Terminology of steel material properties
The terminology of steel material properties is a professional vocabulary that describes the various characteristics of steel materials. Accurately understanding these terms is of great significance for the selection, processing, and application of materials. The properties of steel materials mainly include mechanical properties, physical properties, chemical properties, and process properties. Each type of property has a corresponding terminology system. Mechanical properties refer to the characteristics of steel materials under the action of external forces. Common terms include strength, plasticity, hardness, toughness, fatigue strength, etc.; physical properties include density, melting point, thermal conductivity, electrical conductivity, thermal expansion, etc.; chemical properties mainly refer to corrosion resistance, oxidation resistance, etc.; process properties refer to the properties of steel materials during processing, such as machinability, weldability, forgeability, etc. Mastering the definitions and connotations of these terms will help to correctly evaluate the applicability of steel materials and provide a scientific basis for engineering practice.
Strength is one of the most fundamental terms in the mechanical properties of steel materials, referring to the material’s ability to resist damage from external forces. Common strength indicators include yield strength, tensile strength, and compressive strength. Yield strength, represented by σs and measured in MPa, is the stress at which a material begins to undergo plastic deformation. It is an important factor in material selection during structural design. For example, the yield strength of Q235 steel is 235 MPa. Tensile strength, represented by σb, is the maximum stress a material can withstand before breaking. It reflects the material’s ability to resist fracture. For example, the tensile strength of 45 steel is approximately 600 MPa. Compressive strength is the material’s ability to resist compression failure. For brittle materials such as cast iron, compressive strength is much greater than tensile strength. The level of strength depends on the material’s chemical composition, microstructure, and heat treatment. Alloying and heat treatment can significantly improve the strength of steel.
Plasticity refers to the ability of steel to deform plastically without breaking under external forces. Commonly used plasticity indicators include elongation and reduction of area. Elongation, expressed as δ, is the percentage of the gauge length after the material breaks, expressed as 20%–30% of the original gauge length. For example, the elongation of low-carbon steel can reach 20%–30%, while that of cast iron is almost zero. Reduction of area, expressed as ψ, is the percentage of the fracture area after the material breaks, expressed as ψ, and also reflects the plasticity of the material. Materials with good plasticity are easier to process by pressurizing processes such as rolling, forging, and stamping. When subjected to stress, they absorb energy through plastic deformation, reducing the risk of sudden fracture. The plasticity of steel increases with increasing temperature and decreases with increasing carbon content. For example, high-carbon steel has lower plasticity than low-carbon steel.
Hardness measures a steel material’s ability to resist local deformation, particularly indentation or scratching, and is an important indicator of a material’s surface properties. Commonly used hardness indices include Brinell hardness (HB), Rockwell hardness (HRC, HRB, etc.), and Vickers hardness (HV). Brinell hardness is suitable for measuring lower-hardness steel materials, such as annealed mild steel. During measurement, a carbide ball of a certain diameter is pressed into the material surface under a certain load, and the hardness value is calculated based on the indentation diameter. Rockwell hardness is widely used and has multiple scales. HRC is suitable for measuring high-hardness steel materials after quenching, such as cutting tools and bearings, while HRB is suitable for measuring medium-hardness materials, such as quenched and tempered steel. Vickers hardness offers high measurement accuracy and is suitable for measuring the hardness of thin parts or materials with surface treatments, such as carburized and nitrided layers. There is a certain correlation between hardness and strength; generally, higher hardness corresponds to greater strength.
Toughness refers to the ability of steel to absorb energy before fracture, reflecting its ability to withstand impact loads. Commonly used toughness indicators include impact toughness and fracture toughness. Impact toughness, expressed as αk and measured in J/cm², is the energy absorbed by a material when it breaks under an impact load. It is measured through impact tests such as the Charpy V-notch test. Low-temperature impact toughness is an important indicator for evaluating a material’s low-temperature brittleness. Fracture toughness, expressed as KIC, is a material’s ability to resist crack propagation. It is suitable for evaluating the safety of components containing cracks and is of great significance for steel used in large structural components and high-pressure vessels. The toughness of steel is affected by factors such as temperature, microstructure, and stress state. Low temperatures significantly reduce toughness, resulting in cold brittleness. Refining grain size and reducing inclusions can improve toughness. Understanding these terms helps ensure accurate decisions in material selection, processing technology development, and product design, ensuring product performance and safety.
