The principle of vacuum furnace design is to extract ambient air from the furnace at the beginning of heat treatment, while also introducing process gas to achieve a specific pressure lower than atmospheric pressure. Therefore, the vacuum furnace can effectively control the quantity, type, and quality of gas passing through the furnace throughout the entire process. Gas quenching with inert gas is a widely used technique for controlling quenching rate and results. In addition, as the temperature inside the hot zone increases, adding specific process gases can bring more benefits, thereby improving the results of various heat treatment applications. The first step of vacuum heat treatment process is to remove the air inside the furnace. Heated metal parts are particularly prone to react with common elements in the air, such as water vapor or oxygen, so removing air can significantly reduce unnecessary harmful reactions. However, introducing specific process gases can also generate reactions or conditions required as part of the process. Recharging process gas during the heating process can bring many benefits. Some processes may require the introduction of very small amounts of gas (0.01 Torr to 10 Torr), such as hydrogen to help remove oxides, or the addition of elements such as carbon for surface carburizing and hardening, or the addition of other inert gases to limit the volatilization of elements such as Cr in high-temperature processes. Other processes may require the introduction of sufficient process gas (1 atmosphere or higher) to achieve convective heating by operating internal fans, thereby uniformly heating parts with complex geometries or large cross-sections.

Partial pressure process

Partial pressure process refers to the process of injecting a very small flow of process gas into the furnace after it reaches a vacuum state, in order to generate or control reactions when the temperature rises. The process gas can be an inert gas such as argon or nitrogen, or an active gas such as hydrogen or acetylene. The partial pressure heat treatment process can be used as furnace blowing, which means that most of the residual gas atoms and molecules in the furnace are removed by adding inert gas. When inert gases (such as argon) are added to the furnace during the heating process, they can drive the movement of smaller atoms or molecules such as hydrogen, oxygen, and nitrogen, forcing them to be quickly evacuated outside the furnace during the second vacuum pumping of the system. Inert gases can also slow down the volatilization of elements such as Cr, as Cr and other elements are prone to precipitate from the surface of the workpiece at high temperatures and extremely low pressures. After filling with inert gas, even a very low partial pressure of inert gas helps prevent the precipitation of elements in Cr containing materials. Some materials of steel are particularly prone to oxidation, and oxides may even form on their surface before heat treatment. Therefore, sometimes it is necessary to introduce hydrogen gas * into the heat treatment process at a specific temperature to promote the reaction between certain metal oxides and the introduced hydrogen gas, thereby reducing the oxidation of the workpiece. Although it is common knowledge to be very careful when introducing hydrogen into any system from a safety perspective, it is generally not recommended to use hydrogen in heat treatment processes involving components containing titanium or certain copper alloys. In processes such as low-pressure carburizing, adding process gases such as acetylene at low pressure can help provide surface carburizing and hardening solutions that meet specific requirements. This is particularly effective for parts with very complex geometric shapes, such as those made of powder metal. In addition to these processes, the partial pressure gas system can also be used to avoid diffusion bonding between parts and fixtures, and provide a temporary cooling step to improve the efficiency of vacuum cooling.

Convection heating process

The convection process is to fill an inert gas into a vacuum furnace until the pressure inside the container is equal to or higher than one atmosphere. Pulling the furnace to a near vacuum state and then refilling it with another gas to the same pressure as before vacuuming may not seem very meaningful, but by controlling the elements present in the refilled gas, it can ensure that the processed parts do not react with harmful elements (such as water or oxygen) that may be present in the ambient air. Vacuum furnaces mainly transfer heat through thermal radiation. In a vacuum environment, due to the absence of ambient air, heat will not be transferred through convection. Therefore, operators are able to effectively control the process temperature and uniformity in various key steps of the entire heat transfer process. However, due to the linear transfer of energy by thermal radiation, complex geometric shapes such as curved surfaces and deep holes may seriously affect the consistency of the heat treatment process. Filling the furnace with an inert gas at atmospheric pressure or higher can better control the heating uniformity of complex components. And at lower temperatures, convective heating is more efficient than radiative heating. The airflow generated by the internal convection fan ensures effective heat transfer to all geometric shapes of the entire complex component. Airflow can provide more consistent heat transfer for deep holes or sharp corners, ensuring that the entire component can be uniformly heated and reach the required insulation temperature faster. Thus, while ensuring temperature uniformity, it effectively reduces process time and accelerates the production cycle of complex parts.

Result

Although the main function of a vacuum furnace lies in its gas quenching ability, adding process gases during the heating and insulation process adds more functionality and a wider range of applications to the equipment. Partial pressure and convective heating make the control of the working environment inside the vacuum furnace more precise, making it an ideal tool for stable production and clean workpieces.