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A Comprehensive Guide to Ferritic Stainless Steel
Stainless steel is an alloy made with a combination of chromium, nickel, and other metals that provide excellent resistance to corrosion. Over 100 unique grades of stainless steel are available, with most falling into five broad categories.
- Austenitic Stainless Steel
- Ferritic Stainless Steel
- Martensitic Stainless Steel
- Duplex Grades
- Precipitation-Hardening Stainless Alloys
Stainless steel comes in different types, each with unique characteristics. These properties determine the strengths and weaknesses of each type for specific applications. This article focuses on ferritic stainless steel, providing essential information on how it is classified and what makes it different from other types of stainless steel.
Definition of Ferritic Stainless Steel
Ferritic steel is a stainless steel alloy with more than 12% chromium. It differs from other types of stainless steel in two essential ways: its chemical composition and molecular grain structure. Here are some of the critical things you should know about these variations.
Ferritic stainless steel is classified as a straight chromium, non-hardenable category of stainless alloys with chromium content ranging from 10.5% to 30% and a carbon level under 20%. These steels can’t be hardened by heat treatment and are only slightly hardenable by cold rolling.
The History of Ferritic Stainless Steel
A Canadian engineer, Frederick Mark Becket (1875-1942), industrialized ferritic stainless steel around 1912. He used silicon instead of carbon as a reducing agent in metal production, which made low-carbon ferroalloys and certain steels practical. He discovered a ferrous alloy with 25-27% chromium, the first high-chromium alloy known as heat-resisting stainless steel.
Ferritic stainless steels were discovered early, but their growth was only possible in the 1980s:
- It is possible to achieve extremely low levels of carbon during the steel making process.
- New grades of metal that can be welded were developed.
- Thermomechanical processing eliminated “roping” and “ridging”, which caused inhomogeneous deformation during deep drawing and textured surfaces.
- At a time when there were significant variations in the price of nickel, end-user markets, such as domestic appliances, began demanding less expensive grades of stainless steel with a more stable price. In response, ferritic stainless steel grades became more attractive for specific applications, such as houseware.
Frederick Mark Becket (1875-1942)
Making Process of Ferritic Stainless Steel
Ferritic stainless steel is produced by melting steel and then adding the necessary elements to create the desired alloy. This molten mixture is cast into various shapes, such as slabs, billets, or blooms, using pour-casting or continuous-casting methods. The cast material is then hot or cold rolled or forged into the required shapes, such as plates or sheets. To refine the microstructure and relieve stresses, the material is annealed by heating it to a temperature that dissolves crystal boundaries and then gradually cooling it. Other alloying elements may be added as needed throughout the manufacturing process to achieve the desired composition. The melting and casting processes are typically carried out in an electric arc or induction furnace in an environment that maintains purity, such as an inert atmosphere or a vacuum. Finally, the material is subjected to various final manufacturing processes.
Appearance of Ferritic Stainless Steel
Ferritic alloys are a type of stainless steel with a shiny silver finish. When polished flat, they can reflect light like a mirror without distorting its color. In everyday use, ferritic components have a slightly textured or matte surface or a highly polished and reflective finish.
Chemical Composition of Ferritic Stainless Steel
Please find below the chemical compositions of different elements in Ferritic stainless steel arranged in a tabular format for your reference.
| Element | Percentage |
| Iron (Fe) | 65-72% |
| Chromium (Cr) | 16 – 24% |
| Carbon (C) | < 0.08% |
| Nickel (Ni) | < 2.0% |
| Manganese (Mn) | < 1.0% |
| Silicon (Si) | < 1.0% |
| Phosphorus (P) | < 0.04% |
| Sulfur (S) | < 0.03% |
| Nitrogen (N) | < 0.1% |
| Molybdenum (Mo) | < 2.0% |
| Titanium (Ti) | < 0.1% |
| Copper (Cu) | < 0.5% |
| Selenium (Se) | < 0.5% |
| Aluminum (Al) | < 0.5% |
| Vanadium (V) | < 0.2% |
Ferritic Stainless Steel Grades
Different alloys are utilized in ferritic steel grades depending on the specific application. Ferrites can be classified into five major groups.
Group 1 (Type 409/410 L)
The first group of stainless steels is characterized by having the lowest chromium content and the lowest price. Initially intended for automotive exhaust systems, type 409 is now widely used for catalytic converter casings. Type 410 L is used primarily for LCD monitor frames, containers, and buses.
Group 2 (Type 430)
Type 430 is a popular type of ferritic steel containing chromium in the 16% to 18% range. This makes it more resistant to corrosion compared to group 1 ferrites. Type 430 is commonly used to manufacture washing machine drums, kitchen sinks, and other kitchenware. Due to its similar characteristics, it can also be used as a substitute for type 304 in many applications, such as indoor panels.
Group 3 (Type 430TI, 439, 441)
Group 3 ferritic steels have a chromium content between 17.5% and 18.5%. They possess excellent weldability and formability, sometimes even superior to 304 steels. These properties make them a wonderful substitute for a variety of applications currently using 304 steels. Some of these applications include exhaust systems, sinks, and exchange tubes.
Group 4 (Type 434, 436, 444)
Group 4 ferritic steel types contain molybdenum, increasing their corrosion resistance. These steels are commonly used in hot water tanks and outdoor applications exposed to corrosive environments.
Group 5 (Type 446, 445)
The steels in the last group are typically used in environments that are highly prone to corrosion. These types of ferritic steel contain a high proportion of chromium and molybdenum, which gives them outstanding corrosion resistance properties. In fact, their performance can be compared to that of titanium metal, which makes them the perfect choice of steel for applications such as heat exchangers, water heaters, and boilers.
The Properties of Ferritic Stainless Steel
Mechanical Properties (cold rolled) of Ferritic Stainless Steel
| ASTM A240 | EN 10088-2 | ||||||
| — | UTS (MPa, min) | 0.2% yield stress (MPa, min) | Elongation (%, min) | — | UTS (MPa) | 0.2% yield stress (MPa, min) | Elongation (%, min) |
| 9 | 390 | 170 | 20 | 1.4512 | 380 – 560 | 220 | 25 |
| 410 | 415 | 205 | 20 | 1.4003 | 450 – 650 | 320 | 20 |
| 430 | 450 | 205 | 22 | 1.4016 | 450 – 600 | 280 | 18 |
| 439 | 415 | 205 | 22 | 1.451 | 420 – 600 | 240 | 23 |
| 441 | 415 | 205 | 22 | 1.4509 | 430 – 630 | 250 | 18 |
| 434 | 450 | 240 | 22 | 1.4113 | 450 – 630 | 280 | 18 |
| 436 | 450 | 240 | 22 | 1.4526 | 480 – 560 | 300 | 25 |
| 444 | 415 | 275 | 20 | 1.4521 | 420 – 640 | 320 | 20 |
Physical Properties of the most common Ferritic Stainless Steel
| AISI / ASTM | Young’s Modulus(GPa) | Density(g/cm3) | Electrical Resistance(μΩ·m) |
| 409 / 410 | 220 | 7.7 | 0.58 |
| 430 | 220 | 7.7 | 0.6 |
| 430Ti / 439 / 441 | 220 | 7.7 | 0.6 |
| 434 / 436 / 444 | 220 | 7.7 | 0.6 |
| 447 | 220 | 7.7 | 0.62 |
Thermal Properties of the most common Ferritic Stainless Steel
| AISI / ASTM | Specific Heat 0…100 °C(J/(kg·K)) | Thermal Conductivity at 20 °C (W/(m·K)) | Thermal Expansion 0…600 °C (10−6/K) |
| 409 / 410 | 460 | 25 | 12 |
| 430 | 460 | 25 | 11.5 |
| 430Ti / 439 / 441 | 460 | 25 | 11.5 |
| 434 / 436 / 444 | 460 | 23 | 11.5 |
| 447 | 460 | 17 | 11 |
Main Properties Of Ferritic Stainless Steel
Although ferritic steels cannot be hardened or strengthened through heat treatment, they can be softened by undergoing an annealing process that involves heating and cooling the metal. Ferritic grades have advantageous mechanical properties but are less strong or corrosion-resistant than austenitic steels. Ferritic steels are generally easy to weld, but some grades have limitations that restrict their use to thinner gauges.
Standard ferritic stainless steels are less expensive than austenitic grades because they lack nickel and have lower amounts of chromium. SS430 denotes the most commonly used ferritic stainless steel. Due to its low chromium content, SS409 is less resistant to corrosion. On the other hand, SS439 is more corrosion-resistant and stable at high temperatures. Ferritic stainless steels offer a range of beneficial properties depending on the alloy.
Corrosion Resistance
Stress corrosion cracking (SCC) is a common type of steel damage that occurs due to both tensile stress and a corrosive environment. Austenitic stainless steels are especially prone to SCC when exposed to chlorides. On the other hand, ferritic steels have microstructures that make them highly resistant to SCC. As a result, they are an excellent choice for use in environments that contain chlorides.
Ductility And Formability
Carbon is responsible for providing steel with its hardness, but at the same time, it makes it more brittle and less ductile. Ferritic steel, which contains low levels of carbon, as little as 0.03 percent, is known for its excellent ductility, which allows it to be shaped extensively without any risk of weakening.
The low carbon content in ferritic steel also makes it highly formable, which means it can be transformed into various shapes without cracking or necking.
However, the benefits of ferritic steel’s low carbon content come with inevitable trade-offs. For instance, ferritic steel cannot be hardened through heat treatment, and it may exhibit specific problems during welding, such as unwanted cracks along the heat-affected zone.
Low Thermal Expansion
Ferritic steels possess a crucial advantage in that they have a naturally low coefficient of thermal expansion. This means that as they are heated, they will expand less and retain their fixed dimension more readily. This property is especially useful for metals used in high-temperature applications.
High Thermal Conductivity
Ferritic steels demonstrate excellent thermal conductivity, allowing heat to move efficiently through them. As a result, they are a popular choice for furnace and boiler heat exchangers and other applications involving heat transfer.
Resistance to Oxidation
Ferritic stainless steel is known for its exceptional oxidation resistance, especially at high temperatures. This is because a layer of chromium-oxide forms on the surface of the steel, which acts as a protective film. Manufacturers can enhance this resistance further by adding aluminum or silicon during the production of ferritic steel.
Magnetism
Ferritic stainless steel contains a certain amount of iron, which makes it magnetic. This type of steel is one of the most magnetic materials due to its high iron content, making it more susceptible to magnetism.
The degree of magnetism in stainless steel is directly proportional to the amount of ferrite, which is a form of iron oxide. Therefore, the more ferrite present, the stronger the magnetic properties of the steel.
However, although ferritic steel is magnetic, it may not always be suitable for applications requiring strong magnetic fields, such as MRI scanners. In such cases, austenitic steel is used instead, as it is non-magnetic.
Weldability
Ferritic stainless steel can be welded, but it is more challenging than welding other types of stainless steel. This is because ferritic steel has a higher carbon content and lower melting point, making welding more difficult. Furthermore, ferritic steel is more prone to cracking during welding due to its low ductility.
To minimize the risk of cracking, it is crucial to use the correct welding technique, select a filler material with low carbon content, and preheat the workpiece. By following the appropriate welding techniques, using the proper filler material, and preheating the workpiece, you can ensure a successful weld when working with this specific type of steel.
Common Forms of Ferritic Stainless Steel
- Stainless steel fittings
- Stainless steel tube
- Stainless steel plate
- Stainless steel round bar
- Stainless steel sheet metal
- Stainless steel square bar
- Stainless steel pipe
Applications of Ferritic Stainless Steel
- Automotive:Various ferritic stainless steel grades are commonly used in automotive exhaust systems, mufflers, and catalytic converters.
- Automotive Trim: Ferritic stainless steel alloys are suitable for automotive trim and decorative elements due to their appearance and excellent formability.
- Architecture: Architects use these alloys for decorative trims, cladding, and interior design due to their aesthetic appeal, strength, and affordability.
- Appliances:Ferritic stainless steel is used in kitchen appliances such as stoves, ovens, and refrigerator doors for its aesthetics, durability, and corrosion resistance.
- Heat Exchangers: Ferritic stainless steel is suitable for heat exchanger tubing in moderate and mildly corrosive industrial processes due to its excellent resistance to scaling and corrosion at high temperatures.
- Industrial Equipment: It is commonly used in industrial applications that involve moderate exposure to corrosive environments, such as chemical processing equipment, heat exchangers, and structural components.
- Food Processing: Ferritic stainless steel is commonly used in food-related industries due to its corrosion resistance, stain resistance, and ease of cleaning.
- Energy Sector: Boiler tubing and steam generator tubes are vital in power plants because they require resistance to heat and oxidation during combustion.
- Welded Structural Roles: Ferritic alloys are utilized in welded structures where affordability and high-quality mechanical properties are advantageous.
- Magnetic Applications:Ferritic stainless steel’s magnetic properties are helpful in specific electronic and electromagnetic applications.
Advantages & Disadvantages
Advantages
If you select ferritic stainless steel, it will provide numerous benefits for your fabrication projects and applications.
- High Strength:Ferritic materials are highly valued for their exceptional strength, making them ideal for applications that require superior durability.
- Corrosion Resistance: Ferritic stainless steel typically exhibits good resistance to corrosion in moderately corrosive environments.
- Oxidation Resistance:Its high oxidation resistance makes it ideal for applications requiring protection against environmental conditions.
- Cost-Effective:Ferritic is a lower-cost alternative to other stainless steel grades.
- Low Maintenance:Ferritic stainless steel is low maintenance and requires minimal cleaning, making it an ideal option for applications where upkeep is important.
- Temperature Tolerance:It is highly effective in applications that require extreme temperatures due to its excellent temperature tolerance.
- Formability:Formability and ease of fabrication are essential factors that make it versatile in various manufacturing processes.
- Magnetism: Magnetic properties can provide significant advantages in specific electronic and electromagnetic applications.
Disadvantages
Although ferritic steel has several benefits, it has a few drawbacks.
- Ferritic steel does not harden with heat treatment. Parts made from this steel remain relatively soft, unable to withstand high wear and tear or heavy loads.
- Compared to austenitic stainless steel, the corrosion resistance of this material is relatively low.
- It is important to note that there is a susceptibility to sensitization and intergranular corrosion, which may cause concern.
- The mechanical strength and ductility of non-austenitic grades are generally lower than those of austenitic.
- The material’s formability is limited due to its low ductility.
- During welding, materials can become brittle and sensitive, which poses a challenge.
Comparison Between Ferritic Stainless Steel and Others
Ferritic Stainless Steel & Austenitic Stainless Steel
Ferritic and austenitic stainless steel are two major types of stainless steel. Ferritic stainless steel is composed of a ferrite matrix with a body-centered-cubic (BCC) crystal structure. It is magnetic and has excellent ductility and formability properties.
On the other hand, austenitic stainless steel comprises austenite with a face-centered-cubic (FCC) crystal structure. It has non-magnetic properties and higher strength.
How do they differ from each other?
Both ferritic and austenitic stainless steels have different crystal structures and magnetic properties. Their main differences lie in the applications for which they are best suited.
Ferritic stainless steel is known for its strength and ductility, making it ideal for use in automotive parts, kitchen utensils, and pipelines. When it comes to chemical processing equipment, austenitic stainless steel is a great option thanks to its exceptional strength and corrosion resistance, such as in kitchen appliances and surgical instruments.
Despite their differences, both types of stainless steel have unique applications and benefits.
Ferritic Stainless Steel & Martensitic Stainless Steel
Ferritic stainless steel has a body-centered cubic (BCC) crystal structure and provides excellent corrosion resistance in moderate environments. On the other hand, martensitic alloys have a body-centered tetragonal (BCT) crystalline matrix. Both ferritic and martensitic stainless steels exhibit varying degrees of high magnetic permeability, depending on their alloy composition and work hardening and manufacturing processes.
Martensitic stainless steels can usually be heat/quench hardened and tempered, whereas ferritic alloys are not heat-treatable. Ferritic stainless steels also tend to be more corrosion-resistant than martensitic alloys, although this can vary depending on the specific alloys being compared.
Finally, martensitic alloys are stronger and more abrasion-resistant than ferritics, but this, too, depends on the particular alloys being compared and the manufacturing processes used.
Regular Maintenance
To prevent rust on steel, perform regular maintenance. Here are step-by-step instructions to help you maintain the material.
- Regular cleaning is essential. Use a soft cloth, warm water, and mild detergent to remove debris from the material surface.
- Inspect the steel for any signs of rust or corrosion and address it immediately.
- Prevent rust on ferritic steel by applying a lubricant, such as oil or wax.
- If the steel requires it, apply a rust-inhibiting primer.
- If the steel comes into contact with water, dry it with a clean cloth afterward.
- It is crucial to store ferritic stainless steel in a cool and dry area, free from moisture and extreme temperatures, to hold back corrosion.
- Inspect the steel regularly for any cracks, dents, or other damage and repair it immediately.
Summary
You now understand the significance of ferritic stainless steel, which is generally used in various industries such as food processing, automotive, and chemical. This knowledge will help you decide when and where to utilize it. Additionally, you have learned how to maintain this material to ensure it lasts longer appropriately.
FAQs
What is nitronic 50 hs?
Nitronic 50 is a premium austenitic stainless steel with better corrosion resistance than the 300 or 400 stainless steel series. Due to its superior strength, excellent corrosion resistance, and low magnetic permeability, it is highly preferred for high-risk applications such as medical implants.
What is the difference between 430 and 430F stainless steel?
430F is easier to machine than 430 stainless steel due to its higher sulfur and molybdenum content, but it’s more difficult to form.
