Ferritic stainless steel refers to the chromium content of 10.5% to 30%, with a body-centered cubic grain structure, and ferrite-based stainless steel materials. This type of steel generally does not contain nickel or only a small amount of nickel, and sometimes also includes a small amount of Mo, Ti, Nb, and other elements, Ferritic stainless steel has a high thermal conductivity, low expansion coefficient, good oxidation resistance, excellent resistance to stress corrosion cracking.
Ferritic stainless steel is magnetic and has good corrosion resistance in many environments, but not as good as austenitic stainless steel. It is known for its resistance to high temperatures, making it suitable for high-temperature applications such as exhaust systems and furnace components. Ferritic stainless steel is also commonly used in applications such as automotive trim and appliance parts due to its aesthetic appearance and low cost. However, generally, they have poorer resistance to localized corrosive attack, inferior stretch formability, poor weldability, their tendency to brittle fracture, and the need to constrain their carbon and nitrogen contents to deficient levels. They cannot be hardened or strengthened by heat treatment. It can be cold-worked and softened by annealing. Due to their lower chromium content and lack of nickel, standard ferritic steel grades are usually less expensive than their austenitic counterparts.
Ferritic stainless steel has similar properties to mild steel but has better corrosion resistance due to its high chromium content, which forms a protective oxide layer on the surface to prevent corrosion, but its corrosion resistance is not as good as austenitic stainless steel, because ferritic steel has a body-centered-cubic (BCC) structure, while austenitic steel has a face-centered-cubic (FCC) structure, which provides better corrosion resistance and durability. In addition, ferritic stainless steels usually have a lower nickel content than austenitic stainless steels. Nickel enhances the stability of the austenitic structure and plays a vital role in providing superior corrosion resistance, in contrast, the substantial lack of nickel in ferritic stainless steel limits its corrosion resistance. In addition, alloying elements (special grades of ferritic steels often include alloys such as molybdenum, aluminum and titanium) contribute to the cracking resistance of ferritic stainless steels, especially at high temperatures and in corrosive environments.
Their moderate corrosion resistance and poor fabrication properties can be improved in the higher alloyed grades such as 434 and 444 and in the proprietary grade 3CR12 (a modified version of grade 409 steel). Common uses of ferritic stainless steels are in automotive exhaust systems (11% Cr), automotive trims (17% Cr–1.5% Mo), and hot water tanks (18% Cr–2% Mo–Ti).
Ferritic Stainless Steel Categories
Ferritic stainless steel is available in five grades: three of standard grades (Group 1, Group 2, and Group 3) and two of special grades (Group 4 and Group 5). Groups 1, 2 and 3 are the first, second and third generation grades, respectively. The standard grades constitute over 90% of the ferritic utilization in tonnage and application. Group 1 (typically Type 409/410L) has the lowest chromium content of the entire group and is also the least expensive while Group 2 (Type 430) is the most widely used family of ferritic alloys with a higher chromium content. Group 2 is credited with greater corrosion resistance and behaves most like austenitic Grade 304. In some applications, this grade is suitable to replace Type 304. Group 3 includes Types 430Ti, 439 and 441; chromium content in this grade may be up to 18 wt%. This grade shows better weldability and formability with stabilization using Nb, Ti, Al, Mo. It even presents better corrosion behavior in most cases than the 304 austenitic grades. Group 4 such as Types 434, 436, and 444 contains higher Mo at concentration level greater than 0.5 wt%. These grades are molybdenum alloyed, for extra corrosion resistance while Group 5 has additional chromium and molybdenum for extra corrosion and scaling resistance. Chromium in this grade is in the range of 25–29 wt% and Mo may be up to 3 wt%.
Ferritic Stainless Steel Grades
Grades and Composition
| UNS No | AISI No. | C | Si | Mn | P | S | Cr | Mo | Ni | Others |
|---|---|---|---|---|---|---|---|---|---|---|
| S40300 | 403 | 0.15 | 0.50 | 1.00 | 0.040 | 0.030 | 11.5/13.0 | – | – | – |
| S40500 | 405 | 0.08 | 1.00 | 1.00 | 0.040 | 0.030 | 11.5/14.5 | – | – | Al 0.10./0.30 |
| S40800 | – | 0.08 | 1.00 | 1.00 | 0.045 | 0.045 | 11.5/13.0 | – | 0.50 | Ti 12xC/1.10 |
| S41008 | 410S | 0.08 | 1.00 | 1.00 | 0.040 | 0.030 | 11.5/13.5 | – | 0.60 | – |
| S43000 | 430 | 0.12 | 1.00 | 1.00 | 0.040 | 0.030 | 16.0/18.0 | – | – | – |
| S43400 | 434 | 0.12 | 1.00 | 1.00 | 0.040 | 0.030 | 16.0/18.0 | 0.75/1.25 | – | – |
| S43600 | 436 | 0.12 | 1.00 | 1.00 | 0.040 | 0.030 | 16.0/18.0 | 0.75/1.25 | – | Nb+Ta 5xC/0.80 |
| S44200 | 442 | 0.20 | 1.00 | 1.00 | 0.040 | 0.035 | 18.0/23.0 | – | 0.60 | – |
Chemical Composition % By Mass Max
Typical Grades
430 stainless steel: it is the most commonly used ferritic steel, which can be used as a replacement for the austenitic steel 304 in some applications. 430 stainless steel has a very low concentration of carbon, nickel, and other alloying elements, which makes 430 stainless cheaper. It has good heat and corrosion resistance and can handle organic acids and nitric acid, and of course, as ferritic stainless steel it has excellent resistance to stress corrosion cracking. 430 has good cutting and forming properties and low work hardening rate, but its ductility is low and it is prone to wear and tear. 430 Stainless Steel is commonly used in washing machine drums, kitchen sinks, cutlery, interior panels, automotive trim parts, Nitric acid tanks, etc.
434 stainless steel: it is the most common non-hardenable ferritic stainless steel, which is high in molybdenum to improve its corrosion and heat resistance, and has properties close to those of 430. As a very widely used ferritic stainless steel, 434 is capable of withstanding temperatures up to 815°C. It is also a good choice for the most demanding applications. Due to its excellent resistance to high temperatures, it cannot be hardened by heat treatment and can only be cold formed like mild steel. The main use of 434 stainless steel is as automotive trim. It is also used in furnace chambers, range hoods, steam iron bases, and chemical processing equipment.
409 Stainless Steel: 409 is a ferritic stainless steel stabilized by the presence of titanium and/or niobium, which maintains good mechanical properties and corrosion resistance at elevated temperatures. 409 has good formability and weldability characteristics and can be welded by a variety of methods including arc welding, resistance spot welding and seam welding. However, 409 welding requires preheating to 150-260 degrees Celsius and post-weld annealing to improve ductility. It is not suitable for aesthetic applications as it tends to develop minor surface rust. 409 stainless steel was originally developed for automotive exhaust systems, but its applications have expanded to include catalytic converters, mufflers, fuel filters, heat exchangers and agricultural machinery.
Ferritic Stainless Steel Characteristics
Magnetic
Ferritic stainless steel is magnetic.
High Thermal Conductivity
Ferritic steels have high thermal conductivity, they are suitable materials for constructing boilers, heat exchangers and other applications involving heat transfer.
Excellent Resistance to SCC
Since ferrites also have excellent resistance to stress corrosion cracking, this enables them to withstand chlorides, high humidity and high temperatures. Ferritic steels are therefore used to resist chloride stress corrosion cracking, corrosion in aqueous media, oxidation at high temperatures and pitting and crevice corrosion in chloride media. Applications include automotive exhaust equipment, radiator tanks, catalytic reactors, dry fertilizer tanks and animal guards.
Low Thermal Expansion
Low thermal expansion of ferritic stainless steel (less expansion than austenitic stainless steel when heated).
High Temperature Oxidation Resistance
The high temperature oxidation resistance of ferritic stainless steel is good (compared with austenitic stainless steel, the surface is not easy to form oxide skin).
Good Creep Resistance
Good creep resistance of ferritic stainless steels containing the stabilizing element niobium (less strain under prolonged stress than austenitic stainless steels).
Machining
Ferritic stainless steels are easier to grind and machine than austenitic stainless steels (cutting and machining of austenitic stainless steels requires specialized tools and high-powered machinery, resulting in greater tool wear and tear).
High Yield Strength
The yield strength of ferritic stainless steel is higher than that of 304 stainless steel (comparable to carbon steel).
Why is Ferritic Stainless Steel Not Easy to Weld?
- High thermal conductivity: Ferritic stainless steels have a higher thermal conductivity than austenitic stainless steels. This means they can dissipate heat more efficiently, and maintaining the heat required for welding can be challenging.
- Rapid Cooling Rates: Ferritic stainless steels tend to experience rapid cooling rates during the welding process, especially in thicker sections. This rapid cooling can lead to the formation of brittle phases, such as martensite, in the heat affected zone, which increases the risk of cracking.
- Sensitivity to hydrogen embrittlement: Ferritic stainless steels are more susceptible to hydrogen embrittlement during welding. The welding process introduces hydrogen into the material, which, if not managed properly, can lead to the formation of hydrogen cracks.
- Sensitivity to thermal cracking: ferritic stainless steel is susceptible to thermal cracking, especially in the presence of impurities such as sulfur and phosphorus. The welding process may introduce these impurities, and appropriate welding techniques (e.g. preheating and control of interlayer temperature) are essential to minimize the risk of thermal cracking.
- Limited ductility at low temperatures: ferritic stainless steels exhibit reduced ductility at lower temperatures and the welding process involves localized heating and cooling. Such temperature fluctuations can lead to embrittlement of welded joints and reduced toughness.
- Formation of chromium carbide: ferritic stainless steels may be prone to the formation of chromium carbide at grain boundaries, especially when the carbon content is high. This can lead to sensitisation of the heat affected zone and reduced corrosion resistance.
Applications
Ferritic stainless steel is nowadays used to make condensers and feedwater heaters for power plants, and heat exchangers for desalination. It can be used to make automotive exhaust, purification and treatment devices and mufflers. It can also be used to make the furnace tube, heat exchanger, reactor and pipeline of refinery normal pressure reduction, catalytic cracking, delayed coking, hydrocracking and other devices.
FAQ
Ferritic stainless steel can rust, although ferritic stainless steel has good corrosion resistance, it can still rust under certain specific conditions. The main components of ferritic stainless steels are chromium (≥10.5%) and iron, sometimes with other elements such as molybdenum, titanium and niobium. Their relatively low cost and stability, due to their absence of nickel, give them an advantage in certain applications.
Ferritic stainless steel is magnetic, the main components of ferritic stainless steel include iron, chromium and a small amount of nickel, and because of the large amount of iron in its chemical composition, ferritic stainless steel is usually magnetic.