Weathering steel

From Wikipedia, the free encyclopedia

Cor-Ten steel - Fulcrum (1987) by Richard Serra

Weathering steel, best-known under the trademark COR-TEN steel, is a group of steel alloys which were developed to obviate the need for painting, and form a stable rust-like appearance if exposed to the weather for several years.

United States Steel Corporation (USS) holds the registered trademark on the name COR-TEN.^[1] Although USS sold its discrete plate business to International Steel Group (now Arcelor-Mittal) in 2003,^[2] it still sells COR-TEN branded material in strip-mill plate and sheet forms.

In some areas it may be known without the hyphen as "Corten steel".

The original COR-TEN received the standard designation A 242 ("COR-TEN A") from the ASTM International standards group. Newer ASTM grades are A 588 ("COR-TEN B") and A 606 for thin sheet. All alloys are in common production and use.

1 Properties
- 1.1 ASTM A 588
- 1.2 ASTM A 242
2 Usage
3 Usage challenges
4 References
5 External links

[edit] Properties

Weathering means that due to their chemical compositions CORTEN steels, when utilised unprotected, exhibits increased resistance to atmospheric corrosion compared to unalloyed steels. This is because it forms a protective layer on its surface under the influence of the weather.

The corrosion retarding effect of the protective layer is produced by the nature of its structure components and the particular distribution and concentration of alloying elements in it. The layer protecting the surface develops and regenerates continuously when subjected to the influence of the weather.

The mechanical properties of weathering steels depend on which alloy and how thick the material is. ^[3] ^[4] ^[5]

[edit] ASTM A 588

A 588 has a yield strength of at least 50 ksi (50,000 psi, 345 MPa), and ultimate tensile strength of 70 ksi (480 MPa) for all rolled shapes and plate thicknesses up to 4.0 inches thick. Plates from 4.0 to 5.0 inches have yield strength at least 46 ksi (320 MPa) and ultimate tensile strength at least 67 ksi (460 MPa), and plates from 5.0 to 8.0 inches thick have yield strength at least 42 ksi (290 MPa) and ultimate tensile strength at least 63 ksi (435 MPa).

[edit] ASTM A 242

The older A 242 alloy has a yield strength of 50 ksi (345 MPa) and ultimate tensile strength of 70 ksi (480 MPa) for light-medium rolled shapes and plates up to 0.75 inch thickness. It has yield strength of 46 ksi (320 MPa) and ultimate strength of 67 ksi (460 MPa) for medium weight rolled shapes and plates from 0.75 to 1.50 inch thick. The thickest rolled sections and plates from 1.50 to 4.0 inches thick have yield strength of 42 ksi (290 MPa) and ultimate strength of 63 ksi (435 MPa).

[edit] Usage

Weathering steel is popularly used in outdoor sculptures, such as in the large Chicago Picasso sculpture, and as exterior facades, for its rustic antique appearance.

It has also been used in bridge and other large structural applications such as the New River Gorge Bridge, the newer span of the Newburgh-Beacon Bridge and the creation of the Australian Centre for Contemporary Art (ACCA).

It is very widely used in marine transportation, in the construction of shipping containers.

The first use of COR-TEN for architectural applications was the John Deere World Headquarters in Moline, Illinois. The building was designed by architect Eero Saarinen, and completed in 1964.

COR-TEN was used by St. Louis Car Company to build an order of electric railcars for Illinois Central Railroad in 1971. For uniformity, the next order was built to similar specs, including COR-TEN bodies, by Bombardier in 1979. The use of COR-TEN was seen as a cost-cutting move in comparison with the railcar standard of stainless steel. The durability of COR-TEN railcars did not live up to expectations, and rust holes are appearing in the Illinois Central railcars. Most of these railcars still operate out of Chicago. No other orders of railcars have been made of weathering steel. It's also worth noting that the IC painted their COR-TEN railcars, an unusual practice for a steel designed to remain exposed. See pictures in the "external links" section. Painted weathering steel is no more corrosion-resistant than painted conventional steel, because the protective patina will not form in time to prevent corrosion over a localized area of attack such as a small paint failure.

[edit] Usage challenges

Using weathering steel in construction presents several challenges:

Weld-area corrosion

Ensuring that weld-points weather at the same rate as the other materials may require special welding techniques or material.

Trapped water corrosion

It is not a rustproof material. If water is allowed to accumulate in pockets, those areas will experience higher corrosion rates, so provision for drainage must be made. This led to tragedy in the Fort Worth Water Gardens when uninformed maintenance people placed soil and plants in the mounting pits at the bottom of very tall light poles which rusted through, one falling and killing two people. ^[6]

Salt-Laden Air Environments

Weathering steel is sensitive to salt-laden air environments like Hawaii. In such environments, it is possible that the protective patina may not stabilize but instead continue to corrode. An example of such a structure would be the Aloha Stadium built in 1975 in Hawaii.

Discoloration

Weathering steel's normal surface weathering can lead to rust stains on nearby surfaces. The U.S. Steel Tower in Pittsburgh, Pennsylvania was constructed by U.S. Steel in part to showcase COR-TEN steel. The initial weathering of the material resulted in a discoloration of the surrounding city sidewalks, as well as other nearby buildings. A cleanup effort was orchestrated by the corporation once weathering was complete to undo this damage, but the sidewalks still have a decidedly rusty tinge. This problem has been reduced in newer formulations of weathering steel. Staining can be prevented if the structure can be detailed so that water does not drain from the steel onto concrete where stains would be visible.