SCM440
Chromium-Molybdenum Alloy Structural Steel
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 exceeds 1000 MPa with 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 |
Mechanical Properties After Heat Treatment (Reference)
| Condition | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness (HB) |
|---|---|---|---|---|
| Annealing | — | — | — | ≤ 229 |
| Q+T (Tempered at 550°C) | ≈ 1000–1100 | ≈ 850–950 | ≥ 12 | ≈ 293–331 |
| Q+T (Tempered at 200°C) | ≈ 1350–1500 | ≈ 1200–1400 | ≥ 8 | ≈ 40–45 HRC |
Values above are reference figures after heat treatment. 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 830–870°C with oil quench. Compared to carbon steel, SCM440 has a longer incubation period on the TTT curve — even the center of larger cross-sections fully transforms to martensite, which is the critical reason it replaces S45C for larger parts.
Quench & Temper (Q+T)
Q+T (quench + high-temperature temper at 550–650°C) is the most typical delivery condition for SCM440. HB 293–331 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 (200–250°C) raises hardness above 40 HRC 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 | Full equivalent |
| DIN (Germany) | 42CrMo4 | Full equivalent |
| EN (Europe) | 42CrMo4 (1.7225) | Full equivalent |
| ISO | 42CrMo4 (ISO 683-1) | Full equivalent |
| GB (China) | 42CrMo | Full equivalent |
| BS (UK) | 708M40 | Near equivalent |
SCM440 and SAE 4140 are chemically near-identical, but acceptance criteria for mechanical properties differ across standards. JIS G 4053 does not mandate post-heat-treatment testing; 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.
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.
S45C
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.
SCM415 / SCM420
Carburized surface case required
SCM440's carbon content is too high for carburizing — switch to low-carbon alloy steels for case-hardened parts.
For technical consultation, sample quotations, or mill certificate review, contact the Double Steel sales team via the inquiry form below or LINE customer service.