> SCM420 is a chromium-molybdenum case hardening steel under JIS G4053 with slightly higher carbon than SCM415. After carburizing and quenching, its core tensile strength exceeds 930 MPa with a surface hardness of HRC 58–62 — the workhorse material for medium-load gears and shafts. This guide pr
SCM420 is a chromium-molybdenum case hardening steel under JIS G4053 with slightly higher carbon than SCM415. After carburizing and quenching, its core tensile strength exceeds 930 MPa with a surface hardness of HRC 58–62 — the workhorse material for medium-load gears and shafts. This guide presents strength and hardness data in tables and compares SCM420 with SCM415 and SCM440.
What is SCM420?
SCM420 is a low-carbon Cr-Mo case hardening steel (JIS G4053) with 0.18–0.23% carbon, positioned between SCM415 and medium-carbon Q&T steels. After carburizing, quenching, and tempering, it develops a hard wear-resistant case over a core stronger than SCM415's — making it one of the most common carburizing grades for transmission gears, drive shafts, and sprockets.
Chemical Composition (JIS G4053)
| Element | C | Si | Mn | P | S | Cr | Mo |
|---|---|---|---|---|---|---|---|
| Content (%) | 0.18–0.23 | 0.15–0.35 | 0.60–0.85 | ≤0.030 | ≤0.030 | 0.90–1.20 | 0.15–0.30 |
Mechanical Properties of SCM420 by Heat Treatment
1. Normalized / Annealed State (As-Machined)
| Property | Value |
|---|---|
| Tensile Strength | Approx. 550–700 MPa |
| Brinell Hardness | Approx. HB 150–200 |
| Elongation | Approx. 16–22% |
Features: Good machinability. Gear cutting and drilling should be completed before carburizing.
2. After Carburizing, Quenching & Tempering
| Zone | Reference Value | Notes |
|---|---|---|
| Surface Hardness | HRC 58–62 | Wear and pitting resistant |
| Core Tensile Strength | ≥ 930 MPa (JIS G4053 reference) | Higher than SCM415's ≥ 830 MPa |
| Core Hardness | Approx. HRC 30–40 | Depends on size and hardenability |
| Effective Case Depth | Typically 0.5–1.5 mm | Specify on the drawing |
Features: The stronger core gives better tooth-root bending fatigue capacity than SCM415 — well suited to medium-load gears and shafts.
Toughness, Fatigue Life, and Practical Notes
- Core strength advantage: The higher carbon content yields a notably stronger core than SCM415, allowing higher tooth-root bending stress.
- Contact fatigue: Residual compressive stress in the case improves pitting resistance for long-running drive components.
- Distortion control: Carburizing runs hot and long — grind precision gear teeth after carburizing, and watch distortion on slender shafts.
- Process sequence: Rough machining → carburizing, quenching & tempering → finish grinding.
SCM420 Selection: vs SCM415 and SCM440
| Criteria | SCM415 | SCM420 | SCM440 |
|---|---|---|---|
| Carbon Content (%) | 0.13–0.18 | 0.18–0.23 | 0.38–0.43 |
| Strengthening Route | Carburizing | Carburizing | Q&T (+ induction) |
| Surface Hardness | HRC 58–62 | HRC 58–62 | HRC 50–55 (induction) |
| Core / Body Tensile Strength | ≥ 830 MPa | ≥ 930 MPa | ≥ 980 MPa (Q&T) |
| Typical Uses | Small gears, pins | Medium-load gears, shafts | Drive shafts, bolts, gears |
Rule of thumb: Need an ultra-hard wear surface (gear teeth, cam faces) → carburizing steel SCM415/SCM420. Part carries overall stress without needing HRC 58+ surface → Q&T steel SCM440.
SCM420 Frequently Asked Questions (FAQ)
Q1: How do I choose between SCM420 and SCM415?
A: Surface hardness after carburizing is equivalent (HRC 58–62); the difference is core strength: SCM420's reference core tensile strength is ≥ 930 MPa versus ≥ 830 MPa for SCM415. Choose SCM420 for medium-load and heavier gears and shafts; SCM415 suffices for small or lightly loaded parts and costs slightly less.
Q2: When should I use SCM420 (carburizing) versus SCM440 (Q&T)?
A: It comes down to surface hardness requirements. Parts needing an HRC 58+ wear surface — gear teeth, cam faces — require carburizing steel SCM420. Parts carrying overall stress without an ultra-hard surface — shafts, bolts, connecting rods — are simpler and cheaper in Q&T steel SCM440. Induction hardening can bring SCM440's surface to HRC 50–55, still below carburized HRC 58–62.
Q3: How should surface hardness and case depth be specified for SCM420?
A: Specify surface hardness HRC 58–62 and the effective case depth on the drawing — typically 0.5–1.5 mm, or roughly 0.15–0.2 times the module for gears — and require hardness and case depth inspection reports from the heat treater. Double Steel Corp. stocks SCM420 round bars in a full range of sizes — contact us for a quote!