Selecting the material to be used from a variety of materials is a work restricted by many factors.Therefore, how to choose the material of parts is an important part of parts design.The principle of selecting mechanical parts materials is: the required materials should meet the requirements of the use of parts and have good technology and economy.
Use Requirement
The use requirements of mechanical parts are as follows:
1) Working conditions and load conditions of parts and requirements to avoid corresponding failure forms.
Working condition refers to the environmental characteristics, working temperature and the degree of friction and wear of parts.Parts working in hot and humid environment or corrosive media, their materials should have good rust and corrosion resistance, in this case, can first consider the use of stainless steel, copper alloy.The influence of working temperature on material selection mainly has two aspects: on the one hand, it is necessary to consider the linear expansion coefficient of the materials of the two parts that cooperate with each other should not differ too much, so as not to produce excessive thermal stress or loose fit when the temperature changes;On the other hand, the change of mechanical properties of materials with temperature should also be considered.Parts working under sliding friction, to improve the surface hardness, in order to enhance wear resistance, should choose suitable for surface treatment of hardened steel, carburized steel, nitride steel and other varieties or choose friction reduction and wear resistance of good materials.
Load condition refers to the magnitude and nature of the load and stress on the part.Brittle materials are in principle only suitable for manufacturing parts that work under static loads;In the case of impact, plastic materials should be used as the main material;For the surface of the larger contact stress parts, should choose the surface treatment of materials, such as surface hardening steel;For parts subjected to stress, fatigue resistant materials should be selected;For parts under impact load, materials with high impact toughness should be selected;For the size depends on the strength and the size and quality of the parts are limited, should choose high strength materials;For parts whose dimensions depend on stiffness, materials with large elastic moduli should be selected.
Generally, the properties of metal materials can 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 materials.For the most commonly used modulated steel, the blank with different mechanical properties can be obtained due to the different tempering temperature.The higher the tempering temperature, the lower the hardness and stiffness of the material, and the better the plasticity.Therefore, when selecting the variety of materials, the heat treatment specification should be stipulated at the same time, and indicated on the drawing.
2) Limitations on the size and quality of parts.
Parts size and quality of the size and material variety and blank manufacturing method.Production of casting blank generally can not be limited by the size and mass size;In the production of forging blank, it is necessary to pay attention to the production capacity of forging machinery and equipment.In addition, the size of parts and quality of the size and material weight ratio, should be as far as possible to choose the strong weight ratio of large materials, in order to reduce the size and quality of parts.
3) The importance of parts in the whole machine and components.
4) Other special requirements (such as insulation, diamagnetic, etc.).
Technological Requirements
In order to make parts easy to manufacture, the complexity of parts structure, size and blank type should be considered when selecting materials.For parts with complex shape and large size, if casting blank is considered, good casting performance should be selected;If the welding blank is considered, low carbon steel with good welding performance should be selected.For simple shape, small size, large batch of parts, suitable for stamping and die forging, should choose good plastic materials.For the parts that need heat treatment, the material should have good heat treatment performance.In addition, the workability of the material itself and the workability after heat treatment should also be considered.
Economic Requirement
1) The relative price of the material itself
Under the premise of meeting the requirements of use, low-cost materials should be selected as far as possible.This is especially important for mass-manufactured parts.
2) Processing expenses of materials
When the quality of the part is not large and the amount of processing is large, the processing cost will account for a large proportion of the total cost of the part.Although cast iron is cheaper than steel plate, it is more expensive to weld cast iron than steel plate for certain single or small-volume box parts because the latter saves the cost of mold manufacturing.
3) Save materials
In order to save materials, heat treatment or surface strengthening (shot peening, rolling, etc.) can be used to give full play to and use the potential mechanical properties of materials;Surface coating (chrome plating, copper plating, blackening, blue, etc.) can also be used to reduce the degree of corrosion and wear, prolong the service life of parts.
4) Utilization rate of materials
In order to improve the utilization rate of materials, no cutting or less cutting can be used, such as die forging, investment casting, stamping, etc., which can not only improve the utilization rate of materials, but also reduce the working hours of cutting.
5) Save valuable materials
Through the use of combination structure, can save the price of higher materials, such as combined structure worm gear ring with good friction reduction but expensive tin bronze, and the wheel core is cheap cast iron.
6) Save rare materials
In this respect, manganese-boron alloy steels with abundant resources in China can be used to replace chromium-nickel alloy steels with less resources, and aluminum bronze can be used to replace tin bronze.
7) The supply of materials
In the selection of materials, should choose locally available and easy to supply materials, in order to reduce the cost of procurement, transportation, storage;From the simplified material varieties of the perspective of supply and storage, for small batch production of parts, should reduce as far as possible on the same machine use material varieties and specifications, in order to simplify the supply and management, and in the process of machining and heat treatment much easier to master the most reasonable operation method, thus improve the manufacturing quality, reduce scrap, improve labor productivity.
Post time: Feb-22-2022