Vacuum furnaces are available in both batch and (perhaps less common) continuous styles with the vast majority of furnaces in use categorized as either vertical or horizontal in orientation. In this two-part article, we will discuss the uses and features of batch vacuum furnaces and provide an introduction to continuous furnace design.

Why Use Vacuum?

Let’s briefly review why vacuum technology is so important for heat treatment. The primary reason has to do with air and the reactive constituents contained within it. Air is a gaseous mixture that contains varying amounts of water vapor, oxygen, carbon dioxide, nitrogen and hydrogen and each of these constituents of air are reactive with various metals. At room temperature these chemical reactions occur too slowly to be problematic, however, these reactions are greatly accelerated at the elevated temperatures required for heat treatment. There are changes to the microstructure of a material’s surface when a heated metal is exposed to air. The changes experienced can be either surface contamination or a thin exterior layer that is harder or softer than the interior of the part being heat treated. For example, a piece of steel will discolor when heated above about 200°C (392°F), forming a thin layer of ferrous oxide. This presents a challenge when heat treatment is necessary for applications where part cleanliness or appearance is important.

The type and reliability of instrumentation and process controls used on vacuum furnaces in the heat treatment industry is critical to both the performance of the vacuum furnace itself as well as the results that are achieved when processing critical components. It is not an understatement to say that given the life expectancy of vacuum equipment, instrumentation and controls should be updated every few years to take advantage of the most advanced technology possible (e.g., remote communication and diagnostics, process monitoring and control by Internet-based devices and the like).

Demands on Instrumentation and Control Packages

Temperature control and as a result temperature uniformity can be difficult because of the heat transfer characteristics of the furnace as it moves, for example, from convection to radiant heating and convective/conductive heat transfer during quenching. For example, the ability to vary the furnace heating rate (e.g., 3°C/min – 25°C/min) demands precise and accurate measurement and control, including setpoint program control with guaranteed soak features.

Vacuum furnaces are often used for a variety of products and processes by the heat treater making recipe management an important function. Temperature overshoot of set points is typically not allowed. Setpoint program control is often applied to the temperature, vacuum level and gas pressure with extensive interaction between these programs and also with the logic control.

There is a constant need throughout the industry to produce the highest quality parts to the most stringent product specifications. Both long-established and new materials are being employed to meet the needs of lighter, stronger, smaller and more efficient designs, and the use of vacuum technology in manufacturing is of paramount importance in achieving these goals.

The strategy being adopted by manufacturing to meet these needs relies heavily on vacuum processes and equipment through:

• Process development – New materials, new products, and new applications demand absolute cycle repeatability, flexibility, and control, and as such, designers are specifying vacuum processing over other heat treatment methods.
• Process substitution – Older process technologies and the equipment associated with them are being replaced by vacuum equipment. The justification lies in reduced unit cost achieved by lowering the overall cost of manufacturing and/or through material and efficiency savings.
• Process replacement – Product-performance demands are forcing designers to look toward vacuum processing and its ability to offer a superior product for the same or very similar cost.