In order to avoid the recurrence of similar material problems, I now share my thoughts on some common knowledge of metal materials and the general principles of selection of materials. I will share and share with my colleagues. First, the performance of metal materials The properties of materials include mechanical properties, chemical properties and processing performance. The main mechanical properties of the material - tensile strength, yield strength, elongation, reduction of area, hardness, impact toughness; chemical properties of the material - corrosion resistance, oxidation resistance, chemical stability; - Casting performance, forging performance, welding performance, heat treatment process performance, cold process performance. The processability of the material plays an important role in determining the processing possibilities. Casting process - refers to the fluidity of the liquid material, shrinkage, degree of segregation and the tendency to produce shrinkage and so on. Forging process - refers to the material ductility, hot brittleness and the cold and hot plastic deformation ability. Welding process - refers to the welding performance of the material and weld cracks tendency. Heat treatment process - refers to the hardenability of materials, quenching deformation tendencies and heat treatment of its infiltration capacity and so on. Cold workmanship - refers to the hardness of the material, machinability, degree of cold work hardening and possible surface roughness after cutting. Second, the general choice of materials Principle 1, the chemical properties and corrosion resistance of materials can meet the requirements of medium conditions; 2, the processing of materials can meet the design requirements of the performance; 3, the material has a good price, economic effect is obvious. Third, the corrosion resistance of materials and corrosion-resistant material selection 1, the type and characteristics of metal corrosion: Selection of materials in the corrosive media often involve the corrosion resistance of the material. Corrosion types and characteristics of metallic materials are shown in the following table: Corrosion types and characteristics of metallic materials Corrosion type Characteristics Homogeneous corrosion Chemical and electrochemical reactions occur uniformly over the entire exposed surface or large area of ​​a metallic material, resulting in macro-thinning of the metal. Is a common phenomenon of corrosion. Intergranular corrosion along the metal grain boundary corrosion occurs, the main feature is the same external dimensions of the metal, most still maintain a metallic luster, but the strength and ductility of the metal drop, cracks appear on the surface after the cold bend. Selective corrosion of an alloy or a certain element in the corrosion process is selectively corroded, for example: chromium, manganese, molybdenum and nitrogen duplex steel in industrial acetic acid occurred in the selective corrosion of austenite. Stress corrosion metal brittle fracture under the combined effect of persistent à -stress and specific corrosive medium is characterized by corrosion cracking or even fracture. The origin of the fracture is often at the bottom of small pitting corrosion or corrosion pits. Crystal, transcrystalline and hybrid three, the fracture has the characteristics of brittle fracture. Corrosion Fatigue Metal Corrosion caused by the combined effect of medium and alternating stress damage, which is characterized by the production of corrosion pits and a large number of cracks leading to the loss of fatigue strength of the metal cracks, most of the cracks do not bifurcate. Resulting in fatigue corrosion of the media are: acidic media, chloride, and containing H2S, SO2, O2 and so on. Most of the surface of the pitting corrosion-free metal will not corrode and corrode slightly. In some places, there will be small corrosion and corrosion extending to the deep place. Crevice corrosion Corrosion that occurs in electrolyte solutions, between metals and metals, or between metals and nonmetals, due to a gap sufficient to allow the medium to enter while leaving the medium in a standstill. Galvanic corrosion In the electrolyte solution, two or more different potential of the metal contact make the more negative potential corrosion of the metal. Corrosion often appear grooves, pits and other macro phenomena. Abrasion corrosive corrosive fluid and solid particles and the relative movement of the metal surface, especially the eddy current on the metal surface erosion, but also with the bare metal part of the chemical or electrochemical interaction, causing corrosion, the metal in the form of corrosion products from Metal surfaces fall off, the surface often appear with directional grooves, channels, ripples, holes and other corrosion profiles. Hydrogen corrosion Under high temperature and high pressure, H reacts chemically with the carbon in the steel to cause embrittlement of the steel. The characteristic is that there is no obvious change in the outside, but the mechanical properties are remarkably decreased, the fracture is brittle fracture, and the decarburization phenomenon can be seen in the metallographic observation And intergranular cracks, and sometimes can form a macro bubbling.
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