Basic knowledge of metal materials


1. Meaning

The development of human civilization and the progress of society are closely related to metal materials. After the Stone Age, the Bronze Age and the Iron Age were marked by the application of metal materials. In modern times, a wide variety of metal materials have become an important material basis for the development of human society.

2. Types

Metal materials are usually divided into ferrous metals, non-ferrous metals and special metal materials.

(1) Ferrous metals are also called iron and steel materials, including industrial pure iron containing more than 90% iron, cast iron containing 2% to 4% carbon, carbon steel containing less than 2% carbon, and structural steel, stainless steel, Heat-resistant steel, high-temperature alloy, stainless steel, precision alloy, etc. Black metals in a broad sense also include chromium, manganese and their alloys.

(2) Non-ferrous metals refer to all metals and their alloys except iron, chromium and manganese, which are usually divided into light metals, heavy metals, precious metals, semi-metals, metals and rare earth metals. The strength and hardness of non-ferrous alloys are generally higher than that of pure metals, and the resistance is large and the temperature coefficient of resistance is small.

(3) Special metal materials include structural metal materials and functional metal materials for different purposes. Among them are amorphous metal materials obtained by rapid condensation process, as well as quasicrystal, microcrystalline, nanocrystalline metal materials, etc.; there are also functional alloys such as stealth, anti-hydrogen, superconductivity, shape memory, vibration damping, and metal matrix composites materials, etc.

3. Performance

Generally divided into two categories: process performance and service performance. The so-called process performance refers to the performance of the metal material under the specified cold and hot processing conditions during the processing and manufacturing of mechanical parts. The technological performance of metal materials determines its adaptability to processing and forming in the manufacturing process. Due to different processing conditions, the required process properties are also different, such as casting properties, weldability, forgeability, heat treatment properties, machinability, etc.

The so-called service performance refers to the performance of metal materials under the conditions of use of mechanical parts, including mechanical properties, physical properties, chemical properties, etc. The performance of metal materials determines its range of use and service life. In the machinery manufacturing industry, general mechanical parts are used in normal temperature, normal pressure and very strong corrosive medium, and each mechanical part will bear different loads during use. The resistance of metal materials to damage under load is called mechanical properties (also known as mechanical properties in the past). The mechanical properties of metal materials are the main basis for the design and selection of parts. The properties of the applied loads (such as tensile, compression, torsion, impact, cyclic loads, etc.) are different, and the mechanical properties required for metal materials will also be different. Commonly used mechanical properties include: strength, plasticity, hardness, impact toughness, multiple impact resistance and upper limit of fatigue.

Metal material characteristics

1. Fatigue

Many mechanical parts and engineering components work under alternating loads. Under the action of alternating load, although the stress level is lower than the upper yield limit of the material, after a long time of repeated stress cycling, sudden brittle fracture will occur, which is called fatigue of metal materials. The characteristics of fatigue fracture of metal materials are:

(1) The load stress is alternating;

(2) The action time of the load is longer;

(3) The fracture is instantaneous;

(4) Both plastic and brittle materials are brittle in the fatigue fracture zone. Therefore, fatigue fracture is a common and dangerous fracture form in engineering.

The fatigue phenomenon of metal materials can be divided into the following types according to different conditions:

(1) High-cycle fatigue: refers to fatigue with a stress cycle number of more than 100,000 under the condition of low stress (working stress is lower than the upper yield limit of the material, or even lower than the upper limit of elasticity). It is a common fatigue failure. High cycle fatigue is commonly referred to as fatigue.

(2) Low cycle fatigue: refers to the fatigue in which the number of stress cycles is below 10,000 to 100,000 under high stress (working stress is close to the upper yield limit of the material) or high strain. Since the alternating plastic strain plays a major role in this fatigue failure, it is also called plastic fatigue or strain fatigue.

(3) Thermal fatigue: refers to the fatigue damage caused by the repeated action of thermal stress caused by temperature changes.

(4) Corrosion fatigue: refers to the fatigue failure of machine components under the combined action of alternating loads and corrosive media (such as acid, alkali, seawater, reactive gas, etc.).

(5) Contact fatigue: This refers to the contact surface of machine parts. Under the repeated action of contact stress, pitting or surface crushing and peeling occurs, resulting in failure and damage of the machine parts.

2. Plasticity

Plasticity refers to the ability of metal materials to deform (plastic deformation) without being damaged under the action of external load. When a metal material is stretched, both the length and the cross-sectional area change. Therefore, the plasticity of the metal can be measured by two indicators, the elongation of the length (elongation) and the shrinkage of the section (shrinkage).

The greater the elongation and section shrinkage of the metal material, the better the plasticity of the material, that is, the material can withstand large plastic deformation without damage. Generally, metal materials with elongation greater than 5% are called plastic materials (such as low carbon steel, etc.), and metal materials with elongation less than 5% are called brittle materials (such as gray cast iron, etc.). A material with good plasticity can produce plastic deformation in a large macroscopic range, and strengthen the metal material due to plastic deformation at the same time of plastic deformation, thereby improving the strength of the material and ensuring the use of parts. In addition, materials with good plasticity can be smoothly processed by certain forming processes, such as stamping, cold bending, cold drawing, straightening, etc. Therefore, when selecting metal materials for mechanical parts, some plasticity indexes must be met.

3. Durability

The main forms of construction metal corrosion:

(1) Uniform corrosion. Corrosion of the metal surface results in a uniform thinning of the section. Therefore, the annual average thickness loss value is often used as an indicator of corrosion performance (corrosion rate). Steel generally corrodes uniformly in the atmosphere.

(2) Pitting corrosion. Metal corrodes in spots and forms deep pits. The generation of pitting corrosion is related to the nature of the metal and the medium in which it is located. Pitting corrosion is prone to occur in media containing chloride salts. The upper limit hole depth is often used as an evaluation index for pitting corrosion. Piping corrosion is often considered in the corrosion of pipelines.

(3) Galvanic corrosion. Corrosion at the contact of different metals due to different potentials.

(4) Crevice corrosion. Local corrosion of metal surfaces often occurs in crevices or other hidden areas due to differences in the composition and concentration of media between different parts.

(5) Stress corrosion. Under the combined action of corrosive medium and high tensile stress, the metal surface corrodes and expands into micro-cracks, which often lead to sudden breakage. This failure can occur with high-strength steel bars (steel wires) in concrete.

4. Hardness

Hardness indicates the ability of a material to resist the pressure of a hard object into its surface. It is one of the important performance indicators of metal materials. Generally, the higher the hardness, the better. Commonly used hardness indicators are Brinell hardness, Rockwell hardness and Vickers hardness.

Brinell hardness (HB): Press a part of the hardened steel ball (generally 10mm in diameter) into the surface of the material with a partial load (generally 3000kg) and keep it for a period of time. After the load is removed, the ratio of the load to its indentation area, It is the Brinell hardness value (HB) in kilogram force/mm2 (N/mm2).

Rockwell hardness (HR): When HB>450 or the sample is too small, the Brinell hardness test cannot be used and the Rockwell hardness measurement is used instead. It uses a diamond cone with an apex angle of 120° or a steel ball with a diameter of 1.59 and 3.18mm to press into the surface of the tested material under partial load, and the hardness of the material is obtained from the depth of the indentation. Depending on the hardness of the test material, different indenters and total test pressures can be used to form several different Rockwell hardness scales, and each scale is marked with a letter after the Rockwell hardness symbol HR. The commonly used Rockwell hardness scales are A, B, and C (HRA, HRB, HRC). Among them, the C scale is widely used.

HRA: It is the hardness obtained by using a 60kg load diamond cone indenter, which is used for materials with high hardness (such as cemented carbide, etc.).

HRB: It is the hardness obtained by using a 100kg load and a hardened steel ball with a diameter of 1.58mm. It is used for materials with lower hardness (such as annealed steel, cast iron, etc.).

HRC: It is the hardness obtained by using a load of 150kg and a diamond cone indenter, and is used for materials with high hardness (such as hardened steel, etc.).

Vickers hardness (HV): Use a load within 120kg and a diamond square cone indenter with an apex angle of 136° to press into the surface of the material, and divide the surface area of ​​the material indentation pit by the load value, which is the Vickers hardness value ( HV). Hardness test is a simple and easy test method in mechanical property test. In order to replace some mechanical performance tests with hardness tests, a more accurate conversion relationship between hardness and strength is required in production. Practice has proved that among various hardness values ​​of metal materials, there is an approximate corresponding relationship between the hardness value and the strength value. Because the hardness value is determined by the initial plastic deformation resistance and the continued plastic deformation resistance, the higher the strength of the material, the higher the plastic deformation resistance, and the higher the hardness value.

Properties of metal materials: The properties of metal materials determine the scope of application of the material and the rationality of its application. The properties of metal materials are mainly divided into four aspects, namely: mechanical properties, chemical properties, physical properties, and technological properties.

1. Mechanical properties

(1) The concept of stress, the force on a unit cross-sectional area inside an object is called stress. The stress caused by external force is called working stress, and the stress that is balanced inside the object under the condition of no external force is called internal stress (such as tissue stress, thermal stress, residual stress remaining after the machining process...).

(2) Mechanical properties, when a metal is subjected to external forces (loads) under partial temperature conditions, the ability to resist deformation and fracture is called the mechanical properties (also known as mechanical properties) of metal materials. The load that metal materials bear has many forms, it can be static load or dynamic load, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, friction, vibration, Impact, etc. Therefore, the indicators to measure the mechanical properties of metal materials mainly include the following:

1.1. Strength

This is the upper limit of the ability of the material to resist deformation and damage under the action of external force, which can be divided into upper limit of tensile strength (σb), upper limit of bending strength (σbb), upper limit of compressive strength (σbc), etc. Since metal materials have some laws to follow from deformation to failure under the action of external force, tensile tests are usually used for measurement, that is, metal materials are made into samples of some specifications, and stretched on a tensile testing machine until the test is performed. The samples were fractured, and the strength indicators measured were as follows:

(1) Upper limit of strength: the upper limit stress that the material can resist fracture under the action of external force, generally refers to the upper limit of tensile strength under the action of tensile force, expressed by σb, such as the upper limit of strength corresponding to the high point b in the tensile test curve, the commonly used unit is Megapascal (MPa), the conversion relationship is: 1MPa=1N/m2=(9.8)-1kgf/mm2 or 1kgf/mm2=9.8MPa.

(2) Upper limit of yield strength: When the external force of the metal material sample exceeds the upper limit of elasticity of the material, although the stress does not increase, the sample still undergoes obvious plastic deformation. When the degree of deformation is no longer proportional to the external force, obvious plastic deformation occurs. The stress at the time of yielding is called the upper limit of yield strength, which is represented by σs, and the S point corresponding to the tensile test curve is called the yield point. For materials with high plasticity, there will be an obvious yield point on the tensile curve, while for materials with low plasticity, there is no obvious yield point, so it is difficult to find the upper limit of yield according to the external force at the yield point. Therefore, in the tensile test method, the stress at which 0.2% plastic deformation occurs on the gauge length of the specimen is usually specified as the upper limit of conditional yield, which is represented by σ0.2. The upper yield limit index can be used as a design basis for requiring that parts do not experience significant plastic deformation during operation. However, for some important parts, it is also considered that the yield ratio (ie σs/σb) is smaller to improve its safety and reliability, but the utilization rate of materials is also lower at this time.

(3) Upper limit of elasticity: The material will deform under the action of external force, but the ability to return to its original state without external force is called elasticity. The upper limit stress at which the metal material can maintain elastic deformation is the upper limit of elasticity, which corresponds to the e point in the tensile test curve, expressed in σe, in megapascals (MPa): σe=Pe/Fo where Pe is the time when the elasticity is maintained The upper limit of the external force (or the load when the material is elastically deformed at the upper limit).

(4) Elastic modulus: this is the ratio of the stress σ to the strain δ (unit deformation corresponding to the stress) within the upper elastic limit of the material, expressed by E, in megapascals (MPa): E=σ/δ =tgα where α is the angle between the oe line on the tensile test curve and the horizontal axis ox. The elastic modulus is an index reflecting the rigidity of a metal material (the ability of a metal material to resist elastic deformation when it is stressed is called rigidity).

1.2. Plasticity

The upper limit ability of metal materials to deform without damage under the action of external force is called plasticity, usually the elongation of the gauge length of the sample during the tensile test δ(%) and the shrinkage of the sample section ψ(%) The elongation δ= [(L1-L0)/L0]x100%, this is the difference (increase) between the gauge length L1 and the original gauge length L0 of the specimen after the specimen is broken in the tensile test. ratio of L0. In the actual test, the measured elongation of tensile specimens of the same material but with different specifications (such as square, round, rectangular and gauge length) will be different, so special remarks are generally required, such as For commonly used circular section specimens, the elongation measured when the initial gauge length is 5 times the diameter of the sample is expressed as δ5, and the elongation measured when the initial gauge length is 10 times the diameter of the sample is expressed as δ10. Section shrinkage ψ=[(F0-F1)/F0]x100%, which is the difference between the original cross-sectional area F0 and the small cross-sectional area F1 at the neck of the fracture after the specimen is broken during the tensile test (section reduction) and F0 Ratio. In practice, the commonly used circular cross-section samples can usually be calculated by diameter measurement: ψ=[1-(D1/D0)2]x100%, where: D0- original diameter of the sample; D1- the fracture thickness after the sample is broken Small diameter at neck. The larger the δ and ψ values, the better the plasticity of the material.

1.3. Toughness

The ability of metal materials to resist damage under impact load is called toughness. The impact test is usually used, that is, when a metal sample of some size and shape is broken by an impact load on a specified type of impact testing machine, the impact energy consumed per unit cross-sectional area on the fracture is used to characterize the toughness of the material: αk=Ak/ The unit of F is J/cm2 or Kg·m/cm2, 1Kg·m/cm2=9.8J/cm2αk is called the impact toughness of the metal material, Ak is the impact energy, and F is the original cross-sectional area of ​​the fracture. 5. Under the action of repeated stress or alternating stress for a long time (the stress is generally less than the upper limit of yield strength σs), the phenomenon of fracture without significant deformation is called fatigue.

fatigue failure or fatigue fracture, which is due to a variety of reasons that cause a local stress (stress concentration) greater than σs or even greater than σb on the surface of the part, causing plastic deformation or micro-cracks to occur in the local area. With the increase of the number of repeated alternating stress , the crack gradually expands and deepens (stress concentration at the upper end of the crack) leads to the reduction of the actual cross-sectional area of ​​the part under stress until the local stress is greater than σb and fracture occurs. In practical applications, the sample is generally subjected to repeated or alternating stress (tensile stress, compressive stress, bending or torsional stress, etc.) within a specified number of cycles (generally 106 to 107 times for steel, and 106 to 107 times for non-ferrous metals). Take 108 times) the upper limit stress that can be endured without fracture as the upper limit of fatigue strength, expressed by σ-1, in MPa. In addition to the above five commonly used mechanical performance indicators, for some materials with particularly strict requirements, such as metal materials used in aerospace, nuclear industry, power plants, etc., the following mechanical performance indicators are also required: upper limit of creep: at some temperature and constant tensile load, the phenomenon of material slowly deforming plastically with time is called creep. High temperature tensile creep tests are usually used, that is, under constant temperature and constant tensile load, the creep elongation (total elongation or residual elongation) of the specimen within a specified time or the creep elongation rate is relatively constant In the stage of , the upper limit stress when the creep rate does not exceed a specified value, as the upper limit of creep, expressed in MPa, where τ is the test duration, t is the temperature, δ is the elongation, and σ is the stress; or where V is the creep velocity. The upper limit of tensile strength at high temperature: the upper limit stress of the sample under the action of constant temperature and constant tensile load, which can reach the specified duration without breaking, expressed in MPa, where τ is the duration, t is the temperature, and σ is stress. Metal notch sensitivity coefficient: expressed in Kτ, the ratio of the stress of the notched specimen to the unnotched smooth specimen when the duration is the same (high temperature tensile test): where τ is the test duration, and is the notched specimen is the stress of the smooth specimen. Or use: to represent, that is, the ratio of the duration of the notched specimen to the duration of the smooth specimen under the same stress σ. Thermal Resistance: The resistance of a material to mechanical loads at elevated temperatures.

2. Chemical properties

The properties of metals that cause chemical reactions with other substances are called chemical properties of metals. In practical applications, the corrosion resistance of metals, and the influence of compounds formed between different metals and between metals and non-metals on mechanical properties are mainly considered. Among the chemical properties of metals, especially the corrosion resistance, it is of great significance to the corrosion fatigue damage of metals.

3. Physical properties

The physical properties of metals are mainly considered:

(1) Density (specific gravity): ρ=P/V in grams/cubic centimeter or ton/cubic meter, where P is weight and V is volume. In practical applications, in addition to calculating the weight of metal parts based on density, it is important to consider the specific strength of the metal (ratio of strength σb to density ρ) to help material selection, as well as acoustic impedance in acoustic testing related to non-destructive testing (product of density ρ and speed of sound C) and materials with different densities in ray detection have different absorption capabilities for ray energy and so on.

(2) Melting point: The temperature at which the metal changes from a solid state to a liquid state has a direct impact on the smelting and thermal processing of the metal material, and has a great relationship with the high temperature performance of the material.

(3) Thermal expansion. As the temperature changes, the volume of the material also changes (expands or shrinks), which is called thermal expansion. It is often measured by the coefficient of linear expansion, that is, the ratio of the increase or decrease in the length of the material to its length at 0°C when the temperature changes by 1°C. . Thermal expansion is related to the specific heat of the material. In practical applications, the specific volume should also be considered (when the material is affected by external influences such as temperature, the increase or decrease of the volume of the material per unit weight, that is, the ratio of volume to mass), especially for working in a high temperature environment, or in cold or hot For metal parts working in alternate environments, the influence of their expansion properties must be considered.

(4) Magnetic. The property of attracting ferromagnetic objects is magnetism, which is reflected in parameters such as permeability, hysteresis loss, residual magnetic induction, coercive force, etc., so that metal materials can be divided into paramagnetic and diamagnetic, soft and hard magnetic materials. .

(5) Electrical properties. Its electrical conductivity is mainly considered, and its resistivity and eddy current loss are affected in electromagnetic nondestructive testing.

4. Process performance

The adaptability of metal to various processing methods is called process performance, which mainly includes the following four aspects:

(1) Cutting performance: It reflects the difficulty of cutting metal materials with cutting tools (such as turning, milling, planing, grinding, etc.).

(2) Forgeability: It reflects the difficulty of forming metal materials in the process of pressure processing, such as the level of plasticity when the material is heated to a partial temperature (expressed as the size of plastic deformation resistance), the temperature range that allows hot pressure processing Size, thermal expansion and contraction characteristics, and the limit of critical deformation related to microstructure and mechanical properties, fluidity of metal during thermal deformation, thermal conductivity, etc.

(3) Castability: It reflects the difficulty of melting and casting a metal material into a casting, which is expressed as fluidity, inhalation, oxidization, melting point in the molten state, uniformity and compactness of the microstructure of the casting, and coldness. shrinkage, etc.

(4) Weldability: It reflects the rapid heating of the metal material locally, so that the joint part is rapidly melted or semi-melted (requires pressure), so that the joint part can be firmly joined together and become a whole, which is expressed as melting point, Inhalation, oxidation, thermal conductivity, thermal expansion and contraction characteristics, plasticity, correlation with the microstructure of the seam and nearby materials, and the impact on mechanical properties during melting.

Development prospects of metal materials and metal products industry

The metal products industry includes the manufacture of structural metal products, metal tools, containers and metal packaging containers, containers, stainless steel and similar daily metal products, ships and marine engineering, etc. With the progress of society and the development of science and technology, metal products are used more and more widely in various fields of industry, agriculture and people's life, and they also create more and more value for the society.

The metal products industry also encountered some difficulties in the development process, such as single technology, low technical level, lack of advanced equipment, shortage of talents, etc., which restricted the development of the metal products industry. To this end, we can take measures to improve the technical level of enterprises, introduce advanced technology and equipment, and cultivate suitable talents to improve the development of the metal products industry.

In 2009, the products of the metal products industry will be more and more diversified, the technical level of the industry will be higher and higher, the product quality will be steadily improved, and the competition and market will be further rationalized. In addition to the further regulation of the industry by the state and the implementation of preferential policies for related industries, the metal products industry will have huge development space from 2009 to 2012.

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