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Defects and prevention of heat treatment of graphite mold
In recent years, graphite molds have been continually impacting the mold market with its excellent characteristics, becoming the darling of the mold industry. The following Kebowei graphite products manufacturers tell you about the defects and prevention methods in the heat treatment of graphite molds.
 
1. There are soft spots on the surface of graphite mold
 
After the heat treatment of the graphite mold, there are soft spots on the surface, which will affect the wear resistance of the graphite mold and reduce the service life of the graphite mold.
 
(1) Reasons for production
 
1) The graphite mold has oxide scale, rust spots and partial decarburization on the surface before heat treatment.
 
2) After quenching and heating the graphite mold, the cooling and quenching medium is improperly selected, and the impurities in the quenching medium are excessive or aging.
 
(2) Preventive measures
 
1) Before the heat treatment of the graphite mold, the scale and rust should be removed. When the surface of the graphite mold is properly protected during quenching, the vacuum furnace, salt bath furnace and protective atmosphere furnace should be used for heating.
 
2) When cooling the graphite mold after quenching and heating, select a suitable cooling medium, and filter the long-term cooling medium frequently, or replace it regularly.
 
2, the graphite mold is poorly organized before heat treatment
 
The final spheroidized structure of the graphite mold is coarse and uneven, and the spheroidization is imperfect. The structure has mesh, band and chain carbides, which will make the graphite mold easy to crack after quenching, which will cause the graphite mold to be scrapped.
 
(1) Reasons for production
 
1) There is serious carbide segregation in the original structure of the graphite die steel material.
 
2) The forging process is not good, such as the forging heating temperature is too high, the deformation amount is small, the stop forging temperature is high, the cooling speed after forging is slow, etc., so that the forged structure is coarse and there are mesh, band and chain carbides present, so that the ball It is difficult to eliminate during annealing.
 
3) Poor spheroidizing annealing process, such as too high or too low annealing temperature, short isothermal annealing time, etc., may result in uneven spheroidizing annealing or poor spheroidization.
 
(2) Preventive measures
 
1) Generally, according to the working conditions of the graphite mold, the production batch and the toughening performance of the material itself, try to select a good quality graphite mold steel material.
 
2) Improve the forging process or use normalizing heat treatment to eliminate the non-uniformity of the network and chain carbides and carbides in the raw materials.
 
3) High-carbon graphite die steel with severe segregation of carbides that cannot be forged can be subjected to solution heat treatment.
 
4) Develop a correct spheroidizing annealing process specification for the forged blank, which can be tempered heat treatment and fast uniform spheroidizing annealing.
 
5) Reasonably install the furnace to ensure the uniformity of the temperature of the mold blank in the furnace.
 
Graphite mold
 
3, graphite mold produces quenching crack
 
The cracking of the graphite mold after quenching is the biggest defect in the heat treatment process of the graphite mold, which will cause the processed graphite mold to be scrapped, causing great loss in production and economy.
 
(1) Reasons for the cause
 
1) There is severe network carbide segregation in the graphite mold material.
 
2) There are mechanical processing or cold plastic deformation stresses in the graphite mold.
 
3) Improper heat treatment of graphite mold (heating or cooling too fast, improper selection of quenching cooling medium, low cooling temperature, too long cooling time, etc.).
 
4) Graphite molds have complex shapes, uneven thickness, sharp corners and threaded holes, which cause excessive thermal stress and tissue stress.
 
5) Graphite mold quenching heating temperature is too high to produce overheating or overburning.
 
6) The tempering of the graphite mold after quenching is not timely or the tempering holding time is insufficient.
 
7) When the graphite mold is repaired and quenched, it is quenched again without intermediate annealing.
 
8) The graphite mold is heat treated and the grinding process is improper.
 
9) When the graphite mold is heat-treated after heat treatment, there are high tensile stresses and microcracks in the hardened layer.
 
(2) Preventive measures
 
1) Strictly control the intrinsic quality of graphite mold raw materials
 
2) Improve the forging and spheroidizing annealing process, eliminate the network, ribbon, and chain carbides, and improve the uniformity of the spheroidized structure. ,
 
3) The graphite mold after mechanical processing or after cold plastic deformation shall be subjected to stress relief annealing (>600 ° C) and then heat quenched.
 
4) For graphite molds with complex shapes, asbestos should be used to block threaded holes, and the dangerous section and thin wall should be wrapped, and graded quenching or austempering should be used.
 
5) Annealing or high temperature tempering is required for rework or refurbishment of graphite molds.
 
6) The graphite mold should be preheated during quenching heating, pre-cooling measures should be taken during cooling, and suitable quenching medium should be selected.
 
7) The quenching heating temperature and time should be strictly controlled to prevent overheating and over-burning of the graphite mold.
 
8) After quenching the graphite mold, it should be tempered in time, the holding time should be sufficient, and the high-alloy complex graphite mold should be tempered 2-3 times.
 
9) Choose the right grinding process and the right grinding wheel.
 
10) Improve the graphite mold EDM process and perform stress relief and tempering.
 
4. The microstructure of the graphite mold is coarse after quenching
 
The coarseness of the graphite mold after quenching will seriously affect the mechanical properties of the graphite mold. When used, the graphite mold will be broken, which will seriously affect the service life of the graphite mold.
 
(1) Reasons for the cause
 
1) Graphite mold steel is confused, the actual steel quenching temperature is much lower than the quenching temperature of the required graphite mold material (such as GCr15 steel as 3Cr2W8V steel).
 
2) The graphite ball steel is not subjected to the correct spheroidization process before quenching, and the spheroidized structure is poor.
 
3) The graphite mold quenching heating temperature is too high or the holding time is too long.
 
4) The graphite mold is placed improperly in the furnace and is prone to overheating in the vicinity of the electrode or heating element.
 
5) For graphite molds with large cross-section changes, the quenching heating process parameters are not properly selected, and overheating occurs at thin sections and sharp corners.
 
(2) Preventive measures
 
1) Steel should be strictly inspected before storage, and steel should be prevented from being confused.
 
2) The correct forging and spheroidizing annealing should be performed before the graphite mold is quenched to ensure a good spheroidized structure.
 
3) Properly formulate the specification of quenching heating process for graphite mold, and strictly control the heating temperature and holding time of quenching.
 
4) Regularly detect and calibrate the temperature measuring instrument to ensure the normal operation of the instrument.
 
5) The graphite mold should be kept at an appropriate distance from the electrode or heating element when heated in the furnace.

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