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Basic knowledge of metal materials

by AOMag | post a comment

Metal material refers to the metal element or the metal element mainly composed of a metal material with the general term. Including pure metals, alloys, metallic intermetallic compounds and special metallic materials (Note: metal oxides (such as alumina) are not metal materials).

species---

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

(1) Ferrous metals, also known as ferrous materials, include industrial pure iron with over 90% iron, cast iron with 2% -4% carbon, carbon steel with less than 2% carbon, structural steel, stainless steel, Heat-resistant steel, high temperature alloys, stainless steel, precision alloys. The broader ferrous metals 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 usually divided into light metals, heavy metals, precious metals, semimetals, 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, the temperature coefficient of resistance is small.

(3) Special metal materials include structural metal materials and functional metal materials for different purposes. Among them, amorphous metal materials obtained by rapid condensation process, as well as quasicrystal, microcrystal and nanocrystalline metal materials, and other special functional alloys such as stealth, hydrogen resistance, superconductivity, shape memory, wear resistance and vibration damping As well as metal matrix composites.

performance---

Generally divided into two types of process performance and performance. The so-called process performance refers to the mechanical parts in the manufacturing process, the metal material in the set of cold, hot processing conditions showed performance. The performance of metal materials is good or bad, it determines the manufacturing process adaptability of forming. Due to different processing conditions, the required process performance is different, such as casting performance, weldability, malleability, heat treatment performance, machinability and so on.

The so-called performance refers to the use of mechanical parts in the conditions of use, the performance of the metal material performance, which includes mechanical properties, physical properties, chemical properties. The performance of metal materials is good or bad, determines its use and service life. In the machinery manufacturing industry, the general mechanical parts are used in normal temperature, atmospheric pressure and very strong corrosive medium, and all mechanical parts will be subjected to different loads during use. Metallic materials are resistant to damage under load and are called mechanical properties (formerly known as mechanical properties). The mechanical properties of metal materials is the main basis for the design and selection of parts. Depending on the nature of the applied load (eg, tensile, compressive, torsional, impact, cyclic loading, etc.), the required mechanical properties of the metal material will also vary. Common mechanical properties include: strength, plasticity, hardness, impact toughness, multiple impact resistance and fatigue limits.

Metal material characteristics ---

Fatigue

Many mechanical parts and engineering components, is to withstand alternating load work. Under the action of alternating load, although the stress level is lower than the yield limit of the material, sudden brittle fracture will occur after repeated cyclic stress for a long time. This phenomenon is called fatigue of metal material. Metal fatigue fracture characteristics are: (1) Load stress is alternating;

(2) the role of load longer time;

(3) rupture occurs instantaneously;

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

Fatigue of metal materials, according to different conditions can be divided into the following categories:

(1) High-cycle fatigue: refers to the fatigue under the condition of low stress (the working stress is lower than the yield limit of the material, or even lower than the elastic limit), and the stress cycle number is more than 100,000. It is the most common type of fatigue damage. High cycle fatigue is generally referred to as fatigue.

(2) low cycle fatigue: refers to the stress in the high stress (working stress close to the material yield limit) or high strain conditions, stress cycle weeks in the 10000 to 100000 the following fatigue. Since alternating plastic strain plays a major role in this fatigue failure, it is also called plastic or strain fatigue.

(3) thermal fatigue: Refers to the thermal stress caused by repeated changes in temperature, resulting in fatigue damage. (4) Corrosion Fatigue: refers to the fatigue damage caused by the joint action of machine parts under alternating load and corrosive medium (such as acid, alkali, seawater, active gas, etc.).

(5) Contact fatigue: This refers to the contact surface of the machine parts. Under the repeated action of contact stress, the pitting flakes or surface crushing and peeling appear and the machine parts fail to be destroyed.

Plasticity

Plasticity refers to the ability of metal materials to undergo permanent deformation (plastic deformation) without being damaged by external forces under load. When the metal material is stretched, the length and cross-sectional area should be changed. Therefore, the plasticity of the metal can be measured by the elongation (elongation) and the shrinkage (reduction of area) of the length.

The greater the elongation and reduction of area of ​​the metallic material, the better the plasticity of the material, ie, the material can withstand greater plastic deformation without damage. Metal materials with an elongation of more than 5% are generally referred to as plastic materials (such as mild steel) while those with an elongation of less than 5% are called brittle materials (such as gray cast iron, etc.). Plastic material, it can produce plastic deformation in a larger macro range, and in the plastic deformation of the metal material due to plastic deformation and strengthen, thereby enhancing the strength of the material to ensure the safe use of the parts. In addition, the plastic material can be smooth for some molding process, such as stamping, cold bending, drawing, straightening and so on. Therefore, the choice of metal materials for mechanical parts, must meet certain plastic specifications.

Durability

The main form of construction metal corrosion:

(1) uniform corrosion. Corrosion of the metal surface makes the cross section thinner uniformly. Therefore, the annual average thickness loss value as an indicator of corrosion performance (corrosion rate). Steel is generally uniform corrosion in the atmosphere.

(2) Pitting corrosion. Corrosion of metal points and the formation of deep pit. Pitting corrosion and the nature of the metal and the medium in which it is concerned. Pitting occurs easily in media containing chloride salts. Pits commonly used as the maximum pore depth assessment. Pipe corrosion and more consideration pitting problems.

(3) Galvanic corrosion. Different metal contact, due to the different potential corrosion.

(4) crevice corrosion. Metallic surface cracks or other hidden areas often occur due to the different parts of the medium between the composition and concentration of the local corrosion caused.

(5) stress corrosion. Corrosion in the medium and high tensile stress, the metal surface corrosion and inward expansion into micro-cracks, often leading to sudden rupture. This damage can occur in high-strength steel (steel wire) in concrete.

Hardness

Hardness means the ability of a material to resist hard objects pressing on their surface. It is one of the important performance indicators of metallic materials. The higher the general hardness, the better wear resistance. Commonly used hardness indicators Brinell hardness, Rockwell hardness and Vickers hardness.

Brinell hardness (HB): With a certain load (usually 3000kg) to a certain size (diameter is generally 10mm) hardened steel ball pressed into the material surface, to maintain a period of time, after loading, the load ratio of its indentation area, Brinell hardness is the value (HB), in kilograms force / mm2 (N / mm2).

Rockwell hardness (HR): When HB> 450 or the sample is too small, you can not use the Brinell hardness test instead Rockwell hardness measurement. It is a 120 ° angle diamond cone or 1.59,3.18 mm diameter steel ball, under certain load into the surface of the material under test, indentation depth obtained by the hardness of the material. Depending on the hardness of the test material, several different Rockwell hardness scales can be made using different indenters and total test pressure, with one letter for each gauge, following a Rockwell hardness symbol HR. Commonly used Rockwell hardness scale A, B, C three (HRA, HRB, HRC). C scale which is the most widely used.

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

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

HRC: 150kg load and diamond cone press obtained hardness, for high hardness materials (such as hardened steel, etc.). Vickers hardness (HV): press the surface of the material with a diamond square cone press with a load of 120 kg and a vertex angle of 136 °. The surface area of ​​the indentation of the material is divided by the load value, which is the Vickers hardness value HV). Hardness test is the easiest test method in mechanical properties test. In order to use hardness test instead of some mechanical properties test, the production needs a more accurate conversion of hardness and strength. Practice has proved that the various hardness values ​​of metal materials, hardness and strength values ​​have an approximate corresponding relationship between. Because the hardness value is determined by the initial resistance to plastic deformation and resistance to plastic deformation, the higher the strength of the material, the higher the plastic deformation resistance, the higher the hardness value.

Performance of metal materials ---

The properties of metal materials determine the scope of application of materials and the rationality of the application. The performance of metal materials can be divided into four aspects, namely: mechanical properties, chemical properties, physical properties, process performance.

Mechanical properties

(A) the concept of stress, the internal unit of the cross-sectional area of ​​?? the force borne by the stress. The stress caused by external force is called working stress. The stress that is balanced inside the object without external force is called internal stress (such as the stress of tissue, the thermal stress, the residual stress after the process is finished ...).

(B) mechanical properties, metal under certain temperature conditions to withstand external force (load), the ability to resist deformation and fracture is called the mechanical properties of metal materials (also known as mechanical properties). Metallic materials can be subjected to loads in many forms, either static or dynamic, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, as well as friction, vibration, Impact and so on, so the measure of the mechanical properties of metal materials are mainly the following indicators:

1.1. Strength

This is the maximum capacity of the material to resist deformation and damage under external force. It can be divided into tensile strength limit (σb), bending strength limit (σbb), compressive strength limit (σbc) and so on. As the metal material under the action of external force from deformation to destruction have a certain rule to follow, it is usually measured by the tensile test, the metal material into a certain size of the sample, the tensile test machine stretching until the test Like fracture, the determination of the strength indicators are:

(1) Strength limit: the maximum stress that the material can resist the fracture under the external force, generally refers to the tensile strength limit under the tensile force, expressed as σb. For example, the tensile strength at the highest point b in the tensile test curve corresponds to the common unit Is MPa, the conversion relations are: 1MPa = 1N / m2 = (9.8) -1kgf / mm2 or 1kgf / mm2 = 9.8MPa.

(2) Yield strength limit: When the external force of the metal material specimen exceeds the elastic limit of the material, the specimen will still undergo obvious plastic deformation though the stress will not increase any more. This phenomenon is called yielding, that is, the material withstands the force to a certain extent Degree, the deformation is no longer proportional to the external force and have a significant plastic deformation. The yield stress at yield is called the yield strength limit, denoted by σs, which corresponds to the yield point at point S in the tensile test plot. For ductile materials, a significant yield point appears on the tensile curve, whereas for low plastic materials there is no apparent 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 0.2% plastic deformation of the gage length on the sample is generally defined as the conditional yield limit, which is expressed as σ0.2. Yield limit indicators can be used to require parts in the work does not produce significant plastic deformation of the design basis. However, for some important parts, it is also considered that the yield ratio (that is, σs / σb) should be small to improve its safety and reliability, but the material utilization rate is also lower at this time.

(3) elastic limit: the material will be deformed by external forces, but the ability to restore the status quo after removal of external force called elasticity. The maximum stress that a metal material can maintain its elastic deformation is the elastic limit, corresponding to point e in the tensile test graph, expressed in σe, in MPa (MPa): σe = Pe / Fo where Pe is the elastic modulus The maximum external force (or the material when the maximum elastic deformation of the load).

(4) Modulus of elasticity: This is the ratio of the stress, σ, of the material within the elastic limit, to the strain δ, the amount of deformation per unit of stress, in E, in MPa: E = σ / δ = tgα where α is the angle between the oe line and the horizontal axis ox on the tensile test curve. Elastic modulus is an indicator of the rigidity of metallic materials (the ability of a metallic material to resist elastic deformation when it is stressed is called rigidity).

1.2. Plasticity

The maximum capacity of a metal material to undergo permanent deformation without damage under external force is called plasticity. Usually, the elongation at the gauge length δ (%) and the reduction at the specimen cross-section ψ (%) during the tensile test = [(L1 - L0) / L0] × 100%, which is the difference between the gauge length L1 and the original gauge length L0 (increase) of the specimen after breaking the specimen after breaking the specimen in the tensile test And L0 ratio. In the actual test, the tensile elongation of the same material but different specifications (diameter, cross-sectional shape - such as square, circular, rectangular and gage length) measured the elongation will vary, so generally require special filling, for example The most commonly used circular cross-section specimens, the initial gauge length of 5 times the sample diameter measured elongation was expressed as δ5, while the initial gauge length of 10 times the sample diameter measured elongation was expressed as δ10 . Sectional reduction rate ψ = [(F0-F1) / F0] x100%, which is the difference between the original cross-sectional area F0 and the minimum cross-sectional area F1 of the fracture neck at the time of tensile test (fracture reduction) Ratio. In practice, the most commonly used circular cross-section specimen can usually be calculated by the diameter measurement: ψ = [1- (D1 / D0) 2] × 100%, where: D0- the original diameter of the specimen; D1- Neck minimum diameter. 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 loading is called toughness. The impact test, which is the impact energy used per unit cross-sectional area of ​​a fracture, characterizes the toughness of the material when a metal specimen of a certain size and shape is broken by an impact load on a type-specific impact tester: ak = Ak / F unit 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. Fatigue strength limit Under long-term repeated stress or alternating stress (stress is generally less than the yield strength σs), the phenomenon of rupture without significant deformation is called fatigue failure or fatigue fracture, which is due to For a variety of reasons, the local stress on the surface of parts (stress concentration) greater than σs or even greater than σb causes the plastic deformation or microcracking locally. As the number of repeated alternating stress increases, the crack gradually expands and deepens Where the stress concentration) leads to the reduction of the actual cross-sectional area under stress in the local part until the local stress is larger than σb and the fracture occurs.






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