BASIC THEORETICAL KNOWLEDGE OF HEAT TREATMENT
Microstructure of steel
Crystal structure of metal
Matter is composed of atoms. According to the different arrangement of atoms, matter can be divided into two categories, crystal and amorphous. The arrangement of atoms in a crystal is regular, that is, “orderly arrangement”. This regular arrangement is called the crystal structure; The arrangement of atoms in non crystal is irregular, that is, “disordered arrangement”. The arrangement of atoms is usually called lattice structure. The arrangement of atoms in all metals is regular. Therefore, metals belong to crystals.
Metallographic structure of steel
The concept of “phase”: components with the same chemical composition, crystal structure and physical properties in the structure of metal alloy. In the field of metallography, it is called metallography, which includes solid solutions, metal compounds and pure elements.
The concept of “organization”: it generally refers to the whole composed of one or more phases with different shapes, sizes and distribution modes, as well as various material defects and damages seen by metallographic methods.
Generally, we call the alloy composed of iron (FE) and certain carbon (c) as steel, but carbon exists in the form of iron and carbon compound (Fe3C) in steel. Due to the existence of carbon in steel, it will affect the lattice structure of iron and form different structures. Generally, all kinds of structures in steel are collectively referred to as metallographic structure. The microstructure of steel is different, and its properties are very different. Different structures can be obtained by different heat treatment of steel, and finally the properties we need can be obtained. The basic structures of steel are as follows:
(1) Austenite: solid solution of face centered cubic structure formed by iron and other elements, generally referring to the combination of carbon and other elements γ Interstitial solid solution in iron.
(2) Ferrite: solid solution of body centered cubic structure formed by iron and other elements, generally referring to the combination of carbon and other elements α Interstitial solid solution in iron.
(3) Martensite: metastable phase transformed from austenite by non diffusive transformation. In fact, it is a interstitial solid solution in which carbon is supersaturated in iron. The crystal has a body centered square structure.
(4) Pearlite: the layered microstructure with alternating ferrite and cementite sheets. It is the direct product of eutectoid reaction of undercooled austenite. It can also be understood as the mechanical mixture of ferrite and cementite.
(5) Bainite: the polymerized structure of ferrite and cementite decomposed by undercooled austenite in the range below pearlite transformation temperature and above martensite transformation temperature. The upper bainite decomposed at higher temperature is called upper bainite, which is feathery; The bainite decomposed at lower temperature is called lower bainite, which is similar to the acicular structure of low-temperature tempered martensite.
In addition, in actual production, according to the requirements of product performance and the specific heat treatment process, there will be other structures in the steel, such as sorbite, troostite, granular pearlite, tempered martensite, tempered sorbite, etc., but these structures are not essentially different from the above basic structures.
Transformation of steel during heating
Whether annealing, normalizing, quenching or carburizing, it is necessary to heat the steel parts to the austenitic state first. Austenite is a solid solution of carbon atoms in the face centered cubic lattice gap of iron. The composition, uniformity, grain size and the quantity and distribution of other phases of austenite have a great influence on the decomposition process, decomposition products and properties of austenite during cooling. At the same time, the heating process of steel will also cause changes in surface quality and composition (oxidation and decarburization), which will affect the heat treatment effect of workpiece.
In order to ensure that the heat treatment can achieve the expected purpose, it is necessary to master the laws of Austenite Formation and growth during steel heating, and use these laws to control the heat treatment effect.
(1) Austenite Formation
The temperature range of Austenite Formation in steel during heating can generally be explained according to the iron carbon alloy state diagram . It can be seen from the figure that when the eutectoid steel with pearlite structure is heated from room temperature to below A1 temperature, there is no other structural transformation except that the carbon content of ferrite increases slightly. When the temperature rises slightly above A1, pearlite changes to austenite. Similarly, for hypoeutectoid steel with ferrite and pearlite, when heated slightly above A1, pearlite changes to austenite, while ferrite does not change. With the continuous increase of heating temperature, ferrite continues to change to austenite. When the temperature rises to A3, all ferrite changes to austenite.