The Prevention for Defects in the Heat Treatment of Graphite Molds

Ⅰ. There are soft spots on the surface of the graphite mold

There are soft spots on the surface of the graphite mold after heat treatment, which will affect the wear resistance of the graphite mold and reduce the service life of the graphite mold.

1. Cause

(1) Before the heat treatment of the graphite mold, there are oxide scales, rust spots and local decarburization on the surface.

(2) After the graphite mold is quenched and heated, the cooling and quenching medium is improperly selected, and there are too many impurities or aging in the quenching medium.

2. Preventive measures

(1) Before the heat treatment of the graphite mold, the oxide scale and rust spots should be removed. During the quenching and heating, the surface of the graphite mold should be properly protected. The vacuum electric furnace, salt bath furnace and protective atmosphere furnace should be used as much as possible for heating.

(2) When the graphite mold is cooled after quenching and heating, a suitable cooling medium should be selected, and the cooling medium used for a long time should be filtered frequently or replaced regularly.

2. Poor structure of graphite mold before heat treatment

The final spheroidization structure of the graphite mold is coarse and uneven, the spheroidization is imperfect, the structure has mesh, ribbon and chain carbides, all of which will make the graphite mold prone to cracks after quenching and cause the graphite mold to be scrapped.

1. Cause

(1) The original structure of graphite mold steel material has serious carbide segregation.

(2) Poor forging process, such as high forging heating temperature, small deformation, high temperature of stopping forging, slow cooling rate after forging, etc., which make the forging structure coarse, and there are mesh, ribbon and chain carbides. It is difficult to eliminate these during spheroidizing annealing.

(3) Poor spheroidizing annealing process, such as too high or too low annealing temperature, short isothermal annealing time, etc., can cause uneven spheroidizing annealing structure or poor spheroidization.

2. Preventive measures

(1) Generally, according to the working conditions, the production batch and the strength and toughness of the material itself of the graphite mold, the graphite mold steel material of good quality should be selected as far as possible.

(2) Improve the forging process or adopt normalizing preparation heat treatment to eliminate the unevenness of the mesh and chain carbides in the raw materials.

(3) For high-carbon graphite mold steels with serious carbide segregation that cannot be forged, solid solution refining heat treatment can be carried out.

(4) For the die blank after forging, the correct specification for spheroidizing annealing process should be formulated, and quenched-tempered heat treatment and rapid uniform spheroidizing annealing can be adopted.

(5) Install the graphite crucible furnace reasonably to ensure the uniformity of the die blank temperature in the furnace.

3. Quenching cracks in the graphite mold

The graphite mold cracks after quenching, which is the biggest defect in the heat treatment process of the graphite mold, which will cause the processed graphite mold to be scrapped and cause great losses in production and economy.

1. The reason

(1) The graphite mold material has serious mesh carbide segregation.

(2) There is mechanical processing or cold molding deformation stress in the graphite mold.

(3) Improper heat treatment of graphite mold (heating or cooling too fast, improper selection of quenching cooling medium, cooling temperature is too low, cooling time is too long, etc.).

(4) Graphite molds have complex shapes, uneven thickness, sharp corners and threaded holes, etc., which cause excessive thermal stress and organizational stress.

(5) The quenching heating temperature of the graphite mold is too high, resulting in overheating or overburning.

(6) After the graphite mold is quenched, the tempering is not timely, or the tempering holding time is insufficient.

(7) When the graphite mold is reworked and quenched, it is heated and quenched again without intermediate annealing.

(8) The graphite mold is heat-treated and the grinding process is improper.

(9) During the EDM machining of the graphite mold after heat treatment, there are high tensile stresses and microcracks in the hardened layer.

2. Preventive measures

(1) Strictly control the internal quality of graphite mold raw materials.

(2) Improve the forging and spheroidizing annealing process to eliminate the mesh, ribbon, and chain carbides, and improve the uniformity of the spheroidizing structure.

(3) Graphite molds after mechanical processing or cold plastic deformation should be subjected to stress relief annealing (>600°C) before heating and quenching.

(4) For graphite molds with complex shapes, asbestos should be used to plug the threaded holes, to wrap dangerous sections and thin walls, and hierarchical quenching or isothermal quenching should be adopted.

(5) When repairing or refurbishing graphite molds, annealing or high temperature tempering is required.

(6) When the graphite mold is quenched and heated, it should be preheated; when it is cooled, pre-cooling measures should be taken, and a suitable quenching medium should be selected.

(7) The quenching heating temperature and time should be strictly controlled to prevent the graphite mold from overheating and burning.

(8) After quenching, the graphite mold should be tempered in time, and the holding time should be sufficient. The high-alloy complex graphite mold should be tempered 2 to 3 times.

(9) Choose the right grinding process and the right grinding wheel.

(10) Improve the EDM process of graphite molds and perform stress-relief tempering.

4. The graphite mold has a coarse structure after quenching

The coarse structure of the graphite mold after quenching will seriously affect the mechanical properties of the graphite mold, and the graphite mold will be broken during use, which will seriously affect the service life of the graphite mold.

1. The reason

(1) The graphite mold steel is mixed up. The actual steel quenching temperature is much lower than the required quenching temperature of graphite mold material (for example, GCr15 steel is regarded as 3Cr2W8V steel).

(2) Before the graphite mold steel is quenched, the correct spheroidizing process is not carried out, and the spheroidizing structure is poor.

(3) The quenching heating temperature of the graphite mold is too high or the holding time is too long.

(4) Improper placement of the graphite mold in the furnace may cause overheating in the area close to the electrode de graphite or heating element.

(5) For graphite molds with large cross-section changes, the quenching and heating process parameters are not properly selected, resulting in overheating at thin cross-sections and sharp corners.

2. Preventive measures

(1) Before the graphite material is put into storage, strict inspections should be carried out to prevent the graphite materials from being mixed up.

(2) Before the graphite mold is quenched, the correct forging and spheroidizing annealing should be carried out to ensure a good spheroidizing structure.

(3) Correctly formulate the specifications for graphite mold quenching and heating process, and strictly control the quenching heating temperature and holding time.

(4) Regularly check and calibrate the temperature measuring instrument to ensure the normal operation of the instrument.

(5) When the graphite mold is heated in the furnace, it should keep a proper distance from the electrode or heating element.

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