Mold heat treatment defects and prevention

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

(1) Cause

1) There are oxide scales, rust spots and local decarburization on the surface of the mold before heat treatment.

2) After the mold is quenched and heated, the cooling and quenching medium is not properly selected, and there are too many impurities or aging in the quenching medium.

(2) Preventive measures

1) Before the heat treatment of the mold, the oxide scale and rust spots should be removed, and the surface of the mold should be properly protected during quenching and heating. Vacuum electric furnace, salt bath furnace and protective atmosphere furnace should be used for heating.

2) When the 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 organization of the mold before heat treatment

The final spheroidization structure of the mold is coarse and uneven, and the spheroidization is imperfect. The structure has mesh, band and chain carbides, which will make the mold easy to crack after quenching and cause the mold to be scrapped.

(1) Cause

1) There is severe carbide segregation in the original structure of the die steel material.

2) Poor forging process, such as high forging heating temperature, small deformation, high stop forging temperature, slow cooling rate after forging, etc., so that the forging structure is coarse and there are network, band and chain carbides, which make the ball It is difficult to eliminate during chemical 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 organization or poor spheroidization.

(2) Preventive measures

1) Generally, according to the working conditions of the mold, the production batch and the strength and toughness of the material itself, try to choose high-quality mold steel materials.

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

3) High-carbon die steel with severe segregation of carbides that cannot be forged can be subjected to solid solution refining heat treatment.

4) Formulate the correct spheroidizing annealing process specification for the forging die blank, and adopt quenching and tempering heat treatment and rapid uniform spheroidizing annealing process.

5) Reasonably install the furnace to ensure the uniformity of the mold blank temperature in the furnace.

3. The mold produces quenching cracks

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

(1) Reasons

1) The mold material has serious segregation of network carbides.

2) There is mechanical processing or cold plastic deformation stress in the mold.

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

4) The complex shape, uneven thickness, sharp corners and threaded holes of the mold make the thermal stress and the structural stress too large.

5) Excessive heating temperature of mold quenching will cause overheating or overburning.

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

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

8) Improper grinding process for mold heat treatment.

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

(2) Preventive measures

1) Strictly control the internal quality of mold raw materials

2) Improve the forging and spheroidizing annealing process, eliminating the mesh, band, and chain carbides, and improving the uniformity of the spheroidizing structure. ,

3) After mechanical processing or cold plastic deformation, the mold should be subjected to stress relief annealing (>600℃) before heating and quenching.

4) For molds with complex shapes, asbestos should be used to plug the threaded holes, to wrap dangerous sections and thin walls, and to use graded quenching or austempering.

5) Annealing or high temperature tempering is required when reworking or refurbishing molds.

6) The mold should be preheated during quenching and heating, and pre-cooling measures should be taken when cooling, and suitable quenching medium should be selected.

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

8) After the mold is quenched, it should be tempered in time, and the holding time should be sufficient. The high-alloy complex mold should be tempered 2-3 times.

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

10) Improve the mold EDM process, and carry out stress relief and tempering.

4. Coarse structure after mold quenching

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

(1) Reasons

1) Die steel is confused, the actual steel quenching temperature is much lower than the required quenching temperature of the mold material (such as GCR15 steel as 3CR2W8V steel).

2) The die steel is not properly spheroidized before quenching, and the spheroidizing structure is poor.

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

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

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

(2) Preventive measures

1) The steel should be inspected strictly before entering the warehouse to prevent the steel from being mixed up.

2) Correct forging and spheroidizing annealing should be carried out before die quenching to ensure a good spheroidizing structure.

3) Correctly formulate mold quenching heating process specifications, 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 mold is heated in the furnace, keep a proper distance from the electrode or heating element.