(1) The working condition and loading condition of the parts and the requirements to avoid the corresponding failure forms.

Working condition refers to the environmental characteristics, working temperature, friction and wear degree of the parts. For parts working in hot and humid environment or corrosive medium, the materials shall have good anti rust and corrosion resistance. In this case, stainless steel and copper alloy can be considered first. The influence of working temperature on material selection mainly includes two aspects: on the one hand, it is necessary to consider that the linear expansion coefficient of two parts matched with each other should not be too large, so as not to produce excessive thermal stress or make the fit loose when the temperature changes; on the other hand, it is also necessary to consider the change of material mechanical properties with temperature. In order to improve the surface hardness and enhance the wear resistance of the parts working under sliding friction, the quenching steel, carburizing steel, nitriding steel and other varieties suitable for surface treatment should be selected, or the materials with good antifriction and wear resistance should be selected.

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

Generally, the properties of metal materials can be improved by heat treatment. Therefore, it is necessary to make full use of the means of heat treatment to bring the potential of materials into full play; for the most commonly used modulated 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, the heat treatment specification should be specified at the same time and indicated on the drawing.

(2) Restrictions on size and quality of parts.

The size and quality of parts are related to the variety of materials and the manufacturing method of blank. The production of casting blank is generally not limited by size and quality, while the production capacity of forging machinery and equipment should be paid attention to when producing forging blank. In addition, the size and mass of the parts are also related to the strength weight ratio of the material, so the material with high strength to weight ratio should be selected as far as possible, so as 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 insulation, diamagnetism, etc.).

02 process requirements

In order to make the parts easy to process and manufacture, the complexity of part structure, size and blank type should be considered when selecting materials. For parts with complex shape and large size, if casting blank is considered, the material with good casting performance shall be selected; if welding blank is considered, low carbon steel with good welding performance shall be selected. For the parts with simple shape, small size and large batch, it is suitable for stamping and die forging, and the material with better plasticity should be selected. For parts requiring heat treatment, the material shall have good heat treatment performance. In addition, the machinability of the material itself and the machinability after heat treatment should also be considered.

03 economic requirements

(1) Relative price of material itself

On the premise of meeting the use requirements, low-cost materials should be selected as far as possible. This is particularly important for parts manufactured in large quantities.

(2) Material processing costs

When the quality of parts is small and the processing quantity is large, the processing cost will account for a large proportion in the total cost of parts. Although cast iron is cheaper than steel plate, it is more expensive to use cast iron than steel plate for some single or small batch box parts, because the latter can save the cost of mold manufacturing.

(3) Saving materials

In order to save materials, heat treatment or surface strengthening (shot peening, rolling, etc.) process can be used to give full play to the potential mechanical properties of materials; surface coating (chromium plating, copper plating, blackening, French blue, etc.) can also be used to reduce the degree of corrosion and wear and prolong the service life of parts.

(4) Material utilization

In order to improve the utilization of materials, no cutting or less cutting can be adopted, 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.

(5) Saving valuable materials

For example, the worm gear ring of the combined structure is made of tin bronze which has better antifriction property but is expensive, and the wheel core is made of cheap cast iron.

(6) Saving rare materials

In this respect, we can use Mn-B series alloy steel with rich resources to replace Cr Ni alloy steel with less resources, and use aluminum bronze to replace tin bronze.

(7) Material supply

In the selection of materials, local materials which are 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 parts produced in small batch, the varieties and specifications of materials used on the same machine should be reduced as much as possible, so as to simplify the supply and management, and make it easier in the process of processing and heat treatment Master the most reasonable operation method, so as to improve the manufacturing quality, reduce waste products and improve labor productivity.