JIS G4103 defines 11 nickel-chromium-molybdenum (Ni-Cr-Mo) structural steel grades — SNCM220 through SNCM815. These grades share the same fundamental Cr-Mo base as the SCM series but add nickel, which raises hardenability for larger cross-sections, improves low-temperature toughness, and in carburizing grades, provides higher case impact resistance. The SNCM numbering system encodes useful information: the first two digits after “SNCM” give the approximate carbon content in units of 0.01% (SNCM220 = ~0.20% C; SNCM439 = ~0.40% C; SNCM815 = ~0.15% C). The remaining digit(s) differentiate grades with the same carbon but different Ni levels. Understanding this structure allows engineers to navigate the series without memorizing every grade — the carbon level determines whether a grade is for carburizing (low C) or through hardening (higher C), and the Ni level determines the maximum effective section size.
- Full Grade Table
- Decoding the SNCM Numbering System
- Carburizing Grades (Low-C SNCM)
- Through-Hardening Grades (Medium-C SNCM)
- Section Size vs Grade Selection
- ASTM and EN Equivalents
- Common Mistakes
- FAQ
1. Full Grade Table
| Grade | C (%) | Ni (%) | Cr (%) | Mo (%) | Type | ASTM Nearest |
|---|---|---|---|---|---|---|
| SNCM220 | 0.17–0.23 | 0.40–0.70 | 0.40–0.65 | 0.15–0.25 | Carburizing | AISI 8620 |
| SNCM240 | 0.36–0.43 | 0.40–0.70 | 0.60–1.00 | 0.15–0.30 | Through-hardening | AISI 8640 type |
| SNCM415 | 0.12–0.18 | 1.55–2.00 | 0.40–0.65 | 0.15–0.30 | Carburizing | AISI 4320 |
| SNCM420 | 0.17–0.23 | 1.55–2.00 | 0.40–0.65 | 0.15–0.30 | Carburizing | AISI 4320H |
| SNCM431 | 0.27–0.35 | 1.55–2.00 | 0.60–1.00 | 0.15–0.30 | Through-hardening | — |
| SNCM439 | 0.36–0.43 | 1.55–2.00 | 0.60–1.00 | 0.15–0.30 | Through-hardening | AISI 4340 (approx.) |
| SNCM447 | 0.43–0.50 | 1.55–2.00 | 0.60–1.00 | 0.15–0.30 | Through-hardening | — |
| SNCM616 | 0.12–0.18 | 2.80–3.20 | 1.55–2.00 | 0.40–0.60 | Carburizing | — |
| SNCM625 | 0.20–0.27 | 3.00–3.50 | 1.00–1.50 | 0.15–0.30 | Through-hardening | — |
| SNCM630 | 0.25–0.35 | 2.80–3.20 | 2.55–3.30 | 0.40–0.60 | Through-hardening | — |
| SNCM815 | 0.12–0.18 | 4.00–4.50 | 0.70–1.00 | 0.15–0.30 | Carburizing | — |
Source: JIS G4103:2016. Total alloy content increases from SNCM220 (lowest Ni) to SNCM815 (highest Ni), corresponding to increasing section size capability for full through-hardening or case-depth uniformity.
2. Decoding the SNCM Numbering System
The JIS SNCM numbering convention encodes the approximate carbon content:
- SNCM220: ~0.20% C → carburizing grade (low C for case hardening)
- SNCM415, SNCM420: ~0.15%, ~0.20% C → carburizing grades; the 4/5 prefix distinguishes the Ni level group
- SNCM431, SNCM439, SNCM447: ~0.31%, ~0.40%, ~0.47% C → through-hardening grades in the same Ni group
- SNCM616: ~0.15% C → high-Ni carburizing grade
- SNCM815: ~0.15% C → highest-Ni carburizing grade
Rule of thumb: grades ending in “1x” (SNCM415, SNCM616, SNCM815) are carburizing grades (C ≤ 0.20%). Grades ending in “3x”–”5x” (SNCM431, SNCM439, SNCM447, SNCM625, SNCM630) are through-hardening grades.
3. Carburizing Grades (Low-C SNCM)
| Grade | Ni | Max Section (carburized case uniformity) | Primary Use |
|---|---|---|---|
| SNCM220 | 0.40–0.70% | ~40 mm | General gears, shafts — entry Ni carburizing grade |
| SNCM415 | 1.55–2.00% | ~60 mm | Medium gears, differential components, camshafts |
| SNCM420 | 1.55–2.00% | ~60 mm | Higher load gears (same Ni as 415, slightly higher C) |
| SNCM616 | 2.80–3.20% | ~100 mm | Large-section case-hardened gears, heavy machinery |
| SNCM815 | 4.00–4.50% | ~150+ mm | Largest marine/industrial gears requiring deep, uniform case |
The carburizing grade progression from SNCM220 to SNCM815 is driven by one need: ensuring that the case-core transition is sharp and uniform even in large gear sections. In small gears (under 40 mm body diameter), SNCM220 provides adequate case depth and core hardness. As section size increases, the core cooling rate during quench slows — without sufficient Ni to maintain hardenability, the core transforms to bainite (lower strength, lower hardness) instead of martensite. SNCM415/420 extends this to ~60 mm; SNCM815 handles marine-scale gears where body diameter can exceed 150 mm.
4. Through-Hardening Grades (Medium-C SNCM)
| Grade | Ni | Max Section (full through-HT) | Tensile Strength (Q&T) | Primary Use |
|---|---|---|---|---|
| SNCM240 | 0.40–0.70% | ~50 mm | 780–1030 MPa | Shafts, medium section structural parts |
| SNCM431 | 1.55–2.00% | ~80 mm | 930–1080 MPa | Medium-large shafts, axles |
| SNCM439 | 1.55–2.00% | ~100 mm | 980–1180 MPa | Heavy-duty shafts, propeller shafts, crankshafts |
| SNCM447 | 1.55–2.00% | ~100 mm | 1080–1270 MPa | High-strength shafts requiring maximum tensile strength |
| SNCM625 | 3.00–3.50% | ~200 mm | 880–1030 MPa | Large marine shafts, turbine shafts |
| SNCM630 | 2.80–3.20% | ~200 mm | 1030–1230 MPa | Large high-strength components, pressure vessels |
SNCM439 is the most widely used through-hardening grade in the JIS G4103 series — the closest JIS equivalent to AISI 4340 (though with different Ni/Cr ranges). It covers the majority of heavy-duty shaft and crankshaft applications. SNCM447 provides higher tensile strength at the same section size by raising carbon to 0.43–0.50%. SNCM625 and SNCM630 are used for very large marine and turbine components where section sizes exceed 150 mm and no lower-Ni grade provides full through-hardening.
5. Section Size vs Grade Selection
| Shaft/Gear Diameter | Carburizing Grade | Through-Hardening Grade |
|---|---|---|
| Below 30 mm | SCM415 / SNCM220 | SCM440 / SNCM240 |
| 30–60 mm | SNCM415 / SNCM420 | SNCM240 / SNCM431 |
| 60–100 mm | SNCM420 / SNCM616 | SNCM439 / SNCM447 |
| 100–150 mm | SNCM616 | SNCM625 / SNCM630 |
| Above 150 mm | SNCM815 | SNCM625 / SNCM630 |
6. ASTM and EN Equivalents
| JIS Grade | ASTM/AISI Nearest | EN Nearest | Match Quality |
|---|---|---|---|
| SNCM220 | AISI 8620 | 20NiCrMo2-2 / 1.6523 | ✅ Close match |
| SNCM415 | AISI 4320 | 20NiCrMo7 / 1.6587 (approx.) | ✅ Close match |
| SNCM439 | AISI 4340 (approx.) | 36NiCrMo16 / 1.6773 (higher Ni) or 34CrNiMo6 / 1.6582 | ⚠️ Approximate (different Ni range) |
| SNCM625 | No exact equivalent | 36NiCrMo16 / 1.6773 (approx.) | ⚠️ Approximate |
| SNCM815 | No standard equivalent | 14NiCr14 / 1.5752 (lower Mo) | ⚠️ Approximate (lower Mo) |
7. Common Mistakes
8. FAQ
Q: Is SNCM439 the same as AISI 4340?
Similar but not identical. Both are Ni-Cr-Mo through-hardening steels with comparable carbon (~0.40%) and Mo (~0.20%). The main difference is Ni content: AISI 4340 has 1.65–2.00% Ni with Cr 0.70–0.90%; SNCM439 has 1.55–2.00% Ni with Cr 0.60–1.00%. The ranges overlap substantially, and for most applications the grades are functionally equivalent. For critical applications requiring exact composition compliance, verify both specifications — they are not guaranteed substitutes.
Q: Why are there no JIS G4103 grades with intermediate Ni content between 0.70% and 1.55%?
The JIS series reflects the historical development of structural steels for Japanese industry. The 0.40–0.70% Ni range (SNCM220, SNCM240) was derived from AISI 8xxx series; the 1.55–2.00% Ni range mirrors AISI 43xx and European Ni-Cr-Mo traditions. There is no strong technical argument against intermediate Ni grades — the gap simply reflects standardization history rather than metallurgical necessity.
Summary
- JIS G4103 covers 11 SNCM grades: decode by C level (carburizing ≤ 0.23%, through-hardening ≥ 0.27%) and Ni level (hardenability / section size)
- SNCM220 (8620 equiv.): standard carburizing for sections ≤ 40 mm; SNCM415/420 extends to 60 mm; SNCM815 handles 150+ mm
- SNCM439 (4340 approx.): standard through-hardening for sections up to 100 mm — the most widely used JIS G4103 grade
- SNCM625 / SNCM630: high-Ni for very large sections (150–200 mm) in marine and turbine applications
- Temper embrittlement (250–450°C) applies to all Ni-Cr-Mo grades — always temper above 500°C for structural applications
- Grade selection by section size is the primary criterion — using high-Ni grades for small sections wastes Ni cost with no performance benefit
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