Weathering steel, also known as atmospheric corrosion resistant steel, is a low alloy steel series between ordinary steel and stainless steel. Weathering steel is made of ordinary carbon steel with a small amount of corrosion resistant elements such as copper and nickel added. It has the characteristics of high-quality steel such as toughness, plasticity, forming, welding, abrasion, high temperature, and fatigue resistance; its weather resistance is 2-8 times that of ordinary carbon steel, and its paintability is 1.5-10 times that of ordinary carbon steel. At the same time, it is rust-resistant, making components corrosion-resistant and life-extending, thinning and reducing consumption, saving labor and energy, etc. Weathering steel is mainly used for steel structures such as railways, vehicles, bridges, towers, photovoltaics, and high-speed projects that are exposed to the atmosphere for a long time. It is used to manufacture containers, railway vehicles, oil derricks, port buildings, oil production platforms, and chemical and petroleum equipment containing hydrogen sulfide corrosive media and other structural parts.

1.   Development Overview

Since the beginning of the 20th century, the United States, Germany, Britain, and Japan have conducted in-depth research on weathering steel. As early as 1916, European and American scientists discovered that copper can improve the corrosion resistance of steel in the atmosphere. In 1916, the American Society for Experimental and Materials (ASTM) began atmospheric corrosion research. C. P. Larrabee and others accumulated data on atmospheric corrosion, summarized corrosion laws, and explored corrosion mechanisms. In the 1930s, the U.S. Steel Company in the United States first successfully developed a corrosion-resistant, high-strength, copper-containing low-alloy steel, Corten steel, which was directly used in buildings and bridges without painting in the 1960s. The most commonly used ones are the Corten A series steels with high phosphorus, copper + chromium, and nickel, and the Corten B series steels with chromium, manganese, and copper alloys. This type of weathering steel is also widely used in Europe and Japan. At present, weathering steel has gradually been widely used as a common steel grade abroad, and detailed regulations have also been made on the development, use, design and construction of steel grades.

2. Basic information of steel types

2.1 Brand and chemical composition

The difference between weathering steel and general copper-containing steel is that in addition to copper, it also contains alloy elements such as phosphorus, chromium, nickel, titanium and vanadium. Representative steel types abroad are CORTENA and SPA2H, mainly Cu2P2Ni2Cr series. Considering resource conditions and raw material economy, domestic steel is often added with appropriate amount of rare earth (RE), mainly Cu2P2RE series.

2.2 Mechanical properties of weathering steel

The mechanical properties of weathering steel are basically close to those of high-quality carbon steel or high-quality low-alloy steel, but weathering steel is required to have better cold working properties.

2.3 Metallographic structure, non-metallic inclusions and grain size

The metallographic structure of steel should be ferrite + pearlite, the oxide inclusion level should not exceed level 2, the sulfide inclusion level should not exceed level 215, and the grain size should not be less than level 7.

3. Weldability of weathering steel

Except for P steel, the weldability of weathering steel is not much different from that of general low-alloy hot-rolled steel. The maximum hardness of the heat-affected zone of welding does not exceed 350HV, so the weldability is good, no hot cracks will be generated during welding, and the cold brittle tendency is not large, so the welding process can be formulated in the same way as low-strength (o=343~292MPa) low-alloy hot-rolled steel.

When selecting welding materials, it should be noted that in addition to meeting the strength requirements, the corrosion resistance of the weld metal must also match that of the parent material. If a filler metal without atmospheric corrosion resistance is used, it should be ensured that the weld itself is weather-resistant.

Before welding, the formed surface layer should be removed to a distance of 10mm to 20mm from the edge of the joint. When the phosphorus content of the welding steel grade is very high, special precautions should be taken.

4. Standards related to weathering steel

There are different standards for different uses of weathering steel. The main standards for weathering steel in my country are:

GB/T 4171 "High Weathering Structural Steel", the main steel type is Q××GNH(L), where ×× represents yield strength and L represents the content of Cr and Ni elements.

GB/T 4172 "Welded Structural Weathering Steel", the main steel type is Q××NH, where ×× represents yield strength.

GB/T 18982 "Corrosion-resistant Steel Plates and Strips for Containers", the main steel types are Q××GNH(L)J, Q××NHYJ, where L represents the content of Cr and Ni elements, NHY represents ocean resistance, and J represents containers.

TB/T 1979 "Technical Conditions for Ordering Atmospheric Corrosion Resistant Steel for Railway Vehicles", the main steel types are expressed by the alloy elements contained, such as 08CuPVRE, 09CuPCrNi, etc.

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