SCM415
Chromium-Molybdenum Case-Hardening Alloy Steel
Overview
SCM415 is a chromium-molybdenum low-carbon alloy steel under JIS G 4053, with carbon 0.13–0.18%, chromium 0.90–1.20%, and molybdenum 0.15–0.25%. JIS explicitly classifies it as a case-hardening (carburizing) steel, designed for the combination of a hard wear-resistant surface and a tough, impact-resistant core. Compared to SNCM220, SCM415 contains no nickel, making it more cost-effective. The Cr-Mo combination gives it significantly better hardenability than plain carbon steel, ensuring uniform core hardening even in oil quench — making it one of the most widely used carburizing steels for machine tool, automotive, and general machinery transmission gears in Taiwan.
Double Steel stocks SCM415 as a permanent inventory item, available in SCM415H and SCM415HRCH grades. Every shipment includes a mill certificate (MTC) with full chemistry and heat number traceability.
Chemical Composition (JIS G 4053)
| Element | Spec Range (%) |
|---|---|
| C (Carbon) | 0.13 – 0.18 |
| Si (Silicon) | 0.15 – 0.35 |
| Mn (Manganese) | 0.60 – 0.90 |
| P (Phosphorus) | ≤ 0.030 |
| S (Sulfur) | ≤ 0.030 |
| Ni (Nickel) | ≤ 0.25 |
| Cr (Chromium) | 0.90 – 1.20 |
| Mo (Molybdenum) | 0.15 – 0.25 |
SCM415H is a hardenability-guaranteed (H-grade) steel. Its chemical composition range is slightly wider than standard grade, but every batch is accompanied by Jominy end-quench test results verifying hardenability falls within a specified band — ensuring batch-to-batch consistency for applications requiring tight dimensional tolerances and stable heat treatment results.
SCM415 vs SCM420 Composition Comparison
SCM415 and SCM420 have nearly identical chemistry — the only difference is carbon content. 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% | 0.15–0.25% | Identical |
Mechanical Properties After Heat Treatment (Reference)
| Condition | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness |
|---|---|---|---|---|
| Normalizing | ≈ 700–800 | ≈ 500–600 | ≥ 18 | ≈ HB 170–210 |
| Carburize + Quench + Low Temper (Surface) | — | — | — | 58–62 HRC |
| Carburize + Quench + Low Temper (Core) | — | — | — | ≈ HRC 28–38 |
Values above are reference figures after heat treatment. Core hardness is influenced by case depth, section size, and quench conditions. Contact Double Steel for guidance on specific case depth or core hardness requirements.
Heat Treatment Characteristics
Case Hardening Fundamentals
The primary heat treatment for SCM415 is carburizing followed by quench hardening. The very low base carbon (~0.15%) ensures the core remains tough and impact-resistant after quenching. Cr (0.90–1.20%) provides substantially better hardenability than plain carbon steel, preventing soft unhardened cores in larger sections. Mo (0.15–0.25%) further suppresses temper embrittlement and stabilizes core strength.
Gas Carburizing Process
Gas carburizing at 900–930°C is the most common process. At approximately 926°C for 4 hours, case depth reaches 0.75–1.25 mm. For deeper cases (e.g., heavy gears requiring 1.5 mm+), extend carburizing time or use step-temperature profiles. After carburizing, parts are typically direct quenched (oil) or reheated to 820–860°C for a secondary quench, followed by low-temperature tempering at 150–180°C to stabilize the martensite structure. Surface hardness reaches 58–62 HRC.
Hardenability Guarantee (H Grade)
Batch-to-batch chemical variation can shift hardenability in standard grades. SCM415H and SCM415HRCH are guaranteed to fall within defined Jominy end-quench hardenability bands, giving heat treaters confidence in process parameter settings — especially important for customers with volume production requiring consistent results. The HRCH designation additionally guarantees cold-working hardness (RCH).
Not Suitable for Q+T Application
SCM415's low carbon content limits strength achievable through quench-and-temper alone. For Q+T structural applications, select SCM440. SCM415 is purpose-designed as a carburizing steel — its value lies in the combination of a hard case and a tough core after the carburizing cycle.
Machining Notes
Machinability
SCM415 machines well in the non-carburized condition. Normalized or annealed hardness is approximately HB 170–210 — turning, drilling, and milling pose no difficulty. All forming operations should be completed before carburizing; only grinding stock should remain after the carburizing cycle.
Grinding Stock Allowance
Parts will have slight distortion after carburizing and quenching. Grinding stock for bores and OD surfaces is typically 0.1–0.3 mm (depending on part geometry, length-to-diameter ratio, 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 to relieve forging stress and homogenize grain structure before entering the carburizing cycle.
Weldability
The very low carbon content makes SCM415 more weldable than most alloy steels — low carbon equivalent with minimal HAZ hardening tendency. In practice, SCM415 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) | SCM415 (G 4053) | Reference standard |
| AISI/SAE (USA) | 5115 / 4118 | Near equivalent (composition range varies) |
| DIN (Germany) | 16MnCr5 | Near equivalent (lower Cr range; no Mo) |
| EN (Europe) | 16MnCrS5 (1.7139) | Near equivalent |
| ISO | 16MnCr5 (ISO 683-11) | Near equivalent |
| DIN / EN | 18CrMo4 (1.7243) | Near equivalent (contains Mo; different Cr range) |
| GB (China) | 15CrMo | Near equivalent (Cr upper limit varies slightly) |
Purchasing note: SCM415 has no single exact AISI/SAE equivalent — 4118 has lower Cr while 5115 has no Mo. When specifying import substitutes, confirm the specific Cr and Mo ranges and verify hardenability specifications match. JIS G 4053 does not specify post-carburizing mechanical properties — actual core hardness and case depth must be agreed with the heat treater.
Typical Applications
Machine Tool Transmission Systems
Gear trains inside machine tool headstocks — speed gears, feed gears, and change gears — require surface wear resistance against tooth-contact fatigue and core toughness against tooth-root bending fatigue. SCM415 carburized stock meets both requirements at lower cost than Ni-containing SNCM grades.
Automotive & Motorcycle Power Transmission
Shift gears, idler gears, and reverse gears in manual and automatic transmissions experience concentrated load cycling — SCM415 carburized surface hardness and core strength combination satisfies passenger vehicle design requirements. Cost-sensitive volume production uses SCM415 as the standard material, upgrading to SNCM220 only when impact requirements are higher.
General Machinery Transmission
Reducer gear sets, sprockets, coupling jaws, cams, and pin shafts running under steady load — SCM415 carburized wear resistance and service life are well-proven for this class of components.
Hand Tools & Hardware
Ratchet heads, wrench drive squares, and socket adapters requiring surface wear resistance combined with impact toughness — SCM415 carburized hardness gradient meets the application requirement at lower processing cost than SNCM220.
Agricultural & Construction Machinery
Transmission gears, chain links, and guide wheels requiring wear resistance — SCM415 carburized stock performs reliably in standard environments. Heavy equipment with higher impact requirements warrants consideration of SNCM220.
Supply Specifications
| Form | Size Range | Notes |
|---|---|---|
| Hot-rolled round bar (SCM415) | Ø 13 – 100 mm | Fengxing source |
| Hot-rolled round bar (SCM415) | Ø 14 – 130 mm | CSC source |
| Hot-rolled round bar (SCM415) | Ø 25 – 250 mm | Import source |
| Hot-rolled round bar (SCM415H) | Ø 13 – 100 mm | Hardenability guaranteed |
| Hot-rolled round bar (SCM415HRCH) | 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
SCM415 is the go-to choice for light-to-medium load carburized transmission gears, pins, and shaft components — offering the best balance of cost and performance. Its hardenability exceeds plain carbon steel, and the Cr-Mo combination makes the carburizing process stable and predictable. It is the most commonly stocked carburizing steel at Taiwanese machining facilities.
SCM420
Higher core strength or deeper case depth required
SCM420 has higher carbon (0.18–0.23%), giving better post-carburizing core strength. Appropriate for larger cross-sections or heavy gears requiring case depths exceeding 1.5 mm.
SNCM220
Significant impact loads or low-temperature service
The low-temperature toughness provided by Ni is what SCM415 lacks — upgrade to SNCM220 for these conditions. Double Steel maintains permanent stock.
SCM440
No carburizing — quench-and-temper is the heat treatment path
SCM415's low carbon content is not suitable for Q+T structural applications. Select SCM440 directly.
For technical consultation, sample quotations, or mill certificate review, contact the Double Steel sales team via the inquiry form below or LINE customer service.