JIS S15C is a low-carbon machine structural steel defined under JIS G4051, with a carbon content of 0.13–0.18%. Its ultra-low carbon core delivers exceptional toughness after carburizing, making it the preferred grade for thin-section carburized parts where core fracture resistance is the primary design concern. Weldability is excellent and cold formability is high. Internationally it matches AISI 1015 (USA) and aligns closely with DIN C15 (Germany).
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose S15C
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4051 | S15C | Japan | Reference |
| ASTM / AISI | 1015 | USA | ✅ Exact Match |
| ISO 683-1 | C15 | International | ⚠️ Nearest Equivalent |
| DIN | C15 / 1.0401 | Germany | ⚠️ Nearest Equivalent |
| EN | C15E / 1.1141 | Europe | ⚠️ Nearest Equivalent |
2. Chemical Composition
| Element | JIS S15C | AISI 1015 | DIN C15 |
|---|---|---|---|
| C | 0.13–0.18% | 0.13–0.18% | 0.12–0.18% |
| 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% | ≤ 0.035% |
| S | ≤ 0.035% | ≤ 0.050% | ≤ 0.035% |
Sources: JIS G4051:2016, ASTM A29/A29M, DIN EN 10083-2
3. Mechanical Properties
As-normalized
| Property | Value (Metric) | Value (Imperial) |
|---|---|---|
| Tensile Strength | 370–490 MPa | 53.7–71.1 ksi |
| Yield Point | ≥ 225 MPa | ≥ 32.6 ksi |
| Elongation (GL=5d) | ≥ 27% | ≥ 27% |
| Reduction of Area | ≥ 55% | ≥ 55% |
| Hardness | 100–156 HB | 100–156 HB |
After carburizing + quench + temper (case surface)
| Property | Case (Surface) | Core |
|---|---|---|
| Hardness | HRC 58–62 | HRC 10–20 (very tough) |
| Effective case depth (HV550 basis) | 0.3–1.0 mm (0.012–0.039 in) | — |
4. Physical Properties
| Property | Value (Metric) | Value (Imperial) |
|---|---|---|
| Density | 7.87 g/cm³ | 0.284 lb/in³ |
| Young’s Modulus | 207 GPa | 30,000 ksi |
| Thermal Conductivity | 52 W/(m·K) | 361 BTU·in/(hr·ft²·°F) |
| Thermal Expansion (20–100°C / 68–212°F) | 11.9 × 10⁻⁶ /°C | 6.6 × 10⁻⁶ /°F |
| Specific Heat | ~481 J/(kg·K) | 0.115 BTU/(lb·°F) |
5. Heat Treatment Conditions
| Process | Temperature | Cooling | Purpose |
|---|---|---|---|
| Normalizing | 890–920°C (1634–1688°F) | Air cool | Refine grain |
| Annealing | 860–900°C (1580–1652°F) | Furnace cool | Soften for cold working |
| Gas Carburizing | 900–940°C (1652–1724°F) | Oil quench | Case hardening to HRC 58–62 |
| Tempering (post-carburize) | 150–180°C (302–356°F) | Air cool | Relieve quench stress |
6. Machinability
- Machinability rating: approximately 70–75% relative to AISI 1212 baseline (100%)
- Excellent in normalized or annealed condition — among the easiest G4051 grades to machine
- Built-up edge tendency at low cutting speeds — use higher speeds for finishing
- Post-carburize finishing requires grinding
7. Weldability
S15C has outstanding weldability. Carbon equivalent (Ceq ≈ 0.21–0.27) is among the lowest of any structural steel.
- Preheat: Not required for any practical section size
- Process: All standard welding processes without restriction
- Filler: ER70S-3 or ER70S-6 for GMAW
- Rule: Always weld before carburizing — never after
8. Common Mistakes
S15C’s softer core is advantageous only when core fracture under impact is the specific failure mode. For most standard carburized gears and pins, S20C provides adequate core toughness with slightly better core strength. Default to S20C unless thin sections or extreme impact loading justifies S15C.
The steep carbon gradient from carburized surface to low-carbon core limits achievable case depth in S15C compared to higher-carbon grades. For deep case requirements (> 1.5 mm / 0.059 in), consider S20C or a Cr-Mo grade such as SCM415 with longer cycle times or vacuum carburizing.
9. When to Choose S15C
- ✅ Thin-section carburized parts where core toughness is more critical than core strength (needle rollers, thin pins, small cams)
- ✅ Cold-headed fasteners requiring maximum ductility
- ✅ Welded assemblies with complex geometry requiring easy processability
- ✅ Light-duty deep-drawn parts (bar/rod product — for sheet, use SPCC)
- ❌ Standard carburized gears with moderate loads — S20C is the better default
- ❌ Through-hardening applications — carbon is too low
- ❌ Large sections needing high core strength — use SCM415 or SCM420
10. FAQ
Q: Is S15C the same as AISI 1015?
Yes for practical purposes. Identical carbon and manganese ranges. JIS G4051 has tighter P and S limits, making S15C marginally cleaner. The two grades are interchangeable in carburizing, cold forming, and welding applications.
Q: When should I choose S15C over S20C for carburizing?
Choose S15C when the carburized part is thin-section and the primary concern is that the core must not fracture under impact — for example, needle roller bearings and thin cam followers. For most standard gears, pins, and bushings, S20C’s slightly stronger core is the better engineering choice.
Q: What case depth can I achieve on S15C?
Typically 0.3–1.0 mm (0.012–0.039 in) to HV550, depending on cycle time and temperature. The steep carbon gradient from the carburized case to the very low-carbon core means the transition zone is sharper than in S20C, which can be an advantage (clear hard/soft boundary) or a limitation (shallower maximum practical depth).
Q: Can S15C be induction hardened?
Not effectively. With only 0.13–0.18% C, induction hardening produces negligible hardness increase — HRC 20 at best. Carburizing is the only viable surface hardening process for S15C.
Q: How does S15C compare to SCM415 for carburizing?
SCM415 (Cr-Mo alloy steel) has significantly better hardenability, achieves deeper case depth for the same cycle, and produces a stronger core due to alloy content. S15C is appropriate for small, lightly loaded parts where alloy steel cost is not justified. For high-load carburized gears and shafts, SCM415 or SCM420 is the standard industrial choice.
comment