Ferritic Stainless Steel
Ferritic steels are high-chromium, magnetic stainless steels that have a low carbon content. Known for their good ductility, resistance to corrosion and stress corrosion cracking, ferritic steels are commonly used in automotive applications, kitchenware, and industrial equipment.
Characteristics of Ferritic Stainless Steel
In comparison to austenitic stainless steels, which have a face-centered cubic (FCC) grain structure, ferritic steels are defined by a body-centered cubic (BCC) grain structure. In other words, the crystal structure of such steels is comprised of a cubic atom cell with an atom in the center.
This grain structure is typical of alpha iron and is what gives ferritic steels their magnetic properties. Ferritic steels cannot be hardened or strengthened by heat treatment but have good resistance to stress-corrosion cracking. They can be cold worked and softened by annealing (heating and then slowly cooling).
While not as strong or corrosion-resistant as austenitic grades, the ferritic grades generally have better engineering properties. Though generally very weldable, some ferritic steel grades can be prone to sensitization of the weld heat-affected zone and weld metal hot cracking. Weldability limitations, therefore, restrict the use of these steels to thinner gauges.
Due to their lower chromium content and lack of nickel, standard ferritic steel grades are usually less expensive than their austenitic counterparts. Specialty grades often include molybdenum.
Ferritic stainless steel usually contains 10.5% to 27% chromium.
Groups of Ferritic Stainless Steels
Ferritic stainless steel alloys can generally be classified into five groups, three families of standard grades (Groups 1 to 3) and two families of specialty grade steels (Groups 4 and 5). While standard ferritic steels are, by far, the largest consumer group in terms of tonnage, demand for specialty grade stainless steels is increasing steadily.
Group 1 (Grades 409/410L)
These have the lowest chromium content of all stainless steels and so are the least expensive of the five groups. They are ideal for slightly corrosive environments where localized rust is acceptable. Grade 409 was initially created for automotive exhaust systems silencers but can now be found in automotive exhaust tubing and catalytic converter casings. Grade 410L is often used for containers, buses, and LCD monitor frames.
Group 2 (Grade 430)
The most commonly used ferritic steels are found in Group 2. They have a higher chromium content and are, consequently, more resistant to corrosion by nitric acids, sulfur gases, and many organic and food acids. In some applications, these grades can be used as a replacement for austenitic stainless steel grade 304. Grade 430 is often found in the interiors of appliances, including washing machine drums, as well as kitchen sinks, indoor panels, dishwashers, cutlery, cooking utensils, and food production equipment.
Group 3 (Grades 430Ti, 439, 441, and Others)
Having better weldability and formability characteristics than Group 2 ferritic sheets of steel, Group 3 steel can be used to replace austenitic grade 304 in a wider range of applications, including in sinks, exchange tubes, exhaust systems, and welded parts of washing machines.
Group 4 (Grades 434, 436, 444, and Others)
With a higher molybdenum content, the ferritic stainless steel grades in Group 4 have enhanced corrosion resistance and are used in hot water tanks, solar water heaters, exhaust system parts, electric kettles, microwave oven elements, and automotive trim. Grade 444, in particular, has a pitting resistance equivalent (PRE) that's similar to grade 316 austenitic stainless steel, allowing it to be used in more corrosive outdoor environments.
Group 5 (Grades 446, 445/447, and Others)
This group of specialty stainless steels is characterized by relatively high chromium content and the addition of molybdenum. The result is steel with excellent corrosion and scaling (or oxidation) resistance. In fact, the corrosion resistance of grade 447 is equivalent to that of titanium metal. Group 5 steels are typically used in highly corrosive coastal and offshore environments.