SNCM240 (JIS G4103) is a Ni-Cr-Mo alloy structural steel that occupies a specific position in the JIS alloy steel family: more hardenable and tougher than SCM440 (Cr-Mo only) for 30–60 mm sections, but significantly cheaper and more machinable than SNCM439 (high Ni, deep-hardening) for applications that do not require SNCM439’s large-section capability. Its 0.4% Ni addition to an SCM440-like Cr-Mo base raises Charpy impact by approximately 20–30% at equivalent tensile strength — the Ni effect on toughness — while improving hardenability sufficiently to achieve full through-hardening at 40–50 mm diameter where SCM440 begins to show under-hardened cores. The result is a practical specification for medium-section shafts, gears, bolts, and structural pins where SCM440 is marginally insufficient and SNCM439 is over-specified and over-priced.
- Position in the JIS Ni-Cr-Mo Steel Family
- Chemical Composition
- Mechanical Properties
- Heat Treatment
- Hardenability: Where SNCM240 Fills the Gap
- Ni Addition: Toughness Mechanism
- Common Mistakes
- When to Choose SNCM240
- FAQ
1. Position in the JIS Ni-Cr-Mo Steel Family
| Grade | Ni (%) | Cr (%) | Mo (%) | Max Section (full HT) | Position |
|---|---|---|---|---|---|
| SCM440 | — | 0.90–1.20 | 0.15–0.30 | ~30 mm ∅ | Baseline Cr-Mo, no Ni |
| SNCM240 | 0.40–0.70 | 0.60–1.00 | 0.15–0.30 | ~50 mm ∅ | Low-Ni Ni-Cr-Mo, medium section |
| SNCM431 | 1.60–2.00 | 0.60–1.00 | 0.15–0.30 | ~80 mm ∅ | Medium-Ni Ni-Cr-Mo |
| SNCM439 | 1.60–2.00 | 0.60–1.00 | 0.15–0.30 | ~100 mm ∅ | High-hardenability Ni-Cr-Mo |
| SNCM625 | 3.00–3.50 | 1.00–1.50 | 0.15–0.30 | ~200 mm ∅ | Deep-hardening large section |
The SNCM series progression represents increasing Ni content for increasing section hardenability. SNCM240 is the entry point — the minimum Ni addition that provides meaningful improvement over SCM440’s hardenability while keeping cost close to the base Cr-Mo grade. Note: SNCM439 and SNCM431 have identical Ni, Cr, Mo ranges in JIS G4103; SNCM439 achieves higher strength through a slightly higher carbon range.
2. Chemical Composition
| Element | SNCM240 (JIS G4103) | SCM440 (comparison) | SNCM439 (comparison) |
|---|---|---|---|
| C | 0.36–0.43% | 0.38–0.43% | 0.36–0.43% |
| Si | 0.15–0.35% | 0.15–0.35% | 0.15–0.35% |
| Mn | 0.60–0.90% | 0.60–0.85% | 0.60–0.90% |
| Ni | 0.40–0.70% | — | 1.60–2.00% |
| Cr | 0.60–1.00% | 0.90–1.20% | 0.60–1.00% |
| Mo | 0.15–0.30% | 0.15–0.30% | 0.15–0.30% |
| P (max) | 0.030% | 0.030% | 0.030% |
| S (max) | 0.030% | 0.030% | 0.030% |
Source: JIS G4103:2016. SNCM240’s carbon range (0.36–0.43%) is nearly identical to SCM440 — the mechanical property improvement comes entirely from the Ni addition and slightly different Cr range, not from carbon content changes. The closest international equivalents are in the AISI 86xx series (8640/8740 type: Ni 0.40–0.70%, Cr 0.40–0.60%, Mo 0.15–0.25%), though SNCM240’s Cr range (0.60–1.00%) is slightly wider and higher than the AISI 8640 specification.
3. Mechanical Properties
JIS G4103 specifies properties for test pieces cut from normalized or quenched-and-tempered material. Values below are for Q&T condition (oil quench from 820–880°C, temper at 520–650°C):
| Property | SNCM240 (Q&T) | SCM440 (Q&T, comparison) | SNCM439 (Q&T, comparison) |
|---|---|---|---|
| Tensile strength | 780–1030 MPa (113–149 ksi) | 780–980 MPa | 980–1180 MPa |
| Yield strength (0.2%) | ≥ 590 MPa (86 ksi) | ≥ 590 MPa | ≥ 835 MPa |
| Elongation | ≥ 17% | ≥ 17% | ≥ 15% |
| Reduction of area | ≥ 55% | ≥ 55% | ≥ 50% |
| Charpy impact (20°C) | ≥ 78 J (57 ft·lbf) | ≥ 63 J (46 ft·lbf) | ≥ 98 J (72 ft·lbf) |
| Hardness (typical) | 229–285HBW | 229–285HBW | 285–352HBW |
The Charpy impact improvement — SNCM240 at ≥ 78 J vs SCM440’s ≥ 63 J — is a direct consequence of the Ni addition. For equivalent tensile strength targets, SNCM240 consistently delivers 15–25% higher impact energy than SCM440 across the 780–1000 MPa range. This translates to better resistance to brittle fracture at low temperatures and under impact loading.
4. Heat Treatment
Normalizing
870–920°C (1598–1688°F), air cool. Refines as-received grain structure before Q&T. Not always required if bar stock microstructure is uniform — confirm with supplier before skipping. Target: ≤ 255HBW normalized.
Hardening
Austenitize at 820–880°C (1508–1616°F) for 1 hr per 25 mm section, oil quench. SNCM240’s Ni addition improves hardenability modestly — oil quench is standard; water quenching is generally not required for sections up to 50 mm. As-quenched hardness: 53–58HRC (section-dependent).
Tempering
| Temper Temperature | Approximate Hardness | Tensile Strength | Application |
|---|---|---|---|
| 450–520°C (842–968°F) | 35–42HRC (330–400HBW) | 1100–1300 MPa | High-strength bolts, pins requiring maximum strength |
| 520–580°C (968–1076°F) | 28–35HRC (260–330HBW) | 900–1100 MPa | Shafts, gears — standard Q&T condition |
| 580–650°C (1076–1202°F) | 22–28HRC (210–265HBW) | 780–900 MPa | Maximum toughness; large forgings, structural parts |
5. Hardenability: Where SNCM240 Fills the Gap
Hardenability — the ability to achieve full martensite transformation at depth during quenching — is the primary technical justification for choosing SNCM240 over SCM440:
| Bar Diameter | SCM440 (oil quench) Core | SNCM240 (oil quench) Core | SNCM439 (oil quench) Core |
|---|---|---|---|
| 25 mm (1 in) | Full martensite (~56HRC) | Full martensite (~56HRC) | Full martensite |
| 40 mm (1.6 in) | Partial — core ~45–50HRC (bainite mixed) | Full or near-full (~53–56HRC) | Full martensite |
| 60 mm (2.4 in) | Under-hardened core — 35–42HRC | Partial — core ~47–52HRC | Full or near-full |
| 100 mm (4 in) | Soft core — 25–35HRC | Under-hardened — 38–45HRC | Partial — ~50–54HRC |
The practical implication: for a 45 mm diameter shaft that must achieve ≥ 285HBW at the core after Q&T, SCM440 in oil quench may fall short (under-hardened bainitic core at ~260HBW). SNCM240 reliably achieves the core hardness target at 45 mm. For a 90 mm shaft, SNCM439 is required. The section size determines the grade — not a vague quality preference.
6. Ni Addition: Toughness Mechanism
Nickel improves toughness in alloy steels through two mechanisms:
- Austenite stability: Ni lowers the martensite transformation temperature (Ms), producing finer martensite on quenching. Finer martensite lath structure means more martensite-martensite boundary area — these boundaries arrest crack propagation more effectively than in coarse martensite, raising fracture toughness
- Ferrite toughening: In the tempered condition, Ni in solid solution in ferrite reduces the ductile-to-brittle transition temperature (DBTT). SNCM240 maintains adequate Charpy impact energy to lower temperatures than equivalent-strength SCM440 — relevant for outdoor equipment operating in sub-zero temperatures
7. Common Mistakes
8. When to Choose SNCM240
Section diameter is 30–60 mm and SCM440 shows under-hardened cores in trial heat treatment. Low-temperature toughness is required (below −20°C service) where SCM440’s DBTT is marginal. Moderate step-up from SCM440 in impact energy (20–30%) is needed without the cost of SNCM439. Shafts, pinion shafts, medium-section gears, connecting rods, and structural pins in this section range for outdoor or impact-loaded machinery.
Section diameter is below 30 mm — SCM440 fully hardens in this range and SNCM240’s Ni addition provides minimal benefit. When toughness specification is met by SCM440 and cost is constrained. For high-volume turned parts, fasteners, and hydraulic components where SCM440 is the established standard and SNCM240 offers no technical advantage at the specified section size.
Section diameter exceeds 60 mm and full through-hardening at the core is required. When tensile strength above 1000 MPa is required throughout the section. For large shafts, propeller shafts, heavy-section gears, and structural forgings where SNCM240’s lower Ni content is insufficient for the hardenability requirement. SNCM439 is the standard for large-section, high-strength Ni-Cr-Mo applications in Japan.
9. FAQ
Q: What is the ASTM equivalent of SNCM240?
SNCM240 has no exact ASTM equivalent. The closest grades in the AISI 86xx series (specifically AISI 8640: C 0.38–0.43%, Ni 0.40–0.70%, Cr 0.40–0.60%, Mo 0.15–0.25%) are similar in composition concept but have slightly different Cr ranges. For procurement in North America, confirm composition against the JIS G4103 ranges and select the closest available grade with equivalent hardenability (Jominy end-quench) verification.
Q: Can SNCM240 be carburized?
Not in the standard SNCM240 grade — the 0.36–0.43% carbon range is too high for carburizing. Case carburizing requires base carbon below 0.25% (typically 0.15–0.25%) so the case carbon gradient is meaningful. For Ni-Cr-Mo carburizing grades, specify SNCM220 (equivalent to AISI 8620) or SNCM415 (equivalent to AISI 4320). SNCM240 is a through-hardening grade used in the Q&T condition.
Q: What is the difference between SNCM240 and SNCM439?
Primarily Ni content: SNCM240 has 0.40–0.70% Ni; SNCM439 has 1.60–2.00% Ni. This 3–4× Ni difference produces approximately 2× the hardenability (critical section size for full through-hardening), meaningfully better low-temperature toughness, and higher cost. SNCM240 is the right specification for 30–60 mm sections; SNCM439 for 60–120 mm sections. Using SNCM439 for 30 mm shafts is over-specification — all the extra Ni cost provides no benefit at that section size.
Summary
- SNCM240 (JIS G4103): 0.4% Ni + Cr-Mo alloy steel — fills the hardenability gap between SCM440 (no Ni, ~30 mm max section) and SNCM439 (2% Ni, ~100 mm max section)
- Effective full-hardening section: ~50 mm diameter in oil quench — the primary specification justification over SCM440
- Charpy impact ≥ 78 J at 20°C (vs SCM440 ≥ 63 J): Ni toughening lowers DBTT by ~20–30°C for cold-climate applications
- Avoid tempering in 250–450°C range — temper embrittlement zone for all Ni-Cr-Mo steels; temper above 520°C for structural parts
- No exact ASTM equivalent; closest is AISI 8640/8740 type with Jominy verification for critical applications
- Not a carburizing grade — for Ni-Cr-Mo case hardening, specify SNCM220 or SNCM415
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