The Characteristics Of Ferritic Stainless Steel

Publish Time: 06/23 2021 Author: Site Editor Visit: 15327

1 Atmospheric corrosion resistance of ferritic stainless steel

Since ferritic stainless steel has good atmospheric corrosion resistance, it has been used as the roof and curtain wall of buildings recently. However, the atmospheric environment in areas close to the sea is particularly harsh, especially the suspended particles in the sea and air are quite corrosive substances. Therefore, the high-chromium ferritic stainless steel used in these environments has been developed. Stainless steel resistant to atmospheric corrosion contains high chromium and high molybdenum, and a small amount of niobium and titanium are added. This steel actually contains 22% chromium and 1.2% molybdenum. Sufficient chromium and molybdenum are essential to improve the pitting resistance of stainless steel. Type 304 and 316 austenitic stainless steels have a significant increase in rust area with the increase in the number of cyclic corrosion test cycles. On the contrary, for ferritic stainless steels such as type 444 and R&D steel, the rusted area slightly increased during the first 600 test cycles, and after a longer test cycle, the rusted area was in a saturated state. The developed steel grade (22Cr-1.2Mo-Nb, Ti) shows that the rust area is the least in any test period.

2 Intergranular corrosion resistance of ferritic stainless steel

410L or 409 stainless steel is used as a material for automobile exhaust emission control system due to its good corrosion resistance, formability and heat resistance. In recent years, the design temperature of automobile exhaust has increased. This is because the increase in automobile exhaust temperature can improve the conversion efficiency of catalytic converters and reduce harmful gases such as NOx, SOx, and hydrocarbon (HC) emissions. However, the increase in temperature may lead to worse corrosion conditions of the material. For example, chromium carbide will produce deposits on the muffler at the exhaust temperature, that is, at a temperature of 400 to 500°C, it will cause chromium depletion at the grain boundary and intergranular corrosion. Since the weld area is particularly sensitive to intergranular corrosion, it is necessary to improve the corrosion resistance of ferritic stainless steel containing 12% Cr. Another way to solve this problem is to develop new ferritic stainless steels. One example is the addition of niobium to steel containing 12% Cr. These steels are widely used as intergranular corrosion resistant materials in automobile exhaust emission systems, such as front ducts, central pipes and mufflers. It is well known that reducing the carbon and nitrogen content in steel is quite effective in preventing intergranular corrosion. In this way, adding niobium and titanium to steel can further improve its resistance to intergranular corrosion.

3 The formability of ferrite stainless steel

The uses of ferritic stainless steel are so wide, and the properties of ferritic stainless steel required by each use are different. However, the formability of ferritic stainless steel is worse than that of austenitic stainless steel such as 304 steel. Although the γ value of ferritic stainless steel, that is, the deep drawability index, varies in a wide range of 1.0 to 2.0, the value of n, that is, the ductility index is limited, about 0.2, which is lower than 0.4 to 0.65 of austenitic stainless steel. For drawn products, it is difficult to use ferritic stainless steel to replace austenitic stainless steel. If you want to replace it, you must change the design of the product and design it into a drawn shape.

4 Toughness of ferritic stainless steel

Many studies have been conducted on the effect of titanium and niobium on the press formability, and the focus is mainly on the average γ value. The conclusion is that the appropriate amount of these elements can effectively improve the press formability. However, excessive addition of these elements can also have harmful effects. For example, as the content of titanium and niobium increases, the transformation temperature of longitudinal cracks also increases. Even though ferritic stainless steel has a good average gamma value, the ductile brittleness transition temperature may cause damage to deep drawability. Since the transformation temperature is one of the decisive factors for formability, deformation may be difficult to proceed at a higher transformation temperature.

5 High temperature strength of ferritic stainless steel

409L (11Cr-Ti) stainless steel is used as the material of automobile exhaust manifold, and the exhaust temperature is designed to be about 800℃. When the exhaust temperature is about 900°C, 430J1L (18Cr—0.4Nb—0.5Cu) stainless steel is used. However, the exhaust temperature is still increasing, which requires further improvement of the quality of stainless steel. In this way, the heat resistance of traditional high-chromium ferritic stainless steel cannot meet the requirements of exhaust manifolds. Therefore, there has always been a strong demand for cost-competitive high-temperature ferritic stainless steels. Considering this demand, the effect of adding niobium and molybdenum on high temperature performance has been studied.

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