SCM440 is one of the most widely used chromium-molybdenum (Cr-Mo) alloy steels under JIS G4053. With far better hardenability than carbon steel, it reaches a tensile strength above 980 MPa after quenching and tempering, making it the first choice for drive shafts, high-strength bolts, and gears. This guide presents strength, hardness, and toughness data in tables for the annealed, quenched & tempered, and induction-hardened states, and compares SCM440 with S45C.
What is SCM440?
SCM440 is a structural alloy steel (JIS G4053) alloyed with chromium and molybdenum, which greatly improve hardenability and tempering stability. Compared with the medium-carbon steel S45C, SCM440 has better hardenability and hardens more deeply in larger diameters, delivering superior strength and fatigue life. Typical applications include drive shafts, high-strength bolts, gears, hydraulic components, and mold bases.
Chemical Composition (JIS G4053)
| Element | C | Si | Mn | P | S | Ni | Cr | Mo | Cu |
|---|---|---|---|---|---|---|---|---|---|
| Content (%) | 0.38–0.43 | 0.15–0.35 | 0.60–0.90 | ≤0.030 | ≤0.030 | ≤0.25 | 0.90–1.20 | 0.15–0.30 | ≤0.30 |
Mechanical Properties of SCM440 by Heat Treatment
1. Annealed State
Annealed SCM440 is relatively soft and easy to machine — ideal for blanks awaiting quenching and tempering.
| Property | Value |
|---|---|
| Tensile Strength | Approx. 600–730 MPa (converted from hardness) |
| Brinell Hardness | Approx. HB 179–217 (furnace-cooled from about 830°C) |
Features: Good machinability. Rough machining is best done before quenching and tempering to reduce tool wear.
2. Quenched & Tempered State
Quenching and tempering is the most common service condition for SCM440, giving the best balance of strength and toughness.
| Property | Reference Value (former JIS G 4105) |
|---|---|
| Tensile Strength | ≥ 980 MPa |
| Yield Strength | ≥ 835 MPa |
| Elongation | ≥ 12% |
| Reduction of Area | ≥ 45% |
| Impact Value | ≥ 59 J/cm² |
| Hardness | HB 285–352 (approx. HRC 30–38) |
These are reference values from the former JIS G 4105, for a Ø25 mm test piece oil-quenched from 830–880°C and tempered at 530–630°C followed by rapid cooling. The current JIS G 4053 does not specify mechanical properties.
Features: Suited to drive shafts, gears, connecting rods, and high-strength bolts under cyclic loading. Actual properties depend on section size and tempering temperature — always refer to the mill certificate and heat treatment report.
3. Induction (High-Frequency) Surface Hardening
After quenching and tempering, SCM440 can be induction-hardened to a surface hardness of about HRC 50–60 (depending on part shape and cooling conditions) while the core retains its toughness — ideal for shaft journals and gear faces requiring local wear resistance.
Heat Treatment Overview
| Condition | Tensile Strength (MPa) | Hardness | Elongation |
|---|---|---|---|
| Annealed | Approx. 600–730 | HB 179–217 | — |
| Quenched & Tempered | ≥ 980 | HB 285–352 (HRC 30–38) | ≥ 12% |
| Q&T + Induction Hardened | Core as Q&T | Surface HRC 50–60 | — |
Toughness, Fatigue Life, and Practical Notes
- Hardenability: Cr and Mo give SCM440 far better hardenability than S45C — larger-diameter parts harden more deeply and more uniformly after Q&T.
- Fatigue life: In the Q&T state, the fatigue limit is roughly 45–50% of tensile strength — well suited to long-term cyclic loading.
- Temper embrittlement: Avoid tempering at roughly 250–400°C (low-temperature temper embrittlement). After high-temperature tempering (530–630°C), cool rapidly to prevent high-temperature temper embrittlement.
- Size effect: Very large sections still show slightly lower core hardness; for critical parts, specify the post-Q&T hardness range and request a heat treatment report.
SCM440 vs S45C: Material Selection
| Criteria | SCM440 | S45C |
|---|---|---|
| Classification | Cr-Mo alloy steel (JIS G4053) | Medium-carbon steel (JIS G4051) |
| Hardenability | Excellent; hardens more deeply in large diameters | Limited; core hardness drops in large diameters |
| Tensile Strength after Q&T | ≥ 980 MPa | ≥ 690 MPa |
| Material Cost | Higher | Lower |
| Typical Uses | Drive shafts, high-strength bolts, gears | General shafts and machine parts |
For general medium-duty, cost-sensitive parts, S45C is sufficient; upgrade to SCM440 for larger diameters or higher strength and fatigue requirements.
SCM440 Frequently Asked Questions (FAQ)
Q1: What hardness can SCM440 reach after quenching and tempering?
A: Typical Q&T hardness is HRC 30–38 (HB 285–352), adjustable via tempering temperature. For wear resistance, induction surface hardening can raise the surface to about HRC 50–60 while the core stays tough. Core hardness is slightly lower in large diameters — confirm hardening depth with your heat treater.
Q2: How do I choose between SCM440 and S45C?
A: The key factors are part diameter and strength requirements. For parts above roughly 50 mm diameter that need strength at the core, or parts under high cyclic loads, choose SCM440; for general medium-duty, cost-sensitive parts, S45C is adequate. SCM440's Q&T tensile strength (≥ 980 MPa) clearly exceeds S45C (≥ 690 MPa), with better fatigue life.
Q3: What bar forms and mill certificates are available for SCM440?
A: SCM440 round bars are commonly supplied as black bars or ground/polished bars — the latter eliminates rough OD machining for precision parts. Request a Mill Test Report confirming C 0.38–0.43%, Cr 0.90–1.20%, and Mo 0.15–0.30% per JIS G4053. Double Steel Corp. keeps ample SCM440 stock in a full range of sizes — contact us for a quote!