Weathering steel is a low-alloy steel with excellent atmospheric corrosion resistance. Atmospheric corrosion resistance is based on an oxide layer that forms on the surface, creating a sealed unbroken protective layer over the steel surface. This oxide layer is called patina. The term weathering steel is generally used, sometimes also weather resistant steel and weatherproof steel.
The patina layer of weathering steel differs in structure and composition from the rust of ordinary steel. An ordinary rust layer is fragile and fissured, with material easily coming off. A patina layer contains elements that adhere well to each other and to the steel surface. As a result, a protective layer from which hardly any material comes off is formed, and it effectively prevents water and oxygen from entering the steel surface. Corrosion does not completely stop beneath a protective patina layer, but corrosion slows so much that the steel can be used in outdoor conditions without a protective coating.
For the patina layer to form over weathering steel, a wetting and drying cycle is required. This means alternate rain and sunshine are needed. Alternating wetting and drying cause chemical reactions on the surface which lead to the formation of a patina layer. Since this is based on natural phenomena, the patina layer of weathering steel can be considered as being a natural protective layer. Weathering steel is not suitable for excessively humid or dry conditions, such as the desert, or submergence in water. The parts of weathering steel that are underground must be coated in the same way as ordinary steel, or a sufficient corrosion allowance must be added to steel thickness.
A patina layer will not withstand large amounts of salt, thus weathering steel is not suitable for use near oceans or at the edge of heavily salted road sections. On moderately salted road sections, weathering steel can be used in bridges, for example. The recommended minimum distance from the coast of a high-salinity sea is 1 km. Alternatively, it is possible to determine salinity in the air and follow national or general guideline values regarding distance. The salinity of Baltic Sea is generally so low that weathering steel can be used on its coasts. In the northernmost parts of Baltic Sea, weathering steel can be used in structures spanning the sea, like a bridge linking the mainland and an island.
History of weathering steel
In the early 1900s, it was discovered that adding copper to ordinary non-alloy steels improves corrosion resistance. Copper alloy steels were used in railway vehicles. At the end of 1920s, weathering steel was developed simultaneously in Germany and in the United States. In 1932, US Steel patented COR-TEN steel, which is well-known around the world. COR-TEN steel ( Corrosion Resistant – Tensile Strength) spread around the world through licensed manufacturers. Finnish architect Eero Saarinen used COR-TEN steel in 1960 in a large administrative building that he designed (John Deere Company) in the United States. Nowadays, SSAB manufactures COR-TEN steel under license as the only manufacturer in Europe.
Weathering steel can also be used when paint-coated
Paint-coated steel samples were tested in corrosion category C5 conditions on the island of Bohus-Malmön in Sweden for six and a half years. The island of Bohus-Malmön is located on Sweden’s west coast at the edge of the North Sea. The series of samples consisted of four weathering steels and an ordinary steel sample that was used as a reference sample. The samples were color coated using an epoxy polyurethane painting system, and the total thickness of the paint surface was 160 μm. According to the paint manufacturer, the painting system is suitable for corrosion category C3-M. A 3 x 50 mm cut was made through the paint surface of the samples, and the sample pieces were put on a sample rack at the Bohus-Malmön corrosion test site.
The results indicate that the paint surface in the ordinary steel samples was damaged significantly around the cut. On the other hand, little sign of corrosion was detected visibly in only a few of the weathering steel samples. For the most part, the weathering steel samples remained almost unchanged.
The samples were examined more closely by removing paint from near the cut. Some rust was also found beneath the paint of the weathering steels. The paint was easy to remove from the rusted area on the surface of the ordinary steel. On the rusted surface of the weathering steel, the paint coating still had good adhesion to the steel surface, and force was required to remove the coating. The corrosion had advanced approximately 4 to 5 times further from the cut beneath the paint on the surface of the ordinary steel in comparison with the rusting in the weathering steel samples.
The corrosion test indicated that paint-coated weathering steel worked significantly better in corrosion category C5 atmospheric corrosion conditions than ordinary steel. The corrosion test continues with another series of samples that have been coated with a zinc silicate polyurethane painting system (corrosion category C5 resistance).
Cost and environmental implications of weathering steel
Weathering steel lasts in bridges, for example, for at least 100 to 120 years and even longer in moderate corrosion conditions. The manufacturing costs of the bridge are largely the same as with a paint-coated steel bridge. Cost savings start to accumulate when the bridge is installed. There will be damage to the paint surface of a color-coated bridge during installation, and this must be repaired by painting after installation. In any case, the building project will be faster since the paint-coating phase and repair painting after installation are eliminated from bridge completion. Bridge components made from weathering steel require no maintenance painting every few decades or, in case of concrete bridges, the repair of concrete parts. Nor will microplastics be released from the paint coating into the environment. The carbon footprint is significantly smaller than that of a paint-coated steel bridge because the carbon dioxide emissions arising during paint manufacture and painting work are eliminated. After use, the components made from weathering steel are completely recyclable. Similar benefits can be attained with, for example, building facades and other infrastructure construction.
Different kinds of weathering steels
Weathering steels are suited for different kinds of applications depending on their alloy and strength. Excellent atmospheric corrosion resistance can be achieved by alloying small amounts of copper, chrome, nickel and silicon into steel at an appropriate ratio as well as phosphorus with some steel grades. With regard to alloying, weathering steels can be divided into two types depending on whether they are alloyed with phosphorus. With phosphorus alloying, it is possible to achieve better corrosion resistance, but at the same time impact toughness and welding properties are slightly weaker. For this reason, phosphorus-alloyed weathering steel, such as COR-TEN A, is used for aesthetic applications, such as facades and works of art. The thickness of the sheets used for facades is generally around 1 to 2 mm, which means that the better corrosion resistance of phosphorus-alloyed steel is a clear benefit.
Weathering steels made without phosphorus offer excellent impact toughness and welding properties, which is why they can be used as structural steels in load-bearing structures. These steels are available in different strengths for different purposes in yield strength classes of 345 MPa to 960 MPa. Using stronger steel makes it possible to achieve a longer lasting structure, reduce the structure’s weight and make structure solutions that are not possible with milder steel. COR-TEN A and SSAB Weathering 700 steels are available as hot-rolled products as well as cold-rolled sheets. Pipes and structural hollow sections are also made from weathering steels.
Extension of standard EN 10025-5 and new steel products
A new version of standard EN 10025-5 that specifies weathering steels (hot-rolled structural steels. Part 5: Atmospheric corrosion resistant structural steels) was published in 2019. Previously, the standard included steels of yield strength classes 235 MPa and 355 MPa, and yield strengths of 420 MPa and 460 MPa were added to the updated standard. The thickness range of the 420 MPa and 460 MPa plate products recently developed by SSAB is between 8 and 65 mm. The plate products are thermomechanically rolled and so have excellent welding properties. The new SSAB Weathering 420 ML and 460 ML steels are in the test delivery phase.
Weathering steel with other materials
Most building materials can be used with weathering steel. When using building materials, it is important to take into account that at the beginning of the formation of the protective patina layer, rust water that could stain other materials will come off the steel. It is recommended that the rust waters are collected or conducted into a drain. Alternatively, materials that are easy to clean can be used. The release of rust water typically ends within 2 to 6 years depending on the local conditions.
Hot-dip galvanized components can be joined to a structure made of weathering steel. Weathering steel does not cause significant galvanic corrosion to galvanized components. This means components made of weathering steel can be attached with hot-dip galvanized bolts, and large galvanized components can be joined to weathering steel. Small parts, such as bolts for bridges or powerline poles, made of stainless steel can be used in large structures made of weathering steel. Larger components made of stainless steel must be isolated from weathering steel components. Nobler metals, like copper, must not be used in direct contact due to the risk of galvanic corrosion. Bolted joints must be tight or, for example, sealed with a rubber seal to prevent water from getting into the joint, which could cause the risk of crevice corrosion. Alternatively, the joints can be designed so that the contact surface is minimized and the joint is well ventilated.
Weathering steel applications
Weathering steel has been used in many applications for almost a hundred years. It has been successfully used in infrastructure, such as bridges, facades and powerline poles. It has been a popular material in works of arts, too. Painted weathering steel is used in railway vehicles, such as tram cars and train carriages. SSAB Weathering 550 steel has been used Helsinki’s new tram cars manufactured by Skoda. Paint-coated SSAB Weathering 550 steel extends the service life of the paint surface and reduces the need for maintenance painting. High-strength weathering steel has also been used for ore wagons which require high strength combined with the benefits of corrosion resistance.
Weathering steel is widely used in the United States in powerline poles, and it is well suited for this purpose in the Nordic countries as well. The natural color of weathering steel blends in superbly with nature.
The Sipoonkorpi bridge, built in Vantaa in 2019 and named ’Sudentassu’ (’Wolf Paw’), links the Kuusijärvi outdoor recreational area with Sipoonkorpi National Park. The 20-meter high and 126-meter long pedestrian and bicycle bridge has been made from COR-TEN B steel and has a decking made of pressure impregnated timber. The bridge was designed by WSP Finland. The Finnish Constructional Steelwork Association awarded the bridge with a commendation in the 2020 constructional steelwork competition.
Finland’s longest bridge made of weathering steel takes the road across the Kemijoki river to the town of Tervola. The bridge was completed in 1975, and so has been in use for 46 years. The bridge is a 494-meter long composite steel-concrete structure of which steel beams are made of weathering steel.
In Israel, the roof structure of Tel Aviv railway station consists of steel tubes. Maintaining the structure is challenging and so it was built with tubes made of COR-TEN B steel. Paint-coated COR-TEN B weathering steel ensures that maintenance painting frequency can be extended and the need for maintenance reduced.
The municipal office building in Oulu called the ’Environment Building’, was completed in 2011, and COR-TEN A steel was chosen for its facade. The building was designed by achitectural firm Vauhtiviiva, and the head designer was architect Juha Pasanen. In the design of the building, the focus was on respect for the environment, due to which CORTEN steel was an excellent choice for the facade.
Weldability of weathering steels
The weldability of weathering steels is similar to that of structural steels of the same strength class. When planning welding work, the most significant difference between the steels relates to the selection of a welding consumable, namely the need to consider the weathering resistance of the welding consumable. Other specific considerations are the need for preheating with thicker plates as well as the heat input limits in the case of welding ultra high strength SSAB Weathering 960 steel.
SSAB has recently developed new kinds of weather resistant plate products with a thickness range from 8 to 65 mm trade name SSAB Weathering ML. The ML plates are manufactured in the yield strength classes of 355 MPa, 420 MPa, and 460 MPa. SSAB Weathering ML plates offer customers the benefit of excellent weldability. In the engineering workshop, better weldability means that the need for preheating is reduced in comparison with traditional weathering steels. Weathering ML steels also offer excellent impact toughness at an operating temperature of -50°C both in the base material and in the welded joint. The submerged arc welding (SAW) and metal active gas (MAG) welding test results for SSAB Weathering 420 ML and 460 ML plates are presented later in the article.
|Steel grade||Yield strength (min. MPa)||Thickness range (mm)|
|COR-TEN A||345||0.5 – 12.7|
|COR-TEN B||345||2 – 40|
|SSAB Weathering 355 ML||355||8 – 65|
|SSAB Weathering 420 ML||420||8 – 65|
|SSAB Weathering 460 ML||460||8 – 65|
|SSAB Weathering 550||550||3 – 6.1|
|SSAB Weathering 700||700||0.8 – 6.1|
|SSAB Weathering 960||960||2.5 – 6.0|
|Welding process||Option 1||Option 2||Option 3|
|111||Equivalent||2.5% Ni||1% Cr + 0.5% Mo|
|135||Equivalent||2.5% Ni||1% Cr + 0.5% Mo|
|121||Equivalent||2.0% Ni||1% Cr + 0.5% Mo|
The option ’Equivalent’ contains: ≥ 0.4% Cu and other alloying elements.
*) Source EN 1090-2, table 6.
Selection of welding consumable
The weathering resistance of the welds of weathering steels can be ensured by following the instructions given in the European steel structure standard EN 1090-2: 2018 (Execution of steel structures and aluminum structures. Part 2: Technical requirements for steel structures). The weathering resistance of the welded joint must be of the same level as that of the base material. Under standard EN 1090-2, the chemical composition of the welding consumables must correspond to option 1, 2 or 3 in table 2. As a general guideline, the chemical composition of the welding consumables should correspond to the base material, i.e. equivalent to option 1 in table 2. Welding consumables corresponding to option 2 are needed when a structure is designed for an operating temperature of -50°C (NL, ML). A CrMo alloy filler metal corresponding to option 3 was initially meant for welding heat resisting steel 13CrMo4-5, and in practice it is not needed for welding SSAB’s weathering steels.
The alloying of the welding consumables type corresponding to option 1 is equivalent to the base material. The welding consumables selection for SSAB Weathering steels of strength classes 355–460 and COR-TEN steels is made according to option 1, and welding consumables for these strength grades are easily available. The base material low-temperature impact toughness guarantee 27J / -50°C of SSAB Weathering ML and NL steels makes choosing the welding consumables more challenging. There is a limited number of option 1 welding consumables available with a -50°C impact toughness guarantee. For submerged arc welding, there is currently only one flux-wire combination available with a -50°C impact toughness guarantee, trade name Bavaria BF10 MW (flux) + BA-S2NiCu (wire). Welding test results using the Bavaria flux-wire combination are presented later in the article.
Welding consumables corresponding to option 2 can be found for the submerged arc welding with even -60 °C Charpy- V impact toughness guarantee. An example of a good combination is Esab OK Flux 10.62 (flux) + OK Autrod 13.27 (wire). SSAB Weathering 460 ML steel welding test results with the Esab combination and with impact toughness tested at a temperature of -60°C are presented at the end of this article.
SSAB Weathering 700 and 960 steels are high strenght weather resistant hot rolled strip products. In addition to option 1 undermatching welding consumables, the filler metal is often chosen also based on the base material’s strength grade. For example, welding consumables with a strength class of 890 MPa that generally consist of a relatively high nickel alloy can be considered for the Weathering 960 steel.
Welding consumables corresponding to table 1 must generally be used for the welds in steel structures that are atmospheric corrosion resistant. Alternatively, unalloyed welding consumables can be used for the multi-pass welding of fillet welds or butt welds in both root passes and filling passes. In this case, surface passes are required to be weld using weather-resistant consumables.
Increasing the working temperature in welding slows down the cooling of the joint and reduces hardening. The level of hardness of the welded joint is lowered and the risk of cold cracking is reduced. The need for raising the working temperature is increased with thicker plates and welding with a low welding heat input. Cold cracking also requires there to be hydrogen in the welded joint as well as a state of higher weld residual stresses.
For welding COR-TEN steels, it is not usually necessary to carry out preheating with low material thicknesses when using low-hydrogen (hydrogen scale ≤ H5) welding consumables. In comparison with traditional S355 structural steels, COR-TEN steels consist of slightly higher alloying, which is why preheating of +100…200°C is recommended with plate thicknesses exceeding 20…30mm. Preheating is determined based on standard EN 1011-2 method A instructions. (Recommendations for welding of metallic materials. Part 2: Arc welding of ferritic steels)
With a thermomechanical hot rolling process, the target steel strength can be achieved with a lower alloying. Thanks to their low carbon equivalent, thermomechanically rolled steels are easy to weld. With the recently developed SSAB Weathering 355–460 ML plates, preheating can often be fully avoided when low-hydrogen welding consumables are used and the welding heat input is Q ≥ 1.5kJ/mm. The weldability of Weathering ML plates is discussed in more detail later in this article.
Thin (t ≤ 8 mm) SSAB Weathering 550, 700 and 960 steels do not require preheating when low-hydrogen (hydrogen grade < H5) welding consumables are used and the general welding guidelines for high-strength steels are followed.
|Steel grade||t8/5 minimum||t8/5 maximum|
|SSAB Weathering 355||5 seconds||30 seconds|
|SSAB Weathering 420ML, 460ML||5 seconds||25 seconds|
|COR-TEN B, B-D||5 seconds||25 seconds|
|SSAB Weathering 550||20 seconds|
|SSAB Weathering 700||15 seconds|
|SSAB Weathering 960||15 seconds1, 2)|
1) t8/5 = 15 seconds applies to undermatching welds.
2) Matching strength welds t8/5 = maximum 4 seconds.
Cooling time recommendation t8/5
The welded joint properties depend on the joint cooling speed, which is impacted by heat input Q, plate thickness, the joint type, welding process and the working temperature. Microstructure changes that are significant in terms of the mechanical properties of the weld take place within the cooling temperature range of 800…500°C. Cooling time t8/5 is used as the value that describes the cooling speed, that is, the time it takes to pass this temperature range. The t8/5 time can be determined according to the general instructions of standard EN 1011-2. The t8/5 times recommended for SSAB’s weathering steels are presented in Table 3. Based on the recommended cooling time, it is possible to determine the welding heat input and other welding parameters as well in order to keep mechanical properties of welded joint as a sufficient good level.
Ultra-high-strength SSAB Weathering 960 steel is more sensitive to the heat input of welding. In the case of a matching weld, heat input must be restricted so as to not exceed the t8/5 time by more than four seconds; see Table 3. It must be decided during the design phase whether the Weathering 960 steel structure’s welds should be matching with base material strength or undermatching. The recommended cooling time ≤ 4 seconds means to low heat input, which can be achieved by using low heat input welding processes, such as pulse MAG welding or laser MAG hybrid welding. If a weld can be designed as undermatching, a higher heat input (t8/5 ≤ 15 seconds) and all traditional welding processes can be used.
Excellent weldability of SSAB Weathering ML plates
The manufacturing process of Weathering ML plates recently developed by SSAB makes it possible to manufacture thicker weather resistant plates with excellent weldability. Thanks to the thermomechanical hot rolling process, the hydrogen content and carbon equivalent values (table 4.) of the steels are relatively low in comparison with the traditionally manufactured weathering steels. Thanks to low carbon equivalent values, the steels can be welded even without preheating, when low-hydrogen welding consumables (hydrogen scale ≤ H5) are used and the heat input is at least 1.5 kJ/mm. However, for thick plates (thickness of a single plate ≥ 40mm) it is recommended to use preheating (100…125°C). When the heat input is low (Q < 1.0kJ/mm), the need for preheating must be determined based on the EN 1011-2 standard method A. The preheating recommendations for SSAB Weathering ML plates are presented in Table 5.
|Steel||Thickness range (mm)||CEV typ.||CEV max|
|Weathering 355 ML||8…65||0.37||0.42|
|Weathering 420 ML||8…65||0.41||0.45|
|Weathering 460 ML||8…65||0.43||0.45|
In addition to reduced need for preheating, the new type of weathering ML plates offers excellent heat affected zone (HAZ) Charpy-V toughness at low temperature -50°C. The welding test results for 25 mm thick SSAB Weathering 420ML and 460ML plates are presented in tables 7…9. MAG welding filler metal was Lincoln LNM28 solid wire. Submerged arc welding (SAW) tests were performed with a Bavaria wire-flux combination which impact toughness guarantee is 27 J @ -50°C. Both LNM28 MAG solid wire and Bavaria SAW wire-flux combination comply with EN 1090-2 standard option 1 alloying. The Weathering 460ML plate is also submerged arc welded with an Esab high-nickel SAW wire-flux combination which complies with EN 1090-2 standard option 2. Impact toughness class of SAW wire-flux combination made Esab is -60°C. Table 8 shows that the impact toughness of SSAB Weathering ML plates heat affected zone remains at a good level even up to -60°C and though the welding heat input has been fairly high 3.0kJ/mm, and the calculated t8/5 cooling time is 24 seconds.
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Sakari Tihinen, Senior Welding Specialist, IWE, SSAB Europe Oy
Esa Virolainen, Senior Specialist, SSAB Europe Oy