Choosing the material to be used from a variety of materials is a task restricted by many factors. Therefore, how to choose the material of a part is an important part of part design. The principle of selecting mechanical parts materials is: the required materials should meet the requirements of the parts and have good manufacturability and economy.

 

1 Requirements for use

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

1) The working conditions and loading conditions of the parts and the requirements for avoiding corresponding failure modes.

The working condition refers to the environmental characteristics of the parts, the working temperature and the degree of friction and wear. For parts that work in hot and humid environments or corrosive media, the materials should have good rust and corrosion resistance. In this case, stainless steel, copper alloy, etc. can be considered first. The influence of working temperature on material selection mainly has two aspects: On the one hand, it is necessary to consider that the linear expansion coefficients of the two parts that cooperate with each other should not be too different, so as to avoid excessive thermal stress or loosening of the fit when the temperature changes; on the other hand, On the other hand, we should also consider the change of the material's mechanical properties with temperature. For parts that work under sliding friction, to increase their surface hardness to enhance wear resistance, you should choose quenched steel, carburized steel, nitrided steel, etc. suitable for surface treatment, or choose good anti-friction and wear resistance properties. Material.

 

The load condition refers to the size and nature of the load and stress on the part. In principle, brittle materials are only suitable for manufacturing parts that work under static load; in the case of impact, plastic materials should be used as the main materials; for parts with large contact stress on the surface, surface treatment should be selected Material, such as case-hardened steel; for parts subject to strain stress, fatigue-resistant materials should be selected; for parts subject to impact load, materials with higher impact toughness should be selected; for those whose size depends on strength but the size and quality are limited For parts, materials with higher strength should be selected; for parts whose size depends on stiffness, materials with larger elastic modulus should be selected.

 

The properties of metal materials can generally be improved and improved by heat treatment. Therefore, heat treatment must be fully utilized to develop the potential of the material; for the most commonly used tempered steel, blanks with different mechanical properties can be obtained due to the difference in tempering temperature. The higher the tempering temperature, the lower the hardness and stiffness of the material, and the better the plasticity. Therefore, when selecting the type of material, the heat treatment specification should be specified at the same time, and it should be noted on the drawing.

 

2) Restrictions on the size and quality of parts. 

The size and quality of the parts are related to the type of material and the manufacturing method of the blank. When producing casting blanks, it is generally not restricted by size and quality; when producing forging blanks, attention should be paid to the production capacity of forging machinery and equipment. In addition, the size and quality of the parts are also related to the strength-to-weight ratio of the material. Materials with a large strength-to-weight ratio should be selected as much as possible in order to reduce the size and quality of the parts.

 

3) The importance of parts in the whole machine and components.

 

4) Other special requirements (such as whether insulation, anti-magnetic, etc.) are required.

 

2 Process requirements

In order to make the parts easy to process and manufacture, the complexity of the part structure, size, and blank type should be considered when selecting materials. For parts with complex shapes and larger sizes, if casting blanks are considered, materials with good casting properties should be selected; if welding blanks are considered, low-carbon steels with good welding performance should be selected. For parts with simple appearance, small size, and large batches, they are suitable for stamping and die forging, and materials with better plasticity should be selected. For parts that require heat treatment, the material should have good heat treatment performance. In addition, the ease of processing of the material itself and the ease of processing after heat treatment should also be considered.

 

3 Economic requirements

 

1) The relative price of the material itself

Under the premise of meeting the requirements of use, low-cost materials should be selected as far as possible. This is especially important for high-volume manufactured parts.

 

2) Processing cost of materials

When the quality of the parts is not large and the processing volume is large, the processing cost will account for a large proportion of the total cost of the parts. Although cast iron is cheaper than steel plate, it is more expensive to use cast iron than steel plate welding for certain single-piece or small batch production box parts, because the latter can save mold manufacturing costs.

 

3) Save materials 

In order to save materials, heat treatment or surface strengthening (shot peening, rolling, etc.) can be used to fully develop and utilize the potential mechanical properties of materials; surface coating (chrome plating, copper plating, blackening, French blue, etc.) methods can also be used. In order to reduce the degree of corrosion and wear and extend the service life of parts.

 

4) Utilization rate of materials

In order to improve the utilization rate of materials, no-cutting or less-cutting processing can be used, such as die forging, precision casting, stamping, etc., which can not only increase the utilization rate of materials, but also reduce the man-hours of cutting processing.

 

5) Save valuable materials

By adopting the combined structure, higher-priced materials can be saved. For example, the worm gear ring of the combined structure is made of tin bronze with better anti-friction properties but expensive, and the wheel core is made of cheap cast iron.

 

6) Save rare materials

In this regard, manganese-boron series alloy steels with richer resources in our country can be used instead of chromium-nickel alloy steels with fewer resources, and methods such as using aluminum bronze instead of tin bronze can be used.

 

7) Supply of materials

When selecting materials, materials that are locally available and easy to supply should be selected to reduce the cost of procurement, transportation, and storage; from the perspective of simplifying the supply and storage of material varieties, for the parts produced in small batches, the number of The variety and specifications of materials used on the same machine to simplify supply and management, and make it easier to master the most reasonable operating methods during processing and heat treatment, thereby improving manufacturing quality, reducing waste, and increasing labor productivity.

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