Basic characteristics of metals Properties of metal materials In order to use metal materials more reasonably and give full play to their functions, it is necessary to master the properties of parts made of various metal materials under normal working conditions (serviceability) and the properties of materials during cold and hot processing (process performance). The properties of materials include physical properties (such as specific gravity, melting point, electrical conductivity, thermal conductivity, thermal expansion, magnetism, etc. ), chemical properties (durability, corrosion resistance and oxidation resistance), and mechanical properties are also called mechanical properties. The technological performance of materials refers to the ability of materials to adapt to cold and hot processing methods. The surface area of metallic materials is also important. There is a special specific surface area tester to test the specific surface area. Static nitrogen adsorption method is generally used abroad, while domestic dynamic nitrogen adsorption method is relatively mature. However, most of the existing instruments in China can only be directly compared. The 3H-2000 specific surface area tester of Beijing Huihaihong Nano-technology Co., Ltd. is an instrument that can truly realize the BET detection function (both are direct comparison methods). More importantly, the 3H-2000 specific surface area tester of Beijing Huihaihong Nanotechnology Co., Ltd. is the only fully automatic and intelligent specific surface area testing equipment in China. The test results are highly consistent with international standards and have good stability. At the same time, human errors are reduced and the accuracy of test results is improved. (1) Mechanical properties Mechanical properties refer to the characteristics of metal materials under the action of external forces. 1, strength: the ability of a material to resist deformation and fracture under external force (load). The load per unit area of a material is called stress. 2. Yield point (бs): called yield strength, it means that when the stress of the material reaches a certain critical value during the tensile process, the load continues to increase or produce 0.2%L without increasing the deformation. Time stress value in Newton/mm2 (n/mm2). 3. Tensile strength (бb), also known as strength limit, refers to the maximum stress that a material bears before it breaks. The unit is Newton per square millimeter (n/mm2). 4. Elongation (δ): the percentage of the total elongation of the material after tensile fracture to the original gauge length. 5. Area shrinkage (ψ) The percentage of the maximum reduced area of the cross section to the original area after the tensile fracture of the material. 6. Hardness: refers to the ability of a material to resist the pressure of other hard objects on its surface. Commonly used hardness can be divided into Brinell hardness (HBS, HBW) and Rockwell hardness (HKA, HKB, HRC). 7. Impact toughness (Ak): the ability of materials to resist impact load, in joule per square centimeter (J/cm2). Stress-strain curve analysis of low carbon steel tensile 1. Elasticity: εe=σe/E, index σe, E2. Stiffness: △ L = P L/E F, ability to resist elastic deformation, strength: σs- yield strength, σb- tensile strength 4. Toughness: impact absorption work Ak5. Fatigue strength: alternating load σ- 1 < σ S6. In the stages of hardness HR, HV and HB I, the initial stress-strain curve is a straight line, and the maximum stress limit at this stage is called the proportional limit of the material. When the stress increases to a certain value, a horizontal line segment appears in the stress-strain curve. At this stage, the stress is almost constant, but the deformation increases sharply, and the material loses its ability to resist deformation. This phenomenon is called yield, and the corresponding stress is called yield stress or yield limit, which is expressed by σ s, and the third stage is strengthening stage, after which the material's ability to resist deformation is enhanced. The stress corresponding to the highest point in the strengthening stage is called the strength limit of the material. Expressed by σb, the strength limit is the maximum stress that a material can bear. Stage ⅳ is necking stage. When the stress increases to the maximum σb, a part of the specimen shrinks significantly and finally breaks at the neck. σs and σb are the main indexes to measure the strength of low carbon steel. Stiffness: △ L = P L/E F, the ability to resist elastic deformation. P-tension, l-original length of material, E-elastic modulus, and plastic deformation of F-sectional area: the deformation that cannot be recovered after external force is removed, that is, residual deformation is called plastic deformation. The ability of a material to undergo large plastic deformation without damage is called plasticity or ductility of the material. The two indexes to measure the plasticity of materials are elongation and area shrinkage. Elongation δ=(△l0/l)× 100% area shrinkage ψ = ((a-a1)/a )×100% toughness (impact toughness): usually expressed as impact absorption work Ak, which means that a material absorbs plastic deformation under impact load. Fatigue strength: the strength index that the material can resist infinite stress (107) without fatigue fracture, and the alternating load σ- 1 < σ s is the design standard. Hardness: the hardness of the material. There are many methods to determine the hardness test, which can be generally divided into three categories: elastic recovery method (Shore hardness), indentation method (Brinell hardness, Rockwell hardness and Vickers hardness) and scratch method (Mohs hardness), among which indentation method is the most widely used in production. It is to press a hard indenter with a certain shape and size into the surface of the measured material under a certain load, and calculate the hardness value of the material according to the surface area or depth of the indentation left. Because of the different standards and instruments used in hardness determination, there are many methods to determine the hardness of materials by indentation method. Commonly used methods include Brinell hardness (HB), Vickers hardness (HV) and Rockwell hardness (HR). (2), process performance refers to the ability of materials to withstand all kinds of processing and treatment of those properties. 8. Castability: refers to some technological properties of whether a metal or alloy is suitable for casting, mainly including fluidity and mold filling ability; Shrinkage, the ability of volume shrinkage when casting solidifies; Segregation refers to the heterogeneity of chemical composition. 9. Weldability: refers to the characteristics that two or more metal materials are welded together by heating or heating and pressure, and the interface can meet the purpose of use. 10. Top gas section performance: refers to the performance that metal materials can be allowed to upset without breaking. 1 1, cold bending performance: refers to the performance of metal materials that can withstand bending without cracking at room temperature. The bending degree is generally expressed by the ratio of the bending angle α (outer angle) or the bending center diameter D to the material thickness A. The larger A or the smaller d/a, the better the cold bending performance of the material. 12. Stamping performance: the ability of metal materials to withstand stamping deformation without breaking. Stamping at room temperature is called cold stamping. The test method is cupping test. 13. Forging performance: the ability of metal materials to withstand plastic deformation without cracking during forging. (3) Chemical properties refer to the resistance of metal materials to chemical or electrochemical reactions when they are in contact with surrounding media. 14, corrosion resistance: refers to the ability of metal materials to resist various media corrosion. 15, oxidation resistance: refers to the ability of metal materials to resist the formation of scale at high temperature.