Vacuum heat treatment of high speed steel

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High-speed steel originated at the end of the 19th century. It is a high-carbon and gold-containing cutting tool steel containing a large amount of W, Mo, Cr, V, Co and other alloying elements. After vacuum heat treatment, high-speed steel still maintains high hardness at a high temperature of 600°C, so it can maintain high-speed cutting capability and high wear resistance at higher temperatures! At the same time, it has high enough strength, and has appropriate toughness and Plasticity, which is unmatched by other superhard tool materials. High-speed steel has high hardenability, and small and medium-sized cutting tools can be hardened even when cooled in air, so it is also known as wind steel.

Compared with carbon cutting tool steel and alloy cutting tool steel, the cutting speed of high-speed steel can be increased by 2 to 4 times, and the true life of the blade can be increased by 5 to 18 times.

High-speed steel is widely used to manufacture various machining tools with large size, fast cutting speed, heavy load and high working temperature. Such as turning knives planing knives, broaches, drills, etc. In addition, high-speed steel can also be used in molds and some special bearings. In short, due to the hardness of high-speed steel. Excellent performance in wear resistance, red hardness and other aspects, high-speed steel still accounts for more than 65% of cutting tool materials in modern tool materials. The output value accounts for about 70%.

Vacuum heat treatment of high speed steel

Since high-speed steel contains a large number of refractory alloy carbides, the heating temperature of vacuum quenching must be high enough to dissolve the alloy carbides into austenite, and the alloying elements in martensite after quenching are high enough, and only alloys Martensite with high element content has high red hardness. Therefore, the heating temperature of high-speed steel vacuum quenching is relatively high compared to other steels. Generally speaking, the vacuum quenching temperature of high-speed steel is between 1180 and 1230 °C. Relatively speaking, the higher the vacuum quenching temperature, the better the red hardness, provided that overheating does not occur.

Because high-speed steel contains a large amount of alloying elements, the thermal conductivity of high-speed steel is very poor. If the high-speed steel is directly heated in a high-temperature furnace, it is easy to cause deformation and cracking of the high-speed steel. Therefore, when heating large workpieces, it is generally Preheating is required, which is generally carried out in two stages. The first preheating temperature is between 550 and 650 °C, and the second preheating temperature is between 820 and 870 °C.

And because the vacuum quenching temperature of high-speed steel is very high, the workpiece is easily oxidized in the heating furnace, so the heating of high-speed steel is generally carried out in a salt bath.

Before the vacuum heat treatment of high-speed steel, spheroidizing annealing is generally performed. The annealing temperature is generally between 820 and 900 ° C. The purpose of annealing is not only to reduce the hardness to facilitate cutting, but also to prepare for the subsequent quenching.

Determination of the amount of retained austenite in specimens after three tempering

It can be seen from the data in the table that the amount of retained austenite in the sample is still higher than the theoretical value after normal vacuum quenching and incomplete vacuum quenching of the high-speed steel in the experiment and three times of tempering. In order to continue to reduce the amount of retained austenite, we can perform high temperature vacuum quenching again, or perform a cold treatment at -85 °C. However, the cold treatment is generally carried out immediately after quenching and cooling or after the first tempering. This will have a better cooling effect.

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