Die heat treatment includes annealing after forging, high temperature tempering or low temperature tempering after rough machining, quenching and tempering after finishing machining, stress relief and low temperature tempering after wire cutting.
Only when the hot and cold machining works well with each other, can a good mold life be guaranteed.
When the mold cavity is large and the wall is thin, the upper limit of the normal quenching temperature needs to be used to increase the amount of retained austenite and prevent the mold from swelling. Due to the short heating time, the rapid heating method reduces the tendency of oxidative decarburization, the crystal grains are fine, and the quenching deformation of large carbon tool steel molds is small. It is better to use low quenching and high tempering processes for high-speed steel.
The quenching temperature is low and the tempering temperature is relatively high, which can greatly improve toughness. Although the hardness is reduced, it is extremely effective for increasing the life of the mold due to breakage or fatigue damage.
Usually Cr12MoV steel quenching heating temperature is 1000 ℃, oil cooling, and then tempering at 220 ℃. If the heat treatment can be performed once before this heat treatment, that is, heating to 1100 ℃ for heat preservation, oil cooling, and 700 ℃ high temperature tempering, the life of the mold can be greatly improved.
In the early 1970s, we applied high-quenching and high-recovery heat treatment steel wire clamp hot forging dies to 3Cr2W8V steel and also achieved good results, with a life expectancy of more than 2 times. Using low-temperature nitrocarburizing process, the surface hardness can reach 1200HV, can also greatly improve the life of the mold.
Low temperature electrolytic sulfurization can reduce the frictional force of metal deformation and improve the anti-seizure performance. Using 6W6Mo5Cr4V steel to make cold extrusion punch, after low temperature nitrocarburizing, the service life is increased by more than 1 times on average, and then the low temperature electrolytic sulfurization treatment can further increase the service life by 50%.
There is a large residual stress after the die is quenched, which often causes the die to deform or even crack. In order to reduce the residual stress, the mold should be tempered while it is hot after quenching, and the tempering should be sufficient. Insufficient tempering is prone to cracks before grinding.
For carbon tool steel, the residual stress can be eliminated by about 50% by tempering at 200 ℃ for 1h, and the residual stress can be eliminated by about 75% to 80% by tempering for 2h. If the tempering is performed at 500-600 ℃ for 1h, the residual stress can be eliminated by 90%.
The CrWMn steel punch of a certain factory was tempered for 1 hour after quenching, and it broke shortly after use.
When tempered for 2.5 hours, no fracture was found during use. This shows that the tempering is not uniform. Although the surface hardness meets the requirements, the internal structure of the work is not uniform, the residual stress is not fully eliminated, and the mold is prone to early fracture and failure.
After tempering, it is generally air-cooled. During the tempering and cooling process, new tensile stress may appear inside the material. It should be slowly cooled to 100 ~ 120 ℃ and then released, or add a low temperature tempering after high temperature tempering. PVD and CVD in surface coating hardening technology have made great progress in recent years.
Vacuum evaporation, vacuum sputtering and ion plating are commonly used in PVD. The ion plating layer has strong adhesion, good plating performance and deposition speed. Fast, pollution-free and other advantages.
The ion plating process can plate TiC and TiN on the surface of the mold, and its service life can be extended several times to dozens of times. Ion plating is a deposition technique that combines vacuum evaporation and gas discharge.
The hollow cathode discharge method (HCD method) is to evacuate with a vacuum pump first, and then feed the reaction gas to the vacuum pump, and maintain the vacuum degree in the range of 10-5 ~ 10-2Pa, using low-pressure high-current HCD electron gun to make the evaporated metal The compound is ionized, thereby forming a protective film on the working surface.
In order to improve the plating efficiency, a negative voltage is generally applied to the workpiece.
The surface treatment technology of forging dies is not widely used in China, so it is necessary to develop in this field. Surface hardening support such as carburizing, nitriding, boronizing, carbonitriding of the whole mold cavity and local spraying, brush plating and surfacing of the mold cavity are very promising.
Technology has been greatly improved. After the die failure, the welding repair technology has been researched and applied by factories in the early 1990s in China.
