Under certain temperature conditions and external force, the ability to resist deformation and fracture of the material is called the mechanical property of the material. The conventional mechanical properties of materials for boilers and pressure vessels mainly include strength, hardness, plasticity and toughness.
(1) Strength: strength refers to the resistance of metal materials to deformation or fracture under external force. The strength index is an important basis for determining the allowable stress in design. The commonly used strength indexes are yield strength σ s or σ 0.2 and tensile strength σ B. when working at high temperature, creep limit σ N and rupture strength σ D should be considered.
(2) Plasticity: plasticity refers to the ability of a metal material to deform plastically before fracture. The plasticity index includes: elongation δ, the relative elongation of the sample after breaking; the reduction of area ψ, the relative reduction of the cross-sectional area at the breaking point; and the cold bending (angle) α, which is the angle measured when the specimen is bent to the first crack on the tensile surface.
(3) Toughness: toughness refers to the ability of metal materials to resist impact loads. Toughness is usually expressed by impact energy AK and impact toughness value α K. A k value or α K value not only reflects the impact resistance of the material, but also is sensitive to some defects of the material, which can sensitively reflect the slight changes of the material quality, macro defects and microstructure. Moreover, AK is very sensitive to the brittle transformation of materials, and the cold brittleness of steel can be tested by low temperature impact test. A new index of material toughness is fracture toughness δ, which reflects the resistance of material to crack growth.
(4) Hardness: hardness is a performance index to measure the hardness of materials. There are many methods of hardness test, the principle is not the same, the hardness value and meaning are not exactly the same. The most commonly used is the static load indentation hardness test, that is, Brinell hardness (HB), Rockwell hardness (HRA, HRB, HRC), Vickers hardness (HV), whose values represent the ability of the material surface to resist the impact of hard objects. The shore hardness (HS) belongs to the rebound hardness test, and its value represents the elastic deformation work of metal. Therefore, hardness is not a simple physical quantity, but a comprehensive performance index reflecting the elasticity, plasticity, strength and toughness of materials.
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