Under a certain temperature condition and the action of external force, the resistance of the protective box material to deformation and fracture is called the mechanical properties of the material.The conventional mechanical properties of materials used in 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 the action of external forces.The strength index is an important basis for determining the allowable stress in the design. The commonly used strength indicators are the yield strength sigma S or sigma 0.2 and the tensile strength sigma b. 

(2) plasticity: plasticity refers to the ability of metal materials to undergo plastic deformation before fracture.Plasticity index includes elongation delta, that is, the relative elongation of the specimen after breaking;Reduction of bits, the sample after snap, snap in cross-sectional area is relatively narrow;Cold bending (Angle) a, that is, the Angle at which the specimen is bent until the first crack appears in the tensile surface.

(3) toughness: toughness refers to the ability of metal materials to resist impact load.Toughness is usually expressed by impact work Ak and impact toughness value alpha k.Α k value or alpha k value in addition to reflect the material shock resistance, and is sensitive to some of the pitfalls of materials, sensitive to reflect the material quality, the macroscopic defects and small changes in the microstructure.Moreover, Ak is very sensitive to the brittle transition of materials, and the cold brittleness of steel can be tested by low temperature impact test.A new index of ductility is fracture ductility, which is used to show the material's resistance to crack propagation.

(4) hardness: hardness is a performance index to measure the hardness of materials.Hardness test method is more, the principle is not the same, measured hardness value and meaning is not exactly the same.The most commonly used are the stone-load indentation hardness tests, known as brinell hardness (HB), rockwell hardness (HRA, HRB, HRC), and vickers hardness (HV), which represent the ability of a material's surface to resist the indentation of a hard object.However, shore hardness (HS) is a 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.