S35C and S45C are adjacent grades in the JIS G4051 carbon steel family, separated by only 0.10 % in carbon content — yet that gap creates meaningful differences in weldability, hardenability, and achievable strength. Engineers who choose between them based on habit (“S45C is the standard”) rather than requirements end up with either unnecessary welding restrictions or strength they could have had for the same material cost. This article draws the line between them clearly.
Chemical Composition — Side by Side
| Element | S35C (JIS G4051) | S45C (JIS G4051) | Difference |
|---|---|---|---|
| C (%) | 0.32 – 0.38 | 0.42 – 0.48 | +0.10 % C (higher in S45C) |
| Si (%) | 0.15 – 0.35 | 0.15 – 0.35 | Identical |
| Mn (%) | 0.60 – 0.90 | 0.60 – 0.90 | Identical |
| P (%) | ≤ 0.030 | ≤ 0.030 | Identical |
| S (%) | ≤ 0.035 | ≤ 0.035 | Identical |
| CE (IIW formula) | ≈ 0.40 | ≈ 0.47 | +0.07 → weldability gap |
Mechanical Properties Comparison
| Property | S35C (normalized) | S45C (normalized) | S35C (Q&T) | S45C (Q&T) |
|---|---|---|---|---|
| Tensile strength (MPa) | ≥ 510 | ≥ 570 | ≈ 610 – 690 | ≈ 690 – 780 |
| Yield strength (MPa) | ≥ 295 | ≥ 325 | ≈ 440 | ≈ 490 |
| Elongation (%) | ≥ 23 | ≥ 20 | ≥ 20 | ≥ 17 |
| Hardness (normalized) | ~149HBW | ~167HBW | — | — |
| Induction hardened (surface) | 50 – 55HRC | 55 – 62HRC | — | — |
The strength gap between S35C and S45C in normalized condition is approximately 60 MPa tensile — meaningful, but not dramatic. In Q&T condition the gap is similar: roughly 80–90 MPa. The more significant differences are in induction hardening response and weldability, not raw tensile strength.
Hardenability — Induction Hardening Response
Higher carbon content directly improves the depth and hardness achievable by induction hardening. Both S35C and S45C can be induction hardened, but the surface hardness ceiling differs:
| Steel | Surface hardness (induction hardened) | Effective case depth (typical) | Notes |
|---|---|---|---|
| S35C | 50 – 55HRC | 1 – 3 mm | Adequate for light wear applications |
| S45C | 55 – 62HRC | 1 – 3 mm | Standard choice for shafts and gears |
| S50C | 60 – 64HRC | 1 – 3 mm | Maximum surface hardness in the plain carbon range |
If the drawing specifies ≥ 55HRC after induction hardening, S35C cannot reliably meet that requirement. S45C should be specified. If ≥ 58HRC is required consistently, S50C is the safer choice.
Weldability — The Deciding Factor in Most Cases
When a fabrication or repair welding operation is part of the manufacturing or maintenance process, weldability becomes the primary selection criterion. The CE difference between S35C and S45C translates directly into preheat requirement:
| Steel | CE (approx.) | Preheat required (thin section < 25 mm) | Preheat required (thick section > 25 mm) | PWHT recommended |
|---|---|---|---|---|
| S35C | ~0.40 – 0.44 | Usually not required | 50 – 100 °C | Optional for low-stress joints |
| S45C | ~0.47 – 0.52 | 100 °C minimum | 150 – 200 °C | Recommended for structural joints |
Selection Guide — Which Grade for Which Situation?
Choose S35C when:
- The part is welded during fabrication or may need repair welding in service
- Normalized tensile strength of ≥ 510 MPa is sufficient
- Induction hardening to 50–55HRC is adequate (light wear applications)
- The section is thin (≤ 15 mm) and weld cracking risk must be minimized
- The part is a structural bracket, lever, or frame member where ductility and weldability matter more than hardened surface properties
Choose S45C when:
- The part is a shaft, gear, key, or fastener that will not be welded in service
- Induction hardening to ≥ 55HRC is required at contact or bearing surfaces
- Normalized tensile strength of ≥ 570 MPa is needed
- Q&T to 690–780 MPa is required and section size is ≤ 50 mm
- It is the established standard for the part type in your facility (standardization reduces sourcing and process complexity)
The “close call” scenario — S35C Q&T vs. S45C normalized:
S35C in Q&T condition (≈ 690 MPa tensile) overlaps with S45C normalized (≥ 570 MPa) and S45C Q&T lower end. If you need ≈ 620–680 MPa tensile and also need welding, S35C Q&T is viable — you get the strength without the weld cracking risk. The trade-off: Q&T processing adds cost vs. using normalized S45C. Run the numbers for your production volume.
- Does the part require welding (fabrication or maintenance)? → Prefer S35C
- Is induction hardening to ≥ 55HRC required? → Use S45C (or S50C)
- Is the tensile requirement ≤ 510 MPa (normalized)? → S35C is sufficient
- Is the tensile requirement 570–780 MPa (normalized or Q&T, section ≤ 50 mm)? → S45C
- Does the section exceed 50 mm and require through-hardening? → Neither — use SCM440
Summary
- S35C and S45C differ primarily in carbon content (0.35 % vs. 0.45 % C mean), which drives differences in CE, weldability, and induction hardening response — not just tensile strength.
- S35C: better weldability (CE ~0.40–0.44), lower induction hardness ceiling (50–55HRC), normalized tensile ≥ 510 MPa.
- S45C: requires preheat for welding (CE ~0.47–0.52), better induction hardness (55–62HRC), normalized tensile ≥ 570 MPa.
- Key decision: if the part is welded now or in service, start with S35C. If it needs a hard surface and will never be welded, S45C is the right call.
- For sections > 50 mm requiring through-hardened Q&T properties, upgrade to SCM440 — neither carbon grade will deliver.
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