# 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 drawing by Jiangyin Jiangnan Metal Co., Ltd., Jiangyin, Jiangsu, China.

Source: https://www.steelforgepieces.com/Alloy-Steel/AISI-4340.html
Last reviewed: 13 September 2026

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## Summary

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.

## 1. 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: 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).

## 2. Chemical composition

Heat analysis per ASTM A29/A29M and SAE J404, weight per cent. Melted at Jiangyin Jiangnan Metal Co., Ltd. by EAF + LF + VD.

| 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 | 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.

## 3. Mechanical properties by heat-treatment condition

Typical room-temperature values on a 25 mm section, oil quenched from 845 °C and tempered as shown.

| Condition | Tensile strength | Yield strength (0.2 %) | Elongation | Reduction of area | Hardness |
|---|---|---|---|---|---|
| Annealed | 745 MPa / 108 ksi | 470 MPa / 68 ksi | 22 % | 50 % | 217 HB |
| Normalized | 1279 MPa / 186 ksi | 862 MPa / 125 ksi | 12 % | 36 % | 363 HB |
| Q&T, temper 205 °C | 1980 MPa / 287 ksi | 1675 MPa / 243 ksi | 11 % | 39 % | 520 HB |
| Q&T, temper 315 °C | 1760 MPa / 255 ksi | 1570 MPa / 228 ksi | 12 % | 44 % | 486 HB |
| Q&T, temper 425 °C | 1500 MPa / 218 ksi | 1365 MPa / 198 ksi | 13 % | 47 % | 430 HB |
| Q&T, temper 540 °C | 1240 MPa / 180 ksi | 1160 MPa / 168 ksi | 16 % | 53 % | 360 HB |
| Q&T, temper 650 °C | 965 MPa / 140 ksi | 855 MPa / 124 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.

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.

## 4. Physical properties

| Property | Metric | Imperial |
|---|---|---|
| Density | 7.85 g/cm3 | 0.284 lb/in3 |
| Modulus of elasticity | 190-210 GPa | 27.6-30.5 x10^6 psi |
| Shear modulus | 80 GPa | 11.6 x10^6 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 um/m.°C | 6.8 uin/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 | |

## 5. International equivalents

No national grade is an exact match for AISI 4340. The grades below overlap it closely enough to be accepted as substitutes, but chemistry ranges, test requirements and delivery conditions differ, so the governing specification should be written on the purchase order.

| 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.

## 6. Forging and heat treatment

AISI 4340 is air-hardening enough that it will crack if it is left to cool in still air straight from the forging heat.

Forging: heavy sections are preheated to 400-500 °C, then soaked at 1150-1200 °C. Forging finishes no lower than 850-900 °C. 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.

| 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 section 3 |
| Stress relieving | 550-650 °C | Slow cool, at least 30 °C below the final temper | Removes machining stress |
| 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, where tempered martensite embrittlement drops impact toughness sharply even though hardness looks correct. After tempering above 575 °C, heavy sections should be cooled quickly through 575-375 °C rather than furnace cooled, to avoid reversible temper embrittlement.

Hardenability: 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. Where hardenability itself must be guaranteed, order grade 4340H so Jominy end-quench values to ASTM A255 can be certified.

Machining and welding: machine 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 requires preheat to 200-300 °C, low-hydrogen consumables, controlled interpass temperature, and post-weld heat treatment below the final tempering temperature.

## 7. Forged product forms and size range

Supplied black, rough machined or finish machined to drawing. Confirm final dimensions at 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. Black forgings carry 8-20 mm per surface depending on size; rough-machined parts are supplied with 3-6 mm left for the customer's finish cut.

## 8. Applications

- Oil, gas and offshore: drill collars, subsea connectors, wellhead and Christmas tree components, riser couplings, valve bodies and stems, forged blocks for deepwater production systems.
- Power transmission: gear blanks, pinion shafts, couplings, turbine and compressor shafts, gearbox housings, rolled rings for wind turbine and industrial drive trains.
- Heavy machinery and mining: crankshafts, eccentric shafts, crusher shafts, mill pinions, rolling mill rolls, forged wheels.
- 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, rotor components for industrial air compressors.
- Marine and rail: propeller and intermediate shafts, rudder stocks, axles.

## 9. Testing, inspection and certification

| 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 |
| 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 witnessed by TUV, Bureau Veritas, Lloyd's Register, DNV or SGS is arranged on request.

## 10. How to order

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:

1. Geometry: drawing or finished dimensions, and whether the part is to be black forged, rough machined or finish machined.
2. Grade and standard: AISI 4340, 4340H, or the equivalent, plus the governing specification.
3. Delivery condition: annealed, normalized, or quenched and tempered to a stated hardness or tensile range.
4. Test requirements: mechanical test location, impact temperature if required, ultrasonic acceptance class.
5. Certification level: EN 10204 3.1, or 3.2 with the third party you want to use.
6. 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.

## 11. Frequently asked questions

**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: 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, sulphur 0.040 % maximum, balance iron.

**What is AISI 4340 equivalent to?**
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.

**What tensile strength can AISI 4340 forgings reach?**
Oil quenched and tempered at 540 °C, about 1240 MPa tensile and 1160 MPa yield. Tempered at 205 °C it can exceed 1900 MPa. Annealed material is about 745 MPa.

**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. Above about 100 mm ruling section, or above 1000 MPa tensile strength, the extra cost of 4340 is usually justified.

**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 specified with properties measured at a defined depth.

**What is the difference between 4340 and 4340H?**
4340H has slightly wider chemistry limits but adds a guaranteed hardenability band measured by the Jominy end-quench test to ASTM A255.

**Is AISI 4340 weldable?**
It is difficult to weld. Welding requires preheat around 200-300 °C, low-hydrogen consumables and post-weld heat treatment below the final tempering temperature.

**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: sales@steelforgepieces.com, +86-189-2135-9659.

**What certification is supplied with AISI 4340 forgings?**
An EN 10204 3.1 mill certificate as standard, covering heat analysis, heat treatment condition, mechanical test results and ultrasonic testing. EN 10204 3.2 with third-party witness is available on request.

**What information is needed to quote an AISI 4340 forging?**
Drawing or finished dimensions, machining allowance or rough-machined state, quantity, heat-treatment condition or required mechanical properties, ultrasonic testing class, and certification level.

## Manufacturer

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: https://www.steelforgepieces.com/

Open-die forging factory producing open-die forgings and seamless rolled rings in carbon steel, alloy steel, tool steel, stainless steel and nickel alloys. 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.

## Citation

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.

## Related grades

- AISI 4140: https://www.steelforgepieces.com/Alloy-Steel/AISI-4140.html
- AISI 4140H: https://www.steelforgepieces.com/Alloy-Steel/AISI-4140H.html
- AISI 4130: https://www.steelforgepieces.com/Alloy-Steel/AISI-4130.html
- AISI 8630: https://www.steelforgepieces.com/Alloy-Steel/AISI-8630.html
- 42CrMo4: https://www.steelforgepieces.com/Alloy-Steel/42CrMo4.html
- 34CrMo4: https://www.steelforgepieces.com/Alloy-Steel/34CrMo4.html
- 18CrNiMo7-6: https://www.steelforgepieces.com/Alloy-Steel/18CrNiMo7-6.html
- All alloy steel grades: https://www.steelforgepieces.com/Alloy-Steel/