Second, China Mechanical Parts material selection principles

Choosing the right material from a wide variety of materials is a work that is constrained by many factors. The commonly used materials for mechanical parts in China are steel, cast iron, non-ferrous metals and non-metals. The grades, properties and heat treatment knowledge of commonly used materials can be found in the mechanical design manual. Choosing materials in mechanical design is an important part. With the continuous development of materials science, the requirements for parts in the mechanical manufacturing industry are increasing. Therefore, the designer should fully understand the properties and applicable conditions of the material when selecting materials, and consider the requirements of the use, process and economy of the parts. The following is a brief introduction to the general selection principles of metallic materials (mainly steel).

The principle of selecting Chinese mechanical parts materials is: the required materials should meet the requirements of the parts, have good craftsmanship and economy.

1, use requirements

The requirements for the use of Chinese mechanical parts are as follows:

(1) The working condition and loading condition of the parts, and the required working conditions to avoid the corresponding failure forms refer to the environmental characteristics, working temperature, friction and wear of the parts. For parts working in hot or humid environments or corrosive media, the material should have good corrosion and corrosion resistance, such as stainless steel, copper alloy, etc.

The influence of working temperature on material selection, on the one hand, should consider that the linear expansion coefficients of the materials of the two parts that do not fit together should not be too large, so as to avoid excessive thermal stress or loosening of the joint when the temperature changes; The mechanical properties of a material change with temperature. For parts working under sliding friction, the surface hardness should be increased to enhance the wear resistance. The hardened steel, carburized steel, nitrided steel, etc. suitable for surface treatment should be selected or the friction reducing and wear resistance should be good. material.

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The load condition refers to the magnitude and nature of the load and stress. In principle, brittle materials are only suitable for the manufacture of parts that work under static load; in the case of some impacts, plastic materials should be used as the main material; for parts with large contact stress on the surface, surface treatment should be selected. Materials such as case hardened steel; for fatigue-resistant parts, fatigue-resistant materials should be selected; for impact-loaded parts, materials with higher impact toughness should be selected; for size depending on strength, size and quality For restricted parts, materials with higher strength should be selected; for parts whose size depends on stiffness, materials with large elastic modulus should be used.

The properties of metallic materials can generally be improved and improved by heat treatment. Therefore, it is necessary to make full use of the means of heat treatment to exert the potential of the materials. For the most commonly used quenched and tempered steel, due to the difference in tempering temperature, blanks with different mechanical properties can be obtained. The higher the tempering temperature, the lower the hardness and strength of the material, and the better the plasticity. So when choosing the type of material. The heat treatment specifications shall be specified at the same time and shall be indicated on the drawings.

(2) Restriction on part size and quality The size and quality of the part are related to the type of material and the method of making the blank. When manufacturing blanks from cast materials, it is generally not limited by size and quality. When manufacturing blanks from forged materials, attention must be paid to the production capacity of forging machinery and equipment. In addition, the size and mass of the part are also related to the weight-to-weight ratio of the material. Materials with a large weight-to-weight ratio should be used as much as possible to reduce the size and quality of the part.

(3) The importance of the part in the whole machine or component.