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Points to keep in mind when vacuum heating
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Overview.
The greatest advantage of vacuum heating is that it is easy to obtain an excellent bright surface. However, if the pressure at the time of heating is incorrectly selected, many adverse effects can occur on the surface of the workpiece.
01 What is Vacuum?
A vacuum is any space with a pressure lower than atmospheric pressure (1.013 x 105 Pa ), ranging from a low vacuum to an ultra-high vacuum, depending on the pressure range. The range used for heat treatment is up to the high vacuum range. Especially for heat treatment of general steel materials, a low or medium vacuum is sufficient. The use of a vacuum activates the surface of the workpiece, making it easy to obtain a brilliant surface, and surface reactions are also more active. For this reason, vacuum is now used for a wide range of heat treatments, including quenching heat treatment of metal molds and surface heat treatment of carburizing and nitriding.
| The domain of the vacuum (the quantitative world) | Pressure range |
| Low vacuum [low vacuum] | 105~102Pa |
| Medium vacuum [medium vacuum] | 102~10-1Pa |
| High vacuum | 10-1~10-5Pa |
| Ultra high vacuum | 10-5Pa or less |
02 De-elementing associated with vacuum heating
All substances evaporate when they reach vapor pressure when heated in a vacuum. In other words, elements with high vapor pressure, such as manganese (Mn) and chromium (Cr), evaporate easily, and the higher the heating temperature and the lower the heating pressure, the greater the amount of evaporation.
Rust is said to occur easily on vacuum heat-treated stainless steel because 1) stable oxides on the surface are destroyed and 2) Cr with high vapor pressure evaporates, resulting in a decrease in Cr concentration on the surface. In particular, as shown in the figure below, the lower the heating pressure, the greater the amount of Cr evaporation.
Therefore, if a stainless steel basket is used to quench high-speed tool steel at high quenching temperatures, as shown in the above figure, this evaporated Cr will adhere to the surface and the surface Cr concentration of the high-speed tool steel will be rather high. In such high temperature heating in a vacuum, it is desirable to increase the pressure during heating as much as possible.
03 Surface Roughness and Brightness
Vacuum heating has the following effects on the treated material surface cleaning effect (oxide dissociation) and degreasing effect (decomposition of fats and oils). The lower the pressure, the greater these effects of vacuum heating. However, as shown in the figure below, blindly lowering the pressure will have the opposite effect when used for heat treatment.
