SCM420
Chromium-Molybdenum Case-Hardening Alloy Steel
Overview
SCM420 is a chromium-molybdenum low-carbon alloy steel under JIS G 4053, with carbon 0.18–0.23%, chromium 0.90–1.20%, and molybdenum 0.15–0.30%. JIS classifies it as a case-hardening (carburizing) steel, positioned between SCM415 and SCM440. Its carbon content is approximately 0.05% higher than SCM415, delivering higher core hardness after quenching and better strength after tempering, while still maintaining the low-carbon base needed for effective carburizing. This carbon range makes SCM420 the preferred choice over SCM415 for heavy-duty gears and transmission shafts requiring deeper case depths and higher core strength.
Double Steel stocks SCM420 as a permanent inventory item, available in SCM420H hardenability-guaranteed grade. Every shipment includes a mill certificate (MTC) with full chemistry and heat number traceability.
Chemical Composition (JIS G 4053)
| Element | Spec Range (%) |
|---|---|
| C (Carbon) | 0.18 – 0.23 |
| Si (Silicon) | 0.15 – 0.35 |
| Mn (Manganese) | 0.60 – 0.85 |
| P (Phosphorus) | ≤ 0.030 |
| S (Sulfur) | ≤ 0.030 |
| Ni (Nickel) | ≤ 0.25 |
| Cr (Chromium) | 0.90 – 1.20 |
| Mo (Molybdenum) | 0.15 – 0.30 |
SCM420 vs SCM415 Composition Comparison
SCM415 and SCM420 differ primarily in carbon content, with a minor difference in Mo upper limit. SCM415's lower carbon (max 0.18%) gives better core toughness, improved machinability, and more forgiving carburizing parameters. SCM420's higher carbon (min 0.18%) produces approximately 3–5 HRC higher core hardness under identical quench conditions, providing a larger tooth-root bending fatigue design margin. For most standard-load transmission gears, SCM415 core strength is sufficient — only when design calculations show insufficient core strength, or when deeper case depths are required for heavy-duty parts, is SCM420 the right choice.
| Element | SCM415 | SCM420 | Significance |
|---|---|---|---|
| C (Carbon) | 0.13 – 0.18% | 0.18 – 0.23% | SCM420 core hardness ≈ 3–5 HRC higher |
| Cr (Chromium) | 0.90 – 1.20% | 0.90 – 1.20% | Identical |
| Mo (Molybdenum) | 0.15 – 0.25% | 0.15 – 0.30% | SCM420 has slightly higher Mo upper limit |
Mechanical Properties After Heat Treatment (Reference)
| Condition | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness |
|---|---|---|---|---|
| Normalizing | ≈ 750–850 | ≈ 550–650 | ≥ 16 | ≈ HB 190–230 |
| Carburize + Quench + Low Temper (Surface) | — | — | — | 58–62 HRC |
| Carburize + Quench + Low Temper (Core) | — | — | — | ≈ HRC 32–42 |
Values above are reference figures after heat treatment. Core hardness is influenced by case depth, section size, and quench conditions. SCM420 core hardness reference values are slightly higher than SCM415 (≈ 28–38 HRC) due to the higher base carbon content. Contact Double Steel for guidance on specific requirements.
Heat Treatment Characteristics
Gas Carburizing Process
Gas carburizing is typically performed at 900–930°C. At approximately 926°C for 4 hours, case depth reaches 0.75–1.25 mm. For deeper cases (1.5–2.0 mm), SCM420's slightly higher carbon content produces a smoother carbon gradient through the transition zone — the hardness gradient is shorter than SCM415, making it better suited for large-module gears subject to high Hertzian contact stress.
Quench & Low-Temperature Temper
Oil quenching at 820–870°C is standard. The Cr-Mo combination gives SCM420 hardenability comparable to SCM415 — both fully harden the core of typical section sizes under oil quench. Low-temperature tempering at 150–180°C stabilizes the martensite and relieves quench stress while maintaining 58–62 HRC surface hardness. SCM420's slightly higher post-temper core hardness provides a larger design margin for tooth-root bending fatigue strength.
Hardenability Guarantee (H Grade)
SCM420H is verified via Jominy end-quench testing to fall within specified hardenability bands, giving heat treaters predictable process control. For volume production of automotive and motorcycle gear parts, batch-to-batch hardenability consistency directly affects dimensional tolerance and service life stability — H grade is the standard choice for demanding mass-production customers.
Not Suitable for Q+T Application
SCM420's carbon ceiling (0.23%) approaches the lower boundary for light quench-and-temper use, and some applications use it in a normalized + lightly tempered condition for moderate-strength structural parts. However, Q+T strength is well below SCM440 levels. For structural applications requiring tensile strength above 1000 MPa, select SCM440.
Machining Notes
Machinability in Non-Carburized Condition
SCM420 machines well in the non-carburized condition. Normalized hardness is approximately HB 190–230, slightly harder than SCM415 (HB 170–210), but still within the comfortable range for standard cutting tools — turning, drilling, and gear hobbing pose no difficulty. All forming operations should be completed before carburizing; only grinding stock should remain after the carburizing cycle.
Gear Hobbing & Shaping Timing
Gear parts are typically hobbed or shaped in the normalized condition, then sent for carburizing and quench hardening. For high-accuracy requirements, gear grinding after carburizing is performed with a grinding allowance of 0.10–0.25 mm (depending on module and accuracy grade).
Carburizing Masking
Threaded holes, mating shoulders, keyways, and other surfaces that must not be hardened should be masked before carburizing using copper plating or anti-carburizing paste, preventing hardness in areas that require subsequent machining or precise fits.
Pre-Forging Treatment
If bar stock requires hot forging, work within the 1050–1200°C hot-forging temperature range. Normalize after forging (880–920°C, air cool) to homogenize grain structure and relieve forging stress before entering the carburizing cycle. Post-forge microstructure uniformity directly affects the uniformity of the subsequent carbon concentration distribution.
Distortion Control
Quench distortion after carburizing is similar to SCM415, but the slightly higher carbon content causes marginally greater volumetric expansion during quenching. For dimensionally sensitive thin-walled or long-shaft parts, marquenching (stepped quench) is recommended to reduce distortion, or allow additional grinding stock.
Weldability
Low carbon content makes SCM420 more weldable than most alloy steels. In practice, however, it is almost always used as a carburized component, so welding applications are rare. If welding is necessary, perform it before carburizing and follow with proper post-weld annealing.
International Standard Equivalents
| Standard | Equivalent Grade | Correspondence |
|---|---|---|
| JIS (Japan) | SCM420 (G 4053) | Reference standard |
| AISI/SAE (USA) | 4120 / 5120 | Near equivalent (4120 has Mo but low Cr; 5120 has no Mo) |
| DIN / EN (Germany/Europe) | 20MnCrS5 (1.7149) | Near equivalent (no Mo; higher Mn) |
| ISO | 20MnCr5 (ISO 683-11) | Near equivalent |
| DIN / EN (Germany/Europe) | 20CrMo4 (1.7321) | Near equivalent (contains Mo; different Cr range) |
| GB (China) | 20CrMo | Near equivalent (Cr range varies slightly) |
SCM420 has no single exact equivalent in any international standard system. The key trade-off is between the Cr-Mo combination (JIS) and higher Mn to compensate for the absence of Mo (DIN/EN 20MnCr5 series). The MnCr5 grades achieve comparable hardenability to SCM420's Cr+Mo combination but differ in temper embrittlement resistance. Confirm Mo content requirements against your design specification before substituting.
Typical Applications
Automatic Transmission & Automotive Drive Parts
Drive shaft gears, planetary gear carriers, and torque converter drive parts in automatic transmissions — components under high torque requiring higher core strength. SCM420's core hardness advantage after carburizing translates directly into longer fatigue life for this class of components.
Machine Tool Heavy Spindle Drive
Large-module helical gears, worm gear sets, and heavy feed drive gears inside spindle housings where Hertzian contact stress is high — these require deeper effective case depth and higher core bending strength. SCM420 outperforms SCM415 in this application.
Construction & Agricultural Machinery
Final drive gears, angle drive gearbox gears, and planetary gears for excavator slewing rings — large transmission parts where SCM420's higher core hardness (HRC 32–42) provides greater resilience against occasional heavy impact loads.
Precision Machinery Transmission
Reducer gear sets, camshafts, and spiral bevel gears where surface wear resistance is equivalent to SCM415 but higher core strength is needed. SCM420 is the direct drop-in upgrade when design load calculations show SCM415 core strength is insufficient.
Hand Tools & Power Tools
High-torque drive claws, shift forks, and ratchet seats subject to repeated impact loading. SCM420's higher core hardness provides better resistance to plastic deformation under impact, extending service life.
Supply Specifications
| Form | Size Range | Notes |
|---|---|---|
| Hot-rolled round bar (SCM420) | Ø 13 – 100 mm | Fengxing source |
| Hot-rolled round bar (SCM420) | Ø 14 – 130 mm | CSC source |
| Hot-rolled round bar (SCM420) | Ø 25 – 250 mm | Import source |
| Hot-rolled round bar (SCM420H) | Ø 13 – 100 mm | Hardenability guaranteed |
| 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. H-grade orders include a Jominy hardenability band report on request.
Material Selection Guide
SCM420 is positioned between SCM415 and SCM440. The core selection criterion is: the part requires carburizing, SCM415 core strength is insufficient, but full-section Q+T strength from SCM440 is not needed. When a part requires deeper case depth (over 1.5 mm), large-module or heavy-duty gears, and higher core bending fatigue strength, SCM420 is the right choice.
SCM415
Load is moderate, SCM415 core strength is sufficient
SCM415 is more cost-effective — no upgrade needed. SCM420's core strength advantage is approximately 3–5 HRC; if design calculations show SCM415 has adequate margin, the benefit of upgrading is limited.
SNCM220
Significant impact loads or low-temperature service
The low-temperature toughness provided by Ni is what SCM420 lacks — switch to SNCM220 for these conditions. Double Steel maintains permanent stock.
SCM440
Q+T only — no carburizing in the process route
If the heat treatment path is purely quench-and-temper with no carburizing, SCM420's low carbon limits achievable strength. Select SCM440 for Q+T structural applications.
For technical consultation, sample quotations, or mill certificate review, contact the Double Steel sales team via the inquiry form below or LINE customer service.