Selecting the material to be used from a variety of materials is a task that is constrained by many factors. Therefore, how to select the material of a part is an important part of part design. The principle of selecting the material of a mechanical part is that the required material should meet the use requirements of the part and have good processability and economy.

01 Use requirements

The use requirements of mechanical parts are as follows:

(1) The working condition and load condition of the part and the requirements put forward to avoid the corresponding failure form.

The working condition refers to the environmental characteristics, working temperature, and degree of friction and wear of the part. For parts working in a hot and humid environment or corrosive medium, the material 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 matching parts should not differ too much, so as to avoid excessive thermal stress or loosening of the fit when the temperature changes; on the other hand, it is also necessary to consider the change of the mechanical properties of the material with temperature. For parts working under sliding friction, in order to improve their surface hardness and enhance wear resistance, it is necessary to select hardened steel, carburized steel, nitrided steel and other varieties suitable for surface treatment or materials with good friction reduction and wear resistance.

Loading conditions refer to the size and nature of the load and stress on the parts. In principle, brittle materials are only suitable for manufacturing parts working under static loads; in the case of impact, plastic materials should be used as the main materials; for parts with large contact stress on the surface, materials that can be surface treated should be selected, such as surface hardened steel; for parts subject to strain, fatigue-resistant materials should be selected; for parts subject to impact loads, materials with higher impact toughness should be selected; for parts whose size depends on strength and whose size and mass are limited, 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 enhanced through heat treatment. Therefore, it is necessary to make full use of heat treatment to unleash the potential of materials. For the most commonly used tempered steel, blanks with different mechanical properties can be obtained due to different tempering temperatures. 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, its heat treatment specifications should be specified at the same time and indicated on the drawing.

(2) Restrictions on part size and mass.

The size and mass of a part are related to the type of material and the manufacturing method of the blank. When producing cast blanks, there are generally no restrictions on size and mass; when producing forged blanks, attention should be paid to the production capacity of forging machinery and equipment. In addition, the size and mass of a part 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 to reduce the size and mass of the part.

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

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

02 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 large sizes, if casting blanks are considered, materials with good casting properties should be selected; if welding blanks are considered, low-carbon steel with good welding properties should be selected. For parts with simple shapes, small sizes and large batches, stamping and die forging are suitable, and materials with good plasticity should be selected. For parts that require heat treatment, the material should have good heat treatment properties. In addition, the ease of processing of the material itself and the ease of processing after heat treatment should also be considered.

03 Economic requirements

(1) Relative price of the material

Under the premise of meeting the use requirements, low-priced materials should be used as much as possible. This is especially important for parts manufactured in large quantities.

(2) Processing cost of materials

When the quality of the parts is not large but 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 plates, for some single-piece or small-batch box parts, the cost of using cast iron is higher than that of steel plate welding because the latter can save the manufacturing cost of the mold.

（3） Saving materials

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

（4） Material utilization

In order to improve the utilization of materials, non-cutting or less-cutting processing can be used, such as die forging, precision casting, stamping, etc., which can not only improve the utilization of materials, but also reduce the working hours of cutting processing.

（5） Saving precious materials

By adopting a combined structure, more expensive materials can be saved. For example, the worm gear ring of the combined structure adopts tin bronze, which has good friction reduction but is expensive, while the wheel core adopts cheap cast iron.

（6） Saving rare materials

In this regard, manganese-boron alloy steels, which are relatively abundant in my country, can be used to replace chromium-nickel alloy steels, which are less abundant in China, and aluminum bronze can be used to replace tin bronze.

（7） Material supply

When selecting materials, materials that are available locally 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 small-batch production parts, the variety and specifications of materials used on the same machine should be reduced as much as possible to simplify supply and management, and it will be easier to master the most reasonable operating methods during processing and heat treatment, thereby improving manufacturing quality, reducing waste, and increasing labor productivity.