The development of Ferritic stainless steel
The internal microstructure of Ferritic stainless steel is ferrite, with a mass fraction of the chromium in the range of 11.5% ~ 32.0%. With the increase of chromium content, the acid resistance of the steel will increase as well. By adding molybdenum (Mo), the ability of resistance to acid corrosion
and stress corrosion can be improved. The types of national standard stainless steel grades includes 00Cr12, 1Cr17, 00Cr17Mo, 00Cr30Mo2, ect. This kind of steel is used in absorption tower of manufacturing nitric acid chemical equipment, heat exchangers, storage tanks and tank of nitric acid transport, as well as other components and equipment which do not withstand shock loads.
According to the total conten of carbon and nitrogen, high-chromium ferritic stainless steel is divided into two series of ordinary purity and ultra-high purity. The ordinary purity high-chromium ferritic stainless steel has a mass fraction of carbon about 0.1% , along with a small amount of nitrogen, such as grades of 1Cr17、1Cr17Mo. Compared with generally used Austenitic stainless steel, its drawback is that the material is brittle and welding process is poor. The reason is that it can only be refined through the pressure processing when ferrite grain growth and the steel with more than 17% WCr will generate the brittleness of σ phase (CrmFen) when having a long residence time at high temperatures (e.g. 600 to 820 ℃). Also, 475 ℃ brittleness occurs when the steel stays in a temperature range of 400 ~ 526 ℃ for a long time. The application of this kind of steel is limited, due to the big brittleness.There is a process in the improvement of the quality of the ferritic stainless steel. Originally, the steel,for example 1Cr17 steel, uses chromium as a ferrite stabilizing element, however with high carbon content. Due to its low toughness, if the steel is not subjected to a heat treatment after welding, intergranular corrosion occurs. Later, forming elements of strong ferrite, such as aluminum, is added for reducing the chromium and carbon content in the steel. The process performance and corrosion resistance is superior to the former. Also the manufacturing cost is reduced. However, the improvement of toughness is not obviou enough. Till recent years, new methods of refining are used, such as using vacuum or protective atmosphere refining technology to smelt ultra-high purity high chromium Ferritic stainless steel plate, which is with ultralow total content of carbon and nitrogen. This kind of steel is superior in terms of toughness, corrosion resistance, and weldability than ordinary purity ferritic stainless steels. For the reason, the steel becomes a promising material and are widely used.
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