It is necessary to consider that the materials used can be easily manufactured from blank to finished product. For example, a part with a complicated structure and a large size is difficult to forge, and it can be cast or welded, and the material must have good casting properties or welding properties.
Depending on the process chosen, the material's processing possibilities for the process are to be considered. For casting, the liquidity of the material, the possibility of shrinkage cavities and segregation are considered; for welding, the weldability of the material and the tendency to crack are considered; for forging, the extensibility, hot brittleness and Deformability, etc.for parts requiring heat treatment, consider the hardenability of the material, the tendency of quenching deformation, etc.; for the parts to be machined, consider the hardness, machinability, degree of cold work and after cutting. Surface roughness that can be achieved, etc.
3. Economic requirements
(1) The relative price of the material itself should be as low as possible when the requirements for use are met. This is especially important for parts that are manufactured in large quantities.
(2) Processing cost of materials When the quality of parts is small and the processing volume is large, the processing cost accounts for a large proportion of the total cost of parts. Although cast iron is cheaper than steel, for some box-type parts produced in single or small batches, the use of cast iron is more costly than welding with steel plates, because the latter can save the manufacturing cost of the mold.
(3) The material utilization rate is processed by chipless or less chipping, such as die forging, precision casting, stamping, etc., which can improve the utilization rate of materials.
(4) Local quality principle In many cases, parts have different requirements for materials in different parts. It is virtually impossible to choose a material to meet different requirements, and if possible, the price is very expensive. At this time, according to the principle of local quality, different materials may be used in different China Mechanical Parts or different heat treatment processes may be adopted, so that the requirements of each part are respectively satisfied. For example, the teeth of the worm wheel must have excellent wear resistance and high resistance to gluing. Other parts only need to have general strength. Therefore, a bronze ring gear is used in the cast iron wheel core casing to meet these requirements. Another example is that the sliding bearing only requires friction reducing at the surface where it contacts the journal, so it is only necessary to use the friction reducing material to make the bearing bush without having to manufacture the entire bearing with the friction reducing material.
Local quality can also be obtained by carburizing, surface quenching, surface spraying, surface rolling and the like.
(5) Materials used to save valuable and rare materials Due to supply reasons or economic requirements, other materials may be substituted for the selected materials. For example, when the strength is the main requirement, materials with higher strength and higher price may be used, and materials with lower strength and lower cost may be used instead, and the structural size is appropriately increased; when wear resistance or corrosion resistance is the main requirement When it is not necessary to use materials with good wear resistance or corrosion resistance, poor materials can be used for various surface hardening treatments or anti-corrosion treatments; for rare materials, ordinary materials can also be used instead, for example, aluminum bronze is used instead of tin bronze to make bearing pads. .
(6) Material supply situation Starting from the supply and storage of simplified material varieties, the types and specifications of materials used on the same machine should be reduced as much as possible for small-volume production parts.