SS400 and A36 at a Glance
SS400 is a general-purpose structural rolled steel defined in the Japanese Industrial Standard JIS G3101, and A36 is the corresponding carbon structural steel covered by the American standard ASTM A36/A36M. Both grades are among the most widely used low-cost structural steels in the world, and both are regularly supplied as round, square and rectangular hollow sections for building frames, machine supports, racks, platforms and general fabrication.
The name SS400 stands for 'Steel for General Structure' with a minimum tensile strength of 400 MPa. The name A36 refers to a minimum yield strength of 36 ksi, which equals approximately 250 MPa. Although the two grades are often treated as interchangeable in simple structures, they are governed by different standard systems and differ in composition limits, guaranteed mechanical values and inspection rules, so substitution should always be checked against the governing design code.
Chemical Composition Comparison
Both materials are low-carbon structural steels. SS400 leaves the composition partly open and mainly guarantees mechanical properties, while A36 specifies tighter limits for several elements. The table below lists the maximum or range values given in each standard.
| Element | SS400 (JIS G3101) | A36 (ASTM A36) |
|---|---|---|
| Carbon (C) | ≤0.29% | ≤0.25% |
| Manganese (Mn) | ≤1.40% | 0.80–1.20% |
| Silicon (Si) | ≤0.35% | ≤0.40% |
| Phosphorus (P) | ≤0.050% | ≤0.04% |
| Sulfur (S) | ≤0.050% | ≤0.05% |
| Copper (Cu) | Not specified | ≥0.20% when specified as copper-bearing steel |
The practical effect is small: both grades weld readily with conventional low-hydrogen processes, and both are suitable for painted or galvanized service. The slightly higher carbon ceiling of SS400 means that for very thick sections or heavily restrained welds, preheat should be evaluated more carefully.
Mechanical Properties Comparison
| Property | SS400 (JIS G3101) | A36 (ASTM A36) |
|---|---|---|
| Tensile Strength | 400–510 MPa | 400–550 MPa |
| Yield Strength | ≥245 MPa for thickness ≤16 mm; 235 MPa for 16–40 mm; 215 MPa above 40 mm | ≥250 MPa (36 ksi) for thickness up to 200 mm |
| Elongation After Fracture | Thickness- and specimen-dependent; typically ≥21% for thin sections | ≥20% for 8 in. gauge length |
| Impact Toughness | Not guaranteed by JIS G3101; verified per order when required | Not mandated; tested when specified |
| Hardness | Not specified | Typically 137–155 HB when measured |
A key detail is that SS400 yield values drop as section thickness increases, while A36 holds 250 MPa up to 200 mm thickness. For heavy load-bearing members, the thickness-dependent values of SS400 must be applied rather than the 245 MPa figure quoted for thin sections.
Application Scenarios
A36 structural pipe is a standard choice in construction and infrastructure: building columns and trusses, bridge members, machinery frames, shipbuilding, agricultural machinery, mining equipment and automotive structural parts. Its combination of strength, weldability and predictable elongation makes it the default grade in many American and international projects.
SS400 structural tubes serve the same broad range of applications in projects designed to Japanese or Asian standards: construction, machinery manufacturing, automotive components and shipbuilding. SS400 plate and pipe are also used in the petrochemical sector for pressure vessels, storage tanks and process piping at ambient and moderately elevated temperatures, where design is based on the guaranteed tensile range rather than yield-controlled design.
How to Choose Between SS400 and A36
When a project is engineered to ASTM or international building codes, A36 documentation is usually required; when the design basis is JIS, SS400 is the natural grade. For welded structures the two perform similarly. If the design is yield-critical, remember that SS400 guarantees 245 MPa only up to 16 mm thickness. For cold forming, both grades bend and roll well, but tighter A36 composition control gives slightly more predictable behavior in heavy cold working. Cost and availability in the local market often decide the matter when both are acceptable.
Common Misconceptions
SS400 and A36 are not the same steel. Their chemical limits, guaranteed yield values and inspection clauses differ, and certificates cannot be exchanged freely. SS400 is not an 'impact-tested' grade by default, and A36 impact testing also requires a supplementary order. Finally, the 400 in SS400 is a tensile figure, not a yield figure, so it must not be compared directly with the 36 ksi yield designation of A36.
FAQ
Q1: Can SS400 tubes be used in place of A36 tubes?
In many non-critical structures yes, but substitution must be approved by the design engineer. The standards differ in guaranteed yield values for thick sections, composition limits and testing requirements, so the governing code and the project specification must be checked first.
Q2: Which grade has better weldability?
Both are excellent. With carbon at or below 0.29%, both grades weld without special precautions in ordinary thicknesses. For thick or highly restrained joints, standard preheat practice applies, and A36 gives slightly more margin because of its lower carbon ceiling.
Q3: Does SS400 provide guaranteed impact toughness?
JIS G3101 does not mandate Charpy impact testing for SS400. When low-temperature service is expected, impact energy such as 27 J at 0°C can be specified as a supplementary requirement on the order, or a different grade should be selected.
Q4: What do the numbers 400 and 36 mean?
The 400 in SS400 is the minimum tensile strength in MPa. The 36 in A36 is the minimum yield strength in ksi, equal to about 250 MPa. Comparing the two numbers directly is therefore misleading.
Q5: Which standard covers inspection and tolerances for these tubes?
SS400 tubes are inspected to JIS G3101 together with applicable JIS pipe standards such as JIS G3445 or G3466 depending on product form. A36 tubes are typically supplied to ASTM A36 combined with ASTM A500, A501 or A106 depending on whether the product is a welded, seamless or structural hollow section.
Q6: Are SS400 and A36 suitable for low-temperature service?
Both are intended for ambient-temperature general structures. At low temperatures their toughness is not guaranteed, and impact-qualified grades such as A333 or low-alloy steels should be used for cryogenic or sub-zero design conditions.