Ferritic Stainless Steel: Definition, Composition, Properties, Processing, Grades, Applications, and More

Stainless steel is a flexible material known for its great resistance to rust and long-lasting quality. It comes in various forms, including austenitic, ferritic, martensitic, duplex, and precipitation-hardened stainless steel.

Ferritic stainless steel is different from austenitic steel. It has a body-centered cubic (BCC) arrangement and has less nickel. Ferritic stainless steel is also attracted to magnets and is used in situations where these characteristics are advantageous.

This article will focus on ferritic stainless steel, covering its definition, composition, key properties, processing methods, and common grades. Our goal is to give you a clear overview of when and why ferritic stainless steel might be the right choice for your needs.

What Is Ferritic Stainless Steel?

Ferritic stainless steel is a stainless steel with a body-centered cubic (BCC) crystal arrangement that is similar to pure iron at room temperature. The composition typically contains 10.5% to 18% chromium (but a specific grade can be as high as 30%) with little or no nickel and low carbon content. 

It is one of the five stainless steel classes, which is widely used in automotive exhaust systems, household appliances, and industrial equipment. Common grades include 430, 409, and 439. It is magnetic and cannot be hardened through heat treatment.

Chemical Composition of Ferritic Stainless Steel

Ferritic stainless steel is mainly composed of iron, chromium, and carbon. Iron is the main element, constituting the largest portion of the composition. Chromium, making up 10.5-30%, is the main additive that gives corrosion resistance. The carbon content is low, typically less than 0.1%, which keeps the steel soft and malleable. Other elements like molybdenum, aluminum, and titanium might be present in smaller amounts to improve specific properties.

Iron, Fe	Chromium, Cr	Carbon, C	Manganese, Mn	Silicon, Si	Phosphorus, P	Sulfur, S	Nickel, Ni	Molybdenum, Mo
Balance	10.5-30%	≤ 0.12%	≤ 1.00%	≤ 1.00%	≤ 0.040%	≤ 0.030%	≤ 0.50%	≤ 1.00%

Properties of Ferritic Stainless Steel

The following 3 features are the most prominent characteristics of ferritic stainless steel.

• Corrosion Resistance: Ferritic stainless steel resists corrosion well, especially in chloride environments, due to its 11-30% chromium content.
• Magnetism: It is magnetic, unlike austenitic stainless steel, making it useful in induction heating.
• Formability: It has good formability, similar to carbon steel, ideal for complex parts.

These characteristics are tied to ferritic stainless steel’s chemical, physical, and mechanical properties, which ensure its reliability and effectiveness. Let’s explore these in detail.

Chemical Properties

Ferritic stainless steel’s chemical properties involve its behavior in chemical environments,  most notably corrosion, and oxidation resistance, with the main properties being:

Chemical Property	Performance	Description
Corrosion Resistance	Mild reaction.	Good resistance to oxidation and corrosion.
Resistance to Stress-Related Cracking (SCC)	Very low reaction.	Higher resistance compared to austenitic stainless steels, especially in chloride environments.
Resistance to localized corrosion and crevice corrosion	Moderate reaction.	Moderate resistance in chloride environments.
Low Hydrogen Embrittlement Susceptibility	Minimal reaction.	Lower susceptibility, suitable for hydrogen exposure.
Chemical Stability	No reaction.	Stable in various chemical environments.
Oxidation Resistance	No reaction.	Strong resistance to rusting at high temperatures.

Physical Properties

Ferritic stainless steel’s physical properties are key to selecting it for engineering and industrial uses, particularly in heat exchange and magnetic applications. Next, please review the important physical characteristics in the table below:

Physical Property	Specific Values	Description
Density	7.7-8.1 g/cm³	Relatively high density, providing stability in structural applications.
Thermal Conductivity	16-25 W/m·K	Good heat transfer ability, making it suitable for heat exchange uses.
Thermal Expansion	10-12 µm/m·K	Lower thermal expansion compared to austenitic stainless steels, reducing the risk of thermal distortion.
Heat Resistance	Up to 800°C	Keeps strength and resists rusting at high temperatures.
Electrical Resistivity	600-750 nΩ·m	Moderate electrical resistance, is used in situations where electrical conductivity is not essential.
Magnetic Permeability	500-2000 H/m	High magnetic permeability, ideal for magnetic applications.
Melting Point	1425-1510°C	High melting point, ensuring durability under high-temperature conditions.

Mechanical Properties

Ferritic stainless steel’s physical traits determine its strength and durability in building applications. Please read the table below for key mechanical properties:

Mechanical Property	Specific Values	Description
Tensile Strength	400-600 MPa	The highest force it can handle while being stretched or pulled.
Yield Strength	250-450 MPa	The stress at which it starts to change shape permanently.
Elongation	20-30%	The ability to stretch before breaking.
Hardness	150-200 HB	Resistance to indentation or scratching.
Modulus of Elasticity	200 GPa	The ratio of force to stretching in elastic deformation.
Impact Resistance	Moderate	Ability to absorb energy and withstand impact.
Magnetic Property	High	Strong magnetic properties, making it suitable for magnetic applications.
Toughness	Moderate	The ability to absorb energy and deform permanently without breaking.
Formability	Good	Ease of forming into desired shapes.
Weldability	Good	It can be welded without losing strength or durability.
Fatigue Resistance	Moderate	Resistance to damage under cyclic loading.

Want to know about the characteristics of austenitic steels? Please read this austenitic overview article.

Processing of Ferritic Stainless Steel

Unlike austenitic, martensitic, duplex, and precipitation-hardening stainless steel, ferritic stainless steel requires specific handling during production. Below, we’ll explore these unique processing methods and the issues that require attention.

1. Forming

Ferritic stainless steel starts with a straightforward melting and casting process. It is then easily formed, much like carbon steel. Unlike austenitic stainless steel, ferritic steel doesn’t harden significantly during forming, so it doesn’t require frequent annealing.

2. Heat Treatment

Ferritic stainless steel typically undergoes solution treatment and annealing to relieve stress and improve ductility. Unlike martensitic steel, it is air-cooled rather than quenched. It cannot be strengthened by heating like precipitation-hardening steels.

Care is necessary to avoid 475°C embrittlement, which can occur with prolonged exposure, leading to reduced toughness.

3. Cold Rolling

Cold-rolling ferritic stainless steel helps maintain its magnetic properties, unlike austenitic steel, which may lose magnetism during this process.  The strength and hardness of ferritic steel increase significantly after cold rolling. 

However, its formability is relatively poor, making it prone to wrinkling and deformation during the process. To ensure good forming quality, rolling parameters such as reduction and rolling speed must be carefully controlled.

4. Machining

Working with ferritic stainless steel is usually easier than working with austenitic stainless steel. It experiences less galling, making it more manageable for various machining operations. Its stable microstructure contributes to consistent machining performance.

5. Welding

Welding ferritic stainless steel requires care to prevent grain growth and brittleness. Common methods include TIG, MIG, and spot welding. Unlike austenitic steel, ferritic steel may need post-weld heat treatment to restore toughness. 

Its lower thermal expansion helps reduce welding distortion. To prevent sensitization, it’s important to weld at lower temperatures and minimize heat input.

Phase Transformation of Ferritic Stainless Steel

Phase transformation in ferritic stainless steel, vital in welding and heat treatment, is mainly driven by its alloy composition, especially chromium and other elements. This section covers the key aspects of these transformations.

Stable Ferrite Phase:

Ferritic stainless steel primarily remains in the body-centered cubic (BCC) ferrite phase. This phase is stable from room temperature up to about 912°C.

Chromium Content and Phase Stability:

• High Chromium Content (11-30%): The steel typically resists transforming into other phases, even at high temperatures.
• Phase Transformation Temperature:
  • Typically occurs above 950°C.
  • At this temperature, ferrite may turn into austenite, especially when influenced by certain alloying elements.

Effects of Chromium Levels:

• Around 13% Chromium: The steel shifts from ferrite to austenite and back to ferrite as it cools.
• Rapid Cooling with Carbon: This can form martensite, which can later be tempered back to ferrite.
• Over 17% Chromium: The steel remains in the ferritic phase at all temperatures.
• Above 25% Chromium: The σ phase may form, leading to brittleness at room temperature.

Sensitization in Ferritic Stainless Steel

Sensitization happens when ferritic stainless steel is exposed to 450°C to 850°C. In this range, chromium carbides form at grain boundaries, reducing chromium and leading to lower corrosion resistance. This makes the steel more prone to intergranular corrosion. Preventing this requires careful heat control during welding and heat treatment.

What Are the Advantages and Disadvantages of Ferritic Stainless Steel?

Although ferritic stainless steel has unique advantages that make it suitable for specific applications, it also has certain drawbacks due to its inherent properties. These disadvantages can be addressed through various methods. Understanding these pros and cons is crucial for choosing the right material for your needs.

Advantages of Ferritic Stainless Steel

• Corrosion Resistance: Ferritic stainless steel offers good resistance to oxidation and corrosion, particularly in mild environments.
• Magnetic Properties: It retains magnetic properties, which can be advantageous in certain applications like induction heating.
• Formability: It is easy to shape and can be worked like carbon steel, making it suitable for complex designs.
• Strength and Toughness: Ferritic stainless steel offers a good mix of strength and durability for many uses.
• Cost-Effective: Usually, it costs less than austenitic stainless steels because of lower alloying expenses.

Disadvantages and How to Address Them

• Lower Toughness: Ferritic stainless steel can be less tough at lower temperatures. Improve toughness with alloyed grades or post-weld heat treatment.
• Limited Heat Treatability: It cannot be heat-treated for higher strength. Materials like austenitic steels or martensitic steels are used for such applications.
• Susceptibility to Sensitization: It may become sensitized at specific temperatures. Avoid these temperatures and control heating and cooling rates carefully.
• Weldability Issues: Welding can be difficult due to potential brittleness. Use the right welding methods and apply heat treatment after welding to handle these problems.

Common Grades and Applications of Ferritic Stainless Steel

Ferritic stainless steel includes various grades, each with particular qualities for different applications. Knowing these grades and their applications helps in choosing the right material for various needs.

Ferritic Steel Grade	Application Area	Specific Applications
304	General Use	Kitchen equipment, automotive trim, appliances
430	Home Appliances	Dishwasher interiors, oven linings, kitchen sinks
446	High-Temperature	Furnace parts, heat exchangers, exhaust systems
409	Automotive	Exhaust systems, catalytic converters, automotive trim
444	Corrosive Environments	Chemical processing equipment, marine applications
409L	Automotive & Industrial	Exhaust systems, automotive components, industrial machinery
430F	Machining	Precision shafts, screws, bolts, and other fasteners
441	Automotive & Aerospace	Exhaust systems, engine components, aerospace applications

What You Might Also Concern

Here, we cover other important aspects and frequent questions to help you make informed choices about ferritic stainless steel.

How Do You Identify Ferritic Stainless Steel?

Ferritic stainless steel can be recognized by its magnetic traits and its body-centered cubic (BCC) crystal arrangement. It usually contains chromium and has less nickel compared to austenitic stainless steel.

Is 400 Series Stainless Steel Ferritic?

Yes, the 400 series stainless steels are usually ferritic. Grades like 409 and 430 are common examples of ferritic stainless steel.

Is 300 Series Stainless Steel Ferritic?

No, the 300 series stainless steels are mainly austenitic. They contain higher amounts of nickel and chromium, which result in an austenitic structure rather than ferritic.

What is the Difference Between Ferritic and Austenitic Stainless Steel?

Ferritic stainless steels have a body-centered cubic (BCC) structure, are magnetic, and have lower nickel content. Austenitic stainless steels have a face-centered cubic (FCC) structure, are non-magnetic, and contain more nickel and chromium.

Is Ferrite Stronger Than Austenite?

Ferritic stainless steels are generally less strong than austenitic steels but offer better resistance to stress corrosion cracking. Austenitic stainless steels usually provide higher strength and toughness.

Summary & Furthermore

This article briefly explains the definition, composition, properties, grades, applications, and other important aspects of ferritic stainless steel. To learn more about stainless steel or other steel types, check out our blog or contact our metal experts.

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