Quenching and Tempering of High Speed Steel in Vacuum Furnace

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The characteristics of the quenching process of high-speed steel: mainly the high temperature of heating and quenching. The goal is to dissolve as much carbon and alloy into austenite as possible. The quenching methods of high-speed steel include oil quenching, grading, isothermal, and air cooling.

Take W18Cr4V as an example, the quenching temperature is 1270℃～1290℃, and the quenched structure is composed of (60～70%) martensite and (25～30%) retained austenite and nearly 10% carbides that are not dissolved during heating. Composition, grain size 9-10. The hardness is 63～64HRC. When the quenching temperature is insufficient, at 1240°C to 1260°C, most of the carbides are not dissolved into austenite, and the grain size is 11-12. The hardness is 62～63HRC. When the quenching temperature is too high, at 1300°C to 1310°C, the number of carbides decreases, and the grain size is 7-8. The hardness is 64～65HRC. When the quenching heating temperature reaches about 1320°C, the grain boundaries begin to melt, and eutectic ledeburite and delta eutectoid phases emerge, and overfire at this time. When the two quenching is not fully annealed, it is easy to produce naphthalene-like fractures, which are fish-scale-like white flashes, such as naphthalene light, coarse crystal grains, or uneven size. In order to further reduce deformation and improve toughness, austempering can be used for tools with complex shapes and severe carbide segregation. The isothermal temperature is about 240～280℃, and its structure is 40～50% lower bainite, 20% martensite and 35～45% retained austenite and undissolved carbides.

The characteristics of high-speed steel tempering process are mainly high tempering temperature and many tempering times. When the tempering temperature is 560～570℃, the hardness and strength can reach the maximum.

Because the number of retained austenite in quenched high-speed steel is large, need to be tempered three times .After one tempering,there is still 10% untransformed, with a hardness of 64-65HRC. After two more tempering, the transformation can be basically completed. The first tempering has a tempering effect on the quenched martensite, and new stress is generated when the retained austenite transforms into martensite during the tempering cooling, so a second tempering is required, and the second tempering After the new stress is generated, a third tempering is required to further eliminate the stress, which is conducive to improving the strength and toughness of the steel. Therefore, the typical tempering of high-speed steel is set at 560℃, tempering three times, 1 hour each time, tempering The hardness of the latter steel is slightly higher than that after quenching, about 63-66HRC, and the obtained structure is tempered martensite and undissolved carbides and 1 to 2% retained austenite.

In order to ensure the tempering effect of high-speed steel, the following issues should be paid attention to:

① Tempering treatment should be carried out immediately after quenching, otherwise the retained austenite will tend to stabilize, which is not conducive to eliminating the retained austenite;

②The tempering temperature should be uniform, and it is recommended to be carried out in a vacuum tempering furnace;

③After each tempering, it must be cooled to room temperature before the next tempering can be performed;

④After tempering, it should be cooled to room temperature before cleaning, otherwise it is easy to cause cracking.

Learn more:

Vacuum Gas Quenching Furnace

Vacuum Tempering Furnace

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