Introduction: Why Compare S20C and S45C?
Both S20C and S45C are medium-carbon steels defined by the Japanese Industrial Standard JIS G4051. They sit only 0.25% carbon apart, yet that gap changes machinability, hardness, weldability, and price by margins large enough to ruin a job if the wrong grade lands on your dock. S20C is the softer, more formable cousin; S45C is the harder, stronger workhorse that dominates shaft and axle applications across Asia.
This guide compares the two grades across chemistry, mechanical properties, hardness, international equivalents, and real-world use cases — with the practical details buyers need when sourcing from Chinese and Japanese mills.
Chemical Composition: What the 0.25% Carbon Gap Means
JIS G4051 specifies the following typical composition ranges:
| Element | S20C (%) | S45C (%) | Impact |
|---|---|---|---|
| Carbon (C) | 0.18 – 0.23 | 0.42 – 0.48 | Hardness, strength, and weldability driver |
| Silicon (Si) | 0.15 – 0.35 | 0.15 – 0.35 | Deoxidation; similar for both |
| Manganese (Mn) | 0.30 – 0.60 | 0.60 – 0.90 | Higher Mn in S45C boosts hardenability |
| Phosphorus (P) | ≤ 0.030 | ≤ 0.030 | Kept low for ductility |
| Sulfur (S) | ≤ 0.035 | ≤ 0.035 | Lower is better for machining and welding |
The carbon difference is the headline. At 0.20% C, S20C is on the border of low-carbon formability; at 0.45% C, S45C sits in the heart of the medium-carbon band where heat treatment becomes effective and machinability starts to require planning.
Mechanical Properties Comparison
Typical values for hot-rolled bar in the 16–40 mm range:
| Property | S20C | S45C |
|---|---|---|
| Yield strength | ≥ 245 MPa | ≥ 345 MPa |
| Tensile strength | 410 – 550 MPa | 570 – 700 MPa |
| Elongation | ≥ 25% | ≥ 17% |
| Hardness (annealed) | ≈ 110 – 140 HB | ≈ 160 – 200 HB |
| Hardness (normalized) | ≈ 120 – 150 HB | ≈ 180 – 220 HB |
S45C delivers roughly 40% higher yield and tensile strength, but gives up 8 percentage points of elongation. For static structural members, the strength gain is welcome; for parts involving cold forming, coining, or severe bending, S20C’s extra ductility prevents cracks.
Hardness and Heat Treatment Behavior
Both grades respond to heat treatment, but S45C is the one buyers actually specify treated:
- S20C annealed: 110 – 140 HB. So soft it machines almost like free-machining steel. Normalizing adds little strength; quenching produces only modest hardness because carbon is too low for full martensite.
- S45C annealed: 160 – 200 HB. Already harder than S20C normalized. Quenched and tempered to 28 – 34 HRC, it becomes the standard medium-strength shaft material across Asian manufacturing.
Buyer note: if your drawing says “quench and temper to 28 – 32 HRC,” S45C is the correct choice. S20C cannot reach that window regardless of process. Conversely, if the part needs deep cold forming or spinning, S20C is the safer call.
International Equivalents
| Japan (JIS) | USA (ASTM/SAE) | Europe (EN/DIN) | China (GB) |
|---|---|---|---|
| S20C | 1020 | C22 / 1.0402 | 20# / 20钢 |
| S45C | 1045 | C45 / 1.0503 | 45# / 45钢 |
None of these are exact chemical twins — ASTM 1020 allows up to 0.23% C, matching S20C closely, while EN C22 is slightly lower in Mn. For equivalence in structural or pressure applications, always cross-check the specific standard’s test requirements; for general machining and shafts, the table above is accepted in trade.
Machinability: Chips, Tools, and Cycle Time
In practical shop-floor terms:
- S20C machines at high speeds with standard uncoated carbide. Tool life is long, surface finish is good, and chip breaking is easy. It is the grade you hand to a junior operator on a manual lathe.
- S45C requires slightly lower speeds (10 – 15% reduction) and benefits from coated inserts. Chip breaking is harder because the material is tougher; expect more stringy chips unless you use a chip-breaker geometry. The payoff is a part that needs no heat treatment to achieve reasonable wear resistance.
If a part requires heavy machining followed by induction hardening of a surface (e.g., a shaft with a hardened bearing seat), S45C is preferred — its base carbon supports a good case hardness after induction.
Weldability: What to Expect on the Floor
S20C welds with minimal fuss: J422 (E4303) electrodes, no preheat up to 25 mm, standard interpass control. Carbon equivalent is roughly 0.25 – 0.32.
S45C is weldable but demands respect. Carbon equivalent runs 0.45 – 0.55, placing it in the preheat zone for sections over 12 mm. Use low-hydrogen electrodes (J507 / E5015), preheat to 100 – 150°C, and control interpass temperature. Post-weld stress relief at 550 – 600°C is recommended for restrained joints. Many fabricators avoid welding S45C altogether and design bolted or keyed joints instead.
Price and Availability
In Chinese export markets, S45C typically commands a premium of 3 – 8% over S20C for hot-rolled bar, and the gap widens for quenched-and-tempered supply. Both grades are produced by virtually every major mill, so availability is not an issue; the constraint is certified supply with EN 10204 3.1 or 3.2 documentation for European buyers.
When to Choose Each Grade
Choose S20C when:
- The part involves cold forming, bending, or spinning.
- Machinability and tool life are the top priorities.
- Welding is required without preheat complexity.
- The design stress is modest and weight is not critical.
Choose S45C when:
- Shafts, axles, gears, or pins need medium strength (500 – 700 MPa tensile).
- Heat treatment (Q&T or induction hardening) is part of the process.
- Wear resistance on a running surface is required.
- The drawing specifies 1045, C45, or 45# equivalent.
Buyer Verification Checklist
- Mill test certificate with full chemistry, tensile, and hardness
- Heat number traceability to bar marking
- Dimensional tolerance per JIS G3191 or agreed standard
- Straightness check for long shafts (≤ 1 mm/m typical)
- For Q&T supply: certificate must state tempering temperature and hardness range
Frequently Asked Questions
1. Can S20C be used instead of S45C for a shaft?
Only if the design load is very low. S20C’s yield is roughly 30% lower, so the same geometry will deflect more and carry less load. If the drawing specifies S45C or 1045, substituting S20C requires engineering sign-off.
2. Is S45C the same as 1045 steel?
Close but not identical. S45C (JIS) and 1045 (ASTM/SAE) are both approximately 0.45% carbon steels and are treated as interchangeable in most general engineering. For code-governed work, verify the specific standard’s test and tolerance requirements.
3. Which grade machines faster?
S20C machines faster with longer tool life and better chip control. S45C is still considered good-machining, but expects 10 – 15% slower speeds and more attention to chip breaking.
4. Can I weld S45C without preheat?
On thin sections under 6 mm with low-hydrogen electrodes, possibly. On sections over 12 mm or in restrained joints, preheat to 100 – 150°C is strongly recommended to avoid hydrogen cracking. When in doubt, preheat.
5. How do I verify the grade on delivery?
Check that the heat number on the MTC matches the bar marking, run a portable spectrometer (PMI) spot check for carbon and manganese levels, and verify hardness with a portable Brinell tester. For critical orders, cut a tensile coupon and test at a local lab.
Conclusion
S20C and S45C are not interchangeable — they are adjacent grades with distinct strengths. S20C wins on formability, weldability, and machining ease; S45C wins on strength, hardness, and heat-treatment response. The costly mistake is not choosing one over the other, but failing to verify: a shipment of S20C sold as S45C will machine beautifully and then fail under load.
Huaxia-Steel supplies JIS G4051 S20C and S45C round bar, plate, and forgings with EN 10204 3.1 certificates, third-party inspection, and factory-direct pricing. Send us your grade, size, and quantity for a same-week quotation.
