SMA490AW and the JIS G3114 Family
SMA490AW is a hot-rolled atmospheric corrosion-resistant steel plate for welded structures, standardised in JIS G3114, the Japanese industrial standard for hot-rolled atmospheric corrosion resisting steels for welded structure. The designation carries two practical messages. The number 490 identifies the tensile strength class, that is, a tensile strength of the order of 490 MPa, and it does not mean that the yield strength is 490 MPa. The suffix W marks the grade as a weldable weathering steel, while the letter that follows indicates which subdivision of the standard applies. JIS G3114 covers several related grades, including SMA400AW/BW/CW, SMA490AW/BW/CW and SMA570W/P, so orders should always quote the full designation exactly as it appears in the standard.
How Weathering Steel Protects Itself
What separates SMA490AW from an ordinary structural plate is the rust that forms on its surface. Copper, chromium, nickel and phosphorus are alloyed into the steel in controlled amounts, and when the surface is alternately wetted and dried by rain and air, these elements promote the growth of a dense, tightly adherent oxide layer instead of the loose, flaking rust of plain carbon steel. Once that patina matures, typically after a few years of free exposure, it slows further corrosion substantially and the plate can remain bare, with no paint system to inspect or renew. Two conditions must be respected for this mechanism to work. The steel must be freely exposed so that the surface dries regularly, because a permanently wet or buried surface cannot form a stable patina. And the design should drain freely and avoid crevices where moisture collects and corrosion concentrates.
Mechanical Properties
JIS G3114 specifies minimum mechanical properties for SMA490AW, with values that depend on thickness. The figures below apply to plate in the thinner range of the standard, which covers most bridge and building applications.
| Property | Requirement |
|---|---|
| Tensile strength (MPa) | 490 - 610 |
| Yield strength, min (MPa) | 365 |
| Elongation, min (%) | 15 |
The tensile range confirms the meaning of the 490 in the grade name: it is a tensile class, not a yield value. The minimum yield strength of 365 MPa places SMA490AW well above ordinary carbon structural plate while keeping the yield-to-tensile ratio moderate, which preserves the ductility needed for welded bridge and building work. Elongation and impact requirements are tabulated in the standard by thickness class, so the applicable column must be identified from the plate thickness at the time of ordering.
Composition and Weldability
Weathering performance comes from a small alloy addition package rather than from high alloy content, and the chemistry is balanced so that weldability is not sacrificed.
Copper, chromium and nickel: the principal alloying elements behind the protective patina, added in controlled ranges rather than as residuals.
Phosphorus: contributes to atmospheric corrosion resistance in weathering grades, but is limited because excessive phosphorus embrittles welds.
Carbon, sulphur and phosphorus control: carbon is kept low and sulphur is tightly restricted so that toughness and resistance to cracking in the heat-affected zone are maintained.
Welding: SMA490AW is routinely welded by shielded metal arc, gas metal arc and submerged arc processes using tensile-matched consumables. Matching weathering electrodes preserve the appearance and corrosion behaviour of the joint, while uncontrolled hydrogen must be avoided in thick restrained joints.
Applications and Alternatives
SMA490AW is selected for structures that stand outdoors for decades and that benefit from removing the paint maintenance cycle: bridge girders and parapets, building frames and roofing supports, transmission and communication towers, sign gantries, transportation equipment, and coastal or industrial structures exposed to moisture and airborne pollutants. In these applications the reduction in life-cycle cost comes from avoiding repeated surface preparation and repainting rather than from a lower initial plate price. For designers working with non-Japanese standards, the closest Chinese analogue is the weathering structural steel family of GB/T 4171, for example Q355NH, which occupies a similar strength and durability class. Conversion between standards is never exact, so chemistry, mechanical requirements and impact class should be compared table by table before substitution.
Frequently Asked Questions
Q: Does the 490 in SMA490AW mean a 490 MPa yield strength?
No. The 490 identifies the tensile strength class, with a specified tensile range of 490 to 610 MPa. The minimum yield strength specified for the grade is 365 MPa, so the two values must not be confused in design calculations.
Q: Which standard covers SMA490AW?
JIS G3114, the Japanese standard for hot-rolled atmospheric corrosion resisting steels for welded structure, which also covers the SMA400 and SMA570 grades.
Q: Can SMA490AW be left unpainted?
Yes, that is the normal intent, but the patina only forms where the surface is freely exposed and dries regularly. Surfaces that stay wet, are buried, or sit in crevices will corrode and normally need protection.
Q: Is SMA490AW suitable for marine environments?
It performs well in atmospheres containing moisture, industrial pollutants and airborne chlorides, but it is not a substitute for a corrosion-resistant alloy in permanent immersion or heavy salt spray. Splash and immersion zones need additional protection.
Q: How is SMA490AW welded?
Standard arc processes with tensile-matched, low-hydrogen consumables are suitable. Weathering-type electrodes are preferred where the weld will remain exposed, and preheat is set according to thickness and restraint.
Q: How does SMA490AW compare with ordinary carbon steel plate?
It offers a higher strength class than plain carbon structural plate, and its alloy package slows atmospheric corrosion markedly once the patina matures, which reduces the need for coatings over the life of a structure.
