Stainless steels are essential materials in modern industry, offering exceptional resistance to rust and corrosion, even in the most demanding environments. This special alloy, which must contain at least 50% iron, has become indispensable in numerous applications, from household appliances to the chemical industry. In this blog, we will explore what makes stainless steels so unique, and review the main types of stainless steels and their uses.
What Makes Stainless Steels Corrosion Resistant?
The key element that provides rust resistance is chromium. Even a small addition of chromium (from 12.5%) in the alloy enables the formation of a protective layer on the steel surface, preventing corrosion. This process, known as passivation, creates a thin oxide layer that protects the material from further chemical reactions. It is important to note that this passive layer depends on the presence of oxygen, as oxidation is not possible without it.
In addition to protecting against rust, chromium also contributes to increased steel strength and improves its thermal resistance, meaning that the steel can withstand higher temperatures without deforming. However, chromium is not the only element playing a crucial role in the properties of stainless steel; other alloying components, such as nickel, molybdenum, titanium, and niobium, also significantly enhance various properties of these steels.
Key Alloying Elements in Stainless Steel
Stainless steels derive their unique properties primarily from the various alloying elements added to the base iron alloy. Here are the key elements that contribute to corrosion resistance, strength, and other important characteristics of stainless steels:
- Nickel (Ni): Nickel increases overall corrosion resistance and improves impact toughness, especially at low temperatures. Notably, with at least 7% nickel content, the ferritic structure of the steel transforms into austenitic, further enhancing corrosion resistance.
- Molybdenum (Mo): Molybdenum greatly increases the steel’s resistance to pitting corrosion, which is particularly important in environments where materials are exposed to concentrated chemicals. It also increases steel strength, especially at elevated temperatures. Molybdenum is also a ferrite-forming element, contributing further to the steel's thermal resistance.
- Copper (Cu): Even at a relatively small percentage (about 1.5%), copper increases the steel’s resistance to certain reducing acids, such as sulfuric acid.
- Silicon and Aluminum (Si + Al): These two elements form ferrites and enhance the steel’s resistance to oxidation and scaling, which is especially important for ferritic steels with low carbon content, such as chromium steels.
- Titanium and Niobium (Ti + Nb): Both elements are added to ferritic and austenitic steel as stabilizers. They are strong carbide-forming elements, improving the steel’s resistance to intergranular corrosion, especially in welds. They stabilize the steel’s microstructure, preventing the formation of chromium carbides, which could otherwise cause localized corrosion.
- Sulfur (S): Sulfur is usually present as an impurity in stainless steel, but in small amounts (up to about 0.35%), it is added to improve the machinability of steel by cutting. However, steel alloyed with sulfur is typically difficult to weld, or it can only be welded under certain conditions.
- Nitrogen (N): Nitrogen stabilizes the austenitic microstructure of steel, similar to carbon, but with a smaller impact on other properties. Nitrogen increases the steel’s strength and makes it more resistant to deformation.
- Manganese (Mn): Manganese increases the steel’s strength and wear resistance, which is especially important in applications where materials are exposed to mechanical stresses.
Main Groups of Stainless Steels
Stainless steels are divided into four main groups based on their microstructural composition. Each group has unique properties that determine their use in various industrial sectors.
Austenitic Steels
Austenitic steels are the most common type of stainless steel. They contain between 17% and 26% chromium and 7% to 26% nickel, with carbon content less than 0.12%. These steels have high corrosion resistance due to their composition, are non-magnetic, and cannot be hardened by heat treatment. They are also highly weldable, making them popular in many industrial applications.
Austenitic steels have low yield stress and high toughness at extremely low temperatures, making them suitable for use in environments exposed to extreme conditions. Their strength can be increased through cold working, although this reduces the material’s elongation and can lead to slight magnetism due to deformation-induced martensite formation.
Common types of austenitic steels:
- AISI 304/Steel Grade 1.4301/V2A: This is the most commonly used austenitic steel. It contains between 17% and 19% chromium, 8.5% to 10.5% nickel, and a maximum of 0.07% carbon. It is widely used in various industries, including the food and pharmaceutical industries, chemical and paper industries, as well as in the production of household appliances, heating and cooling systems, and structural components for use in corrosive environments.
- AISI 304L/Steel Grade 1.4306: This is a version of AISI 304 with lower carbon content (maximum 0.03%), allowing for better weldability without carbide precipitation and improved resistance to intergranular corrosion.
- AISI 321/Steel Grade 1.4541: This steel contains titanium as a stabilizing element, preventing the precipitation of chromium carbides and improving resistance to intergranular corrosion.
- AISI 316/Steel Grade 1.4401/V4A: This type of steel contains 16.5% to 18.5% chromium, 10.5% to 13.5% nickel, and 2.0% to 2.5% molybdenum, which increases resistance to pitting corrosion and reducing acids. Molybdenum improves resistance to acids, while the increased nickel content is crucial for maintaining the austenitic microstructure.
- AISI 316L/Steel Grade 1.4435 BN 2: This version of AISI 316 has lower carbon content (maximum 0.03%) and is ferrite-free, allowing welding without further heat treatment and improved resistance to intergranular corrosion.
- AISI 316Ti/Steel Grade 1.4571: This is a version of AISI 316 stabilized with titanium, allowing welding without subsequent heat treatment and greater resistance to intergranular corrosion.
Steels with grades 1.4404 and 1.4435 are commonly used for pipelines and in applications where materials are exposed to soft water, reducing acids, and various chemicals.
Duplex Stainless Steels
Duplex stainless steels have a characteristic ferritic-austenitic microstructure in a 50:50 ratio, combining the properties of both microstructures. They contain approximately 22% chromium and 5% nickel, along with additions of molybdenum and nitrogen. This combination gives the steel high strength, resistance to stress corrosion cracking, and excellent mechanical properties, especially yield strength and toughness.
Duplex stainless steels are often used in chemical and marine-oriented applications where corrosion resistance is crucial.
Ferritic Steels
Ferritic steels are primarily chromium steels containing between 12.5% and 18% chromium and less than 0.1% carbon. These steels are magnetic, cannot be hardened by heat treatment, but are weldable. They are fairly resistant to corrosion and are commonly used in the production of nitric acid, interior architecture, the automotive industry (e.g., wheel covers and trim strips), and household appliances.
Martensitic Steels
Martensitic steels are chromium steels containing between 12% and 18% chromium and carbon ranging from 0.1% to 1.2%. They may also contain nickel (0.5% to 2.5%) and molybdenum (up to approximately 1.2%). These steels are magnetic and can be improved through appropriate heat treatment, such as quenching and tempering. Due to the relatively high carbon content, martensitic steels are conditionally weldable, meaning preheating or annealing is required before welding.
Steels with lower carbon content (up to approximately 0.4%) are typically used for tempering, while steels with higher carbon content (over 0.4%) are used as quenched. By adding carbon, hardness in the quenched state increases (0.1% C = approximately 40 HRC, 0.9% C = approximately 58 HRC). Martensitic steels are used for mechanically highly loaded machine parts, shafts, and tools and blades requiring high resistance to oxidative acids.
Conclusion
Stainless steels are crucial in many industries where corrosion resistance and mechanical strength are key. With various alloying elements and microstructural compositions, they allow for a wide range of applications, from everyday household items to specific industrial applications. Understanding these properties enables better material selection and contributes to the longer lifespan of final products, which is essential for sustainable industrial practices.
