Do you know how to vacuum heat treat high speed steel?

High-speed steel vacuum heat treatment

The superiority of high-speed steel can only be brought into play after correct vacuum hardening and vacuum tempering. Its quenching temperature is generally much higher than that of low-alloy cutting tool steel. For high-speed steel, the higher the vacuum hardening heating temperature, the more alloy elements dissolve into austenite, and the higher the alloy elements of martensite after quenching. Only martensite with high alloy element content has high red hardness. The alloy elements (W, Mo, V) that have the greatest effect on the red hardness of high-speed steel only increase their solubility sharply when the temperature is above 1000℃. When the temperature exceeds 1300℃, although the content of these alloy elements can continue to increase, the austenite grains grow sharply at this time, and even local melting occurs at the grain boundaries. As a result, the toughness of the quenched steel is greatly reduced. Therefore, for the quenching heating temperature of high-speed steel, as long as overheating does not occur, the higher the quenching temperature of high-speed steel, the better its red hardness.

Because high-speed steel has poor thermal conductivity and the quenching temperature is extremely high, it is often heated in two or three stages, that is, preheating at 800~850℃ first and then heating to the quenching temperature. Large blades (tools) and complex blades (tools) should be preheated twice (three-stage heating), the first time at 500~600℃ and the second time at 800~850℃. In addition, the use of preheating for high-speed steel can also shorten the time of high-temperature treatment, which can reduce the risk of oxidation decarburization and overheating.

High-speed steel vacuum hardening

High-speed steel vacuum hardening cooling is usually carried out in oil, but for complex shapes, slender rods or thin parts, graded quenching and isothermal quenching can be used. When the cooling speed is too slow, carbides will precipitate from austenite in the temperature range of 800~1000℃, which will have an adverse effect on the red hardness of the steel; graded quenching can increase the amount of residual austenite by 20%~30%, reduce the tendency of deformation and cracking of the workpiece, and improve the strength and toughness of the steel. The normal quenched structure of high-speed steel is martensite (60%~65%) + carbide (10%) + retained austenite (25%~30%). It must be emphasized that the temperature retention time of graded quenching should not be too long, otherwise secondary carbides may precipitate in large quantities, which is detrimental to the performance of the steel.

In addition to martensite, carbide and retained austenite, the vacuum hardened structure of high-speed steel also contains lower bainite. vacuum hardening can further reduce workpiece deformation and improve toughness.