Welding Structural Weather Resistant Steel (GB/T4172-1984)
Weather-resistant steel (also commonly referred to as ‘weathering steel’) is a group of steel varieties which contain copperclad steel which has been treated and promoted to develop excellent corrosion resistance in the presence of moisture and oxygen. Due to its unique chemical composition and specific surface treatment, the structure of weathering steel results in being less susceptible to rust and so forth, which leads to longer potential service life. Weathering steel has a higher corrosion-resistant than typical normal steel and are widely used in the construction industry where frequently exposed to elements. The GB/T4172-1984 (‘High weather-resistant structural steel’) specification covers the corrosion resistance and mechanical properties of weather-resistant steel.
Corrosion resistance is one of the defining characteristics of weathering steel, and that is all down to the chemical composition of the material which contains a mix of iron, chromium, nickel and copper. This results in a finish with a higher level of oxidation, which produces a unique finish that would otherwise be impossible. GB/T4172-1984 states that the chemical composition of weathering steel should contain between 0.25% and 0.55% copper and between 0.20% and 0.50% chromium. The copper content is what is primarily responsible for the increase in durability, as it helps to promote increased resistance to corrosion caused by water, oxygen, and temperature change.
The mechanical properties of weathering steel are also specified in the GB/T4172-1984. These include tensile strength, yield strength, elongation and hardness. The minimum tensile strength of weathering steel should be between 382 MPa and 586 MPa, with a minimum yield strength of 345 MPa and a maximum elongation of 20%. The minimum hardness should also be 22 HRC.
Welding is an acceptable way of joining together sections of weathering steel, as long as the correct welding procedure and filler materials are used. However, due to the high level of corrosion resistance found in weathering steel, the welding heat-affected zone (HAZ) is susceptible to corrosion. To combat this, it is important to specify a suitable grade of welding filler and to follow specific pre-heating and post-heating instructions in order to reduce the amount of weld shrinkage, which in turn reduces any stress concentration in the HAZ.
Besides meeting the chemical composition and mechanical property requirements of the GB/T4172-1984 standard, it is essential that the weathering steel has been processed in order to be properly weather-resistant. This treatment usually involves the application of electrochemical plasticity or anodizing to increase the surface roughness. It is also necessary to apply a rustproof coating over the steel in order to protect against further attack.
In conclusion, weathering steel is a highly corrosion-resistant material which is a great choice for structures exposed to the elements. Its corrosion-resisting chemical composition gives excellent resistance to oxygen, water and temperature change which makes it ideal for outdoor applications. The weathering steel meets the mechanical property requirements specified in the GB/T4172-1984 and can be welded as long as the correct welding procedure and filler materials are used. It is also essential that the steel has been properly processed in order to make it weather-resistant, with the addition of a rustproof coating to provide further protection.
