Stainless steel: what it is and what it is used for

Stainless steel is a ferrous alloy known for its high corrosion resistance, a distinguishing characteristic due to the significant presence of chromium, which must be at least 10.5 percent. This chemical element forms a passive layer of chromium oxide on the surface of the metal that is extremely thin, adherent and not visible, effectively protecting the underlying iron from further oxidation and corrosion.

Physical and mechanical properties

Stainless steel is known for its exceptional physical and mechanical properties, making it an ideal material for a wide variety of industrial, commercial, and domestic applications.

Stainless Steel Physical Properties

  1. Corrosion Resistance: One of the most remarkable properties of stainless steel is its ability to resist corrosion. The presence of chromium (Cr) in the alloy forms a passive chromium oxide layer that protects the underlying material from further environmental and chemical damage.
  2. Resistance to Extreme Temperatures: Stainless steel alloy maintains its mechanical strength even at very high or very low temperatures, making it suitable for applications operating under extreme thermal conditions.
  3. Density: Stainless steel has a relatively high density, about 8 g/cm³, which varies slightly depending on the specific composition and heat treatment applied.
  4. Thermal and Electrical Conductivity: Generally, stainless steel has low thermal conductivity compared to other metals such as copper or aluminum, which means it is not the best conductor of heat. Its electrical conductivity is also low, about one-sixth that of copper.

Stainless Steel Mechanical Properties

  1. Hardness: Stainless steels can vary significantly in hardness depending on their grade and heat treatment. Martensitic steels, for example, are among the hardest and can be further hardened by heat treatment.
  2. Elasticity and Ductility: Many grades of stainless steel, especially austenitic grades, exhibit excellent ductility, which allows them to be machined, bent or deformed without breaking. This makes stainless steel ideal for processes such as cold forming.
  3. Tensile Strength: Stainless steel has high tensile strength, which can be improved through cold working processes and heat treatments. This property is crucial for applications that require a material that can withstand heavy loads without yielding.
  4. Resilience: The resilience of a material indicates its ability to absorb mechanical energy without suffering permanent damage. Many stainless steels, especially austenitic ones, are known for their high resilience, making them ideal for applications subject to shock or vibration.
  5. Wear Resistance: Although stainless steel is not the hardest of metals, its wear resistance is generally good, especially in those grades that have been optimized for this property by the addition of carbon (C) and other alloying elements.

The combined properties of corrosion resistance, hardness, resilience, and machining capabilities make stainless steel an extremely versatile and indispensable material in many industries. Its ability to maintain performance in harsh environmental conditions and withstand severe mechanical and thermal stresses ensures that it will continue to be a popular choice for engineers and designers.

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Classification and Types of Stainless Steel

Stainless steel can be distinguished into several categories based on its chemical composition and structural characteristics.

Classification Based on Crystal Structure

Stainless steel alloy classification is based mainly on the crystalline structure of the material. Austenitic steels, containing chromium (Cr) and nickel (Ni), are the most common. This group includes types such as 304 stainless steel, which has at least 18 percent chromium and 8 percent nickel. These steels are known for their excellent corrosion resistance and excellent formability, are non-magnetic, and retain their toughness even at low temperatures. They are mainly found in kitchen utensils, biomedical equipment, and architectural components.

Ferritic steels, containing chromium and little or no nickel such as type 430 with about 17 percent chromium, are magnetic and offer good corrosion resistance at a lower cost than austenitics. However, they have lower tensile strength and resilience. They are commonly used in household appliances, automotive components and architectural coatings.

Martensitic steels, similar to ferritics but with a higher carbon (C) content, such as types 410 and 420, are magnetic and can be hardened through heat treatments. They offer good wear resistance but have lower corrosion resistance than austenitics. These steels are suitable for cutting tools, water treatment plants, and some engineering applications.

Duplex steels, combine the characteristics of austenitic and ferritic steels; in fact, they contain a balanced proportion of austenite and ferrite, making them particularly strong and versatile. The presence of chromium (Cr) and nickel (Ni) in significant proportions gives these steels excellent corrosion resistance, comparable or superior to that of austenitics, while the ferrite in their structure provides good weldability and high resistance to corrosion stress, especially in chloride-containing environments.

Classification Based on Chemical Composition

Another way of classification is based on the chemical composition of the stainless steel alloy, which determines corrosion resistance and other mechanical properties. Chromium stainless steels contain at least 10.5 percent chromium and a maximum of 1.2 percent carbon. The chromium forms a chromium oxide layer that protects the underlying metal from corrosion. Chromium-nickel stainless steels, in addition to chromium, include nickel, which further improves corrosion resistance and machinability, making them more ductile and formable than chromium-only steels.

Hardenable precipitant stainless steels contain elements such as aluminum (Al), copper (Cu) and niobium (Nb) that allow heat treatment. They can be hardened and strengthened more significantly than other types of stainless steel, making them ideal for high-strength structural components.

Applications of Stainless Steel

Stainless steel finds use in a wide range of industries because of its unparalleled qualities. In the food industry, it is chosen to build equipment that requires easy cleaning and corrosion resistance. In the medical industry, it is used to manufacture surgical instruments and implants, benefiting from its ability to sterilize without risk of corrosion. The automotive industry uses stainless steel for components that need rust resistance, while in the construction field it is often chosen for facades and interior structures for its long life and aesthetic appeal.

Stainless Steel Machining

Stainless steel is an extremely versatile material that can be processed in many different ways to suit specific applications and requirements.

Cutting stainless steel can be accomplished through various methods depending on the thickness of the material and the precision required. Mechanical cutting, using saws and shears, is effective for coarse cuts or when surface finish is not a primary concern. Laser cutting provides very high precision cutting, ideal for complex shapes and fine cuts while minimizing thermal damage to surrounding areas of the material. Plasma cutting is suitable for cutting greater thicknesses of stainless steel, although it can leave a thermally altered area along the cut.

Stainless steel alloy can be bent or curved into a variety of shapes, ranging from simple angles to more complex configurations. Bending machines apply a force to a specific point on the metal to achieve the desired angle, while bending machines are used to create bends and arcs, ideal for piping and architectural structures.

Welding stainless steel requires specific skills to avoid problems such as intergranular corrosion or deformation due to heat. TIG (Tungsten Inert Gas) welding is one of the most common methods of welding stainless steel, offering a high degree of control to achieve clean and precise welds. MIG (Metal Inert Gas) welding, which is faster than TIG welding, is suitable for projects that require high productivity.

Polishing is metalworking, which is essential to achieve an aesthetically pleasing surface finish and to reduce roughness, thereby improving corrosion resistance. Mechanical polishing employs abrasive wheels or belts to grind the surface, while chemical polishing uses chemical solutions to grind and clean the surface, often used to prepare the material for critical applications such as those in food or pharmaceuticals.

Rolling reduces material thickness and improves surface finish, while drawing is used to reduce the diameter of wires and bars, improving surface finish and dimensional accuracy.

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Stainless Steel: Ceramic Coatings for Metals

ATP Europe offers multiple ceramic protective coatings for metals that significantly improve the wear, corrosion, and heat resistance of stainless steels. These coatings are particularly useful in extreme conditions where corrosives and high temperatures can rapidly degrade metals.

  • Enhanced Protection: They form a resistant barrier against chemical corrosion and mechanical wear and tear.
  • Extension of Useful Life: By protecting the metal substrate, coatings extend the life of instruments and components.

Energy Efficiency: They reduce the need for frequent maintenance and replacement of components, thus contributing to greater energy efficiency.

Stainless Steel: Borosilicate Glass Lubricants for Extrusion

Borosilicate glass lubricants are a revolutionary solution offered by ATP Europe for the metal extrusion industry. These lubricants are particularly effective in reducing friction and wear during the stainless steel extrusion process, thus ensuring higher end-product quality and lower operating costs.

  • Friction Reduction: They minimize the friction between the mold and metal, reducing the risk of adhesion and mold wear.
  • Improved Product Quality: They ensure a smoother and more uniform surface finish, improving the aesthetic and structural properties of the product.
  • Extended Tool Useful Life: Protect extrusion tools from premature wear and tear.

Stainless Steel Alloy: Paints for Markings and Stencils

ATP Europe’s marking and stencil paints are designed for identification and decoration applications on stainless steel. These paints not only provide clear and durable marking, but also offer additional resistance against environmental factors that could affect legibility over time.

  • Durability: Resist fading, abrasion and chemicals.
  • Versatility: Suitable for a variety of marking techniques, including hot stamping and laser engraving.
  • Safety and Compliance: Formulated to be safe and compliant with current environmental standards.

If you are interested in our stainless steel protection and marking products, please do not hesitate to contact us. We are always available to show you the best protective coating for your needs!