What is Ferritic Stainless Steel?
Ferritic steels are magnetic, high-chromium stainless steels with little carbon content. Ferritic stainless steels have a body-centered cubic (BCC) lattice structure and are magnetic by nature. They include a lot of chromium (10.5-27% by weight), minimal carbon (0.03-0.12% by weight), and only trace amounts of nickel, manganese, molybdenum, niobium, and titanium. Ferritic steels are frequently used in industrial equipment, kitchenware, and automotive applications due to their excellent ductility, resistance to corrosion, and stress corrosion cracking.
After austenitic stainless steel, ferritic stainless steels are the type of stainless steel used most frequently. They are renowned for their flexibility, corrosion resistance, and magnetic characteristics, and they have high chromium contents but little to no nickel. They can only be cold worked and softened during annealing because they have a body-centered cubic grain structure and cannot be hardened with heat treatment.
Depending on their composition and intended use, ferritic stainless steels are divided into the 400 series, which is then divided into five groups. Even though ferritic stainless steels were created very early, it wasn’t until the 1980s that they began to replace austenitic stainless steels, which were more expensive due to their nickel content.
Characteristics of Ferritic Stainless Steel
Ferritic steels are distinguished by a body-centered cubic (BCC) grain structure in contrast to austenitic stainless steels, which have a face-centered cubic (FCC) grain structure. In other words, the cubic atom cell with an atom at its core makes up the crystal structure of such steels.
Ferritic steels’ magnetic characteristics are due to the characteristic grain structure of alpha iron. Heat treatment cannot be used to harden or strengthen ferritic steels, but they exhibit superior stress-corrosion crack resistance. Although ferritic stainless steels don’t have as much corrosion resistance as austenitic steels, they nevertheless have excellent corrosion and oxidation resistance. They are generally more ductile and formable than austenitic alloys, have better technical qualities, and exhibit good resistance to stress-corrosion cracking. Although ferritic alloys can be welded, they can also experience issues like cracking along heat-affected zones. They can be annealed and cold worked to soften them (heating and then slowly cooling).
The engineering qualities of ferritic grades are often superior, despite them not being as strong or resistant to corrosion as other metals’ grades.
Despite being typically very weldable, some ferritic steel grades might be vulnerable to weld metal hot cracking and sensitization of the weld heat-affected zone. Therefore, only thinner gauges can be used with these steels due to weldability restrictions.
Standard ferritic steel grades are often less expensive than their austenitic counterparts because they have less chromium and no nickel. Molybdenum is typically found in specialty grades.
Ferritic stainless steels have a naturally occurring corrosion-resistant surface layer, unlike galvanized or coated steel, therefore there is no need to add protective surface layers, and there is no need for corrective work or corrosion risk at cut edges. Ferritic Stainless Steel is also simpler to recycle than galvanized steel, which requires the zinc from the coating to be removed before the steel can be remelted. Ferritic stainless steels are typically low maintenance and can be made with a variety of dull and bright surfaces.
Metallurgical characteristics of Ferritic Stainless Steel: The presence of sufficient chromium and other stabilizers to successfully inhibit the development of austenite at high temperatures is the ferritic alloy’s most significant metallurgical feature. Due to the presence of interstitials, austenite does occur in most grades to some extent.
These steels cannot be toughened by quenching because austenite does not develop and the ferrite is stable at all temperatures up to melting. The soft ferrite can easily accommodate any trace amounts of austenite that are present since it transforms into martensite.
To recover the best corrosion resistance after welding, an annealing treatment at 760-815°C (1400-1500°F) is necessary. Heat treatment cannot significantly strengthen ferritic stainless steel. The annealed state is typically when these steels are used. Depending on the specific alloy, the pace of cooling from the desired annealing temperature will vary. The fact that higher chromium-containing alloys are susceptible to embrittlement by sigma or alpha prime phase if improperly heat-treated emphasizes the significance of effective heat treatment.
All ferrites are prone to severe grain development at temperatures exceeding 927°C (1700°F), which reduces the material’s toughness and can only be remedied by cold-working and annealing.
Applications of Ferritic Stainless Steel
For many of its uses, the ferritic grades’ magnetic characteristics are a significant benefit and enabler. This feature is used, for instance, in the creation of magnetic fasteners and induction heating in induction cookers.
Ferritic alloys are better for heat transfer applications like cooking utensils because they have a higher thermal conductivity and a lower thermal expansion coefficient than austenitic grades.
Some ferritic stainless steels have enough corrosion resistance to take the place of austenitic steel 304 in the manufacture of dishwashers, kitchen sinks, and food processing machinery.
Specialty grade ferritic stainless steels can be used in corrosive seawater applications because they contain more molybdenum and more chromium.
Ferritic stainless steels are advantageous in the automotive and nuclear industries thanks to their special combination of characteristics.
Ferritic stainless steels are excellent for a variety of structural applications due to their mechanical and physical characteristics, such as strong and fairly durable structural parts with appealing metallic surface finishes are required. Although it is similar to structural carbon steel in terms of toughness, the ferritic grade 1.4003 has a modified microstructure that guarantees acceptable toughness for outside structural applications. It has an established track record in several applications across a variety of sectors and may be joined using traditional welding techniques. Additional research is required to determine the restrictions on the use of other ferritic grades in terms of toughness in structural applications.
Some other specific uses of ferritic stainless steel include benchwork, cold-water tanks, white goods, refrigeration cabinets, chemical and food processing, water treatment plants, street furniture, and electrical cabinets.
Types of Groups in Ferritic Stainless Steel and their Uses
In general, there are five groups of ferritic stainless steel alloys, three families of normal grades (Groups 1 to 3), and two families of specialty grade steels (Groups 4 and 5). The market for specialty grade stainless steels is gradually rising, even though normal ferritic steels continue to be the largest consumer group in terms of tonnage.
Group 1 (Grades 409/410L): The least expensive of the five groups is this particular group, which has the lowest chromium concentration of any stainless steel. They work best in situations that are just mildly corrosive and where localized rust is acceptable. Grade 409 is presently used in catalytic converter casings and automobile exhaust tubes after being developed first for automotive exhaust system silencers. Buses, LCD display frames, and containers frequently use grade 410L.
Group 2 (Grade 430): Group 2 contains the ferritic steels that are most frequently utilized. Because they contain more chromium, they are more resistant to corrosion caused by nitric acid, sulfur dioxide, and many other organic and food acids. These grades can take the place of the austenitic stainless steel grade 304 in various applications. Appliance interiors, such as washing machine drums, kitchen sinks, indoor panels, dishwashers, cutlery, cooking utensils, and food processing machinery, are frequently made of grade 430 steel.
Group 3 (Grades 430Ti, 439, 441, and Others): Group 3 steel can be utilized to replace austenitic grade 304 in a larger range of applications, including sinks, exchange tubes, exhaust systems, and welded washing machine parts, thanks to its higher weldability and formability qualities than Group 2 ferritic sheets of steel.
Group 4 (Grades 434, 436, 444, and Others): The ferritic stainless steel grades in Group 4 have improved corrosion resistance due to their greater molybdenum concentration and are utilized in hot water tanks, solar water heaters, exhaust system components, electric kettles, microwave oven elements, and car trim. For usage in more corrosive outdoor settings, grade 444 in particular has a pitting resistance equivalent (PRE) that is comparable to grade 316 austenitic stainless steel
Group 5 (Grades 446, 445/447, and Others): This class of specialty stainless steels is distinguished by their incorporation of molybdenum and relatively high chromium content. Steel that is highly resistant to corrosion and scaling (or oxidation) is the end result. Grade 447’s corrosion resistance is actually on par with titanium metals. In more corrosive coastal and offshore conditions, Group 5 sheets of steel are frequently employed.
Ferritic Grade 404 of Stainless Steel: In most applications, the reliable austenitic grade 304 can be replaced with the next generation general-purpose ferritic grade 404. It is ferritic stainless steel that resists corrosion and has superior strength, toughness, fabrication properties, and weldability. Grade 404 exhibits general corrosion resistance that is at least as excellent as grade 304, with improved resistance to intergranular corrosion, stress corrosion cracking, and atmospheric corrosion.
Grade 304 in comparison to grade 404: The 21% chromium and 0.5% copper in Grade 404 provide it with outstanding corrosion resistance in a variety of conditions. Ferritic Stainless Steel grade 404 is replacing grade 304 in sheet metal applications more frequently since it has at least the same level of corrosion resistance. The ease of fabrication – ferritic grades give less tool & machine wear than 304 – allows many users of grade 404 to save significantly. It allows for crisper, cleaner bends, and higher quality products overall. The outstanding corrosion resistance provided by the high chromium concentration of grade 404 is unaffected by the material’s magnetic nature. Because thermal expansion is less, panels undergo less thermal distortion during production and use.
Cleaning ability: This particular grade is fantastic for food equipment, where cleaning ability is essential for removing microorganisms. After washing, it harbors very low amounts of germs yet is resistant to corrosion from food and cleaning agents. As ferritic stainless steel, grade 404 acts like carbon steel in fabrication and formability (G300). With less spring back and forming loads, bends are neater. Tools for cutting and forming often wear out three to five times less quickly. Similar to carbon steel, cutting tool clearances exist. Bending requires a minimum radius of 1t. Compared to 304, the deep ability to draw is better, but the stretch capability is worse.
Future Trends of Ferritic Stainless Steel
Super-ferritic alloys, which have low carbon/nitrogen content grades, weren’t a popular choice even though they were initially created in the 1970s as a potential replacement for titanium in seawater and high chloride applications due to the rarity and high cost of titanium.
As nickel and molybdenum prices surge as a result of their high demand in the manufacture of austenitic grades, super-ferritic alloys are currently being investigated as a possible replacement for super-austenitic alloys. Some examples of super-ferritic alloys that are offered for sale commercially include E-Brite, Monit, and Sea-cure.
Welding in Ferritic Stainless Steel
Due to grain growth and grain boundary, martensite development in the heat affected zone that is heated above 1100°C, ferritic stainless steels become embrittled during welding. Due to the notch sensitivity of these materials and the fact that grain growth causes the ductile-brittle transition temperature to rise above room temperature, the weldment will become brittle as it cools down. It is advised to pre-heat to 200°C and to post-heat treat at 750–850°C. These precautions don’t prevent the harmful microstructure, but they do help to minimize residual welding stresses and achieve some HAZ softening.
The heat-impacted zone may have more toughness than the matching composition weld metal. Austenitic consumables are therefore chosen unless there is a risk of thermal fatigue in service due to variances in the coefficient of expansion between the parent material and the weld metal. For ferritic grades of stainless steel, TIG welding and manual metal arc welding are most frequently utilized.
In the vicinity of a weld that has been heated to a temperature exceeding 1100°C, ferritic stainless steels are also vulnerable to intercrystalline corrosive assault. A post-weld heat treatment at a temperature of between 700 and 850°C may help to solve the issue. Under some conditions, the sigma phase that results from heat treatment might cause brittleness below 200°C. By annealing at a temperature exceeding 850°C, this can be removed.
The industrial application of fusion welded ferritic stainless steel assemblies is constrained by issues with loss of toughness and corrosion resistance in the weld area. However, compared to austenitic grades, these materials are far more resistant to stress corrosion caused by chloride and can provide good general corrosion resistance.
To get mixed ferrite-martensite or ferrite-austenite microstructures, alloys have been designed that either reduce interstitial elements or have balanced chromium-nickel concentrations. This avoids the welding issues associated with ordinary ferritic stainless steel. In service situations when the use of austenitic steels was prohibited due to the risk of stress corrosion, such materials have been successfully welded in thin gauges.
Heat Treatment Process in Ferritic Stainless Steel
The chromium concentration of ferritic stainless steel is higher than that of martensitic stainless steel. The ferritic stainless steel typically contains 14 to 27 wt% chromium. According to Figure 22’s Fe-C-Cr diagram, which is sectioned at 18% chromium, austenite cannot form at low carbon levels until a very high temperature, or 1200 °C, is reached for a carbon concentration of 0.06%. As a result, it may be inferred that the steel of this composition is ferritic from room temperature to 1200 °C and cannot be hardened. Cold working is the sole method that can harden ferritic stainless steel. Ferritic stainless steel may easily be cold worked and deep drawn due to its ferritic microstructure. This form of stainless steel is employed in the food industry and for architectural trims, among other things, for this reason. Steel that has been annealed is exceedingly ductile, soft, and has excellent formability. Ferritic stainless steels have a substantially higher corrosion resistance than martensitic stainless steels and are much stronger than annealed low-carbon steels. There is no other significant heat-treatment procedure besides annealing that may aid to enhance the characteristics of ferritic stainless steel. However, if kept at a temperature between 400 and 500 °C, completely ferritic steel is quite susceptible to embrittlement. The annealing operation must be performed at a temperature above 500 °C until the temperature at which austenite is likely to occur since this embrittlement causes a dramatic decline in impact characteristics. After holding for an appropriate amount of time, which is determined by whether or not the microstructure of the material contains any carbide phases and whether the carbides may easily dissolve to form a single phase homogenous ferrite. The steel is then gradually cooled to 500 °C; prolonged holding between 400 and 500 °C is not permitted to prevent 400 °C embrittlement. It is best to progressively cool the material in the furnace until it reaches 500 °C, at which point it must cool more quickly to avoid embrittlement at 400 °C. Although the specific cause of 400 °C embrittlement is unknown, the documented observation of increased embrittlement with rising chromium content seems to suggest that 400 °C embrittlement is related to the creation of dangerous carbides while holding the steel at the above temperature area.
Crystallography in Ferritic Stainless Steel
The presence of a primarily ferrite matrix networked by scarce carbide presence in the microstructure sets apart ferritic stainless steel from other types of stainless steel. This is normally acceptable, although it might not be able to explain the reasons behind the variations in some attributes among the different stainless steel classes. For instance, the austenitic grade of stainless steel is not magnetic, although ferritic and duplex stainless steels are. At the atomic level, the distinction between the various stainless steel classes is crucial. In contrast to austenite, the crystal structure of ferrite has a distinct arrangement of atoms. While it is face-centered cubic in austenitic stainless steel, the crystallographic orientation is body-centered cubic (BCC) in ferritic stainless steel. This crystal structure controls the majority of the ferritic grade’s mechanical and physical characteristics. These characteristics include magnetic properties, the temperature at which materials shift from being ductile to brittle, plasticity and deformation, conductivity and thermal expansion, texturing, and grain orientation. Plain carbon steels have comparable crystallographic characteristics. Due to fewer atoms per unit cell compared to the FCC structure, the BCC structure is less capable of alloying elements than the austenitic grade. Additionally, ferritics have fewer slip systems than austenitics, which reduces their capacity to form through plastic deformation.
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