Understand normalizing, annealing, quenching and tempering (2)
4. What is quenching?
Quenching is the process of heat treatment to obtain the martensite structure by heating the metal or parts above the phase change temperature and then rapidly cooling them beyond the critical cooling rate. Quenching is to obtain the martensite structure, and after tempering, the workpiece to obtain good performance, in order to give full play to the potential of materials. Its main objectives are:
1) to improve the mechanical properties of metal products or parts, for example, to improve the hardness and wear resistance of tools, bearings, to improve the elastic limit of springs, to improve the comprehensive mechanical properties of shaft parts, etc.
2) improving the material properties or chemical properties of some special steels, such as improving the corrosion resistance of stainless steels and increasing the permanent magnetism of magnetic steels.
In the process of quenching, there should be a correct quenching method besides the reasonable selection of quenching medium. The commonly used quenching methods include single liquid quenching, double liquid quenching, graded quenching, isothermal quenching, and local quenching.
5.Normalizing, annealing, quenching, tempering, the difference and relationship?
The normalizing fire has the following purposes and purposes:
1. For hypereutectoid steels, normalizing is used to eliminate the overheated coarse grain structure and widmanstatten structure of the castings, forgings, and welds, the banded structure of the rolled products, to refine the grains, and as the prior heat treatment before quenching.
2. For over-eutectoid steels, normalizing can eliminate reticulate secondary cementite and refine pearlite, which not only improves mechanical properties but also is beneficial to spheroidizing annealing.
3. Normalizing can eliminate the free cementite in the grain boundary of low carbon deep-drawing sheet steel to improve its deep-drawing property.
4. for low carbon steel and low carbon low alloy steel, more fine flake pearlite structure can be obtained by normalizing, which can increase the hardness to HB140-190, avoid the phenomenon of “sticking tool”, and improve the machinability. For Medium Carbon Steel, it is more economical and convenient to use normalizing when normalizing and annealing are available.
5. for ordinary medium carbon structural steel, normalizing can be used instead of quenching and high temperature tempering under the condition of low mechanical properties, which not only is easy to operate but also makes the steel structure and size stable.
6. High temperature normalizing (150 ~ 200 °C above AC3) can reduce the component segregation of castings and forgings due to the high diffusion rate at high temperature. The coarse grains after high temperature normalizing can be refined by a second normalizing at a lower temperature.
7. For some low-and medium-carbon alloy steels used in steam turbines and boilers, normalizing is often used to obtain bainite structure, which is tempered at high temperature and has good creep resistance when used at 400 ~ 550 °C.
8. In addition to steel and steel, normalizing is also widely used in heat treatment of nodular cast iron to obtain pearlite Matrix and improve the strength of nodular cast iron.
As normalizing is characterized by air cooling, the ambient temperature, stacking mode, air flow and workpiece size have an effect on the structure and properties of normalizing. The normalizing structure can also be used as a classification method of alloy steel. Alloy steels are usually divided into pearlitic, Bainitic, martensitic and austenitic steels according to the microstructure obtained by Air Cooling after the specimens with a diameter of 25 mm are heated to 900 °C.
Annealing is a metal heat treatment process in which the metal is slowly heated to a certain temperature, held for a sufficient period of time, and then cooled at an appropriate rate. Annealing heat treatment is divided into complete annealing, incomplete annealing, and stress relief annealing. The mechanical properties of annealed materials can be tested by tensile test or hardness test. Many plates of steel are supplied in the state of annealing heat treatment. The hardness of steels can be measured by Rockwell hardness tester and HRB hardness tester. For thin steel plates, strips and thin-walled tubes, the hardness of HRT can be measured by surface Rockwell hardness tester.
6.The purpose of Annealing
To improve or eliminate all kinds of structural defects and residual stresses caused by steel during casting, forging, rolling and welding, and to prevent deformation and to crack of the workpiece.
Soften the workpiece for cutting.
Grain Refinement and microstructure improvement to improve the mechanical properties of the workpiece.
Prepare the microstructure for final heat treatment (quenching, tempering).
Current annealing processes are:
1. Complete annealing. It is used to refine the coarse superheat structure of medium and low carbon steel which has poor mechanical properties after casting, forging, and welding. After the workpiece is heated to the temperature above 30 ~ 50 °C from ferrite to austenite, the steel is kept warm for a period of time, then cooled slowly with the furnace, and the AUSTENITE has transformed again during the cooling process, then the microstructure of the steel is excellent.
2. Spheroidizing annealing. Used to reduce the high hardness of tool steel and bearing steel after forging. When the workpiece is heated to 20 ~ 40 °C above the temperature at which austenite begins to form in the steel, it is cooled slowly after heat preservation. During the cooling process, the lamellar cementite in the pearlite becomes spherical, which reduces the hardness.
3. Isothermal annealing. It is used to reduce the high hardness of some alloy structural steels with a high content of nickel and chromium for cutting. In general, the AUSTENITE is cooled to the most unstable temperature at a relatively fast speed, and the hardness can be reduced by the transformation of austenite to troostite or sorbate at a suitable holding time.
4.Recrystallization annealing. It is used to eliminate the hardening phenomenon of metal wire and sheet in the process of cold drawing and cold rolling (hardness increasing and plasticity decreasing) . The heating temperature is generally 50 ~ 150 °C below the temperature at which the steel begins to form austenite. Only in this way can the work hardening effect be eliminated and the metal softened.
5. graphitization annealing. Used to change a cast iron containing a large amount of cementite into malleable cast iron with good plasticity. The casting is heated to about 950 °C and cooled properly after holding the temperature for a certain time so that the cementite is decomposed into flocky graphite.
6. diffusion annealing. The invention is used for homogenizing the chemical composition of the alloy casting and improving its service performance. The casting was heated to the highest possible temperature without melting and kept for a long time until all the elements in the alloy dispersed evenly and cooled slowly.
7. Stress Relief annealing. To eliminate internal stresses in iron and steel castings and welds. The internal stress can be eliminated by cooling the steel products in the air after heat treatment at the temperature below 100 ~ 200 °C.