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Heat treatment of tool steels in vacuum furnaces

Tool steels for molds (SKS, SKD, SKH) are mostly carbide tool steels. Mold performance not only depends on the internal structure of the steel (carbide shape, size, distribution state), but also the structural changes caused by vacuum heat treatment, especially vacuum quenching and vacuum tempering, also have a great impact on the mold performance. Therefore, vacuum heat treatment technology is very important for the mold to obtain the required characteristics.

Tool steel vacuum quenching method
Vacuum quenching is a heat treatment method in which steel is insulated in a vacuum at high temperatures to make the steel in an austenite state, and then rapidly cooled to cause martensitic transformation of the steel to obtain high hardness. Quenching heating is to heat the steel to a temperature above the A3 transformation point, so that the carbon atoms are dissolved and the structure is transformed from ferrite to austenite. The quenching temperature of carbon tool steel (SK) is 760~820℃, the quenching temperature of alloy tool steel (SKS) is 760~880℃, the quenching temperature of die steel (SKD) is 1000~1050℃, and the quenching temperature of high speed steel (SKH) The quenching temperature is 1050~1300℃. If the quenching temperature is too low, the martensite transformation will be uneven and the normal quenching structure will not be obtained. If the quenching temperature is too high, the austenite grains will coarsen, oxidation and decarburization will occur, and the amount of retained austenite will increase after quenching, resulting in a decrease in the hardness and toughness of the steel. The optimal quenching temperature varies depending on the composition of the steel. Therefore, the quenching temperature corresponding to the quenched steel should be determined.
Parallel to the quenching temperature is the vacuum quenching heating time. Improper heating time can cause the same problems as improper heating temperature. Generally speaking, for mold steel, when the center temperature of the workpiece reaches the specified temperature, it must be kept warm for 30 minutes. In actual vacuum quenching operations, uneven temperatures in the vacuum quenching furnace, excessive loading of vacuum quenching workpieces, and uneven loading of workpieces are the causes of the above-mentioned quenching anomalies. Therefore, inspection and regular maintenance of vacuum heat treatment equipment are very important.
The steel heated to the quenching temperature is rapidly cooled to cause the steel to undergo martensitic transformation and obtain high hardness. Generally speaking, the greater the vacuum quenching cooling rate, the denser the steel structure and the better the toughness. Therefore, rapid cooling technology and corresponding equipment are needed. The quenching temperature of high speed steel is higher than that of mold steel. Generally speaking, the higher the quenching temperature and the longer the heating time, the more solid solution of carbides in austenite and the higher the hardness after quenching. However, the longer the heating time, the grain coarsening, oxidation, decarburization and other phenomena will occur.

Tool steel tempering method
Vacuum tempering is a heat treatment method that uses heating to relieve the accumulated strain in the quenched workpiece, adjust the hardness and improve toughness. During the vacuum tempering process, carbides precipitate from the hard and brittle martensite formed by quenching, and the quenched martensite turns into tough tempered martensite. At the same time, the residual austenite in the steel is decomposed to produce The effect of secondary hardening.
Most tool steels are used at high temperatures, so secondary hardening high-temperature tempering (500~650°C) is generally used to obtain the stability of the mold. However, for tool steels used in cold work dies, low-temperature tempering at around 200°C is sometimes used. The end temperature of martensitic transformation of high carbon steel represented by SKD11 is below room temperature, and about 20% residual austenite exists after vacuum quenching. These retained austenites are relatively stable above 450°C. The retained austenite is relatively soft, so the overall mold has high toughness. The low-temperature tempering treatment at around 200°C precipitates carbides from martensite, turns quenched martensite into tempered martensite, and improves toughness. The hardness of low temperature tempered steel is about 60HRC. If there is no temperature increase during mold use and processing after vacuum heat treatment (electrical discharge machining, surface treatment, etc.), low-temperature tempering can make the mold obtain high hardness and high toughness.