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Research on Vacuum hardening of Aluminum Die Casting Mold

Compared with general molds, the characteristics of die-casting molds are: 1. The shape and structure are complex, the processing is difficult, and the manufacturing cycle is long; 2. Because it is a forming mold, it is heat-treated after the mold trial, so it requires small deformation, and the general deformation requirement is 0.08mm; 3. Requires high thermal fatigue resistance, fracture toughness, and long service life; 3. Using high-quality steel such as 3Cr2W8, H13, Y10, etc., the residual salt in the large grooves deformed by salt bath hardening is difficult to clean, and mild corrosion will occur, making it difficult to polish the surface. Even with gas protection heating, the mold cannot effectively prevent deformation. Improper control of the amount of oil dripping will cause carburization or decarburization of the surface, and even affect the subsequent nitriding effect and surface polishing quality. For this reason, people generally usehardening and tempering treatment to make the mold hardness reach (28~32) HRC, then finish machining, test the mold, and gas nitriding to the layer depth (0.20~0.40) mm, the hardness is (700~800) HV5, and the deformation <0.06mm. When the mold ishardened and tempered to the upper limit of hardness, the tool is difficult to process, which prolongs the finishing cycle. Therefore, the mold is oftenhardened and tempered to the lower limit hardness, and the mold is equivalent to adding a hard nitrided layer on the soft substrate. This kind of mold is subject to repeated thermal shocks of molten aluminum and huge die-casting forces (a combination of tension, compression, and shearing). Surface cracks often appear (especially at the gate with poor working conditions) and then gradually expand, or after a certain number of die-castings. Later, pitting corrosion, peeling or even depression will occur, leading to premature failure.

New vacuum heat treatment process for die-casting molds

The use of vacuum heat treatment with small deformation and good surface quality is beneficial to improving the life of the mold. We use appropriatehardening and tempering pretreatment, vacuum hardening and tempering plus shallow nitrocarburizing, and have achieved good technical and economic benefits. Now we only introduce 3Cr2W8V and H13 steel molds as follows.

Preprocessing

Molds with less complex structures arehardened and tempered, which can reduce deformation by 1/3~1/2, and the hardness is controlled to ≤23HRC to facilitate finishing. In addition tohardening and tempering, molds with complex structures should undergo (600~650) CX4h stress relief annealing before finishing.

Vacuum hardening and tempering

It is preheated in sections in a 65kw vacuum furnace and heated slowly and evenly. It is pre-cooled under vacuum before vacuum oilhardening, which greatly reduces the mold deformation caused by thermal stress. For 3Cr2W8V and H13 steel, sub-temperature heating is used at 70C lower than the normalhardening temperature, which further reduces the solid solubility of alloy elements in austenite, reduces the temperature difference during coldness, and further reduces thermal stress, especially structural stress. caused deformation.

Shallow layer nitrocarburizing

Applying a shallow layer of nitrocarburizing after vacuum hardening and tempering can significantly reduce the mold sticking phenomenon of molten aluminum, and is equivalent to the second tempering of the core, making the structure more stable. The nitrocarburizing process is 570CX (2~3)h, the nitrocarburizing layer depth is (0.05~0.10)mm, and the surface hardness is 750~800HV5.

Mold quality analysis

Compared with the originalhardened and tempered and deep nitrided 3Cr2W8V steel aluminum alloy die-casting mold, the core structure of the former is smaller and the fully tempered hardness is higher (44 ~48) HRC, the mold has sufficient strength and toughness, thermal fatigue resistance, and small deformation, both between 0.02~0.10mm. For large inserts and large inserts, the parting surface (flat type, trapezoidal type, complex type) with high precision requirements can be rolled out if the gap is small. If the gap is large (0.20~0.30) mm, use CO2 shielded welding. By making up for it, you can effectively prevent the leakage of liquid aluminum when the mold is in use. After vacuum heat treatment and shallow nitrocarburizing, the mold will not crack or peel off for a long time, and its service life is 2 to 3 times longer than the original tempering and deep nitriding treatment.