What are the most popular basic engineering materi

2022-08-01
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What are the basic engineering materials

any mechanical part or tool is often subjected to various forms of external forces during use. For example, the steel cable on the crane is under the tension of the suspended object; When transmitting power, the connecting rod on the diesel engine is not only affected by the pulling force, but also by the impact force; Shaft parts are subjected to bending moment, torsion and so on. This requires that metal materials must have the ability to bear mechanical loads without exceeding the allowable deformation or damage. This ability is the mechanical properties of materials. The characteristics of metal, such as elasticity, strength, hardness, plasticity and toughness, are used to measure the mechanical properties of metal materials under external forces

1.1.1 strength

strength refers to the ability of metal materials to resist deformation and fracture under static load. The strength index is generally expressed by the load per unit area, i.e. force, and the symbol is σ, The unit is MPa

yield strength and tensile strength are commonly used in engineering. Yield strength refers to the stress at which the metal material yields under the action of external force, or the minimum stress at the beginning of plastic deformation σ S means. Tensile strength refers to the maximum stress that a metal material can bear before being pulled apart under the action of tensile force σ B means

for most mechanical parts, plastic deformation is not allowed during operation, so the yield strength is the basis for part strength design; For parts that fail due to fracture, the tensile strength is used as the basis for strength design

1.1.2 plasticity

plasticity refers to the ability of metal materials to produce plastic deformation without fracture under the action of external forces

the commonly used plasticity indexes in engineering are elongation and reduction of area. Elongation refers to the percentage of the ratio of the elongation of the specimen after breaking to the original length, with the symbol δ express. Reduction of area refers to the ratio of the reduced area of the section to the original cross-sectional area after the specimen is broken, which is expressed by y

the greater the elongation and reduction of area, the better the plasticity; On the contrary, the worse the plasticity. Good plasticity is a necessary condition for metal materials to be processed under pressure, and it is also a necessary condition to ensure the safety of mechanical parts without sudden brittle fracture

1.1.3 hardness

hardness refers to the ability of the material surface to resist the pressing of harder objects. There are many hardness test methods. There are two commonly used hardness test methods in production: Brinell hardness test method and Rockwell hardness test method

(I) Brinell hardness test method

the Brinell hardness test method uses a quenched steel ball or cemented carbide ball with a diameter of D as the indenter, presses it into the metal surface to be tested under the action of load P, unloads it after holding it for a certain time, measures the indentation diameter D formed on the metal surface, and takes the average pressure borne by the unit area of the indentation as the Brinell hardness value of the metal to be tested

Brinell hardness indicators include HBS and HBW. The indenter used by the former is quenched steel ball, which is suitable for metal materials with Brinell hardness value lower than 450, such as annealed steel, normalized steel, Quenched and tempered steel, cast iron, non-ferrous metals, etc; The latter indenter is hard alloy, which is suitable for metal materials with Brinell hardness of 450~650, such as quenched steel

Brinell hardness test method is not suitable for testing the hardness of finished parts or sheet metal due to large indentation

(II) Rockwell hardness test method

Rockwell hardness test method is to use a diamond cone with a top angle of 120 ° or a quenched steel ball with a diameter of f1.558mm (1/16 inch) as indenter, press it into the surface of the metal material to be tested with an uncertain load, and directly read the hardness value on the indicator plate of Rockwell hardness tester according to the indentation depth. There are three commonly used Rockwell hardness indexes: HRA, HRB and HRC

120 ° diamond cone is used as indenter. When the applied pressure is 600N, it is expressed by HRA. The measuring range is 60 ~ 85, which is suitable for measuring alloy, case hardened steel and thinner parts

using f1.588mm quenched steel ball as indenter, when the applied pressure is 1000N, it is expressed by HRC, and its measured hardness value range is 25 ~ 100, which is suitable for measuring non-ferrous metals, annealed and normalized steels and forged iron

the 120 ° diamond cone is used as the indenter. When the applied pressure is 1500N, it is expressed by HRC, and its measured hardness range is 20 ~ 67. It is suitable for measuring quenched steel, Quenched and tempered steel, etc

Rockwell hardness test is fast and easy to operate, and the indentation is small without damaging the workpiece surface, so it is suitable for finished product inspection

hardness is an important mechanical property index of materials. The higher the hardness of general materials, the better the wear resistance. The higher the strength of the material, the greater the plastic deformation resistance and the higher the hardness value

1.1.4 impact toughness

the ability of metal materials to resist impact load is called impact toughness, which is expressed in AK and the unit is j/cm2

impact toughness is usually measured by one-time pendulum impact bending test, that is, the tested material is made into a standard impact sample, and the pendulum is used for one-time punching to measure the impact a consumed by the punching sample, and the qualified steel ball is selected and replaced with a micrometer; (3) The error of the indentation measuring device is too large K, and then

the impact toughness is expressed by the impact energy AK consumed on the unit cross-sectional area f at the specimen notch

The greater the value of

ak, the better the toughness of the material. The material with low AK value is called brittle material, and the material with high AK value is called ductile material. Many parts, such as gears, connecting rods, etc., are subjected to great impact load during operation, so they should be made of materials with high AK value. The AK value of cast iron is very low, and the AK value of gray cast iron is close to zero, so it can not be used to manufacture parts bearing impact load

1.2 common metal materials

metal materials are abundant in sources and have excellent service and processing properties. They are the most commonly used materials in mechanical engineering. They are commonly used to manufacture mechanical equipment, tools and molds, and are widely used in engineering structures

metal materials can be roughly divided into two categories: ferrous metals. Ferrous metals usually refer to steel and cast iron; Nonferrous Metals refer to metals other than black and their alloys, such as copper alloys, aluminum and aluminum alloys

1.2.1 steel

steel is divided into carbon steel (carbon steel for short) and alloy

it also has obvious competitiveness in product cost performance and production efficiency.

carbon steel refers to iron carbon alloy with carbon content less than 2.11% and containing a small amount of silicon, manganese, sulfur and phosphorus impurities. The carbon content of industrial carbon steel is generally 0.05% ~ 1.35%

in order to improve the mechanical properties,

process properties or some special properties (such as corrosion resistance, heat resistance, wear resistance, etc.), some alloy elements (such as Mn, Si, Cr, Ni, Mo, W, V, Ti, etc.) are purposely added in smelting. This steel is called alloy steel

(I) carbon steel

1. classification of carbon steel

there are many classification methods for carbon steel, including the following three common ones

(1) according to the carbon content of steel, it is divided into three categories:

low carbon steel, with a carbon content of 0.60%

(2) according to the quality of steel (that is, according to the amount of harmful elements s and P in steel), it is divided into three categories:

ordinary carbon steel, the content of S and P in steel is ≤ 0.055% and 0.045% respectively

high quality carbon steel, the s and P contents in the steel are ≤ 0.040%;

high grade carbon steel, the s and P contents in the steel are ≤ 0.030% and 0.03 respectively, although the rock block and rock mass have the same pre geological environment of 5%.

(3) According to the use of steel, it is divided into two categories:

carbon structural steel, which is mainly used to manufacture various engineering components and mechanical parts

carbon tool steel is mainly used for manufacturing various tools, measuring tools and moulds

2. expression method of carbon steel grade

(1) carbon structural steel grade of carbon structural steel is composed of the first letter Q of the Chinese phonetic alphabet of yield point "Qu", yield point value, quality grade symbols (a, B, C, d) and deoxidation method symbols (F, B, Z) in sequence. The quality grade is increased in the order of a, B, C and D. f represents rimmed steel, B represents killed steel, Z represents killed steel, etc. For example, Q235-A · f refers to A-level boiling carbon structural steel with yield strength of 235mpa

(2) high quality carbon structural steel the grade of high quality carbon structural steel is represented by two digits. These two figures represent a few tenths of the average carbon content in steel. For example, 45 steel means high-quality carbon structural steel with an average carbon content of 0.45%. 08 steel, which means high-quality carbon structural steel with an average carbon content of 0.08%

(3) carbon tool steel the brand of carbon tool steel is represented by carbon Chinese phonetic prefix T and numbers. The figure represents a few thousandths of the average carbon content of steel. If it is high quality, add "a" after the number. For example, T12 steel represents a carbon tool steel with an average carbon content of 1.2%. T8 steel refers to carbon tool steel with an average carbon content of 0.8%. T12A refers to high-grade high-quality carbon tool steel with an average carbon content of 1.2%

3. examples of carbon steel applications

q195 and Q215 are used for rivets, cotter pins, stamping parts and welding components

q235, q255, used for bolts, nuts, pull rods, connecting rods and structural members of buildings and bridges

q275, used for high strength rotating shafts, spindles, gears, etc

q345, used for ships, bridges, vehicles and large steel structures

08 steel, with low carbon content and good plasticity, is mainly used to manufacture cold stamping parts

10 and 20 steels are commonly used to manufacture stamping and welding parts. Also commonly used in the manufacture of carburized parts

35, 40, 45 and 50 steels are medium carbon steels, which can obtain good comprehensive mechanical properties after heat treatment. They are mainly used to manufacture gears, sleeves, shaft parts, etc. These steels are widely used in mechanical manufacturing

t7 and T8 steels are used to manufacture tools with high toughness, such as punches and chisels

t9, T10, T11 steels are used as cutting tools requiring medium toughness and high hardness, such as drill bits, taps, saw blades, etc

t12 and T13 steels are used for files and drawing dies requiring higher hardness and wear resistance

(II) alloy steels there are many classification methods for alloy steels, including the following two

(1) it is divided into three categories according to the purpose:

alloy structural steel, which is used to manufacture various mechanical parts and engineering components with higher performance requirements

alloy structural steel is used to manufacture cutting tools, measuring tools and moulds with higher performance requirements

special performance steel, steel with special physical and chemical properties, such as stainless steel, heat-resistant steel, wear-resistant steel, etc

(2) the total content of ammonium alloy elements is divided into three categories:

low alloy steel, the total content of alloy elements is less than 5%

medium alloy steel, the total content of alloy elements is 5% ~ 10%

high alloy steel, the total content of alloy elements is greater than 10%

2. expression method of alloy steel grade

alloy steel is numbered according to the carbon content of steel and the type and quantity of alloy elements contained

① the first part of the steel grade indicates the carbon content and the content of alloy structural steel is the same as that of carbon structural steel, with one ten thousandth of carbon as the unit. If the first figure is 45, it means that the average carbon content is 0.45%; Alloy tool steel takes one thousandth of carbon as the unit. If the first figure is 5, it means that the average carbon content is 0.5%

② after the number indicating carbon content, use the chemical symbol of the element to indicate the alloy elements contained. The content of alloy elements is expressed in percent. When the average content is less than 1.5%, only the element symbol is marked, and the content is not marked. For example, 25mn2v means that the average carbon content is 0.25%, the manganese content is about 2%, and the vanadium content is less than 1.5%

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