Although the application of lost foam casting technology in China has been slow, it has developed rapidly in recent years. In particular, due to the low investment in lost foam casting equipment and short process routes, many original small and medium-sized foundry companies are increasingly adopting this technology.
However, some companies have failed to pay attention to some operational problems. Today we will understand the defects and preventive methods in the heat treatment of lost foam molds:
1. The surface of the mold has soft spots
After the heat treatment of the mold, there are soft spots on the surface, which will affect the wear resistance of the mold and reduce the service life of the mold.
(1) Cause
The mold has scale, rust and local decarburization on the surface before heat treatment. After quenching and heating, the cooling and quenching medium is improperly selected, and the impurities in the quenching medium are excessive or aged.
(2) Preventive measures
Before the heat treatment of the mold, the scale and rust spots should be removed. When the mold is heated during quenching, the surface of the mold should be properly protected. Vacuum furnace, salt bath furnace and protective atmosphere furnace should be used for heating. When quenching and heating, select a suitable cooling medium, and filter the long-term cooling medium frequently, or replace it regularly.
2. The mold is poorly organized before heat treatment
The final spheroidized structure of the mold is coarse and uneven, and the spheroidization is imperfect. The structure has mesh, ribbon and chain carbides, which will cause the mold to be cracked after quenching, resulting in scrapping of the mold.
(1) Cause
There is serious carbide segregation in the original structure of the die steel material. Poor forging process, such as forging heating temperature is too high, deformation is small, stop forging temperature is high, and the cooling speed after forging is slow, so that the forged structure is coarse and there are mesh, band and chain carbides, so that spheroidizing annealing It is difficult to eliminate. The spheroidizing annealing process is not good, such as the annealing temperature is too high or too low, the isothermal annealing time is short, etc., which may result in uneven spheroidizing annealing or poor spheroidization.
(2) Preventive measures
Generally, according to the working conditions of the mold, the production batch and the toughening performance of the material itself, try to select a good quality mold steel material. 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.
High-carbon die steel with severe segregation of carbides that cannot be forged can be subjected to solution heat treatment. The correct spheroidizing annealing process specification for the forged blank can be tempered heat treatment and rapid uniform spheroidizing annealing. The furnace is properly installed to ensure the uniformity of the temperature of the mold blank in the furnace.
3. The mold produces quenching cracks
The 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, causing great loss in production and economy.
(1) Reasons for the cause
There is severe network carbide segregation in the mold material. There are mechanical or cold plastic deformation stresses in the mold. Improper heat treatment (heating or cooling too fast, improper selection of quenching cooling medium, low cooling temperature, too long cooling time, etc.).
The mold has a complicated shape, uneven thickness, sharp corners and threaded holes, which cause excessive thermal stress and tissue stress. The quenching heating temperature is too high to cause overheating or overheating. After quenching, the tempering is not timely or the tempering time is insufficient. When the quenching is heated, the quenching is performed again without intermediate annealing. Heat treatment, improper grinding process. When subjected to electrical discharge machining after heat treatment, high tensile stress and microcracks are present in the hardened layer.
(2) Preventive measures
Strictly control the intrinsic quality of the mold raw materials, improve the forging and spheroidizing annealing process, eliminate the network, ribbon, and chain carbides, and improve the uniformity of the spheroidized structure. The mold after mechanical processing or after cold plastic deformation shall be subjected to stress relief annealing (>600 ° C) and then subjected to heat quenching. For 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.
Annealing or high temperature tempering is required when reworking or refurbishing the mold. Preheating should be adopted during quenching heating, pre-cooling measures should be taken during cooling, and suitable quenching medium should be selected. The quenching heating temperature and time should be strictly controlled to prevent overheating and over-burning of the mold.
After the mold is quenched, it should be tempered in time, the holding time should be sufficient, and the high alloy complex mold should be tempered 2-3 times. Choose the right grinding process and the right grinding wheel. Improve the mold EDM process and perform stress relief and tempering.
4. After the mold is quenched, the structure is coarse.
After the mold is quenched, the coarse structure will seriously affect the mechanical properties of the mold. When used, the mold will be broken, which will seriously affect the service life of the mold.
(1) Reasons for the cause
The mold steel is confused, and the actual steel quenching temperature is much lower than the quenching temperature of the required mold material (such as GCr15 steel as 3Cr2W8V steel). The steel was not properly spheroidized before quenching, and the spheroidized structure was poor. The quenching heating temperature is too high or the holding time is too long. Improper placement in the furnace and overheating in the vicinity of the electrode or heating element area. For 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
Before the steel is put into storage, it should be strictly inspected to prevent the steel from being confused. Proper forging and spheroidizing annealing should be performed before the mold is quenched to ensure good spheroidization. Correctly formulate the mold quenching heating process specification and strictly control the quenching heating temperature and holding time. Regularly test and calibrate the temperature measuring instrument to ensure the normal operation of the instrument. Keep the proper distance from the electrode or heating element when heating in the furnace.
