General principles for selecting metal parts:
- In the machine manufacturing industry, whether it is developing new products or updating old products, in the process of designing and manufacturing mechanical parts, in addition to standard parts that can be selected by designers by referring to the manual, most of them must consider the important issue of how to reasonably select materials. Practice has proved that there are many factors that affect the quality and production cost of products, among which the appropriateness of material selection often plays a key role.
- From the general procedure of mechanical parts design and manufacturing, first select the material according to the requirements of the working conditions of the parts, and then determine the structural shape and size of the parts according to the mechanical properties and process properties of the selected materials. When starting to manufacture parts, the processing technology plan should also be formulated according to the materials used. For example, if the selected material is cast iron, it can only be produced by casting. When selecting mechanical parts, the working conditions of the parts, the process properties of the materials and the cost of the products are mainly considered.
Some basic principles of material selection are described as follows:
(1) The selected materials must meet the requirements of the working conditions of the parts
- The working conditions of the parts are various. For example, the stress state can be tension, compression, bending, torsion, shear, etc.; the load properties can be static, impact, alternating; the working temperature can be room temperature, high temperature, low temperature; the environmental media can be acidic, alkaline, seawater, and lubricants. From the above working conditions, the stress state and load properties reflect the mechanical properties; the working temperature and environmental media belong to the use environment. The mechanical properties of the materials are also various. For example, the yield limit, strength limit, fatigue limit, etc. are indicators that reflect the strength of the material; the elongation, cross-sectional shrinkage, etc. are indicators that reflect the plasticity of the material; the impact toughness, fracture toughness, etc. are indicators that reflect the toughness of the material.
- Since the basic starting point of material selection is to meet the strength requirements of the parts, various strength indicators are usually directly used for the design calculation of the cross-sectional dimensions of the parts. However, δ, ψ, αk, K10, etc. are generally not directly used for design calculations. Sometimes, in order to ensure the safety of parts, they are used for indirect strength verification to determine whether the strength, plasticity and toughness of the selected materials are properly matched. As for the hardness index of the material, although a certain amount of estimation of the strength performance can be made, it is not used for the design and calculation of the parts. However, measuring hardness is relatively simple and is widely used in production.
- As for the use environment, it must also be considered when selecting materials. For example: for parts working at high temperatures, heat-resistant steel can be selected; for parts requiring corrosion resistance, austenitic stainless steel can be used; for parts requiring wear resistance, cemented carbide can be used; for parts requiring high hardness, tool steel can be used; and so on.
(2) The process performance of the material is also one of the important bases for material selection
- Because the production methods of parts are different, their quality and production cost will be directly affected.
- The basic processing methods of metal materials include casting, pressure processing, welding, cutting processing and heat treatment.
(3) When selecting materials, great attention must be paid to the economy of the material
- The selected materials should be both cheap and high-quality, and domestic materials should be selected as much as possible. Generally speaking, if cast iron can meet the requirements, cast steel is not needed; if carbon steel can meet the requirements, alloy steel is not needed. For example, some crankshafts and connecting rods use ductile iron instead of forged steel to produce, which reduces the amount of cutting and reduces costs.
- When selecting materials, we must pay attention to the economic efficiency of materials. We must not only consider the price of the material itself and all the costs required to manufacture parts, but also the function of the material. According to the principle of value engineering: value = function/cost. When comparing the results calculated by it, the value is not only the price of the material itself, but also the function and service life of the material, so it can more comprehensively reflect the economic efficiency of material selection. For example: To manufacture a corrosion-resistant container, there are three material selection options.
- One is to use ordinary carbon steel, with a manufacturing cost of 5,000 yuan and can be used for 1 year; the second is to use austenitic acid-resistant stainless steel, with a manufacturing cost of 40,000 yuan and can be used for 10 years; the third is to use ferritic stainless steel, with a manufacturing cost of 15,000 yuan and can be used for 6 years. According to the principle of value engineering, the value coefficients of the first, second and third options are 1:1.25:2, which shows that the third material selection option is more economical.
Want to learn more about metal parts processing? Please contact sales2@szmechanic.com
