AISI 4340 Forgings
UNS G43400 · SAE 4340 · ASTM A29/A29M
Open-die forged rings, shafts, flanges, discs, blocks and bars in AISI 4340 nickel-chromium-molybdenum steel, forged and heat treated to your drawing by Jiangyin Jiangnan Metal Co., Ltd. in Jiangyin, Jiangsu, China.
- Nominal carbon
- 0.40 %0.38–0.43 per ASTM A29
- Nickel
- 1.65–2.00 %deep hardenability
- Tensile, Q&T 540 °C
- 1240 MPa180 ksi typical
- Annealed hardness
- 217 HBmaximum
- Certification
- EN 102043.1 standard, 3.2 on request
AISI 4340 is a through-hardening low-alloy steel with nominally 0.40 % carbon, 1.8 % nickel, 0.8 % chromium and 0.25 % molybdenum. The nickel and molybdenum additions give it far deeper hardenability than plain chromium-molybdenum grades such as AISI 4140, so a heavy forging can be quenched and tempered to high strength without the core being left soft. It is the usual choice for forged shafts, gears, couplings and pressure components that have to combine 1000 to 1250 MPa tensile strength with good impact toughness.
The forgings described on this page are produced by Jiangyin Jiangnan Metal Co., Ltd., an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. Drawings for quotation: sales@steelforgepieces.com.
Key facts
- AISI 4340 carries the designation UNS G43400 and is specified in ASTM A29/A29M, ASTM A322, ASTM A434 and ASTM A646, and in SAE J404.
- Composition per ASTM A29/A29M is C 0.38–0.43, Mn 0.60–0.80, Si 0.15–0.35, Ni 1.65–2.00, Cr 0.70–0.90, Mo 0.20–0.30 %, with P 0.035 % and S 0.040 % maximum.
- Annealed AISI 4340 is about 745 MPa tensile and 217 HB; oil quenched and tempered at 540 °C it reaches about 1240 MPa tensile and 1160 MPa yield; tempered at 205 °C it can exceed 1900 MPa.
- Oil quenching produces a mainly martensitic structure to about 100 mm of ruling section, which is why 4340 is preferred over 4140 in heavy forgings.
- Near-equivalent grades are EN 40NiCrMo8-4 (1.6565), BS 817M40 (EN24), JIS SNCM439, GB 40CrNiMoA and GOST 40KhN2MA.
- Jiangyin Jiangnan Metal Co., Ltd. melts AISI 4340 by EAF + LF + VD and forges it on open-die hammers and presses, with seamless rolled rings produced on a ring rolling mill.
- Every AISI 4340 forging is supplied with an EN 10204 3.1 mill certificate; EN 10204 3.2 with TUV, BV, LR, DNV or SGS witness is available on request.
- Ultrasonic testing is performed to EN 10228-3, SEP 1921 or ASTM A388, to the acceptance class stated on the order.
What AISI 4340 is
AISI 4340 is a medium-carbon, low-alloy through-hardening steel of the nickel-chromium-molybdenum family. It was developed so that large sections could be hardened to the core, and that is still the reason engineers specify it. At the same tensile strength, a 4340 forging keeps more of its toughness in a thick wall than a chromium-molybdenum steel does.
The three alloying elements each have a separate job. Nickel at 1.65–2.00 % slows the transformation of austenite and lifts low-temperature toughness. Chromium at 0.70–0.90 % adds hardenability and temper resistance. Molybdenum at 0.20–0.30 % suppresses temper embrittlement and holds strength at moderately elevated temperature. Together they allow a 200 mm shaft to be oil quenched and tempered to a uniform 300 HB instead of a hard skin over a soft core.
AISI 4340 is normally supplied in the quenched and tempered condition. It is not a free-machining or a readily weldable steel, and it has no meaningful corrosion resistance, so parts are usually machined in the annealed or normalized state and protected by plating, nitriding or paint after final heat treatment.
Designations covered by this datasheet
AISI 4340, SAE 4340, UNS G43400, ASTM A29 Grade 4340, ASTM A322 Grade 4340, ASTM A434 Class BB/BC/BD, ASTM A646 Grade 4340, AMS 6414, AMS 6415 and the hardenability-controlled variant 4340H (UNS H43400, ASTM A304). 4340H has slightly wider chemistry limits but a guaranteed Jominy hardenability band, and is the correct grade to quote when a customer specifies end-quench values.
Chemical composition of AISI 4340
The table below gives the heat analysis required by ASTM A29/A29M and SAE J404. Product analysis tolerances in ASTM A29 allow small deviations from these limits on the finished forging.
| Element | Minimum % | Maximum % | Function in the steel |
|---|---|---|---|
| Carbon (C) | 0.38 | 0.43 | Sets attainable hardness and strength |
| Manganese (Mn) | 0.60 | 0.80 | Hardenability, ties up sulphur |
| Silicon (Si) | 0.15 | 0.35 | Deoxidiser, small strength gain |
| Nickel (Ni) | 1.65 | 2.00 | Deep hardenability and impact toughness |
| Chromium (Cr) | 0.70 | 0.90 | Hardenability and temper resistance |
| Molybdenum (Mo) | 0.20 | 0.30 | Resists temper embrittlement |
| Phosphorus (P) | — | 0.035 | Residual, kept low for toughness |
| Sulphur (S) | — | 0.040 | Residual, kept low for transverse ductility |
| Iron (Fe) | Balance | Approximately 95.2–96.3 % | |
Tighter residual limits, for example P and S each 0.015 % maximum for low-temperature or sour-service work, can be specified on the order and are controlled at the ladle furnace and vacuum degassing stage.
Mechanical properties by heat-treatment condition
AISI 4340 has no single set of mechanical properties. The values it reaches depend on the tempering temperature used after quenching. The figures below are those obtained on a 25 mm test section oil quenched from 845 °C, and they are the reference points used when agreeing a specification. Values guaranteed on a large forging are agreed case by case, because section size changes the cooling rate at the test location.
| Condition | Tensile strength | Yield strength (0.2 %) | Elongation | Reduction of area | Hardness |
|---|---|---|---|---|---|
| Annealed | 745 MPa108 ksi | 470 MPa68 ksi | 22 % | 50 % | 217 HB |
| Normalized | 1279 MPa186 ksi | 862 MPa125 ksi | 12 % | 36 % | 363 HB |
| Q&T, temper 205 °C | 1980 MPa287 ksi | 1675 MPa243 ksi | 11 % | 39 % | 520 HB |
| Q&T, temper 315 °C | 1760 MPa255 ksi | 1570 MPa228 ksi | 12 % | 44 % | 486 HB |
| Q&T, temper 425 °C | 1500 MPa218 ksi | 1365 MPa198 ksi | 13 % | 47 % | 430 HB |
| Q&T, temper 540 °C | 1240 MPa180 ksi | 1160 MPa168 ksi | 16 % | 53 % | 360 HB |
| Q&T, temper 650 °C | 965 MPa140 ksi | 855 MPa124 ksi | 19 % | 60 % | 280 HB |
The 540 °C row is the condition most often ordered for forged shafts and rings, because it balances about 1200 MPa tensile strength against a Charpy V-notch value of 50 to 70 J at room temperature. Impact values at −20 °C or −40 °C can be guaranteed when specified in advance, since they depend on the tempering route and on the cleanliness of the heat.
Section size matters more than the table. A 600 mm diameter forged shaft cannot be quenched as fast at its centre as a 25 mm test bar. When you order, state the ruling section, and state where the test specimen must be taken from: surface, quarter thickness (T/4) or mid-radius. The guaranteed values are then agreed against that location instead of against a textbook figure.
Physical properties
| Property | Metric | Imperial |
|---|---|---|
| Density | 7.85 g/cm³ | 0.284 lb/in³ |
| Modulus of elasticity | 190–210 GPa | 27.6–30.5 ×106 psi |
| Shear modulus | 80 GPa | 11.6 ×106 psi |
| Poisson's ratio | 0.27–0.30 | 0.27–0.30 |
| Thermal conductivity at 100 °C | 44.5 W/m·K | 309 BTU·in/hr·ft²·°F |
| Mean coefficient of thermal expansion, 20–100 °C | 12.3 µm/m·°C | 6.8 µin/in·°F |
| Specific heat capacity, 50–100 °C | 475 J/kg·K | 0.114 BTU/lb·°F |
| Melting range | approx. 1425 °C | approx. 2600 °F |
| Critical temperature Ac1 | approx. 725 °C | approx. 1340 °F |
| Critical temperature Ac3 | approx. 775 °C | approx. 1430 °F |
| Martensite start, Ms | approx. 285 °C | approx. 545 °F |
| Machinability, annealed | approx. 50–60 % of AISI 1212 free-machining steel | |
International equivalents of AISI 4340
No national grade is an exact match for AISI 4340. The grades below overlap it closely enough that they are normally accepted as substitutes, but chemistry ranges, test requirements and delivery conditions differ, so the governing specification should always be written on the purchase order rather than left to the equivalence table.
| Country / body | Standard | Designation | Note |
|---|---|---|---|
| USA | SAE J404 / ASTM A29 | 4340, UNS G43400 | Reference grade |
| USA | ASTM A304 / SAE | 4340H, UNS H43400 | Hardenability-guaranteed variant |
| USA aerospace | AMS | AMS 6414, AMS 6415 | Vacuum melted and premium options |
| Germany | DIN EN 10083-3 | 40NiCrMo8-4 (1.6565) | Closest European match |
| Germany | DIN EN 10083-3 | 34CrNiMo6 (1.6582) | Lower carbon, commonly cross-referenced |
| United Kingdom | BS 970 | 817M40, formerly EN24 | Widely used in oilfield work |
| France | AFNOR NF A35-552 | 35NCD6 | Near equivalent |
| Italy | UNI | 40NiCrMo7 | Near equivalent |
| Japan | JIS G4103 | SNCM439, formerly SNCM8 | Near equivalent |
| China | GB/T 3077 | 40CrNiMoA | Domestic grade used for this supply |
| Russia | GOST 4543 | 40KhN2MA | Near equivalent |
| Sweden | SS | 2541 | Near equivalent |
Where a drawing calls for 817M40 or 34CrNiMo6 and the customer will accept 4340, Jiangyin Jiangnan Metal Co., Ltd. states both designations on the EN 10204 certificate together with the actual heat analysis, so the receiving inspector can verify the substitution.
Forging and heat treatment practice
AISI 4340 is air-hardening enough that it will crack if it is left to cool in still air straight from the forging heat. The sequence below is the practice followed at Jiangyin Jiangnan Metal Co., Ltd. for open-die forgings and seamless rolled rings, and it is the sequence a buyer should expect to see referenced on the mill certificate.
Forging
Heavy sections are preheated to 400–500 °C before being charged into the furnace, then soaked at 1150–1200 °C. Forging finishes no lower than 850–900 °C; working the steel below that range risks tearing and leaves the grain structure uneven. A minimum forging reduction ratio of 3:1 is applied as standard, and 4:1 or higher can be agreed for critical rotating parts. Immediately after forging the part goes to a controlled cool or straight into a post-forge anneal. A heavy section is never left standing in still air.
Heat treatment cycles
| Operation | Temperature | Cooling | Result |
|---|---|---|---|
| Full annealing | 820–850 °C | Furnace cool at 20 °C/h to 600 °C, then air | Soft and machinable, 217 HB max |
| Normalizing | 860–900 °C | Still air, heavy sections fan assisted | Refines grain before quenching |
| Hardening | 830–860 °C | Oil quench; water is not used | Martensite, approx. 55 HRC as quenched |
| Tempering | 205–650 °C | Air, minimum 1 h per 25 mm of section | Sets final strength per Table 2 |
| Stress relieving | 550–650 °C | Slow cool; always at least 30 °C below the final temper | Removes machining stress without losing strength |
| Nitriding, optional | 500–530 °C | Furnace cool | Case to approx. 0.4 mm, 600–650 HV surface |
Two tempering ranges to avoid. Do not temper AISI 4340 between 250 °C and 400 °C: tempered martensite embrittlement in this window drops impact toughness sharply even though hardness looks correct. Separately, after tempering above 575 °C, heavy sections should be cooled quickly through 575–375 °C rather than furnace cooled, to avoid reversible temper embrittlement. Molybdenum reduces this tendency but does not remove it.
If your drawing specifies a hardness that falls in the 250–400 °C tempering window, say so at enquiry stage and an alternative route will be proposed before the order is placed.
Hardenability and section size
Quenched in oil, AISI 4340 hardens through to a ruling section of about 100 mm, and useful properties are still obtained well beyond that. For sections above about 250 mm, the mechanical test location has to be agreed in advance, because the mid-radius of a large forged block cools far more slowly than its surface. Where a customer needs the hardenability itself guaranteed, grade 4340H should be ordered so that Jominy end-quench values to ASTM A255 can be certified.
Machining and welding
Machine AISI 4340 in the annealed or normalized condition wherever the tolerance allows, then finish after quenching and tempering. Machinability in the annealed state is about 50 to 60 % of free-machining AISI 1212. Welding is possible but not routine: preheat to 200–300 °C, use low-hydrogen consumables, maintain interpass temperature, and post-weld heat treat below the final tempering temperature. Welding on a finished quenched and tempered part should always be agreed with the mill first.
Forged product forms and size range
Jiangyin Jiangnan Metal Co., Ltd. produces AISI 4340 as open-die forgings and as seamless rolled rings, supplied black, rough machined or finish machined to drawing. The ranges below cover normal production; larger or unusual geometry is quoted on enquiry.
| Product form | Size range | Single piece weight | Usual delivery condition |
|---|---|---|---|
| Seamless rolled rings | OD 200–4000 mm, height up to 1000 mm | 20 kg – 15 t | Normalized, or Q&T |
| Forged rings and sleeves | OD 150–2500 mm, wall from 30 mm | 20 kg – 12 t | Normalized, or Q&T |
| Shafts and spindles | Dia. 80–1200 mm, length up to 8000 mm | 50 kg – 20 t | Q&T, rough machined |
| Discs and blanks | Dia. up to 2500 mm, thickness 50–800 mm | 30 kg – 15 t | Normalized, or Q&T |
| Forged flanges | Up to 3000 mm OD, ANSI, DIN, EN or to drawing | 10 kg – 8 t | Normalized, or Q&T |
| Blocks and slabs | Thickness 100–900 mm, width up to 1500 mm | 100 kg – 20 t | Annealed, normalized or Q&T |
| Forged round bar | Dia. 80–1000 mm | Cut to length | Annealed, normalized or Q&T |
| Hollow bar and forged pipe | OD 150–1200 mm, ID from 60 mm | 50 kg – 10 t | Normalized, or Q&T |
| Gear blanks and pinion blanks | To drawing | 20 kg – 12 t | Q&T before tooth cutting |
Machining allowance is agreed part by part. As a guide, black forgings carry 8–20 mm per surface depending on size, and rough-machined parts are supplied with 3–6 mm left for the customer's finish cut.
Where AISI 4340 forgings are used
AISI 4340 is specified where a part is large, heavily loaded and cannot be allowed to fail by fatigue or impact. These are the sectors it is regularly supplied into from Jiangyin.
Oil, gas and offshore
Drill collars, subsea connectors, wellhead and Christmas tree components, riser couplings, valve bodies and stems, and forged blocks for deepwater production systems where toughness at depth is critical.
Power transmission
Gear blanks, pinion shafts, couplings, turbine and compressor shafts, gearbox housings and rolled rings for wind turbine and industrial drive trains.
Heavy machinery and mining
Crankshafts, eccentric shafts, crusher shafts, mill pinions, rolling mill rolls and forged wheels for equipment under continuous shock loading.
Pressure equipment
Forged flanges, tube sheets, nozzles, shell rings and closure components for pressure vessels, heat exchangers, air receivers and process modules.
Pumps, valves and compressors
Pump shafts and plungers, valve blocks, seat rings, bodies for ball, gate, globe, check and plug valves, and rotor components for industrial air compressors.
Marine and rail
Propeller and intermediate shafts, rudder stocks, axles and other heavily loaded parts where 4340 or its AMS variants are the established grade.
Testing, inspection and certification
Every AISI 4340 forging leaves Jiangyin with a mill certificate that records the heat number, heat analysis, heat treatment condition, mechanical test results and non-destructive test results against the standard stated on the order.
| Test | Standard | Notes |
|---|---|---|
| Chemical analysis | ASTM A751, spectrometric | Ladle analysis plus product analysis on request |
| Tensile test | ASTM A370 / EN ISO 6892-1 | Test location agreed: surface, T/4 or mid-radius |
| Impact test | ASTM E23 / EN ISO 148-1 | Charpy V-notch, room temperature or down to −46 °C |
| Hardness | ASTM E10, Brinell | Multiple points per piece on large forgings |
| Ultrasonic testing | EN 10228-3, SEP 1921, ASTM A388, ASTM A745 | Acceptance class stated on the order, for example SEP 1921 Class C/c |
| Magnetic particle | ASTM E1444 / EN 10228-1 | For machined surfaces and critical radii |
| Grain size | ASTM E112 | Typically ASTM 5 or finer |
| Hardenability, 4340H only | ASTM A255 | Jominy end-quench curve certified |
| Dimensional report | To drawing | Issued with the certificate for machined parts |
Certification is issued to EN 10204 3.1 as standard. EN 10204 3.2 certification witnessed by TUV, Bureau Veritas, Lloyd's Register, DNV or SGS is arranged on request, with inspection notice given before the test date so the surveyor can attend.
How to order AISI 4340 forgings
Send a drawing in PDF, DWG or STEP, or the finished dimensions in plain text, to sales@steelforgepieces.com. A quotation normally comes back within 24 hours on a working day. Include as much of the following as you have:
- Geometry. Drawing or finished dimensions, and whether you want the part black forged, rough machined or finish machined.
- Grade and standard. AISI 4340, 4340H, or the equivalent you are working to, plus the governing specification such as ASTM A29, ASTM A434 Class BD or a customer spec.
- Delivery condition. Annealed, normalized, or quenched and tempered to a stated hardness or tensile range.
- Test requirements. Mechanical test location, impact temperature if required, and the ultrasonic acceptance class.
- Certification level. EN 10204 3.1, or 3.2 with the third party you want to use.
- Quantity and delivery date. Prototype quantities of one piece are accepted.
Lead time for a normalized forging is typically 25–35 days from order confirmation, and 35–45 days where quenching, tempering and rough machining are included. Confirm the current lead time at enquiry, since it moves with furnace loading.
Frequently asked questions about AISI 4340
What is AISI 4340 steel?
AISI 4340 (UNS G43400) is a through-hardening low-alloy steel containing nominally 0.40 % carbon, 1.8 % nickel, 0.8 % chromium and 0.25 % molybdenum. The nickel and molybdenum give it deep hardenability and high toughness, so it holds strength through heavy forged sections after quenching and tempering.
What is the chemical composition of AISI 4340?
Per ASTM A29/A29M, AISI 4340 contains carbon 0.38–0.43 %, manganese 0.60–0.80 %, silicon 0.15–0.35 %, nickel 1.65–2.00 %, chromium 0.70–0.90 %, molybdenum 0.20–0.30 %, phosphorus 0.035 % maximum and sulphur 0.040 % maximum, with the balance iron. Full limits are in Table 1 above.
What is AISI 4340 equivalent to?
AISI 4340 is close to EN 40NiCrMo8-4 (1.6565), BS 817M40 (EN24), JIS SNCM439, GB 40CrNiMoA and GOST 40KhN2MA. These are near-equivalents rather than exact substitutes, so the governing specification should be stated on the order. Table 4 lists the full cross-reference.
What tensile strength can AISI 4340 forgings reach?
Oil quenched and tempered at 540 °C, AISI 4340 typically reaches about 1240 MPa tensile strength and 1160 MPa yield strength. Tempering at 205 °C can exceed 1900 MPa tensile strength, while annealed material is about 745 MPa. The strength you can guarantee on a specific forging depends on its ruling section.
How is AISI 4340 different from AISI 4140?
AISI 4340 contains 1.65–2.00 % nickel, which AISI 4140 does not. That nickel raises hardenability and low-temperature toughness, so 4340 is chosen for heavier sections and higher strength levels, while 4140 is the cheaper choice for lighter sections. Above about 100 mm ruling section, or above 1000 MPa tensile strength, the extra cost of 4340 is usually justified. Both grades are forged here; see AISI 4140 forgings.
How thick a section can AISI 4340 be through-hardened?
In oil, AISI 4340 hardens through to about 100 mm of ruling section. Beyond that the core cools more slowly and strength falls off, so heavy forgings are normally specified with properties measured at a defined depth such as quarter thickness or mid-radius. Where hardenability itself must be guaranteed, order grade 4340H and Jominy values to ASTM A255 will be certified.
What is the difference between 4340 and 4340H?
4340H (UNS H43400) has slightly wider chemistry limits than 4340 but adds a guaranteed hardenability band measured by the Jominy end-quench test to ASTM A255. Specify 4340H when your drawing calls out end-quench hardness values or when the part is heat treated after delivery and the response has to be predictable.
Is AISI 4340 weldable?
AISI 4340 is difficult to weld because of its carbon and alloy content. Welding requires preheat around 200–300 °C, low-hydrogen consumables and post-weld heat treatment below the final tempering temperature. Welding on quenched and tempered parts should be agreed in advance, because the weld and heat-affected zone will not match the parent properties without a full re-treatment.
Who manufactures AISI 4340 open-die forgings in China?
Jiangyin Jiangnan Metal Co., Ltd. manufactures AISI 4340 open-die forgings at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. The factory supplies forged rings, seamless rolled rings, shafts, flanges, discs and bars in AISI 4340 with EN 10204 3.1 or 3.2 certification. Enquiries go to sales@steelforgepieces.com or +86 189 2135 9659.
What certification is supplied with AISI 4340 forgings?
An EN 10204 3.1 mill certificate is supplied as standard, covering heat analysis, heat treatment condition, mechanical test results and ultrasonic testing. EN 10204 3.2 certification with third-party witness such as TUV, Bureau Veritas, Lloyd's Register, DNV or SGS is available on request.
What information is needed to quote an AISI 4340 forging?
Send the drawing or finished dimensions, the machining allowance or rough-machined state required, the quantity, the heat-treatment condition or required mechanical properties, the ultrasonic testing class and the certification level. Section 10 above lists the full checklist. Drawings can be sent to sales@steelforgepieces.com.
About the manufacturer
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory located at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, in the Yangtze River delta forging cluster, 150 km from the Port of Shanghai. The works produces open-die forgings and seamless rolled rings in carbon steel, alloy steel, tool steel, stainless steel and nickel alloys, with AISI 4340 among its regular grades.
Steel for AISI 4340 forgings is melted by electric arc furnace with ladle refining and vacuum degassing (EAF + LF + VD), then forged on open-die hammers and presses and rolled on a ring mill. Heat treatment, rough machining, mechanical testing and ultrasonic testing are carried out before shipment, and every piece is supplied with an EN 10204 certificate.
Forgings are shipped to customers in the United States, Canada, Germany, the United Kingdom, France, Italy, the Netherlands, Turkey, India, Japan, Korea, Singapore, Australia, Brazil and the Middle East.
Jiangyin Jiangnan Metal Co., Ltd.No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China
Telephone and WhatsApp: +86 189 2135 9659
Email: sales@steelforgepieces.com
Website: www.steelforgepieces.com
Citing this datasheet
This page is maintained as a reference datasheet. If you are quoting these figures in a specification or a report, please cite the page as follows.
Jiangyin Jiangnan Metal Co., Ltd. (2026). AISI 4340 Forgings: Properties, Equivalents and Sizes. Jiangyin, Jiangsu, China. Retrieved from https://www.steelforgepieces.com/Alloy-Steel/AISI-4340.html
Chemistry and mechanical data compiled from ASTM A29/A29M, ASTM A322, ASTM A434, SAE J404 and mill practice at Jiangyin Jiangnan Metal Co., Ltd. Published 13 September 2026. Last reviewed 13 September 2026. Corrections: sales@steelforgepieces.com.