Two, mechanical parts material selection principle
Selecting suitable materials from a variety of materials is a job restricted by many factors. The commonly used materials for mechanical parts include steel, cast iron, non-ferrous metals and non-metallic metals, etc. For the brand, performance and heat treatment knowledge of commonly used materials, please refer to the mechanical design manual. Selecting materials is an important step in mechanical design. With the continuous development of material science, the requirement of parts in machinery manufacturing industry is increasing. Therefore, when choosing materials, designers should fully understand the properties and applicable conditions of materials, and consider the use of parts, technology and economic requirements. The following is a brief introduction to the general selection principles of metal materials (mainly steel).
The selection of mechanical parts material principle is: the required materials should meet the requirements of the use of parts, have good technology and economy.
1. Use requirements
The use requirements of mechanical parts are as follows:
(1) The working condition and load condition of the parts, as well as the requirements put forward to avoid the corresponding failure form. The working condition refers to the environmental characteristics of the parts, working temperature, degree of friction and wear, etc. In hot and humid environment or corrosive medium working parts, the material should have good corrosion inhibition and corrosion resistance, such as stainless steel, copper alloy, etc.
The influence of working temperature on material selection, on the one hand, we should consider the linear expansion coefficient of the material of the two parts with each other can not be too big, so as not to produce too much thermal stress or loose the fit when the temperature changes; On the other hand, the mechanical properties of materials change with temperature. For parts working under sliding friction, to improve the surface hardness and enhance wear resistance, quenched steel, carburized steel, nitride steel and other varieties suitable for surface treatment should be selected or materials with good antifriction and wear resistance should be selected.
The load condition refers to the magnitude and nature of the load and stress. Brittle materials are in principle only suitable for making parts that operate under static loads; In the case of some impact, plastic materials should be used as the main material; For parts with large contact stress on the surface, materials that can be treated should be selected, such as surface hardened steel; For parts subjected to variable stress, fatigue resistant materials should be selected; For parts under impact load, materials with higher impact toughness should be selected; For the size depends on the strength, and the size and quality of the parts are limited, should choose higher strength materials; For parts whose size depends on stiffness, materials with larger elastic modulus should be selected.
The properties of metal materials can generally be enhanced and improved by heat treatment, so it is necessary to make full use of the means of heat treatment to give full play to the potential of materials. For common tempered steel, the blank with different mechanical properties can be obtained because of its tempering temperature. The higher the tempering temperature, the lower the hardness and strength of the material, and the better the plasticity. So when choosing the type of material. The heat treatment specification should also be specified and indicated on the drawing.
The size and mass of a part depends on the type of material and the method of making the blank. When the casting material is used to make the blank, the size and quality of the blank can not be restricted. The production capacity of forging machinery and equipment should be paid attention to when forging blanks are made with forging materials. In addition, the size of the parts and the size of the mass and the strength to weight ratio of materials, should be as far as possible to choose the strength to weight ratio of large materials, in order to reduce the size and quality of parts.
(3) The importance of parts in the whole machine or components.
2. Process requirements
Consider that the materials used can be easily manufactured from blank to finished product. For example, parts with complex structure and large size are difficult to forge, so casting or welding can be used, and the material needs to have good casting or welding properties.
Depending on the process selected, the processing possibilities of the material for the process should be considered. For casting, it is necessary to consider the liquid fluidity of the material, the possibility of shrinkage and segregation. For welding, the weldability of the material and the tendency to crack should be considered; For forging, it is necessary to consider the material’s extensibility, thermal brittleness and deformation ability. For parts that need heat treatment, the hardenability of materials and the tendency of quenching deformation should be considered; For the parts that need to be machined, it is necessary to consider the hardness of materials, easy machinability, cold hardening degree and surface roughness that can be achieved after cutting.
3. Economic requirements
(1) The relative price of the material itself should be selected as low as possible under the premise of meeting the requirements of use. This is especially important for mass-manufactured parts.
(2) Processing costs of materials When the quality of parts is small and the processing volume is large, the processing costs in the total cost of parts to take a large proportion. Although cast iron is cheaper than steel, it is more expensive to use cast iron than plate welding for some single or small batch box parts, because the latter can save mold manufacturing costs.
(3) The utilization rate of materials using no chip or less chip processing, such as die forging, investment casting, stamping, etc., can improve the utilization rate of materials.
(4) Local quality principle In many cases, parts in their different parts have different requirements for materials. It is virtually impossible to choose one material to meet different requirements, and even if possible, it is very expensive. At this time, according to the principle of local quality, different materials or different heat treatment processes can be used in different parts, so that the requirements of each part can be met. For example, the teeth of the worm gear need to have excellent wear resistance and high gluing resistance, and the other parts only need to have general strength, so the bronze gear ring is used in the cast iron core coat to meet these requirements. For example, the sliding bearing is required to have anti-friction only on the surface where it contacts with the journal, so only the bearing bush is made of anti-friction material, rather than the whole bearing is made of anti-friction material. Local quality can also be obtained by carburizing, surface quenching, surface spraying, surface rolling and other methods.
(5) Substitution of materials to save valuable and rare materials Due to supply reasons or economic requirements, the selected materials can be substituted with other materials. For example, when the strength is the main requirement, can choose higher strength and more expensive materials, can also be used to replace poor strength and cheap materials, and the structure size will be appropriately increased; When wear resistance or corrosion resistance is the main requirement, you can not choose good wear resistance or corrosion resistance materials and choose poor materials for a variety of surface hardening treatment or anticorrosion treatment; For rare materials, common materials can also be used instead, such as aluminum bronze instead of tin bronze to make the bearing bush.
(6) Material availability In order to simplify the supply and storage of material varieties, for small batch production parts, should be as much as possible to reduce the variety and specifications of materials used in the same machine.