# SNCM439 Forgings — JIS G4103 Ni-Cr-Mo Alloy Steel

**Standard:** JIS G4103 · **Closest equivalent:** AISI 4340 · **Page updated:** 18 September 2026

**Supplier:** Jiangyin Jiangnan Metal Co., Ltd., open-die forging factory
No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China
Tel +86 189 2135 9659 · sales@steelforgepieces.com · https://www.steelforgepieces.com/
Canonical page: https://www.steelforgepieces.com/Alloy-Steel/SNCM439.html

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SNCM439 is a nickel-chromium-molybdenum alloy structural steel specified in JIS G4103, closely equivalent to AISI 4340. Quenched and tempered, it reaches a minimum tensile strength of 980 MPa at 293 to 352 HBW. Jiangyin Jiangnan Metal Co., Ltd. forges SNCM439 into rings, shafts, flanges, discs and bars to EN 10204 3.2.

| | |
|---|---|
| Standard | JIS G4103 |
| Material group | Ni-Cr-Mo alloy structural steel |
| Closest equivalents | AISI 4340, 40CrNiMoA, 817M40 |
| Melting route | EAF + LF + VD (ESR on request) |
| Delivery condition | Annealed, N+T or Q+T |
| Tensile strength, Q+T | 980 MPa min |
| Hardness, Q+T | 293 to 352 HBW |
| Elongation, Q+T | 16 % min |
| Certification | EN 10204 3.1 / 3.2 |

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## What SNCM439 is

SNCM439 is a medium-carbon low-alloy engineering steel with roughly 0.40 % carbon, 1.8 % nickel, 0.8 % chromium and 0.2 % molybdenum. It belongs to JIS G4103, nickel chromium molybdenum steels for machine structural use. A common error is to quote JIS G4105 instead. That standard covers the chromium-molybdenum SCM grades such as SCM440 and does not contain SNCM439.

Nickel is what separates SNCM439 from the cheaper Cr-Mo grades. It slows the transformation of austenite during the quench, so the core of a heavy section still forms martensite instead of softer bainite or pearlite. SNCM439 therefore keeps both its strength and its impact toughness in sections where 42CrMo4 or SCM440 have already fallen away. Large forged shafts, heavy gear blanks and shock-loaded drive components are the usual applications.

The grade is used almost entirely in the quenched and tempered condition. It is not a case-hardening steel. For carburised gears the equivalent choice is 18CrNiMo7-6 or 17CrNiMo6, although tooth flanks on a Q+T SNCM439 gear are often induction hardened afterwards.

## Chemical composition of SNCM439

Ladle analysis limits to JIS G4103, in weight per cent.

| Element | Min | Max | Role in the steel |
|---|---|---|---|
| Carbon, C | 0.36 | 0.43 | Sets attainable hardness and strength |
| Silicon, Si | 0.15 | 0.35 | Deoxidiser, small strength contribution |
| Manganese, Mn | 0.60 | 0.90 | Hardenability, fixes residual sulphur |
| Phosphorus, P | — | 0.030 | Residual, limited for toughness |
| Sulphur, S | — | 0.030 | Residual, limited for transverse ductility |
| Nickel, Ni | 1.60 | 2.00 | Deep hardenability, low-temperature toughness |
| Chromium, Cr | 0.60 | 1.00 | Hardenability and temper resistance |
| Molybdenum, Mo | 0.15 | 0.30 | Hardenability, resists temper embrittlement |
| Copper, Cu | — | 0.30 | Residual element limit |

On critical forgings we hold P and S well below the JIS maxima, and we can melt to a customer-specified narrowed range. A limit of S at 0.010 % or lower is common where transverse impact properties or ultrasonic cleanliness matter.

## Mechanical properties of SNCM439

JIS G4103 sets the following minimum properties after quenching and tempering. These are test-piece values, measured on a standard specimen taken from a reference section.

| Property | Symbol | Requirement |
|---|---|---|
| 0.2 % proof strength | Rp0.2 | 885 MPa min (90 kgf/mm²) |
| Tensile strength | Rm | 980 MPa min (100 kgf/mm²) |
| Elongation | A | 16 % min |
| Reduction of area | Z | 45 % min |
| Impact energy (U-notch) | — | 69 J/cm² min (7 kgf·m/cm²) |
| Hardness | HBW | 293 to 352 |
| Annealed delivery hardness | HBW | 250 max |

**Section size changes what you can expect.** A 1,000 mm diameter shaft will not reach 980 MPa at its centre whatever the heat treatment, because the quench cannot pull heat out of the core fast enough. On heavy sections we agree the tested section size and the sampling position before the order is confirmed. That usually means mid-radius and mid-thickness sampling to EN 10083-1, or a test prolongation forged onto the part itself. A supplier who quotes full test-piece figures for a one-metre section without qualifying them is quoting numbers he cannot hit.

## Heat treatment of SNCM439 forgings

| Operation | Temperature | Cooling | Resulting condition |
|---|---|---|---|
| Forging | 1,050 to 1,200 °C | Finish above 850 °C, then slow or furnace cool | As-forged |
| Annealing | 820 to 860 °C | Furnace cool | 250 HBW max, machinable |
| Normalising | 820 to 870 °C | Air cool | Refined, uniform grain |
| Hardening | 820 to 870 °C | Oil quench | Martensitic, as-quenched |
| Tempering | 580 to 680 °C | Rapid cool, water or oil | 293 to 352 HBW, Q+T |
| Stress relieving | About 30 °C below final temper | Slow cool | After rough machining or welding |

### Why tempering must be followed by rapid cooling

Nickel-chromium-molybdenum steels of this type suffer temper embrittlement if they are held in, or cooled slowly through, the range from about 375 to 575 °C. Phosphorus and other tramp elements segregate to the prior-austenite grain boundaries and the impact transition temperature climbs, sometimes by tens of degrees, with no change in hardness or tensile strength to warn you. The part passes its tensile test and then fails in service. Three things prevent it: low phosphorus at the melt, molybdenum towards the top of its range, and a quench straight out of the tempering furnace rather than a slow cool inside it.

## SNCM439 international equivalents

| Country / standard | Grade | Closeness |
|---|---|---|
| Japan, JIS G4103 | SNCM439 | Reference grade |
| USA, ASTM A29 / SAE-AISI | 4340 (UNS G43400) | Very close |
| China, GB/T 3077 | 40CrNiMoA | Very close |
| UK, BS 970 | 817M40 (EN24) | Very close |
| Russia, GOST 4543 | 40KhN2MA | Very close |
| Italy, UNI 7845 | 40NiCrMo7 | Close |
| France, AFNOR | 40NCD7 | Close |
| Germany, EN 10083-3 | 34CrNiMo6 (1.6582), 36CrNiMo4 (1.6511) | Similar, ranges differ |
| Australia, AS 1444 | 4340 | Very close |

Equivalent is not identical. 34CrNiMo6 carries more chromium and less carbon than SNCM439, and 4340 has a narrower chromium band. Where a drawing names one grade and the application is safety-critical or code-governed, ask for a dual-certified chemistry that satisfies both specifications rather than substituting one for the other.

## SNCM439 forged product forms

All produced by open-die forging or radial ring rolling. No castings, no welded fabrications.

| Product form | Typical application |
|---|---|
| Seamless rolled rings | Gear blanks, bearing races, flange rings |
| Forged rings, open die | Slewing rings, retaining rings |
| Forged shafts and spindles | Drive shafts, eccentric shafts, rotor shafts |
| Forged round bars | Machining stock, pins, bolts, valve stems |
| Forged discs and blanks | Gear blanks, coupling hubs, blind flanges |
| Forged flanges | Pressure joints, pump and valve bodies |
| Forged sleeves and bushings | Bearing housings, wear sleeves |
| Forged blocks and rectangles | Die blocks, valve bodies, manifolds |

Sizes are confirmed against your drawing. Send the outside diameter, inside diameter and height for a ring, or the diameter and length for a shaft, and we will confirm by return whether it can be forged in one piece and what the rough weight will be.

## How we make SNCM439 forgings

1. **Melting.** EAF + LF + VD as standard. Vacuum degassing removes the hydrogen that causes flaking in heavy Ni-Cr-Mo sections, and lowers oxygen and inclusion content. ESR is available where the application calls for maximum cleanliness.
2. **Forging.** Open-die forging with a controlled forging ratio, so the as-cast structure is fully broken down and the grain flow follows the contour of the part. Heating 1,050 to 1,200 °C, finish above 850 °C, controlled cooling after forging.
3. **Heat treatment.** Annealing, normalising, or quenching and tempering in our own furnaces. Every charge carries a recorded and traceable time and temperature chart. Tempering is followed by a rapid cool.
4. **Machining.** Rough turning, boring and facing to your machining allowance, or finish machining to drawing.

## Inspection, testing and certification

| Test | Standard | Notes |
|---|---|---|
| Chemical analysis | Optical emission spectrometry | Ladle and product analysis |
| Tensile test | ISO 6892-1, ASTM A370, JIS Z 2241 | Sampling position agreed per order |
| Impact test | ISO 148-1, ASTM E23 | Charpy V or U notch, room or sub-zero |
| Hardness | ISO 6506, ASTM E10 | Brinell, multi-point survey |
| Ultrasonic testing | EN 10228-3, SEP 1921, ASTM A388 | Acceptance class stated on the order |
| Magnetic particle | EN 10228-1, ASTM E709 | Surface and near-surface indications |
| Grain size | ASTM E112 | On request |
| Dimensional report | Per drawing | Issued with every shipment |
| Certificate | EN 10204 3.1 or 3.2 | 3.2 witnessed by TUV, SGS, BV, DNV or LR |

Every SNCM439 forging leaves the works with full heat-number traceability. The certificate ties the part back through the heat treatment charge records and the NDT reports to the original melt analysis.

## SNCM439 applications by industry

**Power transmission.** Gear blanks and forged gear rings, pinion and rotor shafts, gearbox and reducer shafts, coupling hubs and flange couplings.

**Oil, gas and petrochemical.** Valve bodies, stems, seats and blocks; wellhead and Christmas-tree components; drill-string and subsea connectors; pump shafts and plunger components.

**Heavy machinery and mining.** Eccentric shafts for crushers, mill pinions and drive shafts, forged rolls and wheels, slewing and retaining rings.

**Marine, energy and general.** Propeller and intermediate shafts, turbine and compressor shafting, high-tensile bolting and studs, crankshafts and connecting components.

SNCM439 is specified where high tensile strength, deep hardening response and impact toughness are needed in one material, usually on shock-loaded rotating components in heavy sections.

## Frequently asked questions

**What standard covers SNCM439?**
SNCM439 is specified in JIS G4103, the Japanese Industrial Standard for nickel chromium molybdenum steels for machine structural use. It is not covered by JIS G4105, which applies to the chromium molybdenum SCM grades such as SCM440.

**Is SNCM439 the same as AISI 4340?**
They are close equivalents, not identical grades. SNCM439 allows a wider band on carbon at 0.36 to 0.43 % against 0.38 to 0.43, on chromium at 0.60 to 1.00 % against 0.70 to 0.90, and on molybdenum at 0.15 to 0.30 % against 0.20 to 0.30. For most quenched and tempered forgings the two behave the same. Where a drawing calls out 4340 by name, ask for a dual-certified heat that meets both chemistries.

**What is the tensile strength of SNCM439?**
Quenched and tempered, JIS G4103 requires a minimum tensile strength of 980 MPa, a minimum 0.2 % proof strength of 885 MPa, elongation of at least 16 %, reduction of area of at least 45 %, and hardness of 293 to 352 HBW. Those figures apply to the standard test piece. Heavy sections read lower at the core.

**What is the difference between SNCM439 and SCM440?**
SNCM439 carries 1.60 to 2.00 % nickel. SCM440, a chromium molybdenum grade, carries none. The nickel gives SNCM439 deeper hardenability and better toughness at low temperature, so it holds its properties through heavier sections. SCM440 is the cheaper choice up to about 100 mm section. Above that, or where impact toughness governs, SNCM439 is the grade to use.

**Can SNCM439 be welded?**
Only with care. The carbon equivalent is high and the heat-affected zone is prone to hydrogen cracking. Weld in the annealed or normalised condition, use low-hydrogen consumables, hold preheat and interpass temperature at 250 to 350 °C, and follow with post-weld heat treatment about 30 °C below the final tempering temperature. Welding fully hardened and tempered material is not recommended.

**Which material certificates can you supply for SNCM439 forgings?**
EN 10204 3.1 certificates issued by our own inspection department, and EN 10204 3.2 certificates witnessed by a third party such as TUV, SGS, BV, DNV or Lloyd's Register. Certificates cover heat chemistry, mechanical test results, heat treatment records, ultrasonic testing and full heat-number traceability.

**How is SNCM439 heat treated?**
Anneal at 820 to 860 °C with furnace cooling to below 250 HBW for machining. Normalise at 820 to 870 °C with air cooling. Harden from 820 to 870 °C in oil and temper at 580 to 680 °C. Cool quickly out of the tempering furnace. Holding nickel chromium molybdenum steel in the 375 to 575 °C range brings on temper embrittlement.

**What information do you need to quote SNCM439 forgings?**
A drawing or the rough dimensions, the quantity, the delivery condition required as annealed, normalised or quenched and tempered, the machining allowance, the NDT standard and acceptance class, and the certificate level under EN 10204 3.1 or 3.2. Email sales@steelforgepieces.com or call +86 189 2135 9659.

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## About the supplier

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. We produce forged rings, seamless rolled rings, shafts, flanges, discs, bars, sleeves and tube sheets in carbon, alloy, tool, stainless and nickel-alloy grades, including SNCM439 to JIS G4103. Material is melted by EAF + LF + VD, forged and heat treated in house, and released against EN 10204 3.1 or third-party-witnessed EN 10204 3.2 certificates.

| | |
|---|---|
| Company | Jiangyin Jiangnan Metal Co., Ltd. |
| Address | No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China |
| Telephone | +86 189 2135 9659 |
| Email | sales@steelforgepieces.com |
| Website | https://www.steelforgepieces.com/ |