Properties and types of ferritic stainless steel

Ferritic stainless steel is a kind of stainless steel. Ferritic stainless steel has excellent performance. It can resist atmospheric corrosion and intergranular corrosion, but its high temperature strength, toughness and processing and forming performance are slightly insufficient. Let's talk about these characteristics in detail.

Ferritic stainless steel plate
Ferritic stainless steels are resistant to atmospheric corrosion
Because ferritic stainless steel has good atmospheric corrosion resistance, it is often used on roofs and curtain walls of buildings. However, it should be noted that the atmospheric environment near the sea is very harsh, especially the suspended particles in the air from seawater are very corrosive, so high-chromium ferritic stainless steel is used in these environments.
Generally, stainless steels that are resistant to atmospheric corrosion will have high chromium and high molybdenum, and a small amount of niobium and titanium have been added. This steel grade actually contains 22% chromium and 1.2% molybdenum. For 304 and 316 austenitic stainless steels, as the number of cycles of the periodic corrosion test increases, the rusted area will increase significantly.
Intergranular Corrosion Resistance of Ferritic Stainless Steel
410L or 409, two ferritic stainless steels, are used as materials for automobile exhaust emission control systems because of their high corrosion resistance, formability, and heat resistance. As the design temperature of automobile exhaust increases, the corrosion conditions for stainless steel materials are also more severe. For example, intergranular corrosion is prone to occur due to high temperature. Therefore, it is necessary to improve the corrosion resistance of ferritic stainless steel.
Another solution is to develop and produce better ferritic stainless steels, such as adding niobium to ferritic stainless steels with 12% chromium, which can further improve their intergranular corrosion resistance. They are widely used in automobile exhaust systems, such as front pipes, center pipes and mufflers.
Formability of ferrous stainless steel
Ferritic stainless steel has a wide range of uses, and the properties of ferritic stainless steel required for each use are different. However, the formability of ferritic stainless steel is slightly weaker than that of austenitic stainless 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 n value, that is, the ductility index is limited, about 0.2, which is lower than the 0.4 to 0.65 of austenitic stainless steel. For drawn-formed products, it is difficult to replace austenitic stainless steel with ferritic stainless steel. If it is to be replaced, it is necessary to change the design of the product and design it into a drawn-formed shape.
Toughness of ferritic stainless steel
Appropriate addition of titanium and niobium can effectively improve the press formability. However, excessive addition of these elements can also cause adverse effects. For example, as the content of titanium and niobium increases, the transformation temperature of longitudinal cracks will also increase. Even if ferritic stainless steel has an excellent average γ value, the ductile-brittle transition temperature may damage the deep drawability. Since the transformation temperature is one of the decisive factors for formability, deformation will be more difficult at higher transformation temperatures.
High temperature strength of ferritic stainless steel
409L ferritic stainless steel is used as the material of automobile exhaust manifold, and the exhaust temperature design of this part is about 800 ℃. When the exhaust temperature is about 900°C, 430J1L stainless steel can be used. However, the exhaust temperature is still increasing, which requires a further increase in the quality of stainless steel. In this way, the heat resistance of traditional high-chromium ferritic stainless steel does not meet the requirements of the exhaust manifold. Therefore, there is always a strong demand for cost-competitive high-temperature ferritic stainless steel. It is precisely because of this demand that the stainless steel industry has long been investigating the effect of niobium and molybdenum additions on the high temperature properties of ferritic stainless steels.