As a mechanical designer, he must understand the industrial development trend when selecting materials, according to national standards, combining China’s resources and production conditions, and considering the materials and their selection from actual conditions. The most commonly used materials in mechanical manufacturing are steel and cast iron, followed by non-ferrous metals. Metal alloy. Non-metallic materials, such as plastics, rubber, etc., also have unique use value in machinery manufacturing.
Metallic material
1. Cast iron
Cast iron and steel are both iron-carbon alloys, and the main difference between them lies in the carbon content. Iron-carbon alloys with less than 2% carbon are called steel, and those with more than 2% carbon are called cast iron. Cast iron has proper fusibility and good liquid fluidity, so it can be cast into parts with complex shapes. In addition, its shock absorption, wear resistance, and machinability (referring to gray cast iron) are good and low in cost, so it is widely used in mechanical manufacturing. Commonly used cast irons are: gray cast iron, ductile cast iron, malleable cast iron, alloy cast iron, etc. Among them, gray cast iron and ductile iron are brittle materials and cannot be rolled or forged. Among the above cast irons, gray cast iron is the most widely used, followed by ductile cast iron.
2. Steel
Compared with cast iron, steel has high strength, toughness and plasticity, and heat treatment can be used to improve its mechanical properties and processing properties. The blank of steel parts can be obtained by forging, stamping, welding or casting, so its application is extremely wide. According to the purpose, steel can be divided into structural steel, tool steel and special steel. Structural steel is used to manufacture various mechanical parts and engineering structural components; tool steel is mainly used to manufacture various cutting tools, molds and measuring tools; special steel (such as stainless steel, heat-resistant steel, acid-resistant steel, etc.) is used for manufacturing in special environments Work parts. According to chemical composition, steel can be divided into carbon steel and alloy steel. The properties of carbon steel mainly depend on the carbon content. The higher the carbon content, the higher the strength of the steel, but the lower the plasticity. In order to improve the performance of steel, steel specially added with some alloying elements is called alloy steel. 1) Carbon structural steel The carbon content of this type of steel generally does not exceed 0.7%. Low carbon steel with a carbon content of less than 0.25% has low strength limit and yield limit, high plasticity, and good weldability. It is suitable for stamping and welding. It is commonly used to make screws, nuts, washers, shafts, Valve guide rods and welding components, etc. Low-carbon steel with a carbon content of 0.1% to 0.2% is also used to make carburized parts, such as gears, piston pins, and sprockets. Through carburizing and quenching, the surface of the parts is hard and wear-resistant, and the heart is tough and impact-resistant. If higher strength and impact resistance are required, low-carbon gold-containing steel can be used. Medium carbon steel with a carbon content of 0.3% to 0.5% has better comprehensive mechanical properties, not only higher strength, but also a certain degree of plasticity and toughness. It is often used as bolts, nuts, keys, Parts such as gears and shafts. High carbon steel with a carbon content of 0.55% to 0.7% has high strength and elasticity and is mostly used to make ordinary leaf springs, spiral springs or wire ropes.
2) Alloy structural steel
The effect of adding alloying elements to steel is to improve the performance of steel. For example: nickel can increase strength without reducing the toughness of steel; chromium can increase hardness, high temperature strength, corrosion resistance and the wear resistance of high-carbon steel; manganese can improve the wear resistance, strength and toughness of steel; the role of aluminum Similar to manganese, its effect is greater; vanadium can improve toughness and strength; silicon can increase elastic limit and wear resistance, but it will reduce toughness. The influence of alloying elements on steel is very complicated, especially when several alloying elements need to be added at the same time in order to improve the performance of steel. It should be noted that the excellent performance of alloy steel depends not only on the chemical composition, but also on the appropriate heat treatment to a greater extent.
3) Cast steel
The liquid fluidity of cast steel is worse than that of cast iron, so when ordinary sand casting is used, the wall thickness is often not less than 10mm. The shrinkage rate of steel castings is larger than that of iron castings, so the fillet of steel castings and transition parts of different wall thicknesses should be larger than iron castings. When choosing steel, it is necessary to use carbon steel with low price and sufficient supply as far as possible while meeting the requirements of use. When alloy steel is required, silicon, manganese, boron, and vanadium alloy steels should also be preferred.
3. Copper alloy
Copper alloys are divided into bronze and brass. Brass is an alloy of copper and zinc, and contains a small amount of manganese, aluminum, nickel, etc. It has good plasticity and fluidity, so it can be rolled and cast. Bronze can be divided into two types: tin-containing bronze and tin-free bronze. They have good anti-friction and corrosion resistance, and can also be rolled and cast. In addition, there are bearing alloys (or babbitt alloys), which are mainly used to make bearing linings for sliding bearings.
Non-metallic materials
1. Rubber
Rubber is rich in elasticity and can absorb more impact energy. It is often used as an elastic component of a coupling or a shock absorber, and a belt drive belt. Hard rubber can be used to make bearing liners lubricated with water.
2. Plastic
Plastics have a small specific gravity and are easy to be made into parts with complex shapes, and various plastics have different characteristics, such as corrosion resistance, heat insulation, insulation, anti-friction, and high friction coefficient. Therefore, they have been used in machine manufacturing in recent years. Its application is increasingly widespread. The plastic formed by using wood chips, asbestos fibers, etc. as fillers and compacting with thermosetting resin is called bonding plastic, which can be used to make instrument brackets, handles and other parts that are not subject to much force. Laminated fillers such as cloth, asbestos, and thin wood boards are used as the matrix, and the plastic formed by pressing with thermosetting resin is called laminated plastic, which can be used to make silent gears, bearing villages and friction plates. When designing mechanical parts, choosing the right material is a complex technical and economic issue. The designer should consider the use of parts, working conditions, physical, chemical, mechanical and technological properties of materials, and economic factors. This requires designers to have extensive knowledge and practical experience in materials and processes. The foregoing is only a brief description. The chemical composition and mechanical properties of various materials can be found in relevant national standards, industry standards and mechanical design manuals.
