Steel is a foundational material, defined as an alloy of iron and carbon. The addition of carbon, typically up to 2.1% by weight, transforms pure iron into a material with superior strength and hardness. Since a single composition cannot meet all industrial demands, various types of steel exist to balance properties like strength, flexibility, corrosion resistance, and cost. The percentage of carbon and the inclusion of other elements are the primary factors used to engineer these types, tailoring them for specific applications, from structural beams to surgical instruments.
Carbon Steels
Carbon steels are the most common category, characterized primarily by carbon content with minimal other alloying elements. The properties of these steels are directly manipulated by the percentage of carbon introduced. As carbon content increases, the steel becomes harder and stronger, but its ductility and weldability decrease. This relationship forms the basis for classifying carbon steels into three main groups.
Low-carbon steel, often called mild steel, contains 0.05% to 0.25% carbon. This low content makes the material highly ductile, easy to weld and form, though it possesses low tensile strength. It is used for general structural components, sheet metal, and automotive body panels.
Medium-carbon steel contains between 0.30% and 0.60% carbon, balancing the strength of high-carbon steel and the ductility of low-carbon steel. This material is often strengthened through heat treatment and employed for components like gears, axles, and railway wheels.
High-carbon steel, exceeding 0.60% carbon, is the strongest and hardest of the group. This rigidity makes it suitable for applications requiring superior wear resistance, such as springs, cutting tools, and high-strength wires.
Alloy Steels
Alloy steels intentionally include significant additions of elements beyond carbon, manganese, and silicon to enhance specific mechanical or chemical properties. These additions alter the underlying metallic structure of the iron-carbon matrix, allowing engineers to tailor the material for demanding conditions. Elements are added in specific proportions to improve characteristics like toughness, strength at high temperatures, or ductility.
Nickel is a common addition, used to increase toughness and impact resistance without sacrificing ductility or strength. Nickel-containing alloys are used in cryogenic equipment and aerospace components. Molybdenum is incorporated to increase strength, hardness, and resistance to creep, especially when exposed to elevated temperatures. Molybdenum also improves corrosion resistance and is a component in hot-work tool steels.
Chromium, when added in smaller amounts than required for stainless steel, enhances hardenability, wear resistance, and overall strength. By controlling the proportions of these elements, alloy steels are created for specialized uses, such as high-stress engine parts or durable machinery. This category is distinct because the elements modify mechanical characteristics, not solely for the corrosion resistance achieved by stainless steel.
Stainless Steels
Stainless steels are distinguished by exceptional corrosion resistance, derived from a minimum chromium content of 10.5%. When chromium is exposed to oxygen, it instantly forms a thin, dense layer of chromium oxide on the surface, known as the passive film. This self-healing film acts as a barrier that prevents further oxidation and rust.
Stainless steel is categorized into three main microstructural types. Austenitic stainless steel is the most widely used, containing nickel and typically a high percentage of chromium (e.g., 18% Cr and 8% Ni in Grade 304). It is non-magnetic, highly formable, and offers superior corrosion resistance, making it the standard for food processing equipment and medical devices. Molybdenum addition, as seen in Grade 316, enhances resistance, particularly against chlorides and pitting corrosion.
Ferritic stainless steel contains high chromium but little or no nickel, making it cost-effective and magnetic. This type cannot be hardened through heat treatment, but it offers good resistance to stress-corrosion cracking and is used in automotive exhaust systems and kitchen appliances.
Martensitic stainless steel contains less chromium and higher carbon content, allowing it to be hardened through heat treatment. Though it has lower corrosion resistance than austenitic grades, its high strength and hardness make it ideal for applications requiring a sharp edge or high wear resistance, such as surgical instruments and cutlery.
Tool Steels and Specialty Applications
Tool steels are high-performance materials defined by their demanding use in manufacturing processes like cutting, stamping, and molding. These materials are formulated to possess extreme hardness, exceptional wear resistance, and the ability to maintain structural integrity at elevated temperatures. Though chemically they often fall under the alloy steel umbrella, their classification is driven by the specific performance characteristics required for tool applications.
High-Speed Steel (HSS) is a type of tool steel designed to retain hardness even when the cutting edge becomes red-hot from friction. This property, known as red hardness, is achieved through alloying elements like tungsten and molybdenum, allowing for faster machining speeds. Maraging steel is an ultra-high-strength steel that derives its strength from the precipitation of intermetallic compounds rather than carbon. This unique structure results in high strength and toughness combined with good machinability and weldability, making it suitable for aerospace components and specialized tooling.