-chromium-molybdenum family and the "40" gives the nominal 0.40% carbon. Nickel and chromium raise hardenability and toughness. Molybdenum suppresses temper embrittlement and holds strength at elevated temperature.
| Element | Symbol | Min % | Max % | Function in the alloy |
|---|---|---|---|---|
| Carbon | C | 0.38 | 0.43 | Sets attainable hardness and strength after quenching |
| Manganese | Mn | 0.70 | 0.90 | Increases hardenability, binds residual sulphur |
| Phosphorus | P | - | 0.040 | Residual, kept low to avoid cold shortness |
| Sulphur | S | - | 0.040 | Residual, kept low for transverse toughness |
| Silicon | Si | 0.20 | 0.35 | Deoxidiser, raises tempering resistance |
| Nickel | Ni | 0.85 | 1.15 | Improves low-temperature toughness and hardenability |
| Chromium | Cr | 0.70 | 0.90 | Hardenability and wear resistance |
| Molybdenum | Mo | 0.20 | 0.30 | Resists temper embrittlement, holds hot strength |
| Iron | Fe | Balance | Base metal | |
Product analysis tolerances per ASTM A29/A29M apply. Our AISI 9840 is melted in an electric arc furnace with ladle refining and vacuum degassing. The actual heat analysis, including tramp elements, is reported on every mill certificate.
AISI 9840 standards and equivalent grades
The table below lists the specifications that cover AISI 9840 and the nearest grades in other national systems. Cross-references are close matches on composition. They are not guarantees of interchangeability, so confirm the exact specification on your order.
| System and country | Designation | Notes |
|---|---|---|
| AISI / SAE (USA) | 9840, SAE J404, SAE J1249 | Primary designation |
| UNS (USA) | G98400 | Unified numbering |
| ASTM (USA) | A322, A519, A274, A29 | Bar, mechanical tubing, general requirements |
| AMS (aerospace) | AMS 6342 | Bar, forgings and tubing, same composition |
| EN / DIN (Germany) | 36CrNiMo4, 1.6511 | Closest European equivalent |
| BS (UK) | 816M40 | Close equivalent |
| UNI (Italy) | 38NiCrMo4 | Close equivalent |
| JIS (Japan) | SNCM439 (approximate) | Higher nickel, verify before substitution |
| GB (China) | 40CrNiMoA (approximate) | Higher nickel, verify before substitution |
Mechanical properties of AISI 9840
AISI 9840 develops its properties by quenching and tempering. Tempering temperature is the main control. Higher tempering trades strength for ductility and impact toughness. Values below are typical for a 25 mm (1 in.) round oil quenched from 845 C.
| Tempering temperature | Tensile strength | Yield strength (0.2%) | Elongation in 50 mm | Reduction of area | Hardness |
|---|---|---|---|---|---|
| 205 C (400 F) | 1980 MPa | 1760 MPa | 10% | 39% | 578 HB |
| 315 C (600 F) | 1760 MPa | 1610 MPa | 11% | 44% | 507 HB |
| 425 C (800 F) | 1500 MPa | 1365 MPa | 13% | 50% | 429 HB |
| 540 C (1000 F) | 1240 MPa | 1105 MPa | 15% | 55% | 361 HB |
| 650 C (1200 F) | 1000 MPa | 860 MPa | 19% | 60% | 302 HB |
Indicative values from published data for small sections. Heavy forgings cool more slowly and will show lower strength at the same tempering temperature. This mass effect is why properties on large forgings are agreed per order. Guaranteed values, test-piece location and sampling direction are fixed at the enquiry stage and reported on the EN 10204 3.1 certificate.
Typical delivery conditions
| Condition | Hardness | Used for |
|---|---|---|
| Annealed | 235 HB max | Maximum machinability before final heat treatment |
| Normalised | 250-300 HB | Stock removal, uniform structure, stress relief |
| Quenched and tempered | 27-34 HRC, about 269-330 HB | General engineering, shafts and rings in service condition |
| Quenched, low temper | up to 55-59 HRC as quenched | Special cases, usually tempered back for toughness |
Physical properties
| Density | 7.85 g/cm3 (0.284 lb/in3) |
|---|---|
| Modulus of elasticity | 205 GPa (29.7 x 10^6 psi) |
| Poisson ratio | 0.29 |
| Thermal conductivity | about 44 W/m.K |
| Specific heat capacity | about 475 J/kg.K |
| Coefficient of thermal expansion, 20-100 C | about 11.6 x 10^-6 per K |
| Magnetic | Yes, ferromagnetic |
Physical properties vary only slightly with heat-treatment condition and are typical values for low-alloy steels of this class.
AISI 9840 heat treatment
A forged AISI 9840 component normally passes through the route below. Steps 1 and 2 condition the structure after forging. Steps 3 and 4 set the final mechanical properties.
- Anneal, 830 C, slow cool Austenitise at 1525 F (830 C), then cool slowly from 1280 F to 1200 F (693 C to 649 C) as an isothermal anneal. This produces the coarse pearlite structure that gives the best machinability, and it doubles as a dehydrogenation treatment against white-spot defects.
- Normalise, 870 C, air cool Heat to 1600 F (870 C) and air cool. Refines the grain after forging and evens out structure through the section before hardening.
- Harden, 830-860 C, quench Austenitise at 1525-1575 F (830-860 C). Oil quench thin or intricate shapes. Water quench large, simple shapes. As-quenched hardness reaches about 55-59 HRC.
- Temper, 540-680 C, immediately after quenching Temper to the required properties. A 550-650 C temper gives roughly 27-34 HRC. Do not leave parts untempered. Quenching stresses will crack heavy sections.
Soak times follow section thickness, typically one hour per 25 mm at temperature. Because AISI 9840 is a white-spot sensitive steel, annealing should be arranged immediately after forging rather than allowing heavy sections to air cool.
Forging practice for AISI 9840
AISI 9840 forges well over a wide window. Heat uniformly and start forging at 2150 F (1175 C). Finish above 1700 F (925 C) and reheat whenever the workpiece falls below roughly 950 C, since continuing to work cold steel tears the grain and opens internal defects.
Cooling after the last blow matters as much as the forging itself. AISI 9840 is sensitive to flake cracking, also called white spot, caused by hydrogen. Cool in sand or in the furnace rather than in still air, and run the anneal straight away. Skipping this step is the most common cause of rejected 9840 forgings. The damage usually shows up at ultrasonic inspection, sometimes not until machining.
| Start forging temperature | 1175 C (2150 F) |
|---|---|
| Finish forging temperature | 925 C (1700 F) |
| Reheat below | 950 C |
| Post-forging cooling | Sand or furnace cooling, never still air on heavy sections |
| Post-forging treatment | Anneal immediately, for stress relief and dehydrogenation |
| Typical forging ratio | 3:1 minimum, 4:1 or higher for critical shafts and rings |
Machining and welding AISI 9840
Machinability
AISI 9840 machines readily, and best from a coarse pearlitic structure. That is what the isothermal anneal in step 1 produces. Rough machining is normally done in the annealed or normalised condition, with final machining after quenching and tempering. Expect machinability around 55-60% of AISI 1212 free-cutting steel in the annealed state.
Weldability
AISI 9840 can be welded, but its carbon and alloy content make it crack-sensitive. Pre-heat and post-weld stress relief are both recommended. Do not weld in the hardened and tempered condition. The heat-affected zone loses its tempered properties and becomes a crack initiation site. Where welding is unavoidable on a heat-treated part, agree the procedure and the post-weld treatment before production starts.
Nitriding
AISI 9840 responds to gas nitriding. A 525 C nitride over a 540 C tempered core gives a surface around 88 HR15N at a case depth of 0.6-0.8 mm, with the core holding about 35 HRC. This suits journals and gear teeth that need wear resistance without losing core toughness.
What AISI 9840 is used for
AISI 9840 is specified where a component is large in section and highly stressed, and where the cost of full 4340 is not justified. Common applications:
| Industry | Typical components |
|---|---|
| Aerospace | Landing-gear components, forged propeller shafts, structural fittings |
| Automotive and heavy vehicle | Crankshafts, connecting rods, drive shafts, steering knuckles |
| Power transmission | Gear shafts, pinion shafts, gear blanks, couplings, spindles |
| Oil, gas and drilling | Drill collars, sub assemblies, valve bodies, pressure housings |
| Heavy machinery | Press components, mill shafts, hydraulic cylinder parts, large sleeves |
| Marine | Propeller and tail shafts, rudder stocks, heavy-duty fasteners |
AISI 9840 compared with 4340, 4140 and 8740
All four are Cr-Mo or Ni-Cr-Mo through-hardening steels at roughly 0.40% carbon. They differ almost entirely in nickel and chromium content, which sets hardenability, meaning the maximum section that will harden right through.
| Grade | C % | Ni % | Cr % | Mo % | Relative hardenability | Relative cost |
|---|---|---|---|---|---|---|
| AISI 9840 | 0.38-0.43 | 0.85-1.15 | 0.70-0.90 | 0.20-0.30 | High | Medium |
| AISI 4340 | 0.38-0.43 | 1.65-2.00 | 0.70-0.90 | 0.20-0.30 | Very high | Highest |
| AISI 4140 | 0.38-0.43 | - | 0.80-1.10 | 0.15-0.25 | Medium | Lowest |
| AISI 8740 | 0.38-0.43 | 0.40-0.70 | 0.40-0.60 | 0.20-0.30 | Medium-high | Low-medium |
How to choose
4140 is the cheapest of the four and the usual first choice under about 100 mm section. Above that it starts to run out of hardenability, and 9840 takes over: most of the performance of 4340 at a lower price, which makes it the common answer for shafts and rings in the 100-200 mm range. 4340 earns its premium on the heaviest sections, at the highest strength levels, and whenever an aerospace or oilfield specification names it outright. 8740 is a leaner Ni-Cr-Mo option that sometimes wins on availability.
We forge all four grades, so if you are open on grade, send the section size and the required properties and we will tell you which is the cheapest steel that will actually meet them.
AISI 9840 forged forms and sizes we make
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory, so AISI 9840 parts are forged to your drawing or to a machining envelope rather than cut from stock. There is no tooling cost and no minimum order quantity. Single prototype pieces and repeat production runs are both normal work.
| Product form | Size range | Notes |
|---|---|---|
| Round bars and rods | Dia. 80-1200 mm, length to 8000 mm | Rough turned, peeled or black as required |
| Square, rectangular, flat and hex bars | Up to 1000 mm across flats | Cut to length, saw-cut ends |
| Hollow bars, sleeves, bushes, bushings | OD 200-2500 mm, ID from 100 mm | Bored and trepanned, wall to drawing |
| Seamless rolled rings | OD 200-4000 mm, height to 1000 mm | Ring rolled, rectangular or profiled section |
| Forged rings and flanges | OD to 3000 mm | Saddle-forged or mandrel-forged |
| Discs, disks, blocks and plates | Dia. or width to 2500 mm, thickness to 800 mm | Upset forged for grain flow across the face |
| Pipes, tubes, cylinders, casings, shells, barrels, housings | OD to 2500 mm | Forged and bored, not welded |
| Gear shafts, pinion shafts, spindles, step shafts | Length to 8000 mm, dia. to 1200 mm | Stepped and pre-machined to drawing |
Single-piece weight range 50 kg to 30 tonnes. Sizes outside these ranges are often still possible. Send the drawing and we will confirm.
Testing and certification
Every AISI 9840 forging is released by our QC department against the following checks, and all individual values are reported on an EN 10204 3.1 mill test certificate:
| Check | Standard or method | Reported |
|---|---|---|
| Melting route and heat number | EAF plus LF plus vacuum degassing | Heat number and melting type |
| Chemical analysis | Spectrometric, ASTM A29 product tolerances | Full analysis including tramp elements |
| Mechanical tests | ASTM A370 or ISO 6892 tensile, ISO 148 Charpy impact | All individual values, test-piece location |
| Hardness | Brinell, ASTM E10 | Individual readings |
| Heat-treatment record | Charted furnace cycle | Full cycle followed, including quench medium |
| Ultrasonic testing | EN 10228-3 or ASTM A388, acceptance class on order | UT report |
| Surface and visual | Magnetic particle to ASTM E709 where specified | MT report |
| Dimensional inspection | Against drawing or agreed machining allowance | Dimensional report and marking check |
| Grain size and macro | ASTM E112, macro-etch where specified | On request |
Third-party inspection by SGS, BV, TUV, Lloyd's Register or your own inspector is welcome and can be arranged at the enquiry stage.
How to order AISI 9840 forgings
Send a drawing in PDF, DWG or STEP format, or just the dimensions. A quotation normally comes back within 24 hours. To price it in one pass rather than three, include:
- Grade and specification. AISI 9840, plus any standard the order must meet, such as AMS 6342, ASTM A322 or a customer spec.
- Dimensions. Finished sizes plus machining allowance, or the raw forging envelope you want.
- Quantity. Prototype, batch or annual volume.
- Heat-treatment condition. Annealed, normalised, or Q+T with the hardness or mechanical properties you need.
- Machining level. As forged, rough turned, or finish machined.
- Testing. UT class, impact test temperature, third-party inspection, certificate type.
- Destination port. For FOB, CIF or DDP pricing.
- Manufacturer
- Jiangyin Jiangnan Metal Co., Ltd.
- Plant type
- Open-die forging factory
- Address
- No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China
- Phone and WhatsApp
- +86 189 2135 9659
- sales@steelforgepieces.com
- Website
- www.steelforgepieces.com
- Ships to
- Worldwide, from Shanghai and Zhangjiagang ports
AISI 9840 frequently asked questions
What is AISI 9840 steel?
AISI 9840 (SAE 9840, UNS G98400) is a medium-carbon nickel-chromium-molybdenum alloy steel containing nominally 0.40% carbon, 1.0% nickel, 0.80% chromium and 0.25% molybdenum. It is a through-hardening grade with high hardenability, used for highly stressed components of large cross section such as propeller shafts, connecting rods, gear shafts, crankshafts and landing-gear parts.
What is the chemical composition of AISI 9840?
0.38-0.43% C, 0.70-0.90% Mn, 0.20-0.35% Si, 0.85-1.15% Ni, 0.70-0.90% Cr and 0.20-0.30% Mo, with phosphorus and sulphur each limited to 0.040% maximum. The balance is iron.
What is the difference between AISI 9840 and AISI 4340?
Carbon, chromium and molybdenum are almost identical. The difference is nickel: 9840 carries 0.85-1.15% against 1.65-2.00% in 4340. Lower nickel makes 9840 cheaper and slightly less hardenable in very heavy sections, while keeping most of the strength and toughness of 4340. For sections up to roughly 150 mm, AISI 9840 is often a direct cost-saving substitute for 4340.
What are the equivalent grades for AISI 9840?
The closest published equivalents are DIN/EN 36CrNiMo4 (1.6511), BS 816M40 and UNI 38NiCrMo4, with JIS SNCM439 and GB 40CrNiMoA as approximate matches carrying more nickel. Aerospace specification AMS 6342 covers the same composition. Cross-references are indicative, so confirm the required specification on the order.
At what temperature is AISI 9840 forged?
Between about 1175 C and 925 C (2150-1700 F). Reheat if the workpiece drops below roughly 950 C. Because 9840 is sensitive to flake or white-spot cracking, cool slowly in sand or in the furnace after forging and anneal immediately rather than air-cooling heavy sections.
How is AISI 9840 heat treated?
Normalise at 870 C and air cool. Anneal by austenitising at 830 C and cooling slowly through 693-649 C for coarse pearlite and optimum machinability. Harden from 830-860 C, oil quench for thin or intricate shapes and water quench for large simple shapes, reaching about 55-59 HRC as quenched. Temper between 540 C and 680 C to the required properties. A 550-650 C temper gives roughly 27-34 HRC.
Can AISI 9840 be welded?
Yes, with care. Pre-heat and post-weld stress relief are both recommended. Do not weld in the hardened and tempered condition, because the heat-affected zone loses its tempered properties and may crack.
Who supplies AISI 9840 forgings?
Jiangyin Jiangnan Metal Co., Ltd., an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, supplies AISI 9840 forgings worldwide. Forms include round and flat bars, hollow bars and sleeves, seamless rolled rings, forged rings, discs, blocks, plates, gear shafts, pinion shafts and spindles, with EN 10204 3.1 certification. Contact sales@steelforgepieces.com or +86 189 2135 9659.
What product forms and sizes are available in AISI 9840?
Round bars 80-1200 mm diameter, flat and square bars, hollow bars and sleeves OD 200-2500 mm, seamless rolled rings OD 200-4000 mm, discs and blocks to 2500 mm, and shafts to 8000 mm long. Single-piece weights run from 50 kg to 30 tonnes.
What certification comes with AISI 9840 forgings?
An EN 10204 3.1 mill test certificate reporting the heat number and melting route, chemical analysis including tramp elements, the full heat-treatment cycle, mechanical and hardness results, dimensional inspection, and ultrasonic examination to EN 10228-3 or ASTM A388 where specified.
Is there a minimum order quantity for AISI 9840 forgings?
No. Open-die forging needs no dedicated tooling, so a single prototype piece is as feasible as a production batch. Send the drawing and the quantity and we will quote both.
About the manufacturer
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. The plant sits in the Yangtze River delta forging cluster, within two hours of Shanghai port. We forge to customer drawings in carbon steel, alloy steel, tool steel, stainless steel and nickel alloys, including AISI 9840, AISI 4340, AISI 4140, 42CrMo4, Inconel and Nimonic grades.
Products are supplied as forged bars, seamless rolled rings, forged rings and flanges, hollow bars and sleeves, discs, blocks and plates, and gear shafts, pinion shafts and spindles, in single-piece weights from 50 kg to 30 tonnes, with EN 10204 3.1 certification and third-party inspection on request.
Data on this page is compiled from SAE J404, ASTM A322 and AMS 6342 together with our own production records. Published typical values are given for guidance. Order-specific values are confirmed on the mill certificate. Last updated .