JIS S25C is a low-to-medium carbon machine structural steel defined under JIS G4051, with a carbon content of 0.22–0.28%. It occupies a practical middle ground: weldable enough for fabricated assemblies, carburizable to full case hardness, and capable of modest through-hardening on small sections. Internationally it matches AISI 1025 (USA) exactly and aligns closely with DIN C25 (Germany).
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose S25C
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4051 | S25C | Japan | Reference |
| ASTM / AISI | 1025 | USA | ✅ Exact Match |
| ISO 683-1 | C25 | International | ⚠️ Nearest Equivalent |
| DIN | C25 / 1.0406 | Germany | ⚠️ Nearest Equivalent |
| EN | C25E / 1.1158 | Europe | ⚠️ Nearest Equivalent |
2. Chemical Composition
| Element | JIS S25C | AISI 1025 | DIN C25 |
|---|---|---|---|
| C | 0.22–0.28% | 0.22–0.28% | 0.22–0.29% |
| Si | 0.15–0.35% | 0.10–0.35% | ≤ 0.40% |
| Mn | 0.30–0.60% | 0.30–0.60% | 0.40–0.70% |
| 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 | ≥ 440 MPa | ≥ 63.8 ksi |
| Yield Point | ≥ 275 MPa | ≥ 39.9 ksi |
| Elongation (GL=5d) | ≥ 23% | ≥ 23% |
| Reduction of Area | ≥ 50% | ≥ 50% |
| Hardness | 123–183 HB | 123–183 HB |
After through-hardening + temper (small sections, ≤ 25 mm / 1 in)
| Property | Water Quench | Oil Quench |
|---|---|---|
| Surface Hardness | HRC 45–50 | HRC 35–45 |
| Core Hardness (after temper 550–650°C) | HRC 20–30 | HRC 18–28 |
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 | 50 W/(m·K) | 347 BTU·in/(hr·ft²·°F) |
| Thermal Expansion (20–100°C / 68–212°F) | 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 | 870–910°C (1598–1670°F) | Air cool | Refine grain, baseline properties |
| Annealing | 840–880°C (1544–1616°F) | Furnace cool | Soften for machining |
| Gas Carburizing | 900–940°C (1652–1724°F) | Oil quench | Case hardening to HRC 58–62 |
| Tempering (post-carburize) | 150–200°C (302–392°F) | Air cool | Relieve quench stress |
| Through-Hardening (quench) | 850–890°C (1562–1634°F) | Water or oil quench | Moderate full-section hardening |
| Tempering (post through-harden) | 550–650°C (1022–1202°F) | Air cool | Restore toughness, reduce brittleness |
6. Machinability
S25C machines similarly to AISI 1025 and is comparable to S20C in setup requirements. The slightly higher carbon content marginally increases cutting resistance relative to S20C but remains well within the easy-to-machine range.
- Machinability rating: approximately 65–70% relative to AISI 1212 baseline (100%)
- Built-up edge (BUE) tendency is moderate at low speeds — use higher cutting speeds for finishing
- Positive-rake tooling reduces cutting forces in soft (normalized) condition
- Carburized case requires grinding; do not turn the hardened layer with standard tooling
7. Weldability
S25C offers good weldability, with a carbon equivalent (Ceq) of approximately 0.32–0.36 — lower than S35C and significantly lower than S45C.
- Preheat: Generally not required for sections under 25 mm (1 in); recommended at 75–100°C (167–212°F) for thicker sections
- Process: SMAW, GMAW, GTAW, and SAW all compatible
- Filler: ER70S-6 or equivalent for GMAW
8. Common Mistakes
The two grades look similar on a datasheet, but S25C’s higher carbon ceiling (0.28% vs. 0.23%) gives it meaningfully different through-hardening potential and a slightly higher carburized core hardness. Do not substitute one for the other in applications where hardness or hardenability is part of the design basis without checking the actual impact.
S25C can reach HRC 45–50 on the surface of a small bar (≤ 25 mm) when water-quenched — but the core of a 50 mm bar will be well below HRC 20. Engineers who specify S25C for through-hardened shafts over 30 mm in diameter will be disappointed. Upgrade to SCM420 or SCM440 for larger cross-sections.
C25 allows Mn up to 0.70% vs. S25C’s 0.60% maximum. In most applications this is inconsequential. However, for parts with tight hardenability specifications or fatigue-sensitive service, the broader Mn range in C25 could produce slightly different heat treatment response. Verify mill certification data when precision matters.
9. When to Choose S25C
- ✅ Carburized gears, pins, and bushings — same surface hardness as S20C, marginally tougher core
- ✅ Small through-hardened parts (≤ 25 mm / 1 in) where HRC 35–50 is sufficient
- ✅ Cold-headed fasteners and bolts requiring moderate strength without alloy steel cost
- ✅ Fabricated assemblies that require welding before heat treatment
- ✅ When S20C is slightly under-strength and S35C introduces unwanted weldability constraints
- ❌ Large cross-sections requiring deep through-hardening — use SCM420 or SCM440
- ❌ High-load gears demanding maximum fatigue life — use a Cr-Mo carburizing grade (SCM415/SCM420)
- ❌ Corrosive environments — apply appropriate surface protection or switch to stainless
10. FAQ
Q: Is S25C exactly the same as AISI 1025?
Yes, for practical purposes. Carbon and manganese ranges are identical. JIS G4051 applies tighter P and S limits (≤0.030% / ≤0.035%) than ASTM A29 (≤0.040% / ≤0.050%), making S25C marginally cleaner. For most mechanical and structural applications, the two grades are fully interchangeable.
Q: How does S25C differ from S20C and S30C?
S20C (0.18–0.23% C) is the better carburizing substrate for applications demanding maximum core toughness. S30C (0.27–0.33% C) begins to approach through-hardening territory and has lower weldability. S25C sits between them: it carburizes well and can be lightly through-hardened on small sections — a versatile combination when the application doesn’t clearly favor either extreme.
Q: What through-hardness can I expect from S25C?
On sections ≤ 25 mm (1 in), water quenching from 860–890°C (1580–1634°F) yields approximately HRC 45–50 at the surface. Oil quenching produces HRC 35–45. After tempering at 550–650°C (1022–1202°F), expect HRC 20–30. For larger sections, core hardness drops significantly — upgrade to an alloy grade if consistent through-hardness is required.
Q: Can S25C replace S35C in a cost-reduction exercise?
Only if the design allows for it. S35C (0.32–0.38% C) has better through-hardening response and higher normalized strength. Substituting S25C will reduce cost but may compromise performance in applications designed around S35C’s properties. Review the design basis — specifically tensile strength, hardness, and hardenability requirements — before making the switch.
Q: Is S25C suitable for induction hardening?
Yes, though surface hardness is limited to approximately HRC 40–50, lower than the HRC 55–62 achievable with S45C or S50C. For applications requiring maximum induction-hardened surface hardness, S45C or S50C is the preferred choice. S25C induction hardening is viable for lightly loaded wear surfaces.
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