What Does SS400 Mean?
SS400 is a structural carbon steel designation defined in JIS G3101, the Japanese Industrial Standard for rolled steels for general structure, widely referenced in construction and machinery export projects. The name encodes its identity: each "S" stands for Steel and Structural, while "400" refers to the guaranteed minimum tensile strength of 400 MPa. SS400 is supplied mainly as hot-rolled plate, bar, section and tube, and is valued for its balanced combination of strength, formability and economy.
Chemical Composition and Mechanical Properties
Under the current edition of JIS G3101, SS400 is a low-carbon structural steel with the following ladle analysis limits:
| Element | Maximum Content (%) |
|---|---|
| Carbon (C) | 0.21 (0.25 for heavy sections) |
| Manganese (Mn) | 1.40 |
| Phosphorus (P) | 0.050 |
| Sulfur (S) | 0.050 |
Because the carbon ceiling is low, SS400 offers good weldability and machinability. The guaranteed mechanical properties for plate and flat products are:
| Thickness | Yield Strength (min) | Tensile Strength |
|---|---|---|
| Up to 16 mm | 245 MPa | 400 - 510 MPa |
| 16 to 40 mm | 235 MPa | 400 - 510 MPa |
| 40 to 100 mm | 215 MPa | 400 - 510 MPa |
Elongation requirements vary with thickness and product form, typically between 17% and 26%. SS400 carries no guaranteed impact toughness, which matters for low-temperature design.
SS400 as a Pipe and Tube Material
A frequent question is whether SS400 is itself a pipe specification. JIS G3101 covers general structural rolled steel and does not define pressure-rated pipe. When a project requests "SS400 pipe," the practical interpretation is structural tubing with SS400-equivalent strength: STK400 under JIS G3444 for electric-resistance-welded structural tubing, or STPG370 under JIS G3454 where pressure service is intended. Buyers should confirm which product standard applies, because welding method, test requirements and dimensional tolerances differ between them.
Fabrication and Welding Considerations
SS400 welds readily with common processes such as SMAW, GMAW and SAW using low-carbon filler metals.
Preheating is generally unnecessary for thin sections; for sections above about 25 mm, a modest preheat of 50 to 100 C reduces hydrogen-cracking risk in cold conditions.
The steel can be cut, drilled, punched and cold-formed without special tooling, which keeps fabrication cost low.
Because no impact toughness is guaranteed, avoid SS400 in service below about -10 C unless supplementary testing is specified.
Typical Application Scenarios
Building and plant structures: columns, beams, platforms, stairs and handrails.
Mechanical frames: machine bases, conveyor supports and non-standard equipment skids.
General piping supports and structural tubing where pressure ratings are not required.
Export machinery and steel structures where Japanese or equivalent specifications are named in the contract.
Common Misconceptions
"SS400 pipe is rated for 400 MPa working pressure." Incorrect: 400 MPa is the minimum tensile strength, not a pressure rating.
"SS400 is a JIS pipe standard." JIS G3101 is a structural steel standard; pipe products follow JIS G3444, G3454 or similar standards.
"Higher carbon content makes better SS400." The 0.21% carbon ceiling is what gives SS400 its weldability; only heavy sections relax the limit slightly.
FAQ
Q1: What does the 400 in SS400 stand for?
A: It stands for the minimum tensile strength of 400 MPa guaranteed by JIS G3101 for the grade.
Q2: Is SS400 the same as Q235B?
A: They are close but not identical. Q235B per GB/T 700 has a minimum yield of 235 MPa and tensile of 370 to 500 MPa, while SS400 guarantees 400 MPa minimum tensile, so SS400 is usually treated as a slightly stronger counterpart. Equivalence should be verified against the specific project requirement.
Q3: Can SS400 be used for pressure piping?
A: Not directly. SS400 has no pressure-design data in JIS G3101. For pressure service, use pressure tube standards such as JIS G3454 STPG370 or an equivalent ASTM specification.
Q4: What filler metal is used to weld SS400?
A: Common low-carbon electrodes such as E6013 or E7018 work well for structural welds; the choice depends on strength requirements and the welding position.
Q5: Does SS400 require impact testing?
A: JIS G3101 does not require impact testing for SS400. If the design temperature is low, specify supplementary Charpy V-notch testing in the purchase order.
Q6: What thickness range is SS400 commonly available in?
A: Hot-rolled SS400 plate is commonly available from about 3 mm to 100 mm; heavier sections up to 200 mm are produced on special order.