Vacuum heat treatment technology heating speed is slow

Characteristics of vacuum heat treatment technology

        The heating speed is slow. In vacuum, gas molecules are thin, and the average free energy of gas molecules increases significantly with the decrease of pressure. The heating of the workpiece will mainly rely on radiation heat transfer. Due to the small effect of convection heat transfer, the heating rate of the workpiece on the side close to the heating element is relatively fast. Therefore, in the furnace loading method and furnace structure configuration of the workpiece, the "back view" part should be avoided as much as possible, or the heating and holding time should be extended appropriately to Ensure that the heating is even.

        The heating speed in vacuum is much slower than heating in salt bath furnace, preliminary estimate is about 6:1. Especially when the furnace temperature is below 600℃, the radiation heat transfer effect is very weak. When heating in a very thin atmosphere, it takes a long time to rely on convection heat transfer, and it is not easy to be uniform. Therefore, tempering in a vacuum furnace often needs to be filled with inert gas and forced to circulate. In view of this, some units use vacuum quenching, nitrate tempering or air furnace tempering.

        Someone once measured the heating rate of GCr15 steel samples in different media. The time required for the core of the sample to be heated to 850℃ are: 8 min in salt bath furnace, 35 min in air furnace, and 50 min in vacuum furnace, that is, the workpiece is in vacuum The diathermy time in the furnace is 6 times that of a salt bath furnace and 1.5 times that of an air furnace.