Selection principles of parts materials

• 1 Requirements for use

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

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 the corresponding failure modes.

The working condition refers to the environmental characteristics of the parts, the working temperature and the degree of friction and wear. The materials of parts working in hot and humid environment or corrosive medium should have good antirust and corrosion resistance. In this case, stainless steel, copper alloy, etc. can be considered first. There are two main aspects of the influence of working temperature on material selection: on the one hand, it should be considered that the linear expansion coefficients of the materials of the two parts that cooperate with each other should not be too different, so as not to cause 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 the mechanical properties of the material with temperature. For parts working under sliding friction, to increase the surface hardness to enhance wear resistance, hardened steel, carburizing steel, nitrided steel and other varieties suitable for surface treatment should be selected or selected with good anti-friction and wear resistance. Material.

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

The performance of metal 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 develop the potential of the material; 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 specification should be specified at the same time, and it should be indicated on the drawing.

(2) Limitations on part size and mass.

The size and quality of the parts are related to the type of material and the manufacturing method of the blank. The production of casting blanks is generally not limited by size and quality; while the production of forging blanks, it is necessary to pay attention 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 high strength-to-weight ratio should be selected as much as possible 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 is required, anti-magnetic, etc.

• 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 large sizes, if casting blanks are considered, materials with good casting performance should be selected; if welding blanks are considered, low-carbon steel with good welding performance should be selected. For parts with simple shape, small size and large batch size, suitable for stamping and die forging, materials with better plasticity should be selected. For parts requiring 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.

• 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 much as possible. This is especially important for parts manufactured in high volumes.

(2) Processing costs 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 plates, for some single-piece or small-batch production box parts, the cost of using cast iron is higher than that of steel plate welding, because the latter can save the cost of mold manufacturing.

(3) Save material

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

(4) Utilization of materials

In order to improve the utilization rate 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 rate of materials, but also reduce the working hours of cutting processing.

(5) Save precious materials

By adopting the combined structure, it is possible to save expensive materials. For example, the worm gear ring gear of the combined structure adopts tin bronze with better anti-friction but expensive price, while the wheel core adopts cheap cast iron.

(6) Save rare materials

In this regard, manganese-boron series alloy steel, which is rich in resources in our country, can be used to replace chromium-nickel alloy steel with less resources, and aluminum bronze can be used instead of tin bronze.

(7) Availability 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 parts produced in small batches, the The varieties and specifications of materials are used on the same machine to simplify supply and management, and it is easier to grasp the most reasonable operation method in the process of processing and heat treatment, so as to improve manufacturing quality, reduce waste products, and increase labor productivity.