Detailed introduction and related knowledge of alloys
Detailed introduction and related knowledge of alloys
Overview of alloys
Alloys-are substances with metallic properties that are synthesized by two or more metals and metals or non-metals through certain methods. They are usually obtained by melting into a uniform liquid and solidifying. According to the number of constituent elements, they can be divided into binary alloys, ternary alloys and multi-element alloys. The earliest production of alloys by humans began with the production of bronze ware. The earliest alloy producers in the world were the ancient Babylonians, who began to refine bronze [alloys of red copper and tin] 6,000 years ago. China is also one of the earliest countries in the world to study and produce alloys. In the Shang Dynasty [more than 3,000 years ago], the bronze [copper-tin alloy] technology was very developed; around the 6th century BC [late Spring and Autumn Period], sharp swords were forged [also heat-treated].
Alloy production and synthesis
Metal materials with metallic properties that are often formed by alloying two or more metal elements or adding other non-metallic elements to a metal base through an alloying process (smelting, mechanical alloying, sintering, vapor deposition, etc.) are called alloys. But alloys may contain only one metal element, such as steel. (Steel is a general term for iron alloys with a carbon content of 0.02% to 2.00% by mass)
Here we should note that alloys are not mixtures in the general sense, but can even be pure substances, such as single-phase intermetallic alloys. The added alloying elements can form solid solutions, compounds, and produce endothermic or exothermic reactions, thereby changing the properties of the metal matrix.
The formation of alloys often improves the properties of the elemental substance. For example, the strength of steel is greater than that of its main component iron. The physical properties of alloys, such as density, reactivity, Young's modulus, electrical conductivity and thermal conductivity, may be similar to those of the constituent elements of the alloy, but the tensile strength and shear strength of alloys are usually very different from the properties of the constituent elements. This is due to the great difference in the atomic arrangement in alloys and single substances.
A small amount of a certain element may have a great effect on the properties of the alloy. For example, impurities in ferromagnetic alloys will change the properties of the alloy.
Unlike pure metals, most alloys do not have a fixed melting point. When the temperature is within the melting temperature range, the mixture is in a state of coexistence of solid and liquid. Therefore, it can be said that the melting point of the alloy is lower than that of the component metals. See eutectic mixture.
Among the common alloys, brass is an alloy of copper and zinc; bronze is an alloy of tin and copper, used for statues, decorations and church bells. Some countries use alloys (such as nickel alloys) for their currencies.
Physical classification of alloys
According to the name of the main metal with a larger content in the alloy, it is classified as a certain alloy, such as a copper alloy with a high copper content, and its properties mainly maintain the properties of copper.
General properties of alloys
All types of alloys have the following general properties:
1. The melting point of most alloys is lower than the melting point of any of its component metals;
2. The hardness is generally greater than the hardness of any of its component metals; (Special case: sodium potassium alloy is liquid and used as a thermal conductor in atomic reactors)
3. The electrical conductivity and thermal conductivity of the alloy are lower than any of its component metals. Using this property of the alloy, high resistance and high thermal resistance materials can be manufactured. Materials with special properties can also be manufactured.
4. Some have strong corrosion resistance (such as stainless steel). For example, adding 15% chromium and 9% nickel to iron can produce a corrosion-resistant stainless steel suitable for the chemical industry.
Types of alloys
1. Mixture alloy (eutectic mixture), when the liquid alloy solidifies, the components of the alloy are crystallized separately, such as solder, bismuth-cadmium alloy, etc.;
2. Solid solution alloy, when the liquid alloy solidifies, the alloy forms a solid solution, such as gold-silver alloy, etc.;
3. Intermetallic alloy, alloy in which the components form compounds with each other, such as brass composed of copper and zinc (β-brass, γ-brass and ε-brass), etc.
Many properties of alloys are better than pure metals, so alloys are mostly used in application materials (see ferroalloys, stainless steel).
Common alloys are as follows
Ductile iron, manganese steel, stainless steel, brass, bronze, nickel silver, solder, duralumin, 18K gold, 18K platinum, etc.
Ferromanganese alloy
Ferromanganese is made from manganese ore. It is smelted in a blast furnace or electric furnace. Ferromanganese is also a commonly used deoxidizer in steel. Manganese also has the function of desulfurization and reducing the harmful effects of sulfur. Therefore, almost all kinds of steel and cast iron contain a certain amount of manganese. Ferromanganese is also an important alloying agent. It is widely used in structural steel, tool steel, stainless heat-resistant steel, wear-resistant steel and other alloy steels.
Aluminum is a widely distributed element. Its content in the earth's crust is second only to oxygen and silicon, and it has the highest content among metals. Pure aluminum has a low density of 2.7 g/cm3, has good thermal conductivity and electrical conductivity (second only to Au, Ag, Cu), good ductility, high plasticity, and can be processed in various mechanical processes. Aluminum has active chemical properties and quickly oxidizes in the air to form a dense and firm oxide film, so it has good corrosion resistance. However, the strength of pure aluminum is low, and various aluminum alloys that can be used as structural materials can only be obtained through alloying.
Features and properties
The outstanding characteristics of aluminum alloys are low density and high strength. Al-Mn and Al-Mg alloys formed by adding Mn and Mg to aluminum have good corrosion resistance, good plasticity and high strength. They are called rust-proof aluminum alloys and are used to make fuel tanks, containers, pipes, rivets, etc. The strength of hard aluminum alloys is higher than that of rust-proof aluminum alloys, but the corrosion resistance is reduced. Such alloys include Al-Cu-Mg and Al-Cu-Mg-Zn. The newly developed high-strength hard aluminum has further improved strength, while the density is 15% lower than that of ordinary hard aluminum, and it can be extruded and formed. It can be used as components such as motorcycle frames and rims. Al-Li alloys can be used to make aircraft parts and advanced sports equipment that bear loads.
Pig iron alloys
Pig iron is hard and brittle, but it is resistant to pressure and wear. Gray iron and ductile iron. The carbon in white iron is Fe3C with a silvery white fracture. It is hard and brittle and cannot be machined. It is the raw material for steelmaking, so it is also called steelmaking pig iron. Gray iron with carbon distributed in the form of flake graphite is called gray iron. The fracture is silver-gray, easy to cut, easy to cast, and wear-resistant. If carbon is distributed in the form of spherical graphite, it is called ductile iron, and its performance and processing properties are close to those of steel. Special cast iron can be obtained by adding special alloy elements to cast iron. For example, if Cr is added, the wear resistance can be greatly improved, and it has very important applications under special conditions.
Ferrosilicon alloy
Ferrosilicon is made of coke, steel scraps, and quartz (or silica) by smelting in an electric furnace. Silicon and oxygen can easily combine to form silicon dioxide. Therefore, ferrosilicon is often used as a deoxidizer in steelmaking. At the same time, since a large amount of heat is released when SIO2 is generated, it is also beneficial to increase the temperature of molten steel while deoxidizing. Ferrosilicon is used as an alloying element addition agent. It is widely used in low-alloy structural steel, alloy steel, spring steel, bearing steel, heat-resistant steel and electrical silicon steel. In addition, ferrosilicon is often used as a reducing agent in ferroalloy production and the chemical industry. The silicon content is 95%--99%. Pure silicon is often used to manufacture single crystal silicon or prepare non-ferrous metal alloys.
Steel alloy
Steel is an alloy composed of iron and C, Si, Mn, P, S and a small amount of other elements. Among them, except for Fe, the content of C plays a major role in the mechanical properties of steel, so it is collectively called iron-carbon alloy. It is the most important and largest metal material in engineering technology.
Classification and properties
According to the carbon content, iron-carbon alloys are divided into two categories: steel and pig iron. Steel is an iron-carbon alloy with a carbon content of 0.03% to 2%. Carbon steel is the most commonly used ordinary steel. It is easy to smelt, easy to process, low in price, and can meet the use requirements in most cases, so it is widely used. According to the carbon content, carbon steel is divided into low carbon steel, medium carbon steel and high carbon steel. As the carbon content increases, the hardness of carbon steel increases and the toughness decreases. Alloy steel is also called special steel. One or more alloying elements are added to carbon steel, so it has some special properties, such as high hardness, high wear resistance, high toughness, corrosion resistance, etc. The alloying elements often added to steel include Si, W, Mn, Cr, Ni, Mo, V, Ti, etc. my country's alloy steel resources are quite rich. Except for the shortage of Cr and Co and the low grade of Mn, the reserves of W, Mo, V, Ti and rare earth metals are very high.
Application of alloys
High-strength aluminum alloys are widely used in the manufacture of aircraft, ships and trucks, etc., which can increase their load capacity and improve their running speed, and have the characteristics of resisting seawater erosion and avoiding magnetism.
Zinc alloy
An alloy composed of other elements added to zinc. Commonly added alloying elements include aluminum, copper, magnesium, cadmium, lead, titanium, etc. Zinc alloy has a low melting point, good fluidity, easy to weld, braze and plastic processing, corrosion resistance in the atmosphere, and waste materials are easy to recycle and remelt; but the creep strength is low and it is easy to cause dimensional changes due to natural aging. Prepared by melting method, die-casting or pressure processing. According to the manufacturing process, it can be divided into cast zinc alloy and deformed zinc alloy.
Application and Others
The main additive elements of zinc alloys are aluminum, copper and magnesium. Zinc alloys can be divided into two categories according to the processing technology: deformation and casting zinc alloys. Casting zinc alloys have good fluidity and corrosion resistance and are suitable for die-casting instruments, automobile parts shells, etc.
[Zinc alloy composition and casting quality]
I. Characteristics of zinc alloys
1. High specific gravity.
2. Good casting performance, can die-cast complex and thin-walled precision parts, and the casting surface is smooth.
3. Surface treatment can be performed: electroplating, spraying, and painting.
4. It does not absorb iron during melting and die-casting, does not corrode the die, and does not stick to the mold.
5. It has good room temperature mechanical properties and wear resistance.
6. It has a low melting point, melts at 385℃, and is easy to die-cast.
Copper alloy
Pure copper is purple-red, so it is also called red copper. It has excellent thermal conductivity and electrical conductivity. Its electrical conductivity is second only to silver and ranks second among metals. Copper has excellent chemical stability and corrosion resistance, and is an excellent metal material for electrical engineering.
Classification
Copper alloys widely used in industry include brass, bronze and white copper.
The alloy of Cu and Zn is called brass, in which Cu accounts for 60% to 90% and Zn accounts for 40% to 10%. It has excellent thermal conductivity and corrosion resistance and can be used as various instrument parts. For example, adding a small amount of Sn to brass is called naval brass, which has good resistance to seawater corrosion. Adding a small amount of lubricating Pb to brass can be used as a sliding bearing material.
Bronze is the metal material with the longest history of human use. It is an alloy of Cu and Sn. The addition of tin significantly increases the strength of copper, improves its plasticity, and enhances its corrosion resistance. Therefore, tin bronze is often used to manufacture wear-resistant parts and corrosion-resistant accessories such as gears. Sn is more expensive, and Al, Si, and Mn have been used in large quantities to replace Sn to obtain a series of bronze alloys. The corrosion resistance of aluminum bronze is better than that of tin bronze. Beryllium bronze is the strongest copper alloy. It is non-magnetic and has excellent corrosion resistance. It is a spring material that can compete with steel.
White copper is a Cu-Ni alloy with excellent corrosion resistance and high resistance, so it can be used as a material for parts and resistors working under harsh corrosion conditions.
Brass contains zinc and a small amount of tin, lead, aluminum, etc.
Lead-tin alloy
Classification
Lead-tin alloys are divided into the following categories according to their uses:
1. Lead-based or tin-based bearing alloys. Together with lead-based bearing alloys, they are collectively referred to as Babbitt alloys. Contains 3% to 15% antimony, 3% to 10% copper, and some alloy varieties also contain 10% lead. Antimony and copper are used to improve the strength and hardness of the alloy. It has a small friction coefficient, good toughness, thermal conductivity and corrosion resistance, and is mainly used to manufacture sliding bearings.
2. Lead-tin solder. It is mainly tin-lead alloy, and some tin solders also contain a small amount of antimony. Tin alloy containing 38.1% lead is commonly known as solder, with a melting point of about 183℃.
It is used for welding components in the electrical instrument industry, as well as sealing of automobile radiators, heat exchangers, food and beverage containers, etc.
3. Lead-tin alloy coating. Using the corrosion resistance of tin alloy, it is applied to the surface of various electrical components, which is both protective and decorative. Commonly used are tin-lead and tin-nickel coatings.
4. Lead-tin alloy (including lead-tin alloy and lead-free tin alloy) can be used to produce various exquisite alloy ornaments and alloy crafts, such as rings, necklaces, bracelets, earrings, brooches, buttons, tie clips, hat ornaments, craft ornaments, alloy photo frames, religious emblems, miniature statues, souvenirs, etc.
Features
Characteristics of lead-tin alloy (used as alloy ornaments and alloy crafts materials)
1. Lead-tin alloy has stable performance, low melting point, good fluidity and low shrinkage.
2. Lead-tin alloy has fine grains, good toughness, suitable hardness and softness, smooth surface, no sand holes, no defects, no cracks, and good polishing and electroplating effects.
3. Lead-tin alloy has good centrifugal casting performance and strong toughness. It can cast complex and thin-walled precision parts with smooth casting surface.
4. Lead-tin alloy products can be surface treated: electroplating, spraying, and painting.
5. The crystal structure of lead-tin alloy is dense, and the raw materials ensure that the casting size tolerance is small, the surface is exquisite, and there are few post-processing defects.
Special alloys
There are thousands of alloys used in industry, and only a few major categories are briefly introduced.
Heat-resistant alloys
Heat-resistant alloys are also called high-temperature alloys. They are of great significance to industrial sectors and application technology fields under high temperature conditions.
Generally speaking, the higher the melting point of a metal material, the higher its usable temperature limit. This is because as the temperature rises, the mechanical properties of metal materials decrease significantly, and the tendency of oxidation corrosion increases accordingly. Therefore, general metal materials can only work for a long time at 500 ℃ to 600 ℃. Metals that can work at high temperatures above 700 ℃ are generally called heat-resistant alloys. "Heat-resistant" means that it can maintain sufficient strength and good oxidation resistance at high temperatures.
There are two ways to improve the oxidation resistance of steel: one is to add alloying elements such as Cr, Si, and Al to steel, or to perform Cr, Si, and Al alloying treatment on the surface of steel. They can quickly form a dense oxide film in an oxidizing atmosphere and firmly attach to the surface of steel, thereby effectively preventing the continued oxidation. The second is to use various methods to form high-temperature resistant coatings such as high-melting-point oxides, carbides, and nitrides on the surface of steel.
There are many ways to improve the high-temperature strength of steel. From the chemical point of view of structure and properties, there are roughly two main methods:
One is to increase the bonding force between atoms in steel at high temperatures. Studies have shown that the bonding force in metals, that is, the strength of metal bonds, is mainly related to the number of unpaired electrons in atoms. From the periodic table, the metal bond of VIB elements is the strongest in the same period. Therefore, adding atoms such as Cr, Mo, and W to steel has the best effect.
The second is to add elements that can form various carbides or intermetallic compounds to strengthen the steel matrix. Carbides generated by several transition metals and carbon atoms belong to interstitial compounds. On the basis of metal bonds, they have added covalent bond components, so they have great hardness and high melting points. For example, adding W, Mo, V, and Nb can generate carbides such as WC, W2C, MoC, Mo2C, VC, and NbC, thereby increasing the high-temperature strength of steel.
In addition to iron-based heat-resistant alloys, nickel-based, molybdenum-based, niobium-based, and tungsten-based heat-resistant alloys can also be obtained by alloying methods. They have good mechanical properties and chemical stability at high temperatures. Among them, nickel-based alloys are the best super-heat-resistant metal materials. The matrix in the organization is a solid solution of Ni?Cr?Co and Ni3Al metal compounds. After treatment, its use temperature can reach 1 000 ℃ ~ 1 100 ℃.
Corrosion-resistant alloys
The ability of metal materials to resist corrosion in corrosive media is called the corrosion resistance of metals. Pure metals with high corrosion resistance usually meet one of the following three conditions:
1. Metals with high thermodynamic stability. It can usually be judged by its standard electrode potential. The more positive the value, the higher the stability; the more negative the value, the lower the stability. Precious metals with good corrosion resistance, such as Pt, Au, Ag, Cu, etc., belong to this category.
2. Metals that are easy to passivate. Many metals can form a dense oxide film with protective effect in oxidizing media. This phenomenon is called passivation. The most easily passivated metals are Ti, Zr, Ta, Nb, Cr and Al.
3. Metals that can form a corrosion product film that is insoluble and has good protective performance on the surface. This situation only occurs when the metal is in a specific corrosive medium, for example, Pb and Al in H2SO4 solution, Fe in H3PO4 solution, Mo in hydrochloric acid, and Zn in the atmosphere.
Therefore, according to the above principle, the industry uses alloying methods to obtain a series of corrosion-resistant alloys. Generally, there are three corresponding methods:
1. Improve the thermodynamic stability of metals or alloys, that is, add alloying elements with high thermodynamic stability to the original non-corrosion-resistant metals or alloys to form solid solutions and increase the electrode potential of the alloys to enhance their corrosion resistance. For example, adding Au to Cu and adding Cu and Cr to Ni belong to this category. However, this method of adding a large amount of precious metals has limited application in industrial structural materials.
2. Adding easily passivated alloying elements, such as Cr, Ni, Mo, etc., can improve the corrosion resistance of the base metal. Chromium-based stainless steel can be obtained by adding an appropriate amount of Cr to steel. Experiments have shown that in stainless steel, the Cr content should generally be greater than 13% to play a corrosion-resistant role. The higher the Cr content, the better its corrosion resistance. This type of stainless steel has good corrosion resistance in oxidizing media, but poor corrosion resistance in non-oxidizing media such as dilute sulfuric acid and hydrochloric acid. This is because non-oxidizing acids do not easily cause alloys to form oxide films, and they also dissolve oxide films.
3. Adding alloy elements that can promote the formation of a dense corrosion product protective film on the alloy surface is another way to prepare corrosion-resistant alloys. For example, steel can resist atmospheric corrosion because a dense compound oxyhydroxide [FeOx·(OH)23-2x] is formed on its surface, which can play a protective role. Adding Cu and P or P and Cr to steel can promote the formation of this protective film, so low-alloy steel resistant to atmospheric corrosion can be made of Cu, P or P, Cr.
Metal corrosion is the most harmful spontaneous process in industry, so the development and application of corrosion-resistant alloys have great social significance and economic value.
Magnetic alloys
In an external magnetic field, materials can show three situations: ① those that are not attracted by the magnetic field are called antimagnetic materials; ② those that are weakly attracted by the magnetic field are called paramagnetic materials; ③ those that are strongly attracted by the magnetic field are called ferromagnetic materials, and their magnetism increases sharply with the strengthening of the external magnetic field, and they can still retain their magnetism after the external magnetic field is removed. Among metal materials, most transition metals are paramagnetic; only a few metals such as Fe, Co, and Ni are ferromagnetic.
The main elements that make up permanent magnetic materials in metals are Fe, Co, Ni, and some rare earth elements. The permanent magnetic alloys used are rare earth-cobalt series, iron-chromium-cobalt series, and manganese-aluminum-carbon series alloys.
Magnetic alloys are increasingly widely used in emerging technology fields such as electricity, electronics, computers, automatic control, and electro-optics.
Titanium alloys
Titanium is a Class IVB element in the periodic table. It looks like steel and has a melting point of 1,672°C. It is a refractory metal. Titanium is abundant in the earth's crust, much higher than common metals such as Cu, Zn, Sn, and Pb. my country's titanium resources are extremely rich. In the super-large vanadium-titanium magnetite discovered in Panzhihua, Sichuan alone, the associated titanium metal reserves are about 420 million tons, which is close to the total proven titanium reserves abroad.
Pure titanium has strong mechanical properties, good plasticity, and is easy to process. If there are impurities, especially O, N, and C, it will increase the strength and hardness of titanium, but it will reduce its plasticity and increase its brittleness.
Titanium is a metal that is easily passivated, and in an oxygen-containing environment, its passivation film can heal itself after being damaged. Therefore, dry corrosion media are stable. Titanium and titanium alloys have excellent corrosion resistance and can only be corroded by hydrofluoric acid with a concentration of 2.5. In particular, it is stable. After titanium or titanium alloy is released, it is still as bright as before, which is far better than stainless steel.
Another important characteristic of titanium is its low density. Its strength is 3.5 times that of stainless steel and 1.3 times that of aluminum alloy, which is the highest among all industrial metal materials at present.
Liquid titanium can dissolve almost all metals to form various alloys such as solid solutions or metal compounds. The addition of alloying elements such as Al, V, Zr, Sn, Si, Mo and Mn can improve the performance of titanium to meet the needs of different departments. For example, Ti-Al-Sn alloy has high thermal stability and can work for a long time at a relatively high temperature; superplastic alloys represented by Ti-Al-V alloy can be elongated by 50% to 150% and its maximum elongation can reach 2,000%. The maximum elongation of plastic processing of general alloys does not exceed 30%.
Due to the above excellent properties, titanium enjoys the reputation of "the metal of the future". Titanium alloy has been widely used in various sectors of the national economy. It is an indispensable material for rockets, missiles and space shuttles. Titanium alloys are widely used in ships, chemicals, electronic devices and communication equipment as well as several light industrial sectors, but the high price of titanium limits its widespread use.
Potassium-sodium alloy
[English] Sodium Potaddium Al
[Other] Sodium-potassium alloy
[Abbreviation] JNHJ
[Chemical structure]
4K-Na
[Chemical properties]
Silver soft solid or liquid. It reacts violently with acid, carbon dioxide, moisture and water, releasing hydrogen, immediately self-igniting, and sometimes even exploding. Density: 0.847 g/ml (100℃) (K78%, Na22%); 0.886 g/ml (100℃) (K56%, Na44%) Melting point: -11℃ (K78%, Na22%); 19℃ (K56%, Na44%);
[Limiting parameters]
Boiling point: 784℃ (K78%, Na22%); 825℃ (K56%, Na44%);
[Application] The coolant used in liquid metal nuclear reactors is sodium-potassium alloy, which is liquid at room temperature.
Melting point of sodium potassium alloy
Sodium Potassium Melting point
20% 80% -10 ℃
22% 78% -11 ℃
24% 76% -3.5 ℃
40% 60% 5 ℃
New alloys
With the development of science and technology, the types of new alloys are increasing day by day. Here are the main ones.
Hydrogen storage alloys
Due to the limited reserves of oil and coal, and the environmental pollution caused by their use, especially the global oil crisis in the 1970s, hydrogen energy has become a research hotspot as a new clean fuel. In the process of hydrogen energy utilization, the storage and transportation of hydrogen is an important link. In 1969, Philips of the Netherlands developed LaNi5 hydrogen storage alloy, which has the property of reversibly absorbing and releasing a large amount of hydrogen. The density of hydrogen in its alloy hydride LaNi5H6 is equivalent to that of liquid hydrogen, which is about 1,000 times the density of hydrogen.
Hydrogen storage alloys are alloys composed of two specific metals, one of which can absorb a large amount of hydrogen to form a stable hydride, while the other metal has a low affinity for hydrogen, but hydrogen can easily move in it. Mg, Ca, Ti, Zr, Y and La belong to the first metal, and Fe, Co, Ni, Cr, Cu and Zn belong to the second metal. The former controls the amount of hydrogen storage, and the latter controls the reversibility of hydrogen release. By rationally combining the two and adjusting the hydrogen absorption and desorption properties of the alloy, an ideal hydrogen storage material that can reversibly absorb and desorb hydrogen at room temperature is obtained.
Lightweight alloy
Aluminum-lithium alloy has the characteristics of high specific strength (fracture strength/density), high specific stiffness and low relative density. If used as a modern aircraft skin material, a large passenger aircraft can reduce its weight by 50 kg. Taking the Boeing 747 as an example, every 1 kg reduction can make a profit of $2,000 a year. Titanium alloy is lighter than steel, corrosion-resistant, non-magnetic, and high-strength. It is an ideal material for aviation and ships.
Shape memory alloy
They have the characteristics of high elasticity, metal rubber properties, and high strength. After plastic deformation under stress at a low temperature, they return to their shape before heating after heating. Alloys such as Ni-Ti, Ag-Cd, Cu-Cd, Cu-Al-Ni, Cu-Al-Zn, etc. can be used for elastic elements of regulating devices (such as clutches, throttle valves, temperature control elements, etc.), thermal engine materials, medical materials (dental correction materials), etc.
The shape memory effect comes from a thermoelastic martensitic phase transformation. The general martensitic phase transformation as a method of quenching and strengthening steel is to heat the steel to a certain critical temperature and keep it warm for a period of time, and then cool it quickly, such as directly inserting it into cold water (called quenching). At this time, the steel is transformed into a martensitic structure and hardens the steel. Later, another so-called thermoelastic martensitic phase transformation different from the above was found in some alloys. Once the thermoelastic martensite is produced, it can continue to grow as the temperature decreases. On the contrary, when the temperature rises, the grown martensite can shrink again until it returns to its original state, that is, the martensite can reversibly grow or shrink with the change of temperature. The thermoelastic martensitic phase transformation is accompanied by a change in shape.
In addition to the above categories, new metal functional materials also include vibration-damping alloys that can reduce noise; biomedical materials that can replace, enhance and repair human organs and tissues; smart materials that have sensors, signal processors, communications and controllers, and actuators implanted in materials or structures, so that the materials or structures have intelligent functions and life characteristics such as self-diagnosis, self-adaptation, and even self-healing of damage, etc.
Super heat-resistant alloys
Nickel-cobalt alloys can withstand temperatures as high as 1,200 °C and can be used in components of jet aircraft and gas turbines. Nickel-cobalt-iron non-magnetic heat-resistant alloys still have high strength and good toughness at 1,200 °C and can be used in components of space shuttles and control rods of atomic reactors. Finding alloy materials that meet the requirements of high temperature resistance, long-term operation (more than 10,000 h), corrosion resistance, and high strength will remain the direction of future research.
Alloy processing
1. Medium carbon steel: Representative steel grades include 30, 35, 40, 45, as well as ML30, ML35, ML40, and ML45. They have relatively stable room temperature properties and are used for small and medium-sized structural parts, fasteners, transmission shafts, gears, etc. [3].
2. Manganese steel: Representative steel grades are 40Mn2 and 50Mn2. They are overheat sensitive, high temperature temper brittle, easy to crack when water quenched, and have higher hardenability than carbon steel.
3. Silicon manganese steel: Representative steel types 35SiMn and 42SiMn. High fatigue strength, decarburization and overheating sensitivity and temper brittleness. Used to manufacture gears, shafts, shafts, connecting rods, worms, etc. with medium speed, medium and high loads but small impact, and can also be used to manufacture fasteners below 400℃.
4. Boron steel: Representative steel types 40B, 45B, 50BA, ML35B. High hardenability, comprehensive mechanical properties are higher than carbon steel, equivalent to 40Cr, used to manufacture parts and fasteners with small cross-sectional dimensions.
5. Manganese boron steel: Representative steel type 40MnB. Hardenability is slightly higher than 40Cr, with high strength, toughness and low-temperature impact toughness, and temper brittleness. 40MnB is often used to replace 40Cr to manufacture large-section parts and 40CrNi to manufacture small parts; 45MnB replaces 40Cr and 45Cr; 45Mn2B replaces 45Cr and partially replaces 40CrNi and 45CrNi as important shafts, and ML35 MnB is also used in the production of fasteners.
6. Manganese vanadium boron steel: Representative steel types 20 MnVB, 40MnVB,. The quenching and tempering performance and hardenability are better than 40Cr, the overheating tendency is small, and there is temper brittleness. It is often used to replace 40Cr, 45Cr, 38CrSi, 42CrMo and 40CrNi to manufacture important quenched and tempered parts, and there are also small and medium-sized bolts below grade 10.9, ML20 MnVB.
7. Manganese tungsten boron steel: Representative steel type 40MnWB. Good low-temperature impact performance, no temper brittleness. Equivalent to 35CrMo and 40CrNi, used to manufacture parts below 70mm.
8. Silicon manganese molybdenum tungsten steel: Representative steel type 35SiMn2MoW. It has high hardenability, calculated based on 50% martensite, water quenching diameter 180, oil quenching diameter 100; quenching cracking tendency and temper brittleness tendency are small; it has high strength and high toughness. It can replace 35CrNiMoA and 40CrNiMo, and is used to manufacture large-section, heavy-load shafts, connecting rods and bolts.
9. Silicon manganese molybdenum tungsten vanadium steel: Representative steel type 37SiMn2MoWVA. Water quenching diameter 100, oil quenching diameter 70; good tempering stability, low-temperature impact toughness, high high-temperature strength, and low tempering brittleness, and is used to manufacture large-section shaft parts.
10. Chromium steel: Represented by 40Cr alloy steel pipe and ML40Cr. Good hardenability, water quenching 28-60mm, oil quenching 15-40mm. High comprehensive mechanical properties, good low-temperature impact toughness, low notch sensitivity, and temper brittleness. Used to manufacture shafts, connecting rods, gears and bolts.
11. Chrome silicon steel: Representative steel type 38CrSi. Hardenability is better than 40Cr, strength and low temperature impact are higher, tempering stability is better, and temper brittleness is more likely. Commonly used to manufacture 30-40mm shafts, bolts and gears with small modulus.
12. Chrome molybdenum steel: Representative steel types 30CrMoA, 42CrMo, ML30CrMo, ML42CrMo. Water quenching 30-55mm, oil quenching 15-40mm; high room temperature mechanical properties and high high temperature strength, good low temperature impact; no temper brittleness. Used to manufacture parts with larger cross-sections, high-load bolts, gears and flanges and bolts below 500℃; pipes and fasteners below 400℃. 42CrMo has higher hardenability than 30CrMoA and is used to manufacture parts with higher strength and larger cross-sections.
13. Chrome manganese molybdenum steel: Representative steel type 40CrMnMo. Oil quenching diameter 80mm, with high comprehensive mechanical properties and good tempering stability. Used to manufacture heavy-duty gears and shaft parts with large cross-sections.
14. Manganese-molybdenum-vanadium steel: Representative steel type 30Mn2MoWA. Has good hardenability: water quenching reaches 150mm, the core structure is upper and lower bainite plus a small amount of martensite; oil quenching 70mm, more than 95% of the core is martensite; good low-temperature impact toughness, low notch sensitivity and high fatigue strength. Used to manufacture important parts below 80mm.
15. Chromium-manganese-silicon steel: Representative steel type 30CrMnSiA. Water quenching 40-60mm (95% martensite), oil quenching 25-40mm. High strength, impact toughness, temper brittleness. Used to manufacture high-pressure blower blades, valve plates, clutch friction plates, shafts and gears, etc.
16. Chromium-nickel steel: Representative steel types 40CrNi and 45CrNi. Water quenching reaches 40mm, oil quenching 15-25mm; good comprehensive mechanical properties, good low-temperature impact toughness, and low temper brittleness tendency. 30CrNi3A has high hardenability, good comprehensive mechanical properties, white spot sensitivity and temper brittleness. It is used to manufacture crankshafts, connecting rods, gears, shafts and bolts with larger cross-sections.
17. Chromium-nickel-molybdenum steel: Representative steel type 40CrNiMoA. It has excellent comprehensive mechanical properties, high low-temperature impact toughness, low notch sensitivity, and no temper brittleness. It is used to manufacture larger crankshafts, shafts, connecting rods, gears, bolts and other parts with large forces and complex shapes.
18. Chromium-nickel-molybdenum-vanadium steel: Representative steel type 45CrNiMoVA. High strength, good tempering stability, oil quenching reaches 60mm (95% martensite). It is used to manufacture heavy-duty automobile elastic shafts and torsion shafts under vibration loads.
Casting properties
The casting properties of alloys refer to the process properties of alloys during casting, mainly the fluidity of alloys and the shrinkage of alloys. These properties are very important for obtaining sound castings.
Fluidity
Fluidity refers to the ability of liquid alloy to fill the mold.
The alloy liquid has good fluidity, which makes it easy to fill the mold cavity and obtain castings with clear contours and complete dimensions. On the contrary, if the alloy has poor fluidity, it is easy to produce defects such as insufficient pouring, cold shut, pores and slag inclusions.
Among the commonly used alloys, gray cast iron and silicon brass have the best fluidity, while cast steel has the worst fluidity.
There are many factors that affect fluidity, among which the main ones are the chemical composition of the alloy, pouring temperature and filling conditions of the mold.
Shrinkage
The phenomenon that the volume and size of liquid alloys continue to decrease during cooling and solidification is called contraction. Contraction is the physical property of the casting alloy itself and is the basic cause of many defects in castings (shrinkage, shrinkage, internal stress, deformation and cracks, etc.). The alloy liquid goes through three stages from being poured into the mold cavity to cooling to room temperature:
1. Liquid contraction: contraction from the pouring temperature to the liquidus temperature where crystallization begins.
2. Solidification contraction: contraction from the temperature where crystallization begins to the solidus temperature where crystallization is completed.
3. Solid contraction: contraction from the temperature where crystallization is completed to room temperature.
The liquid contraction and solidification contraction of the alloy are manifested as the volume reduction of the alloy, which is usually expressed by the volume shrinkage rate. They are the basic reasons for shrinkage cavities and shrinkage defects in castings. Although the solid contraction of the alloy is also a volume change, it only causes changes in the external dimensions of the casting. Therefore, it is usually expressed by the linear shrinkage rate. Solid contraction is the root cause of defects such as internal stress, deformation and cracks in castings.
The chemical composition of the alloy, pouring temperature, casting conditions and casting structure are the main factors affecting the shrinkage of the alloy. The actual shrinkage varies with the shape, size and process conditions of the casting.
In addition, the uneven chemical composition of each part of the alloy liquid during cooling into castings, namely segregation, absorptivity and oxidizability, all have an adverse effect on casting performance.