JIS S20C is a low-carbon machine structural steel defined under JIS G4051, with a carbon content of 0.18–0.23%. Its low carbon level gives excellent weldability and cold workability, while carburizing raises surface hardness to HRC 58–62 — making it the standard choice for gears, pins, and bushings that need a hard surface over a tough core. Internationally it aligns closely with AISI 1020 (USA) and DIN C22 (Germany).
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose S20C
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4051 | S20C | Japan | Reference |
| ASTM / AISI | 1020 | USA | ✅ Exact Match |
| ISO 683-1 | C22 | International | ⚠️ Nearest Equivalent |
| DIN | C22 / 1.0402 | Germany | ⚠️ Nearest Equivalent |
| EN | C22E / 1.1151 | Europe | ⚠️ Nearest Equivalent |
2. Chemical Composition
| Element | JIS S20C | AISI 1020 | DIN C22 |
|---|---|---|---|
| C | 0.18–0.23% | 0.18–0.23% | 0.17–0.24% |
| Si | 0.15–0.35% | 0.10–0.35% | ≤ 0.40% |
| Mn | 0.30–0.60% | 0.30–0.60% | 0.30–0.60% |
| P | ≤ 0.030% | ≤ 0.040% | ≤ 0.045% |
| S | ≤ 0.035% | ≤ 0.050% | ≤ 0.045% |
Sources: JIS G4051:2016, ASTM A29/A29M, DIN EN 10083-2
3. Mechanical Properties
As-normalized (typical values)
| Property | Value (Metric) | Value (Imperial) |
|---|---|---|
| Tensile Strength | ≥ 400 MPa | ≥ 58 ksi |
| Yield Point | ≥ 245 MPa | ≥ 35.5 ksi |
| Elongation (GL=5d) | ≥ 25% | ≥ 25% |
| Reduction of Area | ≥ 50% | ≥ 50% |
| Hardness | 111–163 HB | 111–163 HB |
| Charpy Impact (V-notch) | — (not specified) | — |
After carburizing + quench + temper (case surface)
| Property | Case (Surface) | Core |
|---|---|---|
| Hardness | HRC 58–62 | HRC 15–25 |
| Effective case depth (HV550 basis) | 0.5–1.2 mm (0.020–0.047 in) | — |
4. Physical Properties
| Property | Value (Metric) | Value (Imperial) |
|---|---|---|
| Density | 7.85 g/cm³ | 0.284 lb/in³ |
| Young’s Modulus | 206 GPa | 29,900 ksi |
| Thermal Conductivity | 51 W/(m·K) | 354 BTU·in/(hr·ft²·°F) |
| Thermal Expansion (20–100°C) | 11.7 × 10⁻⁶ /°C | 6.5 × 10⁻⁶ /°F |
| Specific Heat | ~486 J/(kg·K) | 0.116 BTU/(lb·°F) |
5. Heat Treatment Conditions
| Process | Temperature | Cooling | Purpose |
|---|---|---|---|
| Normalizing | 880–920°C (1616–1688°F) | Air cool | Refine grain, relieve stress |
| Annealing | 850–890°C (1562–1634°F) | Furnace cool | Soften for machining |
| Gas Carburizing | 900–950°C (1652–1742°F) | Oil quench | Case hardening |
| Tempering (post-carburize) | 150–200°C (302–392°F) | Air cool | Relieve quench stress |
| Induction Hardening | 900–950°C surface (1652–1742°F) | Water or oil quench | Localized surface hardening |
6. Machinability
S20C machines similarly to AISI 1020. The low carbon level promotes built-up edge (BUE) on cutting tools, particularly at low speeds. Recommended practices:
- Use higher cutting speeds to minimize BUE in finishing passes
- Positive-rake geometry tools reduce cutting forces
- Machinability rating: approximately 65–70% relative to AISI 1212 baseline (100%)
- After carburizing, the hardened case requires grinding — do not attempt turning the case with standard tooling
7. Weldability
S20C is one of the most weldable grades in JIS G4051, owing to its low carbon equivalent (Ceq ≈ 0.28–0.32).
- Preheat: Generally not required for sections under 25 mm (1 in)
- Process: Compatible with SMAW, GMAW, GTAW, and SAW
- Filler: ER70S-6 or equivalent for GMAW
8. Common Mistakes
Engineers specify S20C and request “quench and temper to HRC 50.” The core carbon is insufficient — actual hardness peaks around HRC 25. Redesign for carburizing, or switch to S45C / SCM440 if through-hardness is required.
For most applications the two are interchangeable. However, if your application is sensitive to sulfur inclusions (e.g., fatigue-critical rotating parts), note that JIS G4051 caps S at 0.035% vs. ASTM A29’s 0.050%. Using JIS S20C in place of 1020 is safe; the reverse should include a cleanliness check.
Even tack welds on a carburized surface can cause case cracking. Plan the manufacturing sequence so all joins are made before heat treatment.
9. When to Choose S20C
- ✅ Parts requiring surface hardness HRC 58–62 with a tough core (gears, pins, bushings, camshafts)
- ✅ Components that will be welded before heat treatment
- ✅ Cold-formed or cold-headed fasteners and rivets
- ✅ Budget-conscious carburizing applications where SCM415 (Cr-Mo) is over-specified
- ❌ Applications requiring through-hardness — use S45C or SCM440
- ❌ High-section parts where hardenability depth matters — use SNCM220 (8620 equiv.) or SCM420
- ❌ Corrosive environments — use stainless or apply appropriate coating
10. FAQ
Q: Is S20C exactly the same as AISI 1020?
For most engineering purposes, yes. The carbon and manganese ranges are identical. The only differences are tighter P and S limits in JIS G4051, which make S20C marginally cleaner. Substituting S20C for 1020 is straightforward; using 1020 in place of S20C is acceptable unless the application demands JIS-level cleanliness.
Q: How does S20C compare to S15C and S25C?
S15C (C: 0.13–0.18%) offers even higher core toughness after carburizing but shallower effective case depth. S25C (C: 0.22–0.28%) sits above S20C and approaches the upper carburizing range — it can be through-hardened to about HRC 30. S20C is the pragmatic middle ground for standard carburized gear and pin applications.
Q: Can S20C be induction hardened?
Yes, but surface hardness is limited to approximately HRC 40–50, compared to HRC 58–62 achievable with carburizing. If high surface hardness is critical, specify carburizing. Induction hardening on S20C is more useful for localized stress relief or modest wear resistance on low-load components.
Q: What is the typical effective case depth for S20C after gas carburizing?
Under standard conditions (gas carburizing at 920°C / 1688°F for 4–6 hours), the effective case depth to HV 550 is typically 0.5–1.2 mm (0.020–0.047 in). Adjust time and temperature to hit your target depth per drawing requirements.
Q: Is S20C RoHS-compliant?
S20C itself contains no RoHS-restricted substances (Pb, Cd, Hg, Cr⁶⁺, PBBs, PBDEs). It is suitable for use in RoHS-regulated products. Verify surface treatments (plating, coatings) separately, as these may introduce restricted substances.
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