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What is metal material?

What is metal material?

Metal material refers to the general designation of metal elements or materials with metal characteristics mainly composed of metal elements. Including pure metal, alloy, intermetallic compound of metal material and special metal material. (Note: metal oxides (such as alumina) do not belong to metal materials)

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Significance

The development of human civilization and social progress are closely related to metal materials. After the stone age, the bronze age and iron age are marked by the application of metal materials. Nowadays, various kinds of metal materials have become an important material basis for the development of human society.

Type

Metal materials are usually divided into ferrous, non-ferrous 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% – 4% carbon, carbon steel with less than 2% carbon, as well as structural steel, stainless steel, heat-resistant steel, high-temperature alloy, stainless steel, precision alloy, etc. for various purposes. In a broad sense, 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, which are usually divided into light metals, heavy metals, precious metals, semi metals, rare metals and rare earth metals. The strength and hardness of non-ferrous alloy are generally higher than that of pure metal, and the resistance is large and the resistance temperature coefficient is small.
  • (3) Special metal materials include structural metal materials and functional metal materials for different purposes. Among them, there are amorphous metal materials obtained by rapid condensation process, quasicrystal, microcrystalline, nanocrystalline metal materials, etc.; there are stealth, hydrogen resistance, superconductivity, shape memory, wear resistance, damping and other special functional alloys and metal matrix composite materials.

Eight common metal materials

Cast iron – fluidity
As an unimportant part of our daily living environment, sewer covers are rarely noticed. The reason why cast iron has such a large and wide range of uses is mainly because of its excellent fluidity and its easy pouring into various complex forms. Cast iron is actually the name for a mixture of many elements, including carbon, silicon and iron. The higher the carbon content is, the better the flow characteristics are. Carbon can be found here in the form of graphite and iron carbide.

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The existence of graphite in cast iron makes the sewer cover have excellent wear resistance. Rust usually only appears on the top layer, so it is usually polished. In spite of this, there are also special measures to prevent rusting during the pouring process, that is, adding a layer of asphalt coating on the surface of the casting, and the asphalt penetrates into the fine holes on the surface of the cast iron, so as to play a role of rust prevention. Wechat for metal processing, good content, worthy of attention. The traditional process of producing sand mold castable materials is now used by many designers in other newer and more interesting fields.
Material characteristics: excellent fluidity, low cost, good wear resistance, low solidification shrinkage, very brittle, high compression strength, good machinability.
Typical use: cast iron has been used for hundreds of years, involving construction, bridges, engineering components, home furnishings, Kitchenware and other fields.
Stainless steel — the revolution of stainless steel
Stainless steel is an alloy made of chromium, nickel and other metal elements. The non rusting characteristic of chromium comes from the composition of chromium in the alloy. Chromium forms a strong and self repairing chromium oxide film on the surface of the alloy, which is invisible to the naked eye. The ratio of stainless steel to nickel we usually mention is usually 18:10.
At the beginning of the 20th century, stainless steel began to be introduced into the field of product design as a meta talent. Designers have developed many new products around its tenacity and corrosion resistance, involving many areas that have never been involved before. This series of design attempts are very revolutionary: for example, equipment that can be reused after disinfection appears in the medical industry for the first time.
Stainless steel is divided into four main types: austenite, ferrite, ferrite austenite (composite), martensite. Stainless steel used in household products is basically austenitic.
Material characteristics: health care, anti-corrosion, fine surface treatment, high rigidity, molding by various processing technologies, and difficult cold processing.
Typical uses: austenitic stainless steel is mainly used in household products, industrial pipes and building structures; martensitic stainless steel is mainly used to make knives and turbine blades; ferritic stainless steel has corrosion resistance, mainly used in durable washing machines and boiler parts; composite stainless steel has stronger corrosion resistance, so it is often used in corrosive environment.
Zinc – 730 pounds in life
Zinc, silvery and bluish gray, is the third most widely used non-ferrous metal after aluminum and copper. An average person consumes a total of 331 kg of zinc in his or her lifetime, according to the U.S. Bureau of mines. The melting point of zinc is very low, so it is also an ideal pouring material.
Zinc castings are very common in our daily life: materials, faucets, electronic components, etc. under the surface of doorknob watch layer, zinc has a very high corrosion resistance, which makes it have another basic function, that is, as the surface coating material of steel. In addition to these functions, zinc and copper are also alloy materials for the synthesis of brass. Its corrosion resistance is not just applied to steel coatings – it also helps strengthen our immune system.
Material characteristics: health care, anti-corrosion, excellent castability, excellent anti-corrosion, high strength, high hardness, cheap raw materials, low melting point, creep resistance, easy to form alloy with other metals, health care, fragile at room temperature, ductility around 100 ℃.
Typical use: electronic components. Zinc is one of the alloy materials to form bronze. Zinc also has the characteristics of cleanness and corrosion resistance. Zinc is also used in roof materials, photo engraving plates, mobile phone antennas, and shutter devices in cameras.
Aluminum (AL) – current materials
Compared with gold, which has been used for 9000 years, aluminum, a white metal with a little blue light, can only be regarded as a baby in metal materials. Aluminum came out in the early 18th century and was named. Unlike other metal elements, aluminum does not exist in nature in the form of direct metal elements, but is extracted from bauxite containing 50% alumina (also known as bauxite). Aluminum in this form is also one of the most abundant metal elements on earth.
When aluminum was the first metal, it was not immediately applied to people’s lives. Later, a number of new products aiming at their unique functions and characteristics came out gradually, and this high-tech material also gradually has a wider market. Although the application history of aluminum is relatively short, the output of aluminum products on the market has far exceeded the total of other non-ferrous metal products.
Material characteristics: flexible and plastic, easy to make alloy, high strength weight ratio, excellent corrosion resistance, easy to conduct electricity and heat, recyclable.
Typical uses: vehicle framework, aircraft parts, kitchen appliances, packaging and furniture. Aluminum is also often used to reinforce large structures, such as the statue of Eros on Piccadilly square in London, and the top of Chrysler automobile building in New York.
Magnesium alloy ultra thin aesthetic design
Magnesium is a very important non-ferrous metal. It is lighter than aluminum and can form a high-strength alloy with other metals. Magnesium alloy has the advantages of light weight, high specific strength and specific rigidity, good thermal conductivity and conductivity, good damping and electromagnetic shielding performance, easy processing and molding, easy recovery, etc. However, for a long time, due to the high price and technical limitations, magnesium and magnesium alloys are only used in aviation, aerospace and military industries, so they are called “noble metals”. Nowadays, magnesium is the third largest metal engineering material after steel and aluminum. It is widely used in aerospace, automobile, electronics, mobile communication, metallurgy and other fields. It can be predicted that magnesium will become more important in the future due to the increase of production cost of other structural metals. Sex gets bigger. Sex gets bigger.
Magnesium alloy accounts for 68% of aluminum alloy, 27% of zinc alloy and 23% of steel. It is commonly used in automobile parts, 3C product shell, building materials, etc. Most ultrathin laptops and mobile phone cases are made of magnesium alloy. Since the last century, human beings still have indelible love for metal texture and luster. Although plastic products can form metal like appearance, their luster, hardness, temperature and texture are still different from metal. As a new type of metal material, magnesium alloy gives people a feeling of high-tech products.
The corrosion resistance of magnesium alloy is 8 times that of carbon steel, 4 times that of aluminum alloy, and more than 10 times that of plastic. The corrosion resistance of magnesium alloy is the best one among the alloys. The commonly used magnesium alloy is nonflammable, especially when it is used in the parts of steam turbine car and building materials, which can avoid instantaneous combustion. The reserves of magnesium in the earth’s crust rank the eighth. Most of the magnesium raw materials are extracted from seawater, so its resources are stable and sufficient.
Material characteristics: lightweight structure, high rigidity and impact resistance, excellent corrosion resistance, good thermal conductivity and electromagnetic shielding, good non flammability, poor heat resistance and easy recovery.
Typical application: widely used in aerospace, automobile, electronics, mobile communication, metallurgy and other fields.
Copper: a partner of human beings
Copper is an incredible universal metal. It is so closely related to our life. Many of mankind’s early tools and weapons were made of copper. Its Latin name “cuprum” originated in a place called Cyprus, which is an island rich in copper resources. People use the abbreviation of the island name Cu to name this metal material, so copper has the current code name.
Copper plays a very important role in modern society: it is widely used in building structures as the carrier of power transmission. In addition, for thousands of years, it has been used as raw materials for making body decorations by people of different cultural backgrounds. From the initial simple decoding transmission to the later key role in complex modern communication applications, this malleable, orange red metal has been accompanied by our development and progress. Copper is a kind of excellent conductor, whose conductivity is second only to silver. From the point of time and history of people using metal materials, copper is the longest used metal for human beings after gold. This is largely due to the fact that the copper mine is easy to mine and the copper industry is relatively easy to separate from the copper mine.
Material characteristics: good corrosion resistance, excellent heat conduction, conductivity, hardness, flexibility, ductility, unique effect after polishing.
Typical use: wire, engine coil, printed circuit, roofing material, pipe material, heating material, jewelry, cooking utensil. It is also one of the main alloy components for making bronze.
Chromium – post treatment of high finish
The most common form of chromium is used as an alloy element in stainless steel to enhance the hardness of stainless steel. Chromium plating process is generally divided into three types: decorative coating, hard chromium coating and black chromium coating. Chromium coating is widely used in the field of engineering. Decorative chromium coating is usually plated on the outside of the nickel layer as the top layer, and the coating has a polishing effect as delicate as a mirror. As a decorative post-treatment process, the thickness of chromium coating is only 0.006 mm. When we plan to use chromium plating process, we must fully consider the risk of this process. In recent ten years, the trend of hexavalent decorative chromium water being replaced by trivalent chromium water is more and more obvious, because the former has very strong carcinogenicity, while the latter is considered to be relatively less toxic.
Material characteristics: very high finish, excellent corrosion resistance, hard and durable, easy to clean, low friction coefficient.
Typical use: decorative chrome plating is the coating material of many automotive components, including door handle and buffer, etc. in addition, chrome is also used in bicycle parts, bathroom faucet, furniture, kitchen utensils, tableware, etc. Hard chrome plating is more widely used in industry, including ram in job control block, jet engine components, plastic mold and shock absorber. Black chrome plating is mainly used in musical instrument decoration and solar energy utilization.
Titanium – light and strong
Titanium is a very special metal, very light texture, but also very tough and corrosion-resistant, at room temperature to maintain its own color for life. The melting point of titanium is not much different from that of platinum, so it is often used in aerospace and military precision components. With the addition of electric current and chemical treatment, different colors will be produced. Titanium has excellent acid-base corrosion resistance. Titanium soaked in “aqua regia” for several years is still shiny and shiny. If titanium is added to stainless steel, only about one percent of it will greatly improve the ability of anti rust.
Titanium has the advantages of low density, high temperature resistance and corrosion resistance. The density of titanium alloy is half of that of iron and steel, and its strength is almost the same as that of iron and steel. High strength can be maintained in a wide temperature range of – 253 ℃ ~ 500 ℃. Wechat for metal processing, good content, worthy of attention. These advantages are essential to space metal. Titanium alloy is a good material for making rocket engine shell, artificial satellite and spaceship, which is called “space metal”. Because of these advantages, titanium has become a prominent rare metal since the 1950s.
Titanium is a kind of pure metal. Because of its “purity”, no chemical reaction will occur when the substance contacts it. That is to say, because of its high corrosion resistance and stability, titanium will not affect its essence after long-term contact with people, so it will not cause allergy to people. It is the only metal that has no impact on human plant nerves and taste, which is called “biomimetic metal”.
The biggest disadvantage of titanium is that it is difficult to refine. This is mainly because titanium can combine with oxygen, carbon, nitrogen and many other elements at high temperature. So people used to regard titanium as a “rare metal”. In fact, the content of titanium accounts for about 6 ‰ of the weight of the earth’s crust, more than 10 times more than the sum of copper, tin, manganese and zinc.
Material characteristics: very high strength, good corrosion resistance by weight, difficult to cold work, good weldability, about 40% lighter than steel, 60% heavier than aluminum, low conductivity, low thermal expansion rate, high melting point.
Typical uses: golf clubs, tennis rackets, portable computers, cameras, suitcases, surgical implants, aircraft skeletons, chemical appliances and maritime equipment. In addition, titanium is also used as a white pigment for paper, painting and plastics.

Performance

Generally, it can be divided into process performance and service performance. The so-called technological performance refers to the performance of metal materials under the specified cold and hot processing conditions during the processing and manufacturing of mechanical parts. The technological performance of metal material determines its adaptability in the manufacturing process. Due to different processing conditions, the required technological properties are different, such as casting performance, weldability, malleability, heat treatment performance, cutting processability, etc.
The so-called service performance refers to the performance of metal materials under the service conditions of mechanical parts, including mechanical properties, physical properties, chemical properties, etc. The performance of metal material determines its service range and service life. In the mechanical manufacturing industry, general mechanical parts are used in normal temperature, atmospheric pressure and very strong corrosive medium, and in the use process, each mechanical part will bear different loads. The ability of metal materials to resist failure under load is called mechanical property (also known as mechanical property in the past). The mechanical properties of metal materials are the main basis of parts design and material selection. The mechanical properties required for metal materials will be different with different properties of applied load (such as tension, compression, torsion, impact, cyclic load, etc.). Common mechanical properties include: strength, plasticity, hardness, impact toughness, multiple impact resistance and fatigue limit.

Characteristics of metal materials

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 stress cycling, there will be sudden brittle fracture, which is called fatigue of metal materials. The characteristics of fatigue fracture of metal materials are as follows:

  • (1) The load stress is alternating;
  • (2) The action time of load is longer;
  • (3) The fracture is instantaneous;
  • (4) Both plastic materials and brittle materials are brittle in the fatigue fracture zone. Therefore, fatigue fracture is the most common and dangerous fracture form in engineering.

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

  • (1) High cycle fatigue: refers to the fatigue with the number of cycles of stress above 100000 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). It is the most common fatigue failure. High cycle fatigue is generally referred to as fatigue.
  • (2) Low cycle fatigue: refers to the fatigue with high stress (the working stress is close to the yield limit of the material) or high strain, and the number of cycles of stress is less than 10000-100000. Because the alternating plastic strain plays an important role in the 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 change.
  • (4) Corrosion fatigue: refers to the fatigue damage of machine components under the combined action of alternating load and corrosive medium (such as acid, alkali, sea water, active gas, etc.).
  • (5) Contact fatigue: This refers to the contact surface of machine parts, under the repeated action of contact stress, there is pitting peeling or surface crushing peeling, resulting in failure and damage of parts.

Plasticity

Plasticity refers to the ability of metal materials to produce permanent deformation (plastic deformation) without damage under the action of external load. When a metal material is stretched, its length and cross-sectional area will change. Therefore, the plasticity of the metal can be measured by two indexes: the elongation of the length (elongation) and the shrinkage of the section (shrinkage of the section).
The greater the elongation and the reduction of area of a metal material, the better the plasticity of the material, that is, the material can withstand large plastic deformation without damage. Generally, metal materials with an elongation greater than 5% are called plastic materials (such as low carbon steel), while metal materials with an elongation less than 5% are called brittle materials (such as gray cast iron). The material with good plasticity can produce plastic deformation in a large macro scope, and at the same time make the metal material strengthen due to plastic deformation, so as to improve the strength of the material and ensure the safe use of the parts. In addition, the materials with good plasticity can be processed smoothly in some forming processes, such as stamping, cold bending, cold drawing, straightening, etc. Therefore, when selecting metal materials as mechanical parts, certain plasticity index must be met.

Durability

Main forms of corrosion of building metal:

  • (1) Uniform corrosion. The corrosion of the metal surface makes the section even and thin. Therefore, the annual average thickness loss value is often used as the index of corrosion performance (corrosion rate). The steel is generally corroded uniformly in the atmosphere.
  • (2) Pitting. The corrosion of metal is pitted and forms deep pit. The formation of pitting is related to the nature of metal and its medium. In the medium containing chloride salt, it is easy to have cavitation. The maximum hole depth is often used as the evaluation index. The problem of pitting corrosion is often considered in pipeline corrosion.
  • (3) Galvanic corrosion. Corrosion at the contact of different metals due to different potentials.
  • (4) Crevice corrosion. The local corrosion of metal surface in crevices or other concealed areas is often caused by the difference of medium composition and concentration between different parts.
  • (5) Stress corrosion. Under the action of corrosive medium and high tensile stress, the metal surface corrodes and develops into microcracks, which often leads to sudden fracture. This kind of damage may occur to high-strength steel bars (steel wires) in concrete.

Hardness

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

  • Brinell hardness (HB): press a certain size (diameter is generally 10 mm) of hardened steel ball into the material surface with a certain load (generally 3000 kg), and keep it for a period of time. After unloading, the ratio of load to indentation area is Brinell hardness value (HB), unit: kilogram force / mm2 (n / mm2).
  • Rockwell hardness (HR): when HB > 450 or the sample is too small, Rockwell hardness measurement can not be used instead of Brinell hardness test. It uses a diamond cone with a top 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 a certain load, and the hardness of the material is calculated from the depth of the indentation. According to the hardness of test materials, different indenters and total test pressure can be used to form several different Rockwell hardness scales. Each scale is indicated with a letter after the Rockwell hardness symbol hr. The commonly used Rockwell hardness scales are a, B and C (HRA, HRB and HRC). C scale is the most widely used.
  • HRA: it is the hardness obtained by using a diamond cone press with a load of 60kg. It is used for materials with extremely high hardness (such as cemented carbide, etc.).
  • HRB: it is a 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: the hardness obtained by 150kg load and diamond cone press is used for materials with high hardness (such as hardened steel, etc.).
  • Vickers hardness (HV): a diamond square cone indenter with a load less than 120kg and a top angle of 136 ° is used to press into the surface of the material. The surface product of the indentation pit of the material divided by the load value is the Vickers hardness value (HV). Hardness test is the most simple and easy test method in mechanical property test. In order to replace some mechanical property tests with hardness tests, a more accurate conversion relationship between hardness and strength is needed in production. It has been proved that there is an approximate relationship between the hardness values and the strength values of metal materials. Because the hardness value is determined by the initial plastic deformation resistance and the continuous plastic deformation resistance, the higher the strength of the material, the higher the plastic deformation resistance, and the higher the hardness value.

Properties of metallic materials

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

Mechanical properties

(1) In the concept of stress, the force on the unit cross-sectional area of an object is called stress. The stress caused by the action of external force is called working stress, and the stress balanced in the object without the action of external force is called internal stress (such as tissue stress, thermal stress, residual stress remaining after the end of processing…).
(2) Mechanical properties, the ability of metal to resist deformation and fracture under the action of external force (load) under certain temperature conditions is called the mechanical properties of metal materials (also known as mechanical properties). There are many kinds of loads that metal materials bear, which can be static load or dynamic load, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, friction, vibration, impact and so on. Therefore, the indicators to measure the mechanical properties of metal materials are as follows:

Strength

It can be divided into tensile strength limit (σ b), flexural strength limit (σ BB), compressive strength limit (σ BC), etc. Because metal materials have certain rules from deformation to failure under the action of external force, the tensile test is usually used to determine, that is, the metal materials are made into a certain size of sample, and the tensile test machine is used to stretch until the sample breaks. The main strength indexes to be determined are as follows:

  • (1) Strength limit: the maximum stress that the material can resist fracture under the action of external force, generally refers to the tensile strength limit under the action of tensile force, expressed in σ B, such as the strength limit corresponding to the highest point B in the tensile test curve, commonly used unit is MPa, conversion relationship is: 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, although the stress will not increase any more, the specimen still has obvious plastic deformation. This phenomenon is called yield, that is, when the material bears the external force to a certain extent, its deformation will no longer be directly proportional to the external force to produce obvious plastic deformation. The stress at which yield occurs is called the yield strength limit, which is represented by σ s, and the s point corresponding to the tensile test curve is called the yield point. For the material with high plasticity, there will be obvious yield point on the tensile curve, but for the material with low plasticity, there is no obvious yield point, so it is difficult to calculate the yield limit according to the external force of the yield point. Therefore, in the tensile test method, the stress when the gauge length on the specimen produces 0.2% plastic deformation is usually specified as the conditional yield limit, which is expressed by σ 0.2. The yield limit index can be used as the design basis for requiring no obvious plastic deformation in the work of parts. However, for some important parts, the yield strength ratio (σ s / σ b) is also considered to be small to improve their safety and reliability, but the utilization rate of materials is also low at this time.
  • (3) Elastic limit: the ability of the material to deform under the action of external force, but to return to its original state after removing the external force is called elasticity. The maximum stress that metal materials can maintain elastic deformation is the elastic limit, corresponding to point E in the tensile test curve, expressed in σ e, in megapascals (MPA): σ e = PE / fo where PE is the maximum external force (or the load when the material has the maximum elastic deformation).
  • (4) Modulus of elasticity: This is the ratio of stress σ to strain δ (the unit deformation corresponding to the stress) in the elastic limit range of the material, which is expressed in E, unit MPa: e = σ / δ = TG α, where α is the angle between the O-E line on the tensile test curve and the horizontal axis o-x. The modulus of elasticity is an index reflecting the rigidity of metal materials (the ability to resist elastic deformation when metal materials are stressed is called rigidity).

Plasticity

The maximum ability of metal materials to produce permanent deformation without damage under the action of external force is called plasticity. Generally, the elongation of specimen gauge length δ (%) and the reduction of specimen area ψ (%) in tensile test is δ = [(l1-l0) / l0] X100%, which is the difference between the gauge length L1 after the specimen fracture is aligned and the original gauge length l0 (increase amount) )Ratio to l0. In the actual test, the elongation measured by tensile specimens of the same material but of different specifications (diameter, section shape – such as square, circular, rectangular and gauge length) will be different, so special notes are generally needed, such as the most commonly used round section specimen, the elongation measured when the initial gauge length is 5 times the diameter of the specimen is expressed as δ 5, while the initial gauge length is the test When the sample diameter is 10 times, the measured elongation is expressed as δ 10. The reduction of area ψ = [(f0-f1) / F0] X100%, which is the ratio of the difference between the original cross-sectional area F0 and the minimum cross-sectional area F1 at the fracture neck after tensile test (reduction of area) and F0. In practice, the most commonly used round section specimen can be calculated by diameter measurement: ψ = [1 – (D1 / D0) 2] X100%, where d0 is the original diameter of the specimen; D1 is the minimum diameter of the fracture neck after the specimen is pulled. The larger the δ and ψ values are, the better the plasticity is.

Toughness

The ability of metal materials to resist damage under impact load is called toughness. Generally, impact test is used, that is, when a certain size and shape of metal sample is broken by impact load on a specified type of impact test machine, the impact energy consumed on the unit cross-sectional area of the fracture surface is used to characterize the toughness of the material: α k = AK / F unit J / cm2 or kg · M / cm2, 1kg · M / cm2 = 9.8j/cm2 α K is called the impact toughness of the metal material, AK is the impact energy, f is the original section of the fracture surface Product. 5. Under the action of long-term repeated stress or alternating stress (the stress is generally less than the yield strength σ s), the phenomenon of fracture without significant deformation is called fatigue failure or fatigue fracture, which is caused by the local stress (stress concentration) greater than σ s or even greater than σ B on the surface of parts due to various reasons, so that the local plastic occurs With the increase of the number of times of repeated alternating stress action, the crack gradually expands and deepens (stress concentration at the crack tip), resulting in the reduction of the actual cross-sectional area of the local stress, until the local stress is greater than σ B, resulting in the fracture. In practical application, the maximum stress that the specimen can withstand without fracture under repeated or alternating stress (tensile stress, compressive stress, bending or torsional stress, etc.) within the specified cycle number (generally 106-107 times for steel, 108 times for non-ferrous metal) is taken as the fatigue strength limit, which is expressed by σ – 1, in MPa. In addition to the above five most commonly used mechanical performance indexes, for some materials with special requirements, such as aerospace, nuclear industry, power plant and other metal materials, the following mechanical performance indexes are also required: creep limit: under a certain temperature and constant tensile load, the phenomenon that the material slowly produces plastic deformation with time is called creep. Generally, high temperature tensile creep test is adopted, that is, under constant temperature and constant tensile load, the creep elongation (total elongation or residual elongation) of the specimen within the specified time or the maximum stress when the creep speed does not exceed a specified value at the stage of relatively constant creep elongation speed, as the creep limit, expressed in MPa, where τ is the test duration, t is the temperature , δ is the elongation, σ is the stress; or in, V is the creep speed. High temperature tensile endurance strength limit: under the action of constant temperature and constant tensile load, the maximum stress that the specimen reaches the specified duration without fracture, expressed in MPa, where τ is the duration, t is the temperature, and σ is the stress. Notch sensitivity coefficient of metal: K τ is the ratio of stress of notched specimen and smooth specimen without notch when the duration is the same (high temperature tensile endurance test): where τ is the duration of test, the stress of notched specimen and the stress of smooth specimen. Or: is the ratio of notch specimen duration to smooth specimen duration under the same stress σ. Heat resistance: resistance of material to mechanical load at high temperature.

Chemical properties

The characteristics of chemical reactions between metals and other substances are called the chemical properties of metals. In practical application, the corrosion resistance and oxidation resistance of metals (also known as oxidation resistance, which refers to the resistance or stability of metals to oxidation at high temperature), as well as the influence of compounds formed between different metals and between metals and nonmetals on mechanical properties, etc. are mainly considered. In the chemical properties of metals, especially the corrosion resistance is of great significance to the corrosion fatigue damage of metals.

Physical properties

The physical properties of metals are mainly considered as follows:

  • (1) Density (specific gravity): ρ = P / V unit G / cm3 or T / m3, where p is the weight and V is the volume. In practical application, in addition to calculating the weight of metal parts according to the density, it is very important to consider the specific strength of metal (the ratio of strength σ B to density ρ) to help material selection, as well as the acoustic impedance (the product of density ρ and sound speed c) in acoustic testing related to non-destructive testing and the different absorptive capacity of materials with different density in ray testing.
  • (2) Melting point: the temperature at which a metal changes from a solid state to a liquid state, which has a direct impact on the melting and hot working of a metal material and has a great deal to do with the high temperature performance of the material.
  • (3) Thermal expansion. With the change of temperature, the volume of material also changes (expansion or contraction), which is called thermal expansion. It is usually measured by linear expansion coefficient, that is, the ratio of the increase or decrease of material length to the length at 0 ℃ when the temperature changes by 1 ℃. The thermal expansion is related to the specific heat of the material. In practical application, the specific volume (when the material is affected by temperature and other external factors, the volume of the material per unit weight increases or decreases, that is, the ratio of volume to mass), especially for the metal parts working in the high temperature environment, or in the cold and hot alternative environment, the influence of its expansion performance must be considered.
  • (4) Magnetism. The properties that can attract ferromagnetic objects are magnetism, which is reflected in permeability, hysteresis loss, residual magnetic induction strength, coercive force and other parameters, so that metal materials can be divided into paramagnetic and reverse magnetic, soft magnetic and hard magnetic materials.
  • (5) Electrical performance. The electrical conductivity is mainly considered, which affects the electrical resistivity and eddy current loss in electromagnetic nondestructive testing.

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) Malleability: it reflects the difficulty of forming metal materials in the process of pressure processing. For example, when the material is heated to a certain temperature, its plasticity (expressed as the size of plastic deformation resistance), the allowable temperature range of hot pressure processing, the characteristics of thermal expansion and cold shrinkage, the boundary of critical deformation related to microstructure and mechanical properties, and the fluidity of metal during hot deformation , thermal conductivity, etc.
  • (3) Castability: it reflects the degree of difficulty for metal materials to be melted and cast into castings. It shows the fluidity, air absorption, oxidation, melting point, uniformity and compactness of casting microstructure, as well as the rate of cold shrinkage, etc. in the melting state.
  • (4) Solderability: it reflects the degree of difficulty for metal materials to be rapidly heated locally, melted or semi melted (pressurized) at the joint site, so that the joint site can be firmly combined and become a whole, which is manifested in melting point, inspiratory property, oxidation property, thermal conductivity, thermal expansion and contraction property, plasticity at the time of melting, and the correlation with the microstructure of the joint site and adjacent materials , impact on mechanical properties, etc.

Technological properties of metal materials

Castability

It refers to that metal materials can obtain the properties of qualified castings by casting. Castability mainly includes fluidity, shrinkage and segregation. Fluidity refers to the ability of liquid metal to fill the mold. Shrinkage refers to the degree of volume shrinkage when the casting solidifies. Segregation refers to the inhomogeneity of chemical composition and structure in the metal due to the difference of crystallization sequence during the process of cooling and solidification.

Malleability

It refers to the ability of metal materials to change shape without cracks during pressure processing. It includes hammering, rolling, drawing, extrusion and other processing in hot or cold state. The malleability is mainly related to the chemical composition of metal materials.

Machinability

It refers to the degree of difficulty for metal materials to become qualified workpieces after being cut by cutting tools. The machinability is usually measured by the surface roughness of the workpiece after machining, the allowable cutting speed and the wear degree of the tool. It is related to many factors such as chemical composition, mechanical properties, thermal conductivity and work hardening degree of metal materials. Generally, hardness and toughness are used to judge the machinability. Generally speaking, the higher the hardness of metal materials, the more difficult it is to cut. Although the hardness is not high, the toughness is large and the cutting is difficult.

Weldability

It refers to the adaptability of metal materials to welding processing. It mainly refers to the difficulty of obtaining high-quality welding joints under certain welding process conditions. It includes two aspects: one is the binding property, that is, the sensitivity of certain metal to form welding defects under certain welding process conditions, and the other is the serviceability, that is, the serviceability of certain metal welded joints to use requirements under certain welding process conditions.

Heat treatment

  • (1) Annealing: refers to the heat treatment process in which the metal material is heated to a proper temperature, kept for a certain period of time, and then cooled slowly. Common annealing processes are: recrystallization annealing, stress relief annealing, spheroidizing annealing, complete annealing, etc. The purpose of annealing is mainly to reduce the hardness of metal materials, improve plasticity, facilitate cutting or pressure processing, reduce residual stress, improve the homogenization of structure and composition, or prepare for the later heat treatment.
  • (2) Normalizing: refers to the heat treatment process of heating steel or steel parts to 30-50 ℃ above AC3 or ACM (upper critical point temperature of steel) and cooling in still air after holding for a proper time. The main purpose of normalizing is to improve the mechanical properties of low carbon steel, improve the machinability, refine the grains, eliminate the structural defects, and prepare for the later heat treatment.
  • (3) Quenching: refers to the heat treatment process of heating the steel to a certain temperature above AC3 or AC1 (lower critical point temperature of steel), holding for a certain time, and then obtaining martensite (or bainite) structure at a proper cooling rate. The common quenching processes include salt bath quenching, martensite classification quenching, bainite isothermal quenching, surface quenching and local quenching. The purpose of quenching is to obtain the required martensite structure, improve the hardness, strength and wear resistance of the workpiece, and prepare the structure for the subsequent heat treatment.
  • (4) Tempering: refers to the heat treatment process in which the steel is hardened, then heated to a certain temperature below AC1, kept for a certain time, and then cooled to room temperature. Common tempering processes include: low temperature tempering, medium temperature tempering, high temperature tempering and multiple tempering. The purpose of tempering: it is mainly to eliminate the stress produced during quenching of steel parts, so that the steel parts have high hardness and wear resistance, as well as the required plasticity and toughness.
  • (5) Quenching and tempering: refers to the compound heat treatment process of quenching and tempering steel or steel parts. The steel used for quenching and tempering is called quenched and tempered steel. It generally refers to medium carbon structural steel and medium carbon alloy structural steel.
  • (6) Chemical heat treatment: refers to the heat treatment process in which a metal or alloy workpiece is placed in an active medium at a certain temperature for heat preservation, so that one or several elements penetrate into its surface to change its chemical composition, structure and performance. Common chemical heat treatment processes are: carburizing, nitriding, carbonitriding, aluminizing, boronizing, etc. The purpose of chemical heat treatment is to improve the hardness, wear resistance, corrosion resistance, fatigue strength and oxidation resistance of the steel surface.
  • (7) Solution treatment: refers to the heat treatment process in which the alloy is heated to a high temperature single-phase area and kept at a constant temperature, so that the excess phase can be fully dissolved in the solid solution and then cooled quickly, so as to obtain supersaturated solid solution. The purpose of solution treatment is to improve the plasticity and toughness of steel and alloy, and prepare for precipitation hardening treatment.
  • (8) Precipitation hardening (precipitation strengthening): refers to a heat treatment process in which the metal atoms in supersaturated solid solution are metamerized and / or the desolved particles are dispersed in the matrix to cause hardening. For example, after solution treatment or cold working, precipitation hardening treatment can be carried out at 400-500 ℃ or 700-800 ℃ for Austenitic precipitation stainless steel to obtain high strength.
  • (9) Aging treatment: refers to the heat treatment process in which the properties, shape and size of alloy workpieces change with time after solution treatment, cold plastic deformation or casting and forging, placed at a higher temperature or kept at room temperature. If the aging treatment process of heating the workpiece to a higher temperature and aging treatment for a long time is adopted, it is called artificial aging treatment. If the workpiece is placed at room temperature or under natural conditions for a long time, the aging phenomenon is called natural aging treatment. The purpose of aging treatment is to eliminate the internal stress of the workpiece, stabilize the structure and size, and improve the mechanical properties.
  • (10) Hardenability: refers to the characteristics that determine the hardening depth and hardness distribution of steel under specified conditions. The hardenability of steel is good or bad, which is usually expressed by the depth of harden layer. The greater the depth of the hardened layer, the better the hardenability of the steel. The hardenability of steel mainly depends on its chemical composition, especially the alloying elements and grain size, heating temperature and holding time. For the steel with good hardenability, the uniform mechanical properties of the whole section of the steel can be obtained, and the quenchant with small quenching stress can be selected to reduce the deformation and cracking.
  • (11) Critical diameter (critical quenching diameter): the critical diameter refers to the maximum diameter when the steel is quenched in a certain medium, and the whole martensite or 50% martensite structure is obtained in the center. The critical diameter of some steels can generally be obtained through the hardenability test in oil or water.
  • (12) Secondary hardening: some FERROCARBON alloys (such as high-speed steel) must be tempered several times before further improving their hardness. This hardening phenomenon, called secondary hardening, is caused by the precipitation of special carbides and / or the transformation from austenite to martensite or bainite.
  • (13) Temper embrittlement: refers to the embrittlement phenomenon of quenched steel tempered in certain temperature range or cooled slowly from tempering temperature through this temperature range. Temper embrittlement can be divided into the first and the second. The first kind of temper brittleness is also called irreversible temper brittleness, which mainly occurs when the temper temperature is 250-400 ℃. After the reheat brittleness disappears, repeat the tempering in this range, and no brittleness will occur again. The second kind of temper brittleness is also called reversible temper brittleness, which occurs at 400-650 ℃. When the reheat brittleness disappears, it should be cooled rapidly, not in 400-650 ℃ for a long time Stay or cool down slowly, otherwise catalysis will occur again. The occurrence of temper embrittlement is related to the alloy elements in steel, such as manganese, chromium, silicon and nickel, while molybdenum and tungsten tend to weaken the temper embrittlement.

Development prospect of metal materials and metal products industry

Metal products industry includes structural metal products manufacturing, metal tool manufacturing, container and metal packaging container manufacturing, container, stainless steel and similar daily metal products manufacturing, ship and marine engineering manufacturing, etc. With the progress of society and the development of science and technology, metal products are used more and more widely in industry, agriculture and people’s life, and also create more and more value for the society.
There are also some difficulties in the development of metal products industry, such as single technology, low technology level, lack of advanced equipment, talent shortage, etc., which restrict the development of metal products industry. Therefore, we can improve the development of China’s metal products industry by improving the technical level of enterprises, introducing advanced technology and equipment, and cultivating suitable talents.

Source: China Metal Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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