Ferritic stainless steel (400 series) is a type of steel with ferrite as the main structure, which has a ferrite crystal structure, usually a body-centered cubic (BCC) crystal structure. Ferritic stainless steel contains 15% to 30% chromium, generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. This type of steel has the characteristics of high thermal conductivity, small expansion coefficient, good oxidation resistance, excellent stress corrosion resistance, etc., and is mostly used to manufacture parts resistant to atmospheric, water vapor, water and oxidizing acid corrosion. Compared with austenitic stainless steel, ferritic stainless steel has better corrosion resistance and oxidation resistance, and exhibits significant magnetism at room temperature.
Chemical Composition And Structural Characteristics Of Ferritic Stainless Steel:
The main chemical components of ferritic stainless steel are iron and chromium, and the chromium content is usually between 15% and 30%. Chromium is an important element for forming a passivation film, which can significantly improve the corrosion resistance of steel. Unlike austenitic stainless steel, ferritic stainless steel has a lower carbon content, usually less than 0.1%, which helps prevent the formation of chromium carbides and thus avoid the risk of intergranular corrosion.
The structural characteristics of ferritic stainless steel are mainly composed of body-centered cubic (BCC) crystals. This structure makes it magnetic at room temperature and slightly less tough than austenitic stainless steel, but it performs well in some specific environments, such as high chloride corrosion environments.
Performance Characteristics Of Ferritic Stainless Steel:
- Corrosion resistance：
The most significant advantage of ferritic stainless steel is its superior corrosion resistance. Due to the high chromium content, ferritic stainless steel can form a passivation film in a variety of corrosive environments, which effectively resists oxidation and corrosion. Especially in chloride-containing environments, ferritic stainless steel has excellent resistance to pitting and crevice corrosion.
- Mechanical properties：
The mechanical properties of ferritic stainless steel vary at different temperatures. At room temperature, it has high strength and hardness, but relatively low toughness. Ferritic stainless steel may exhibit brittleness in low temperature environments due to its body-centered cubic structure. However, its low temperature toughness can be improved through proper heat treatment and alloy design.
- Magnetic and Resistivity：
Ferritic stainless steel is magnetic at room temperature, which is determined by its body-centered cubic structure. In contrast, austenitic stainless steel is usually non-magnetic. Ferritic stainless steel has a low resistivity, usually between 60 and 80 μΩ·cm, which makes it advantageous in some electromagnetic applications.
- Welding Performance：
Due to the low thermal expansion coefficient of ferritic stainless steel, the deformation and stress during welding are relatively small. However, the welding performance of ferritic stainless steel is not as good as that of austenitic stainless steel, and it is prone to welding cracks and embrittlement of the weld zone. For this reason, appropriate heat treatment is usually performed after welding to restore its mechanical properties.
- Heat Treatment Performance：
Ferritic stainless steel has a limited response to heat treatment. Due to its crystal structure, ferritic stainless steel cannot usually be hardened by quenching, but its strength and toughness can be improved by annealing and cold working. Stabilization treatment is also a common heat treatment method, which can effectively prevent intergranular corrosion.
Main Types And Grades Of Ferritic Stainless Steel:
400 series stainless steel is the most common ferritic stainless steel series, mainly including grades 409, 410, 430, etc. These steels are characterized by high chromium content and low carbon content, which make them have good corrosion resistance and oxidation resistance, and are widely used in automotive exhaust systems, home appliance housings, architectural decoration and other fields.
Chromium-containing ferritic stainless steel usually has a chromium content of 12% to 18%, which is the most basic type of ferritic stainless steel. Due to their lower alloy cost, these steels are usually used in less demanding environments, such as household kitchen utensils, washing machine inner barrels, etc. Although their corrosion resistance is relatively low, they can still provide sufficient protection in low chloride environments.
Adding molybdenum can significantly improve the resistance of ferritic stainless steel to pitting and crevice corrosion. Typical molybdenum-containing ferritic stainless steel grades include 444 and 446, which are usually used in chemical equipment and marine environments.
Stabilized ferritic stainless steel has stabilizing elements such as titanium and niobium added to the alloy, which can effectively prevent intergranular corrosion and carbide precipitation, thereby improving welding performance. Typical stabilized ferritic stainless steel grades include 439, 441, etc., which are widely used in automobile exhaust systems and heat exchangers.
Production And Processing Technology Of Ferritic Stainless Steel:
- Smelting process：
The smelting process of ferritic stainless steel usually uses electric arc furnace (EAF) or vacuum smelting technology (such as vacuum induction melting, VIM) to refine molten steel to ensure that the impurity content in the steel reaches the lowest level. The main alloying element of ferritic stainless steel is chromium. In order to improve its corrosion resistance and mechanical properties, a small amount of molybdenum, titanium or niobium is sometimes added. The smelted molten steel is usually subjected to secondary refining, such as vacuum degassing (VOD) treatment, to further reduce the content of carbon, nitrogen and oxygen, thereby improving the quality of the steel.
- Heat treatment and cold working：
Heat treatment is one of the key steps that determine the performance of ferritic stainless steel. Annealing is the most common heat treatment method, which eliminates internal stress and improves its toughness by heating the steel to an appropriate temperature (usually between 700°C and 850°C) and then slowly cooling it. Annealing can also restore the hardening caused by cold working, making the steel have better ductility and plasticity.
In the cold working process, ferritic stainless steel exhibits a high work hardening rate, which makes its strength and hardness significantly improved after cold rolling or cold drawing. However, excessive cold working may cause embrittlement of the material, so it is usually lightly annealed after cold working to restore its toughness.
- Surface treatment and quality control：
In order to improve the surface finish and corrosion resistance of ferritic stainless steel, a series of surface treatments such as pickling, polishing, electroplating, etc. are usually carried out. Pickling can remove the oxide scale and impurities on the surface of the steel, while polishing can give it a higher surface brightness and beauty. For some special applications, electroplating treatments such as zinc plating or chrome plating can also be performed to further enhance its corrosion resistance.
Quality control is crucial in the production of ferritic stainless steel. Common quality control measures include chemical composition analysis, mechanical property testing (such as tensile strength, ductility testing) and non-destructive testing (such as ultrasonic testing, X-ray testing). These measures can ensure that the steel meets strict quality standards in each production link, and the products finally delivered to customers meet the expected performance requirements.
