The Basis of Metal Heat Treatment
Metal Heat Treatment
1. What is heat treatment
A process in which solid metal or alloy is heated in an appropriate manner, kept for a certain period of time, and cooled at a certain cooling rate to change its structure, thereby obtaining the required performance.
2. What is the purpose of heat treatment
The internal structure of steel is changed by proper heat treatment process to control the degree of structure transformation and the form of transformation products during phase transformation, thereby improving the properties of steel.
3. What are the conditions for heat treatment
The alloy must have a solid phase transformation before it can be heat treated.
4. What is the process of heat treatment
(1) Heating: critical point + △T value
(2) Heat preservation
(3) Cooling: critical point-△T value, certain cooling rate
5. What are the main parameters
(1) Heating temperature T
(2) Holding time t
(3) Cooling speed V, the cooling medium determines the cooling speed, such as water, brine, alkaline water, air
6. Which types can be divided according to the processing stage and purpose
The purpose is to eliminate segregation and internal stress and obtain a balanced structure for the final heat treatment or subsequent processing.
(2) Final treatment
As the final process of workpiece processing, the final organization is obtained.
7. According to the heat treatment process parameters, which types can be divided into
(1) Ordinary heat treatment
This is the most commonly used heat treatment process in production, such as annealing, normalizing, quenching, and tempering. This type of heat treatment generally does not add other elements, mainly through the transformation of its own structure to obtain the required performance.
(2) Chemical heat treatment
This type of heat treatment is often used on wear parts such as gears and shafts. When the workpiece is subjected to chemical heat treatment, other elements will penetrate into the surface layer, and will not have any effect on the composition of the core. Generally, what element is infiltrated, we call it infiltration × treatment, such as surface infiltration, N infiltration, C and N co-infiltration, etc.
(3) Surface heat treatment
The characteristics of the above two types of heat treatment are combined, that is, no other elements are added during the heat treatment, and the heat treatment is only for the surface, and does not affect the structure of the core, such as surface quenching, but it requires a higher carbon content of the workpiece.
8. What is annealing
Annealing is a heat treatment process in which metals and alloys are heated to an appropriate temperature, kept for a certain period of time, and then slowly cooled. After annealing, the structure of hypoeutectoid steel is ferrite plus lamellar pearlite; eutectoid steel or hypereutectoid steel is granular pearlite. In short, the annealed structure is a structure close to equilibrium.
9. What is the purpose of annealing
(1) Reduce the hardness of steel and increase plasticity to facilitate cutting and cold deformation processing.
(2) Refine the grains, eliminate the structural defects caused by casting, forging, and welding, uniform the structure and composition of the steel, improve the performance of the steel or prepare for the subsequent heat treatment.
(3) Eliminate internal stress in steel to prevent deformation and cracking.
10. What are the types of annealing process?
There are mainly homogenization annealing, complete annealing, incomplete annealing, isothermal annealing, spheroidizing annealing, recrystallization annealing, stress relief annealing
11. What is homogenization annealing
Homogenization annealing is to reduce the segregation of the chemical composition of metal ingots, castings or forging billets and the unevenness of the structure. It is heated to a high temperature, maintained for a long time, and then slowly cooled, in order to homogenize the chemical composition and structure. The annealing process.
The heating temperature of homogenization annealing is generally Ac3+ (150～200℃), that is, 1050～1150℃, and the holding time is generally 10～15h to ensure the full progress of diffusion and eliminate or reduce the unevenness of composition or organization. Because the heating temperature of diffusion annealing is high, the time is long, and the grains are coarse, for this reason, complete annealing or normalizing is performed after diffusion annealing to re-fine the structure
12. What is complete annealing
Complete annealing, also known as recrystallization annealing, is an annealing process in which the iron-carbon alloy is completely austenitized and then slowly cooled to obtain a structure close to the equilibrium state.
Complete annealing is mainly used for hypoeutectoid steels, generally medium-carbon steel and low- and medium-carbon alloy structural steel forgings, castings and hot-rolled sections, and sometimes also used for their welding component
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