Choosing the material to use from a variety of materials is a task limited by several factors. Therefore, how to select the material of the parts is an important part of the design of the parts.
The principle of material selection for mechanical parts is that the required material meets the use requirements of the parts and has good processability and economy.
I. Usage requirements
The requirements for the use of mechanical parts are reflected in the following points:
1) The working and loading conditions of the parts, and the requirements presented to avoid corresponding forms of failure.
Working conditions refer to the environmental characteristics of the parts, the working temperature and the degree of friction and wear. For parts that work in humid and hot environments or corrosive environments, the material must have good resistance to rust and corrosion.
In this case, stainless steel and copper alloys can be considered first.
The impact of working temperature on material selection mainly has two aspects: on the one hand, the linear expansion coefficients of the two parts cooperating with each other should not differ too much, so as to avoid excessive thermal stress or loosen cooperation when the temperature changes; on the other hand, the change in the mechanical properties of the material with temperature must also be considered.
For parts working under sliding friction, surface hardness must be improved to increase wear resistance. Suitable materials should be selected for surface treatment, such as hardened steel, carburized steel and nitrided steel, or materials with good friction reduction and wear resistance should be used.
Load condition refers to the size and nature of the load and stress on the parts.
In principle, brittle materials are only suitable for manufacturing parts that work under static load; under impact, mainly plastic materials should be used; for parts with high contact stress on the surface, materials that can be surface treated should be selected, such as surface hardened steel; for parts under deformation force, fatigue-resistant materials should be selected; for parts under impact load, materials with high impact resistance must be selected; for parts whose size depends on strength and whose size and weight are limited, materials with greater strength should be selected; for parts whose size depends on stiffness, materials with a higher modulus of elasticity should be selected.
The performance of metallic materials can often be improved and enhanced through heat treatment. Therefore, the potential of materials must be fully utilized through heat treatment.
For the most commonly used hardened steel, blanks with different mechanical properties can be obtained due to different tempering temperatures. The higher the tempering temperature, the lower the hardness and rigidity of the material and the better the plasticity.
Therefore, when selecting the variety of materials, the heat treatment specifications must be specified at the same time and indicated on the drawing.
2) Restrictions on size and weight of parts.
The size and weight of the parts are related to the type of material and the manufacturing method of the blank. When producing castings, there is generally no restriction on size and weight; When producing forging blanks, attention should be paid to the production capacity of forging machines and equipment.
Furthermore, the size and weight 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 to reduce the size and weight of parts.
3) The importance of parts in the entire machine and components.
4) Other special requirements (such as the need for insulation, anti-magnetic, etc.).
II. Process Requirements
To facilitate part manufacturing, the complexity of the part structure, size and type of blank must 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 blanks are considered for welding, steel with low carbon content and good welding performance should be selected.
For parts with simple shapes, small sizes and large quantities, they are suitable for stamping and forging, and materials with good plasticity should be selected. For parts that require heat treatment, the material must have good heat treatment performance.
In addition, the machinability of the material itself and the ease of machining after heat treatment must also be considered.
III. Economic Requirements
1) The relative price of the material itself
Under the premise of meeting the usage requirements, materials with a lower price should be selected as much as possible. This is particularly important for parts manufactured in large quantities.
2) The cost of processing the material
When the quality of parts is not great, but the processing volume is large, the processing cost will occupy a large proportion of the total cost of parts.
Although cast iron is cheaper than steel plate, for some single or small production batches of box-type parts, the cost of using cast iron is higher than that of welding steel plate, because the latter can save the cost of manufacturing the mold.
3) Save material
To save materials, heat treatment or surface strengthening (shot peening, lamination, etc.) can be used to fully utilize and exploit the potential mechanical properties of materials; Surface coating methods (chrome plating, copper plating, blackening, bluing, etc.) can also be used to reduce corrosion and wear, thereby extending the life of parts.
4) Use of materials
In order to improve the utilization of materials, no-cut or less-cut processing can be used, such as forging, precision casting, stamping, etc. processing.
5) Save precious materials
By using a combined structure, more expensive materials can be saved. For example, the helical gear ring of the combined structure uses tin bronze, which has better friction reduction but is more expensive, while the wheel core uses cheaper cast iron.
6) Save rare materials
In this regard, manganese-boron series alloy steel, which is abundant in resources in our country, can be used to replace chromium-nickel series alloy steel, which has fewer resources, and aluminum bronze can be used to replace tin bronze, etc. .
7) Supply of materials
When selecting materials, materials that are locally available and easy to supply should be selected to reduce procurement, transportation and storage costs; From the point of view of simplifying the supply and storage of varieties of materials, for small batch production parts, the variety and specifications of materials used in the same machine should be minimized as much as possible to simplify supply and management, and it is easier to understand the most reasonable operation method in the processing and heat treatment process, thereby improving manufacturing quality, reducing waste and improving labor productivity.
