Basic knowledge of metal materials

Metallic materials refer to the general name of metallic elements or materials with metallic characteristics mainly composed of metallic elements. Including pure metals, alloys, metal materials, intermetallic compounds and special metal materials. (Note: Metal oxides (such as alumina) are not 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 all used the application of metal materials as a significant symbol of their times. In modern times, a wide variety of metal materials have become an important material basis for the development of human society.

2. Type
Metal materials are generally divided into ferrous metals, non-ferrous metals and special metal materials.

(1) Ferrous metals, also known as iron and steel materials, include industrial pure iron with more than 90% iron, cast iron with 2% to 4% carbon, carbon steel with less than 2% carbon, and structural steel, stainless steel, Heat resistant steel, high temperature alloy, stainless steel, precision alloy, etc. Ferrous 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. They are generally divided into light metals, heavy metals, precious metals, semi-metals, rare metals, and rare earth metals. The strength and hardness of non-ferrous alloys are generally higher than 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 through the rapid condensation process, as well as quasicrystalline, microcrystalline, nanocrystalline metal materials, etc.; and special functional alloys such as stealth, hydrogen resistance, superconductivity, shape memory, wear resistance, vibration damping, etc. And metal-based composite materials.

3. Performance
Generally divided into two types of process performance and performance. The so-called process performance refers to the performance of metal materials under the specified cold and hot processing conditions during the processing of manufacturing parts. The quality of the metal material’s process performance determines its adaptability to processing and forming during the manufacturing process. Due to different processing conditions, the required process performance is also different, such as casting performance, weldability, forgeability, heat treatment performance, cutting machinability, etc.

The so-called performance refers to the performance of metal materials under the conditions of use of mechanical parts, which includes mechanical properties, physical properties, chemical properties and so on. The quality of the metal material’s performance determines its range and service life. In the mechanical manufacturing industry, general mechanical parts are used in normal temperature, normal pressure and very strong corrosive media, 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 parts design and material selection. The properties of the applied load are different (such as tension, compression, torsion, impact, cyclic load, etc.), and the mechanical properties required for the metal materials will also be different. Commonly used mechanical properties include: strength, plasticity, hardness, impact toughness, multiple impact resistance and fatigue limit.

Characteristics of metal materials

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 yield limit of the material, but after a long period of repeated stress cycles, sudden brittle fracture will also occur, this phenomenon is called fatigue of metal materials. The characteristics of metal material fatigue fracture are:

(1) The load stress is alternating;
(2) The action time of the load is longer;
(3) The fracture occurs instantaneously;
(4) Both plastic and brittle materials are brittle in the fatigue fracture zone. Therefore, fatigue fracture is the most common and dangerous form of fracture 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 of more than 100,000 under low stress (working stress is below the yield limit of the material or even below the elastic limit). It is the most common type of fatigue damage. High cycle fatigue is generally referred to as fatigue.

(2) Low cycle fatigue: refers to fatigue under high stress (the working stress is close to the yield limit of the material) or high strain, and the number of stress cycles is less than 10000~100,000. Since 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 due to temperature changes.

(4) Corrosion fatigue: refers to the fatigue damage of machine parts under the combined action of alternating load and corrosive medium (such as acid, alkali, seawater, active gas, etc.).

(5) Contact fatigue: This refers to the contact surface of machine parts. Under the repeated action of contact stress, pitting peeling or surface crushing and peeling occur, which causes the failure of the machine parts.

2. Plasticity
Plasticity refers to the ability of a metal material to generate permanent deformation (plastic deformation) without being destroyed under the action of an external force under load. When a metal material is stretched, both its length and its cross-sectional area change. Therefore, the plasticity of a metal can be measured by two indicators: length elongation (elongation) and section shrinkage (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 greater plastic deformation without destruction. Generally speaking, metal materials with elongation greater than 5% are called plastic materials (such as low carbon steel, etc.), while metal materials with elongation less than 5% are called brittle materials (such as gray cast iron, etc.). A plastic material with good plasticity can generate plastic deformation in a larger macroscopic range, and at the same time plastic deformation, the metal material is strengthened due to plastic deformation, thereby improving the strength of the material and ensuring the safe use of parts. In addition, materials with good plasticity can be successfully processed by certain forming processes, such as stamping, cold bending, cold drawing, and straightening. Therefore, when selecting metal materials as mechanical parts, certain plasticity indexes must be met.

3. Durability
The main forms of corrosion of construction metals:
(1) Even corrosion. Corrosion of the metal surface makes the cross section evenly thin. Therefore, the annual average thickness loss value is often used as an index of corrosion performance (corrosion rate). Steel generally corrodes uniformly in the atmosphere.

(2) Porosity. The metal corrosion is spot-shaped and forms deep pits. The occurrence of pitting corrosion is related to the nature of the metal and its medium. Porosity easily occurs in media containing chlorine salts. The maximum hole depth is usually used as the evaluation index for pitting corrosion. The corrosion of pipelines mostly considers the problem of pitting corrosion.

(3) Galvanic corrosion. Corrosion caused by different potentials at the contact of different metals.

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

(5) Stress corrosion. Under the combined action of corrosive medium and higher tensile stress, the metal surface corrodes and expands into micro-cracks, often leading to sudden breakage. High-strength steel bars (steel wires) in concrete may cause such damage.

4. Hardness
Hardness indicates the ability of a material to resist the pressing of hard objects into its surface. It is one of the important performance indexes of metal materials. Generally, the higher the hardness, the better the wear resistance. Commonly used hardness indexes are Brinell hardness, Rockwell hardness and Vickers hardness.

Brinell hardness (HB): Press a hardened steel ball of a certain size (generally 10mm in diameter) into the surface of the material with a certain load (generally 3000kg), and keep it for a period of time. After unloading, the ratio of the load to the indentation area, That is the Brinell hardness value (HB), the unit is 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 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 be pressed into the surface of the material under certain 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, each of which 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). The C scale is the most widely used.

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

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

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

Vickers hardness (HV): Press the load within 120kg and a diamond square cone indenter with an apex angle of 136° into the material surface, and divide the surface area of ​​the material indentation pit by the load value, which is the Vickers hardness value ( HV). Hardness test is the most simple and easy test method in mechanical performance 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 between various hardness values ​​of metal materials, there is an approximate corresponding relationship between hardness values ​​and strength values. 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, the higher the hardness value.

Performance of metal materials: The performance of metal materials determines the scope of application and the rationality of applications. The performance of metal materials is mainly divided into four aspects, namely: mechanical properties, chemical properties, physical properties, process performance.

1. Mechanical properties
(1) The concept of stress. The force on the unit cross-sectional area of ​​an object is called stress. The stress caused by the external force is called the working stress, and the stress balanced in the interior of the object without external force is called the internal stress (such as tissue stress, thermal stress, residual stress left after the processing process…).

(2) Mechanical properties. When a metal is subjected to external forces (loads) under certain temperature conditions, its ability to resist deformation and fracture is called the mechanical properties (also called mechanical properties) of the metal material. There are many forms of load that a metal material can bear. It can be a static load or a dynamic load, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, friction, vibration, Impact and so on, so the indicators to measure the mechanical properties of metal materials mainly include the following:

1.1. Strength
This is the maximum ability to characterize the material’s resistance to deformation and damage under the action of external forces. It can be divided into tensile strength limit (σb), flexural strength limit (σbb), and compressive strength limit (σbc). Since metal materials have certain rules to follow from deformation to destruction under the action of external force, they are usually measured by tensile test, that is, the metal material is made into a sample of a certain specification and stretched on a tensile testing machine until the test The sample breaks, and the measured strength indicators are:

(1) Strength limit: The maximum stress that a material can resist fracture under external force, generally refers to the tensile strength limit under the action of tension, expressed by σb, such as the strength limit corresponding to the highest point b in the tensile test graph, commonly used units For megapascals (MPa), the conversion relationship is: 1MPa=1N/m2=(9.8)-1kgf/mm2 or 1kgf/mm2=9.8MPa.

(2) Yield strength limit: When the external force that the metal material sample bears exceeds the elastic limit of the material, although the stress no longer increases, the sample still undergoes significant plastic deformation. This phenomenon is called yielding, that is, the material bears the external force to a certain level Degree, its deformation is no longer proportional to the external force and produces obvious plastic deformation. The stress at the time of yielding is called the yield strength limit, expressed by σs, and the point S corresponding to the tensile test curve is called the yield point. For materials with high plasticity, obvious yield points will appear on the tensile curve, while for low plastic materials there is no obvious yield point, making it difficult to find the yield limit based on the external force at the yield point. Therefore, in the tensile test method, the stress at the time when the gauge length on the sample is 0.2% plastically deformed is usually defined as the conditional yield limit, which is expressed as σ0.2. The yield limit index can be used as a design basis for requiring parts to not produce significant plastic deformation during work. But for some important parts, it is also considered that the yield ratio (ie, σs/σb) should be smaller to improve its safety and reliability, but the material utilization rate is also lower at this time.

(3) Elastic limit: The material will deform under the action of external force, but the ability to return to its original state after removing the external force is called elasticity. The maximum stress that the metal material can maintain the elastic deformation is the elastic limit, which corresponds to the e point in the tensile test graph, expressed as σe, and the unit is megapascal (MPa): σe=Pe/Fo where Pe is when the elasticity is maintained The maximum external force (or load when the material is elastically deformed).

(4) Elastic modulus: This is the ratio of the stress σ and strain δ (unit deformation corresponding to the stress) of the material in the elastic limit range, expressed by E, unit MPa (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 that reflects 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 maximum ability of a metal material to produce permanent deformation without breaking under the action of external force is called plasticity, usually based on the sample gauge length elongation δ (%) and sample section shrinkage ψ (%) elongation δ during tensile test =[(L1-L0)/L0]x100%, this is the difference between the gauge length L1 after the specimen is broken and the original gauge length L0 of the specimen after the specimen is pulled off during the tensile test (increase) Ratio with L0. In the actual test, the elongation of the tensile test specimens of the same material but different specifications (diameter, cross-sectional shape-such as square, round, rectangular and gauge length) will be different, so it is generally necessary to add special, such as For the most commonly used circular cross-section specimens, the elongation measured when the initial gauge length is 5 times the specimen diameter is expressed as δ5, and the elongation measured when the initial gauge length is 10 times the specimen diameter is expressed as δ10 . The section shrinkage rate ψ=[(F0-F1)/F0]x100%, which is the difference between the original cross-sectional area F0 and the minimum cross-sectional area F1 at the fracture neck of the specimen after the tensile test (the reduction in section) and F0 Ratio. In practice, the most commonly used round cross-section specimens can usually be calculated by diameter measurement: ψ=[1-(D1/D0)2]x100%, where: D0- original diameter of the specimen; D1- fracture of the specimen after breaking The smallest diameter at the thin neck. The larger the values ​​of δ and ψ, the better the plasticity of the material.

1.3. Resilience
The ability of a metal material to resist damage under impact load is called toughness. The impact test is usually used, that is, when a metal sample of a certain 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 characterizes 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 metal materials, Ak is the impact energy, and F is the original cross-sectional area of ​​the fracture. 5. Fatigue strength limit Metal materials under long-term repeated stress or alternating stress (stress is generally less than the yield limit strength σs), the phenomenon of fracture without significant deformation is called fatigue failure or fatigue fracture, which is due to Various reasons make the part of the surface of the part cause stress (stress concentration) greater than σs or even greater than σb, causing plastic deformation or micro-cracks in this part. As the number of repeated alternating stress increases, the crack gradually expands and deepens (crack tip Local stress concentration) causes the actual cross-sectional area of ​​the local stress to be reduced 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, for non-ferrous metals Take 108 times) The maximum stress that can withstand without fracture is taken as the fatigue strength limit, which is expressed by σ-1 and the unit is MPa. In addition to the above five most commonly used mechanical performance indicators, for some particularly strict materials, such as metal materials used in the aerospace and nuclear industry, power plants, etc., the following mechanical performance indicators will also be required: Creep limit: in a certain Under temperature and constant tensile load, the phenomenon that the material slowly undergoes plastic deformation over time is called creep. Usually high temperature tensile creep test is used, that is, under constant temperature and constant tensile load, the creep elongation (total elongation or residual elongation) of the sample within a specified time or the creep elongation speed is relatively constant The maximum stress when the creep speed does not exceed a certain value is expressed as the creep limit in MPa, where τ is the test duration, t is the temperature, δ is the elongation, and σ is the stress; or In terms of, V is the creep speed. High temperature tensile endurance strength limit: the maximum stress at which the specimen reaches a specified duration without breaking under constant temperature and constant tensile load, expressed in units of MPa, where τ is the duration, t is the temperature, and σ For stress. Metal notch sensitivity coefficient: Kτ is the ratio of stress between a notched specimen and a smooth specimen without notch when the duration is the same (high-temperature tensile endurance test): where τ is the test duration, which is the notch test Such stress is the stress of a smooth specimen. Or, it can be expressed as: the ratio of the duration of a notched specimen to the duration of a smooth specimen under the same stress σ. Heat resistance: The resistance of materials to mechanical loads at high temperatures.

2. Chemical properties

The property of a chemical reaction between a metal and other substances is called the chemical properties of the metal. In practical applications, the corrosion resistance and oxidation resistance of metals (also known as oxidation resistance, which refers specifically to the resistance or stability of metals to oxidation at high temperatures), and between different metals, metals and The effect of compounds formed between non-metals on mechanical properties and so on. In the chemical properties of metals, especially corrosion resistance 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 unit gram/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 (the ratio of strength σb to density ρ) to help material selection and acoustic impedance in acoustic testing related to nondestructive testing (The product of the density ρ and the speed of sound C) and the materials with different densities in ray detection have different absorption capacities for ray energy and so on.

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

(3) Thermal expansion. With the temperature change, the volume of the material also changes (expansion or contraction) is called thermal expansion, which is often measured by the coefficient of linear expansion. That is, when the temperature changes by 1℃, the ratio of the increase and decrease of the length of the material to its length at 0℃ . Thermal expansion is related to the specific heat of the material. In practical applications, the specific volume must also be considered (when the material is affected by temperature and other external influences, the volume of the material per unit weight increases or decreases, that is, the ratio of volume to mass), especially for working in a high temperature environment, or in cold or hot Metal parts working in alternating environments must consider the effect of their expansion properties.

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

(5) Electrical performance. Mainly consider its electrical conductivity, which affects its resistivity and eddy current loss in electromagnetic nondestructive testing.

4. Process performance
The adaptability of metal to various processing methods is called process performance, which mainly has the following four aspects:
(1) Cutting performance: reflects the difficulty of cutting metal materials with cutting tools (such as turning, milling, planing, grinding, etc.).

(2) Forgeability: Reflects the difficulty of forming metal materials in the process of pressure processing, such as the plasticity of the material when it is heated to a certain temperature (represented by the resistance to plastic deformation), the temperature range that allows hot pressure processing Size, thermal expansion and contraction characteristics and critical deformation limits related to microstructure and mechanical properties, metal fluidity and thermal conductivity during thermal deformation.

(3) Castability: Reflects the ease of melting and casting of metal materials into castings, and shows the fluidity, gettering, oxidation, melting point in the molten state, the uniformity, compactness, and coldness of the microstructure of the casting Shrinkage, etc.

(4) Weldability: reflecting the rapid heating of the metal material in the local area, which quickly melts or semi-melts the joints (need to be pressurized), so that the joints are firmly bonded together to form a whole. Inhalation, oxidation, thermal conductivity, thermal expansion and contraction characteristics, plasticity, correlation with the microstructure of the joints and nearby materials, influence on mechanical properties, etc. during melting.

Development prospects of metal materials and metal products industry
The metal product industry includes structural metal product manufacturing, metal tool manufacturing, container and metal packaging container manufacturing, container, stainless steel and similar daily metal product manufacturing, shipbuilding and marine engineering manufacturing, etc. With the progress of society and the development of science and technology, metal products are more and more widely used in various fields of industry, agriculture and people’s lives, and also create greater and greater value for society.

The metal products industry also encountered some difficulties in the development process, such as single technology, low technical level, lack of advanced equipment, and shortage of talents, 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 technical equipment, and cultivate suitable talents to improve the development of China’s metal products industry.

In 2009, the products of the metal products industry will become 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. Coupled with the country’s further regulation of the industry and the implementation of relevant industry preferential policies, from 2009 to 2012, the metal products industry will have huge room for development.