SCM440
Chromium-Molybdenum Alloy Structural Steel
SCM440 is a JIS G 4053 chromium-molybdenum alloy steel with carbon 0.38–0.43%, equivalent to AISI 4140 (SAE/ASTM), 42CrMo4 (DIN EN 10083-3, material no. 1.7225), and 42CrMo (GB/T 3077). Tensile strength reaches 980 MPa or more after Q+T; far superior hardenability to carbon steel, widely used for high-strength shafts, gears, and hydraulic components.
Overview
SCM440 is a chromium-molybdenum alloy structural steel under JIS G 4053, with carbon 0.38–0.43%, chromium 0.90–1.20%, and molybdenum 0.15–0.30%. These alloying elements give SCM440 far superior hardenability compared to plain carbon steel — after quench-and-temper, tensile strength reaches 980 MPa or more, with more uniform properties across larger cross-sections. It is the most widely consumed alloy steel in Taiwan's industrial sector: wherever high strength, high fatigue life, and larger section sizes are required, SCM440 is typically the engineer's first choice.
Double Steel has specialized in bar steel for over 30 years. SCM440 is a permanent stock item, available immediately in black bar and ground bar. Sourced from authorized Fengxing and Weizhi mills, with a mill certificate (MTC) provided for every shipment.
Chemical Composition (JIS G 4053)
| Element | Spec Range (%) |
|---|---|
| C (Carbon) | 0.38 – 0.43 |
| 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 – 0.30 |
| Cu (Copper) | ≤ 0.30 |
Mechanical Properties After Heat Treatment (Reference)
| Condition | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness (HB) |
|---|---|---|---|---|
| Annealing | — | — | — | ≈ 179–217 |
| Q+T (Tempered at 530–630°C) | ≥ 980 | ≥ 835 | ≥ 12 | 285–352 |
| Q+T (Tempered at 200°C) | — | — | — | ≈ 52–55 HRC (small sections) |
Values above are reference figures after heat treatment (Q+T values are the former JIS G 4105 reference values for a Ø25 mm test piece). Actual results vary with cross-section size and heat treatment parameters. Contact Double Steel to arrange specific mechanical property testing if required.
Heat Treatment Characteristics
Hardenability Advantage
The key advantage of SCM440 is the high hardenability provided by Cr and Mo. Austenitizing temperature is typically 830–880°C with oil quench. Compared to carbon steel, SCM440 has a longer incubation period on the TTT curve — larger cross-sections harden more deeply, which is the critical reason it replaces S45C for larger parts.
Quench & Temper (Q+T)
Q+T (quench + high-temperature temper at 530–630°C) is the most typical delivery condition for SCM440. HB 285–352 hardness with adequate toughness suits parts under dynamic and impact loading. Mo effectively suppresses temper embrittlement, giving more stable post-temper toughness than plain chromium steels.
Low-Temperature Temper (High Hardness)
Low-temperature tempering (about 150–200°C) gives about 52–55 HRC in small, fully hardened sections; hardness falls as the section grows (about 42 HRC in a 100 mm class section). It is used for high wear-resistance applications. Toughness decreases accordingly — impact load conditions should be carefully evaluated in design.
Nitriding
Cr and Mo form stable nitrides during gas nitriding, achieving surface hardness above HV 650 at low treatment temperatures (480–550°C) with minimal distortion — ideal for final surface treatment of precision parts. SCM440's carbon content is too high for carburizing; use SCM415/SCM420 for case-hardened applications.
International Standard Equivalents
| Standard | Equivalent Grade | Correspondence |
|---|---|---|
| JIS (Japan) | SCM440 (G 4053) | Reference standard |
| AISI/SAE (USA) | 4140 | Equivalent grade |
| DIN / EN (Germany/Europe) | 42CrMo4 (1.7225) | Equivalent grade |
| ISO | 42CrMo4 (ISO 683-2) | Equivalent grade |
| GB (China) | 42CrMo | Equivalent grade |
| BS (UK) | 708M40 (BS 970, superseded by EN 10083) | Near equivalent |
SCM440 and SAE 4140 are chemically similar, but acceptance criteria for mechanical properties differ across standards. JIS G 4053 does not specify mechanical properties after heat treatment; EN and ISO standards require explicit tensile and impact tests. Contact Double Steel technical support to determine the correct delivery condition and documentation for your specific standard. Note: SAE 4140 has slightly offset Mn and Cr ranges compared to SCM440 — 4140 specifies Mn 0.75–1.00% and Cr 0.80–1.10%. Verify procurement specification requirements when strict acceptance criteria apply.
Typical Applications
Machine Tool Industry
Spindles, ball screws, heavy guideways, and turret bodies — core structural parts bearing high torque and fatigue loads — require Q+T high strength-toughness and dimensional stability. SCM440 is the standard choice.
Automotive & Motorcycle Parts
Crankshafts, drive shafts, steering racks, connecting rods, and high-strength bolts — critical safety parts — widely use Q+T SCM440. Mo-induced temper stability maintains performance in high-temperature operating environments.
Mechanical Components
Hydraulic cylinders, medium-heavy gears, couplings, and fixture bodies requiring uniform full-section strength — SCM440 is the standard upgrade from plain carbon steel.
Hand Tools & Hardware
High-strength wrenches, ratchet mechanisms, and pneumatic tool housings demanding impact resistance with sustained hardness benefit from SCM440's higher strength-toughness margin.
Defense & Aerospace
Drive shafts, load-bearing structures, and landing gear links — SCM440 (AISI 4140) is one of the most commonly specified baseline materials for high-specification structural components.
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. Non-standard sizes can be sourced — please inquire. Every shipment includes a mill certificate (MTC) with full chemistry and heat number traceability.
Material Selection Guide
SCM440 is the go-to choice for medium-to-high load applications requiring uniform strength-toughness across the full cross-section, or where section size exceeds the hardenability limits of plain carbon steel. Its Q+T strength far exceeds S45C while remaining readily machinable.
Moderate load, cost-sensitive
S45C remains the more economical choice — no need for SCM440. Also stocked by Double Steel.
SNCM439
Higher impact toughness or low-temperature toughness needed
Ni-containing SNCM439 outperforms SCM440 in low-temperature impact applications. Double Steel can assist with sourcing.
For technical consultation, sample quotations, or mill certificate review, contact the Double Steel sales team via the inquiry form below or LINE customer service.
Related Grades
For cross-standard composition and mechanical property tables, see the technical reference tables.
FAQ
- What steel grades is SCM440 equivalent to?
- SCM440 is equivalent to AISI 4140 (SAE), 42CrMo4 (DIN EN 10083-3, material no. 1.7225), and 42CrMo (GB/T 3077).
- What is the main difference between SCM440 and S45C?
- SCM440 contains chromium (0.90–1.20%) and molybdenum (0.15–0.30%), giving far superior hardenability — larger cross-sections harden more uniformly than with S45C. Q+T strength is also significantly higher (reference value ≥ 980 MPa, against ≥ 690 MPa for S45C).
- Are 42CrMo4 and SCM440 the same?
- They are equivalent grades: 42CrMo4 (DIN EN 10083-3) and SCM440 (JIS G 4053) have similar but not identical chemical compositions (42CrMo4 allows carbon up to 0.45%). They are generally interchangeable; for strict acceptance, follow the purchase specification.
- What is SCM440 tensile strength after quench-and-temper?
- After Q+T, the reference tensile strength is 980 MPa or more with hardness HB 285–352 (former JIS G 4105 reference values, Ø25 mm test piece). Actual values vary with section size and tempering temperature.
- What are the typical applications of SCM440?
- Spindles, ball screws, crankshafts, drive shafts, hydraulic cylinders, high-strength bolts, and medium-to-heavy gears — structural parts requiring high strength, toughness, and fatigue life. SCM440 is the most widely consumed alloy steel in Taiwan's industrial sector.
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