SNCM220
Nickel-Chromium-Molybdenum Case-Hardening Steel
Overview
SNCM220 is a Ni-Cr-Mo low-carbon alloy steel under JIS G 4053, with carbon 0.17–0.23%, Ni 0.40–0.70%, Cr 0.40–0.60%, and Mo 0.15–0.25%. JIS explicitly classifies it as a case-hardening (carburizing) steel. The design intent is "hard surface, tough core" — the low-carbon matrix provides good core toughness and impact absorption, while the high-carbon carburized case delivers 58–62 HRC wear resistance. This dual-property profile makes SNCM220 a standard choice for heavy-duty transmission gears, pins, and camshafts.
SNCM220 is a regularly stocked grade. A mill certificate (MTC) is included with every shipment, with full chemistry and heat number traceability.
Chemical Composition (JIS G 4053)
| Element | Spec Range (%) |
|---|---|
| C (Carbon) | 0.17 – 0.23 |
| Si (Silicon) | 0.15 – 0.35 |
| Mn (Manganese) | 0.60 – 0.90 |
| P (Phosphorus) | ≤ 0.030 |
| S (Sulfur) | ≤ 0.030 |
| Ni (Nickel) | 0.40 – 0.70 |
| Cr (Chromium) | 0.40 – 0.60 |
| Mo (Molybdenum) | 0.15 – 0.25 |
Mechanical Properties After Heat Treatment (Reference)
| Condition | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness |
|---|---|---|---|---|
| Normalizing | ≈ 700–800 | ≈ 500–600 | ≥ 18 | ≈ 180–220 HB |
| Carburize + Quench + Low Temper | — | — | — | 58–62 HRC (surface) |
| Q+T (secondary use) | ≈ 800–950 | ≈ 650–800 | ≥ 14 | ≈ 230–280 HB |
Values above are reference figures. Actual results vary with case depth, cross-section size, and heat treatment parameters. Contact Double Steel to specify case depth or mechanical property requirements.
Heat Treatment Characteristics
Gas Carburizing & Quench
Typical gas carburizing at 900–930°C; approximately 4 hours at 926°C yields a case depth of 0.75–1.25 mm (actual depth set per part specification). After carburizing, direct quench or reheat quench, followed by low-temperature temper at 150–180°C. Surface hardness reaches 58–62 HRC; core maintains 30–45 HRC.
Role of Nickel
Ni does not form carbides — it dissolves in the ferrite matrix, improving low-temperature toughness and impact strength so the core resists fracture under heavy impact loads. This is why SNCM grades cannot be fully replaced by pure SCM-series case-hardening steels in high-impact applications.
Role of Molybdenum
Mo suppresses temper embrittlement after carburizing quench, ensuring toughness is not degraded by low-temperature tempering. It also improves core hardenability for consistent hardening across larger cross-sections.
Q+T (Secondary Use)
If only moderate strength is needed without a high-hardness surface, SNCM220 can be quench-and-tempered (quench + high-temperature temper above 550°C) for use as a general alloy structural steel — but this is not its primary design intent.
Machining & Processing Notes
Machine Before Carburizing
The low carbon content gives SNCM220 good machinability in the pre-carburized state — hardness is typically HB 180–220 after annealing or normalizing, suitable for turning, milling, and drilling. Complete all major machining before carburizing; leave only finish grinding after case hardening.
Masking Against Carburizing
Surfaces that must not be case-hardened (threaded holes, mating faces) are typically masked by copper plating or anti-carburizing paint. Plan masked zones early in design to avoid costly post-hardening material removal.
Distortion From Quench
Quenching after carburizing inevitably introduces some dimensional change. High-precision parts require grinding allowance (typically 0.1–0.3 mm or more, depending on part geometry and tolerance class).
Weldability
Low carbon content gives SNCM220 better weldability than medium-carbon alloy steels like SCM440, but the presence of Ni, Cr, and Mo still requires attention to post-weld treatment to avoid heat-affected zone hardening. Welding is rarely the primary process for SNCM220 parts — most weld repairs occur before carburizing.
International Standard Equivalents
| Standard | Equivalent Grade | Correspondence |
|---|---|---|
| JIS (Japan) | SNCM220 (G 4053) | Reference standard |
| AISI/SAE (USA) | 8620 | Full equivalent |
| DIN (Germany) | 21NiCrMo2 | Full equivalent |
| EN (Europe) | 20NiCrMo2-2 (1.6523) | Full equivalent |
| ISO | 20NiCrMo2 | Full equivalent |
| GB (China) | 20CrNiMo | Near equivalent |
| BS (UK) | 805M20 | Near equivalent |
SNCM220 and SAE 8620 are the most common case-hardening steel equivalent pair in the Taiwan market. Case depth and core hardness specifications are typically agreed between buyer and supplier; JIS G 4053 does not specify post-carburizing mechanical properties. Contact Double Steel technical support for export compliance documentation.
Typical Applications
Automotive & Motorcycle Drivetrain
Transmission gears, differential planet gears, and driveshaft universal joints requiring "wear-resistant surface + impact-tough core" — carburized SNCM220 is virtually a standard material in global automotive production.
Machine Tools & Mechanical Drives
Camshafts, cam followers, roller chain link plates and pins, and heavy guideway sliders operating under high contact stress — high surface hardness is needed to resist Hertzian contact fatigue spalling. Carburized SNCM220 is the mainstream choice.
Heavy Construction Equipment
Excavator travel gears, bulldozer drive links, and mining equipment gears under heavy load and impact — Ni-induced low-temperature toughness gives SNCM220 an advantage over pure Cr-Mo grades in harsh environments.
Aerospace & Defense
Helicopter transmission gear sets, armored vehicle gearbox components — SNCM220 (SAE 8620) is a long-established, proven material for applications demanding extreme reliability.
Hand Tools & Hardware
Ratchet mechanisms of high-torque tools, wrench drive squares, and pneumatic tool main drive parts — carburized SNCM220 delivers the combination of wear resistance and impact toughness required.
Supply Specifications
| Form | Size Range | Source |
|---|---|---|
| Hot-rolled round bar | Ø 13 – 100 mm | Fengxing |
| Hot-rolled round bar | Ø 14 – 130 mm | CSC |
| Hot-rolled round bar | Ø 25 – 250 mm | Import |
| Ground bar | Ø 6 – 100 mm | — |
Length 5–12 m; cut-to-length available on request. Every shipment includes a mill certificate (MTC) with full chemistry and heat number traceability.
Material Selection Guide
SNCM220 is the preferred choice for case-hardened transmission parts requiring both a wear-resistant surface and impact-tough core — especially for heavy-load, high-speed, or low-temperature gear and pin applications.
SCM415 / SCM420
Lighter load, core impact toughness not critical
Lower cost and wider availability. SCM-series is sufficient for general carburized gear applications; SNCM220's advantage is mainly realized where high core impact demand exists.
SCM440 / SNCM439
Q+T use rather than carburizing
SNCM220's low carbon limits Q+T strength. Switch to SCM440 or SNCM439 for quench-tempered applications requiring strength above 1000 MPa.
For technical consultation, sample quotations, or mill certificate review, contact the Double Steel sales team via the inquiry form below or LINE customer service.