1. Structure and crystallization of pure metal
1. Crystal structure of metal
Metals are crystals in solid state.
The properties, plastic deformation and heat treatment phase transformation of metals are all related to the crystal structure.
The most common lattices in metals are three clocks: body centered cubic lattice, face centered cubic lattice and closely packed hexagonal lattice.
Crystal defects can be divided into point defects, line defects and plane defects according to their geometrical shapes.
2. Crystallization of metals
The process by which a metal changes from a liquid state to a solid (crystalline) state is called metal crystallization.
(1) Cooling curve and supercooling phenomenon
The relation curve between temperature and time in the process of material cooling is called cooling curve.
The cooling curve of metal crystal can be measured by thermal analysis method.
The measurement process is as follows: first melt the metal and make the temperature as uniform as possible, then cool it at a certain speed, record the data of temperature change with time, and draw it in the temperature time coordinate to obtain the cooling curve as shown in Fig. 1.
Since the latent heat of crystallization released during crystallization compensates the heat lost by the metal to the outside, a horizontal line appears on the cooling curve, and the temperature corresponding to this horizontal line is the actual crystallization temperature of the metal.
Experiments show that the actual crystallization temperature T1 of the metal is always lower than the theoretical crystallization temperature (equilibrium crystallization temperature) T0.
This phenomenon is called supercooling.
Supercooling is a necessary condition for crystallization.
The difference △T between T1 and T0 is called supercooling degree, that is, △T = T0-T1.
Fig. 1 cooling curve of pure iron crystal
(2) Crystallization process
The crystallization process is a process of nucleation and nucleation growth.
3. Isomeric transformation of metals
The phenomenon that a metal changes from one lattice to another with the change of temperature in the solid state is called isomorphic transformation.
Metals with isomorphic transformation include iron, cobalt, titanium, tin and manganese.
Crystals of the same metal element existing in different lattice forms are called allotropic crystals of the metal.
2. Structure and crystallization of alloy
Phase: refers to the uniform components in an alloy (or pure metal) that have the same composition, structure and performance and are separated from each other by an interface.
1. Phase structure of alloy
According to the interaction between the elements constituting the alloy, the phase structure in the alloy can be divided into two types: solid solution and metal compound.
(1) Solid solution
When the liquid alloy solidifies, the components can still dissolve with each other to form a phase in which atoms of other elements are dissolved in the lattice of a certain element. This phase is called solid solution.
(2) Metal compound
2. State diagram of binary alloy
The alloy state diagram, also known as the alloy equilibrium diagram or the alloy phase diagram, is a diagram showing the relationship between the state of the alloy and the temperature and composition under the equilibrium condition.
It reflects the structure change law of the alloys with different compositions in the alloy system when they are slowly heated or cooled infinitely.
It is an important basis for selecting the alloy composition, analyzing the microstructure of the alloy, studying the properties of the alloy and formulating the casting, forging and heat treatment processes.
(1) Homogeneous state diagram: a state diagram in which two components can be infinitely miscible in both liquid and solid state.
This kind of alloy crystallizes solid solution from liquid phase during solidification, and this crystallization process is called homogeneous transformation.
(2) Eutectic state diagram: the two components are completely miscible in the liquid state and have a eutectic transition state diagram.
Eutectic Transformation: the transformation in which two solid phases with a certain composition are crystallized simultaneously from a uniform liquid phase with a certain composition at a certain temperature.
(3) Peritectic state diagram: the two components are infinitely miscible in the liquid state, forming a finite solid solution in the solid state, and there is a state diagram of peritectic transformation.
Peritectic Transformation: under constant temperature, the liquid phase of a certain component reacts with the solid phase of a certain component that has been crystallized and forms a new solid phase of another component.
3. Iron carbon phase diagram
1. Iron carbon phase diagram
Steel is an iron-carbon alloy with a certain composition range.
The iron-carbon alloy phase diagram shows the different equilibrium structures of iron-carbon alloys with different compositions at different temperatures, as shown in the Fe-Fe3C phase diagram.
From the Fe-Fe3C phase diagram, we can find out the temperature at which the equilibrium phase transformation occurs in the iron carbon alloy with certain composition, that is, the critical point;
It is possible to predict the phase transformation process in different temperature regions and the possible equilibrium structure when cooling to normal temperature.
See several characteristic points in the Fe-Fe3C phase diagram for the description of each characteristic point in the iron carbon alloy phase diagram, and see the characteristic lines in the Fe-Fe3C phase diagram for the description of each characteristic line.
According to the iron carbon alloy phase diagram, carbon steel with carbon content less than 2.11% and cast iron with carbon content greater than 2.11%.
According to the structure characteristics, the iron carbon alloy is divided into seven categories according to the carbon content in the iron carbon alloy phase diagram:
(1) Industrial pure iron, carbon content < 0.0218%;
(2) Eutectoid steel, carbon content 0.77%;
(3) Hypoeutectoid steel, carbon content 0.0218% ~ 0.77%;
(4) Hypereutectoid steel with carbon content of 0.77% ~ 2.11%;
(5) Eutectic white cast iron, carbon content 4.30%;
(6) Sub crystalline white cast iron with carbon content of 2.11% ~ 4.30%;
(7) Super crystalline white cast iron with carbon content of 4.30% ~ 6.69%;
2. Metal structure
Metal: a material with good thermal conductivity and conductivity, which is opaque and metallic luster, and whose conductivity decreases with the increase of temperature, and is rich in ductility and expansibility.
A solid (i.e., a crystal) in which atoms in a metal are arranged regularly.
Alloy: a substance with metallic properties composed of two or more metals or metals and nonmetals.
Solid solution strengthening: since the solute atoms enter the gap or node of the solvent lattice, the lattice will be distorted and the hardness and strength of the solid solution will be increased. This phenomenon is called solid solution strengthening phenomenon.
Compound: a new crystalline solid structure with metallic properties is formed by the combination of alloy components.
Mechanical mixture: an alloy composition composed of two crystal structures.
Although it is a two-sided crystal, it is a component with independent mechanical properties.
Ferrite: interstitial solid solution of carbon in a-Fe (iron with body centered cubic structure).
Austenite: interstitial solid solution of carbon in g-fe (face centered cubic iron).
Cementite: a stable compound (Fe3C) formed by carbon and iron.
Pearlite: mechanical mixture composed of ferrite and cementite (F + Fe3C contains 0.8% carbon)
Ledeburite: mechanical mixture composed of cementite and austenite (containing 4.3% carbon)
(the above figure shows the metal structure corresponding to each stage of the iron carbon phase diagram)
Metal heat treatment is one of the important processes in mechanical manufacturing.
Compared with other processing processes, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but endows or improves the service performance of the workpiece by changing the microstructure inside the workpiece or changing the chemical composition of the surface of the workpiece.
Its feature is to improve the internal quality of the workpiece, which is generally not visible to the naked eye.
In order to make the metal workpiece have the required mechanical properties, physical properties and chemical properties, in addition to the reasonable selection of materials and various forming processes, the heat treatment process is often essential.
Steel is the most widely used material in the mechanical industry.
The microstructure of steel is complex and can be controlled by heat treatment.
Therefore, the heat treatment of steel is the main content of metal heat treatment.
In addition, aluminum, copper, magnesium, titanium and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different service properties.