JIS S40C is a medium-carbon machine structural steel defined under JIS G4051, with a carbon content of 0.37–0.43%. It sits between the widely specified S35C and S45C, offering a useful blend of weldability and through-hardening capability. Induction hardening to HRC 52–58 is practical on small sections, and through-hardening gives 550–700 MPa tensile strength. Internationally it matches AISI 1040 (USA) and aligns with DIN C40 (Germany).
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose S40C
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4051 | S40C | Japan | Reference |
| ASTM / AISI | 1040 | USA | ✅ Exact Match |
| ISO 683-1 | C40 | International | ⚠️ Nearest Equivalent |
| DIN | C40 / 1.0511 | Germany | ⚠️ Nearest Equivalent |
| EN | C40E / 1.1186 | Europe | ⚠️ Nearest Equivalent |
2. Chemical Composition
| Element | JIS S40C | AISI 1040 | DIN C40 |
|---|---|---|---|
| C | 0.37–0.43% | 0.37–0.44% | 0.37–0.44% |
| Si | 0.15–0.35% | 0.10–0.35% | ≤ 0.40% |
| Mn | 0.60–0.90% | 0.60–0.90% | 0.50–0.80% |
| 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
| Property | Value (Metric) | Value (Imperial) |
|---|---|---|
| Tensile Strength | ≥ 570 MPa | ≥ 82.7 ksi |
| Yield Point | ≥ 345 MPa | ≥ 50.0 ksi |
| Elongation (GL=5d) | ≥ 17% | ≥ 17% |
| Reduction of Area | ≥ 45% | ≥ 45% |
| Hardness | 163–229 HB | 163–229 HB |
After quench + temper (sections ≤ 25 mm / 1 in)
| Temper Temperature | Tensile Strength | Hardness |
|---|---|---|
| 400°C (752°F) | ~900–1000 MPa (130–145 ksi) | HRC 32–38 |
| 550°C (1022°F) | ~700–800 MPa (101–116 ksi) | HRC 22–30 |
| 650°C (1202°F) | ~550–650 MPa (80–94 ksi) | HRC 15–22 |
Induction hardening (surface, sections ≤ 30 mm / 1.2 in)
| Property | Value |
|---|---|
| Surface hardness | HRC 52–58 |
| Effective case depth | 1.0–3.0 mm (0.039–0.118 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 | 50 W/(m·K) | 347 BTU·in/(hr·ft²·°F) |
| Thermal Expansion (20–100°C / 68–212°F) | 11.3 × 10⁻⁶ /°C | 6.3 × 10⁻⁶ /°F |
| Specific Heat | ~486 J/(kg·K) | 0.116 BTU/(lb·°F) |
5. Heat Treatment Conditions
| Process | Temperature | Cooling | Purpose |
|---|---|---|---|
| Normalizing | 850–880°C (1562–1616°F) | Air cool | Refine grain, baseline strength |
| Annealing | 820–860°C (1508–1580°F) | Furnace cool | Soften for machining |
| Through-Hardening (quench) | 820–860°C (1508–1580°F) | Water or oil quench | Full-section hardening |
| Tempering | 400–650°C (752–1202°F) | Air cool | Adjust strength / toughness balance |
| Induction Hardening | 860–910°C surface (1580–1670°F) | Water or oil quench | Surface hardening HRC 52–58 |
6. Machinability
- Machinability rating: approximately 55–60% relative to AISI 1212 baseline (100%)
- Normalized condition preferred for rough machining; annealed for precision work
- Hardened and tempered condition requires carbide tooling for finishing
- Induction-hardened surface requires grinding
7. Weldability
S40C is conditionally weldable. Ceq ≈ 0.43–0.50 requires careful procedure control for reliable joints.
- Preheat: 100–150°C (212–302°F) recommended for sections over 20 mm (0.8 in)
- Process: SMAW with E7018 low-hydrogen; GMAW with ER70S-6
- Post-weld: Stress relief at 550–600°C (1022–1112°F) recommended for restrained joints
- Recommendation: Complete all welding before any heat treatment
8. Common Mistakes
S45C is the industry default, but S40C offers slightly better weldability (lower Ceq) with only marginally lower hardness ceiling. For applications where welding is involved and HRC 52–55 surface hardness is adequate, S40C reduces weld process complexity without sacrificing functional performance.
S40C’s Ceq sits in a range where cold cracking is a real risk in thicker sections without preheat. Engineers who treat S40C like S35C (which rarely needs preheat) will encounter HAZ cracking on sections over 20 mm (0.8 in).
9. When to Choose S40C
- ✅ Shafts, keys, and pins requiring moderate through-hardness with some weldability retained
- ✅ Induction-hardened components where HRC 52–58 surface is sufficient (vs. S45C’s HRC 55–62)
- ✅ Applications where S35C is slightly under-strength and S45C’s weldability constraints are a concern
- ✅ Cold-formed medium-strength fasteners
- ❌ Maximum induction hardness required — use S45C or S50C
- ❌ Large sections requiring deep through-hardness — upgrade to SCM440
- ❌ Free-welding in service without preheat facilities — use S35C or structural grades
10. FAQ
Q: Is S40C the same as AISI 1040?
Yes, for most purposes. The carbon and manganese ranges are virtually identical. JIS G4051’s tighter P and S limits make S40C slightly cleaner. Both grades are interchangeable in shafting, fastener, and through-hardening applications.
Q: Why is S40C less commonly specified than S35C or S45C?
S35C and S45C are the default choices for weldable and hardenable carbon steels respectively — they are widely stocked and well understood. S40C occupies the middle ground and is chosen deliberately when the designer needs a specific combination of properties that neither S35C nor S45C delivers optimally. It is less visible in procurement catalogues but readily available from most bar steel suppliers.
Q: What induction hardness can I achieve on S40C?
HRC 52–58 on sections up to approximately 30 mm (1.2 in) in diameter. This is 3–5 HRC points below S45C in the same conditions. For applications specifying HRC 55 minimum surface hardness, S45C is the safer choice; S40C may not reliably meet the spec across all heats.
Q: Can S40C replace S45C in a cost reduction exercise?
Only if the hardness specification allows it. If the drawing calls for HRC 55–60 induction hardened surface, S40C may not consistently deliver. If the drawing calls for normalized or lightly tempered tensile strength in the 550–700 MPa range, S40C is a valid and sometimes more weldable alternative to S45C.
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