SM400 (JIS G3106) is Japan’s weldable structural steel standard — the grade to specify when guaranteed composition control and Charpy impact toughness matter. This guide compares SM400 against ASTM A36 and EN S275JR, explains the critical difference between SM400 and SS400, and provides sub-grade selection guidance for welded structures and cold-climate fabrication.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose SM400
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G3106 | SM400A / SM400B / SM400C | Japan | Reference |
| ASTM A36/A36M | A36 | USA | ✅ Nearest Exact |
| EN 10025-2 | S275JR / 1.0044 | Europe | ⚠️ Nearest Equivalent |
| ISO 630-2 | S275JR | International | ⚠️ Nearest Equivalent |
SM400 vs SS400 — the critical distinction: SM400 (JIS G3106) specifies an upper carbon limit and optional Charpy impact testing, making it the correct grade for welded structures. SS400 (JIS G3101) has no composition limit — it is a general structural grade specified only by tensile properties. For any welded structural application, SM400 is the appropriate specification.
2. Chemical Composition
SM400 is produced in three sub-grades (A, B, C). SM400B is the most commonly procured sub-grade for general welded fabrication.
| Element | SM400B | ASTM A36 | EN S275JR |
|---|---|---|---|
| C | ≤ 0.20% (t ≤ 50 mm) | ≤ 0.25–0.29% | ≤ 0.21% |
| Si | ≤ 0.35% | ≤ 0.40% | ≤ 0.40% |
| Mn | ≤ 1.40% | 0.80–1.20% | ≤ 1.50% |
| P | ≤ 0.035% | ≤ 0.040% | ≤ 0.040% |
| S | ≤ 0.035% | ≤ 0.050% | ≤ 0.040% |
Sources: JIS G3106:2020, ASTM A36/A36M, EN 10025-2
Note: For SM400B with thickness t > 50 mm (2 in), the carbon limit increases to ≤ 0.22%. Confirm with the mill certificate for thick plate orders.
SM400 Sub-Grade Comparison
| Sub-grade | C Limit | Charpy Test | Through-thickness |
|---|---|---|---|
| SM400A | None specified | None | None |
| SM400B | ≤ 0.20% | ≥ 27 J at 0°C (≥ 20 ft·lbf at 32°F) | None |
| SM400C | ≤ 0.18% | ≥ 47 J at 0°C (≥ 35 ft·lbf at 32°F) | Z25 requirement |
3. Mechanical Properties
| Property | SM400 | ASTM A36 | EN S275JR |
|---|---|---|---|
| Tensile Strength | 400–510 MPa (58–74 ksi) | 400–550 MPa (58–80 ksi) | 410–560 MPa (59–81 ksi) |
| Yield Point (t ≤ 16 mm) | ≥ 245 MPa (≥ 35.5 ksi) | ≥ 250 MPa (≥ 36 ksi) | ≥ 275 MPa (≥ 40 ksi) |
| Elongation (GL = 5.65√A) | ≥ 22% | ≥ 20% | ≥ 23% |
SM400 has a capped tensile strength of 510 MPa (74 ksi). ASTM A36 allows tensile strengths up to 550 MPa (80 ksi). In design calculations that use the upper-bound tensile strength for connection design or overstrength calculations, this 40 MPa (6 ksi) difference must be accounted for when substituting between the two grades.
4. Physical Properties
| Property | Value |
|---|---|
| Density | 7.85 g/cm³ (0.284 lb/in³) |
| Young’s Modulus | 200 GPa (29,000 ksi) |
| Thermal Conductivity | 50 W/(m·K) (347 BTU·in/(hr·ft²·°F)) |
| Thermal Expansion (20–100°C) | 11.7 × 10⁻⁶ /°C (6.5 × 10⁻⁶ /°F) |
| Specific Heat (approx.) | ~490 J/(kg·K) (0.117 BTU/(lb·°F)) |
5. Heat Treatment Conditions
| Process | Temperature | Cooling | Purpose |
|---|---|---|---|
| As-rolled | Rolling finish ~900°C (1,652°F) | Air cool | Standard supply condition |
| Normalizing | 880–920°C (1,616–1,688°F) | Air cool | Refine grain for improved toughness (optional) |
| Stress Relief | 550–650°C (1,022–1,202°F) | Furnace cool | Post-weld residual stress reduction |
SM400 is not heat treated for strength in service. Material is supplied in the as-rolled or normalized condition. Through-hardening is not applicable for this grade.
6. Machinability
- Machinability rating: approximately 70% relative to AISI 1212 baseline
- Standard HSS or carbide tooling suitable for all operations
- Produces good chip formation; continuous chips at low feed rates, broken chips at higher feeds
- No special precautions required compared to general mild steel machining
- Drilling, milling, turning, and sawing all performed with standard parameters
- Surface finish achievable: Ra 1.6–3.2 µm with carbide tooling in good condition
7. Weldability
- Weldability: Excellent — SM400B carbon equivalent (Ceq) typically < 0.40, well below the 0.45 threshold for preheat-free welding
- Preheat: Not required for t ≤ 25 mm (1 in) in ambient temperature conditions
- For t > 40 mm (1.6 in), cold weather (below 5°C / 41°F), or highly restrained joints: preheat to 50°C (122°F) minimum
- Recommended processes: SMAW (E7016/E7018), GMAW, FCAW, SAW — all standard structural welding processes suitable
- Low-hydrogen electrodes recommended as standard practice for structural joints
- Post-weld heat treatment: not normally required for standard structural applications
- For SM400C with Z25 through-thickness requirement: through-thickness test (Z-direction tensile) must be verified on plate
8. Common Mistakes
SS400 (JIS G3101) is a general structural grade that specifies only tensile strength (400–510 MPa) with no carbon limit and no Charpy impact requirement. When a design calls for a welded structural steel — particularly for frames in seismic zones, cold climates, or bridge applications — specifying SS400 instead of SM400B leaves the carbon content uncontrolled. Mill heats of SS400 can reach C ≥ 0.25%, which significantly increases cold cracking risk during welding. For any welded structural fabrication, SM400B is the minimum correct specification.
While SM400 and A36 are near-equivalent at the nominal yield and tensile level, SM400 has a tensile strength ceiling of 510 MPa (74 ksi) while A36 allows up to 550 MPa (80 ksi). In connection design or overstrength calculations (common in seismic design), using SM400 mill data where A36 upper-bound values were assumed can underestimate connection demand by up to 8%. Verify which tensile bound is used in the design calculation before approving material substitution.
9. When to Choose SM400
✅ Choose SM400 when:
- ✅ Welded structural steel frames, building skeletons, and shipbuilding structures
- ✅ Applications requiring guaranteed Charpy impact toughness — specify SM400B (27 J at 0°C) or SM400C (47 J at 0°C)
- ✅ Cold-climate welded fabrication where uncontrolled carbon creates cracking risk
- ✅ Procurement from Japanese mills where JIS G3106 is the standard
- ✅ Bridge components where controlled composition is required by design code
❌ Do not choose SM400 when:
- ❌ Non-welded general structural use where SS400 is available at lower cost
- ❌ High-strength applications — use SM490 (yield ≥ 325 MPa), SM570 (yield ≥ 460 MPa), or SN490 (seismic)
- ❌ Applications requiring A992-style Fy/Fu ratio control for seismic ductility — use SN400B or SN400C
10. FAQ
Q: Is SM400 the same as SS400?
No. SM400 (JIS G3106) is the weldable structural steel standard, while SS400 (JIS G3101) is a general structural grade. SM400B has an upper carbon limit (≤ 0.20%), a mandatory Charpy impact test requirement, and tighter composition control overall. For any welded fabrication — especially in seismic zones, cold climates, or safety-critical applications — SM400 is the correct specification. SS400 should not be used as a substitute for SM400 in welded structures.
Q: Is SM400 equivalent to ASTM A36?
Closely equivalent in yield and tensile strength — both grade 245–250 MPa minimum yield. However, SM400 has a tensile strength ceiling of 510 MPa while A36 has no upper limit (typically reaches 550 MPa in practice). SM400 also has tighter sulfur limits (≤ 0.035% vs ≤ 0.050%). For most structural engineering purposes they are interchangeable; for calculations using upper-bound tensile strength, verify which value was assumed in the design.
Q: Does SM400 require preheat for welding?
For SM400B (C ≤ 0.20%, Ceq typically < 0.40), no preheat is required for thicknesses under 25 mm (1 in) in ambient conditions above 5°C (41°F). For thicker sections, cold weather, or highly restrained joints, preheat to 50–100°C (122–212°F) is recommended. SM400C with its lower carbon (≤ 0.18%) offers even better weldability in demanding conditions.
Q: When should I specify SM400C instead of SM400B?
SM400C adds two requirements beyond SM400B: (1) higher Charpy impact toughness (47 J at 0°C vs 27 J), and (2) through-thickness Z25 ductility requirement for lamellar tearing resistance. Specify SM400C when: the joint is a T-butt weld with high through-thickness stress, the operating temperature may drop below 0°C (32°F), or the connection design is highly restrained and susceptible to lamellar tearing.
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