Ferritic stainless steel, which belongs to the chromium steel grades, is characterized by a ferrite microstructure. Therefore, ferritic steel possesses a body-centered cubic crystal structure. In contrast to austenitic steel grades, which are typified by a face-centered cubic structure, ferritic steel grades are highly magnetic. Further characteristics are the chromium content which varies between 10.5% and 27% and the very little to no nickel content in the alloy. Generally, the carbon content is lower than 0.1%. Due to the low carbon to chromium ratio, the microstructure does not change through temperature. This means that on one hand ferritic steel grades can not be hardened or strengthened by heat treatment but on the other hand can be cold worked.
Ferritic stainless steels show good mechanical properties and tend to have higher yield strength austenitic steel grades.
The advantages and disadvantages
Due to the low or even no amount of nickel, ferritic stainless steel grades are usually less expensive and more price stable than austenitic steels. Although ferritic steels have a mediocre overall corrosion resistance, the resistance to stress-corrosion cracking is higher than in other steel grades. To improve the corrosion resistance, Molybdenum can be added to some special types of ferritic steel. In general, such types of steels possess good weldability attributes, although not as good as the austenites. To stabilize the ferrites and achieve even better weldability, niobium and/or titanium can be added as alloying elements.
Applications for ferritic stainless steel
In general, austenitic steel can be replaced by ferritic steel for many applications, which is already true for the food and drink industry. Due to the high chromium content, ferritic stainless steels are very cleanable. For this reason, it is frequently used in industries that value high standards of hygiene, such as restaurants and catering businesses.
Due to the good resistance to corrosion and the lower price of ferritic steel, many industries replaced austenites with ferrites. For example, the automotive industry in the USA and Europe rely heavily on this type of steel. Further applications for ferrites are construction and household equipment.
Depending on the application, different alloys are applied on ferritic steel grades. In total, ferrites can be divided in five groups.
Group 1 (type 409/410 L)
The first group is characterized by the lowest chromium content and the lowest price of all stainless steels. Originally designed for automotive exhaust systems, type 409 is nowadays used for catalytic converter casings. The main applications for type 410 L are LCD monitor frames, containers and buses.
Group 2 (type 430)
Type 430 is the most common ferritic steel grade and is characterized by a chromium content range from 16% to 18%. Therefore, it is more corrosion resistant than group 1 ferrites and is found in washing machine drums, kitchen sinks and other kitchenware. Due to similar characteristics, type 430 can replace 304 in many applications, such as indoor panels etc.
Group 3 (type 430Ti, 439, 441)
Group 3 ferrite steels have a chromium content between 17.5% and 18.5%. They show an excellent weldability and formability, which in some cases is even superior to 304. These properties make group 3 ferrites an exceptional replacement for a wide range of applications where 304 steels are used. This includes exhaust systems, sinks and exchange tubes.
Group 4 (type 434, 436, 444)
Group 4 is defined by an increased corrosion resistance, which is achieved through the addition of molybdenum. Group 4 ferritic steel types find application in corrosive environments like hot water tanks and outdoor applications.
Group 5 (type 446, 445)
The steels in the last group are used in highly corrosive environments. Due to the high amount of chromium and molybdenum, this types of ferritic steel are characterized by a superior corrosion resistance. In fact, it is comparable to titanium metal, why makes it the ideal steel grade for heat exchangers, water heaters and boilers.
HARDENING OF FERRITE STEELS
Ferritic stainless steels cannot be hardened by conventional heat treatments, but the BORINOX® PROCESS FOR HARDENING STAINLESS STEEL offers exactly this possibility. Furthermore, the process has the advantage that the CORROSION RESISTANCE of the materials is not reduced during the process. In addition to providing 5 times HIGHER HARDNESS, the treatment safely protects against ABRASION, CAVITATION and fatigue. For professional advice, contact our materials experts today.