No matter what field you study, an accurate knowledge of its vocabulary is essential to understanding the subject. In the field of vacuum heat treatment, considerable emphasis is placed on the proper use, meaning, and interpretation of certain words or phrases. A brief summary of the terminology including common processes being run is presented below.
In our previous discussion, we looked at the reasons vacuum technology is important to the thermal processing industry and discussed a few of the most common applications for vacuum furnaces. Here, we will talk about the major components of a batch vacuum furnace and briefly consider continuous (aka semi-continuous) vacuum furnaces. Batch vacuum furnaces are made up of various subsystems (Fig. 1): vacuum pumps; a hot zone complete with a heating source, insulation, hearth/load support structure; vessel or shell with structural support system; power components; and instrumentation.
The purpose of vacuum pumps and the related components is to remove the air from the heating chamber. Although the vacuum pumping system does not remove all the air, the goal is to remove enough of the remaining air so that any oxygen present will not react with the material being processed in the furnace. There are two categories of pumps employed on typical vacuum furnaces: a primary pump with or without an optional booster pump (aka blower) and an optional secondary diffusion pump (Fig. 2).
For most batch vacuum systems, a combination of a mechanical pump (i.e., wet or dry pump) and booster pump will reduce the pressure to roughly 10-3 mbar (millibar) which is considered a medium to high vacuum. For reference, 1 bar is atmospheric pressure, whereas a millibar is 1 thousandth (0.001) of current atmospheric pressure. Therefore, 10-3 mbar is 1 millionth (0.000001) of atmospheric pressure. The primary pump is used for the initial pump-down from atmospheric pressure via the opening of a roughing valve. When the efficiency of the primary pump or primary pump plus booster pump drops off, the roughing valve is then closed and the poppet valve is opened to the secondary (diffusion) pump. The secondary pump is designed to reduce the pressure to very high or ultra-high vacuum, as low as 10-10 mbar.
Cleaning in a solvent offers a level of simplicity and forgiveness not seen in aqueous methods. At one time solvent cleaning was considered mandatory for successful vacuum processing but environmental concerns (VOC and other emissions) and improvements to aqueous systems including drying technology has seen the industry shift to aqueous cleaning as the norm. Today, however, with the advent of vacuum technology, vacuum vapor degreasing has emerged as a viable alternative to aqueous processing.
Solvent cleaning involves three basic steps: wash, rinse and dry. Washing is where the parts are immersed in or placed in contact with a (typically boiling) solvent to assist with the contaminant removal process. The purpose of rinsing is to bring “fresh” or clean solvent in contact with the parts. The aim is to dilute the contaminated solvent present on the surface of the parts from washing. It is important to remember that the rinse solvent must be kept clean. Contaminated solvent is a very common problem and will only reintroduce contaminants back onto the surface. The drying step evaporates the solvent and separates the rinse solvent from the parts.
Today, the maintenance of heat treatment equipment is a point of major emphasis and this is especially true for vacuum furnaces. This article will explore various aspects of vacuum furnace maintenance providing useful tips and practical techniques to simplify the work and make sure that it is done correctly. Let’s begin by understanding the importance of the role of maintenance, and more specifically, how planned preventative maintenance is helping to manage the overall cost of equipment operation.
Accepting the Inevitable
Maintenance is a fact of life for heat treat equipment. In general, the cost of maintenance increases dramatically as the operating temperature increases and/or the process environment becomes more severe (e.g. carburizing versus hardening). This remains true in vacuum furnaces despite the fact that they are often operated below their maximum temperature ratings. As with all equipment, some styles and designs require more attention than others. It is interesting to note, however, that construction of heat treat equipment can often be classified as “heavy duty” or “light duty” by the amount of maintenance required. Of course, if any furnace is operated outside their design limitations, this almost always translates to a need for more extensive maintenance. A great deal of money can be spent – and wasted – if careful thought and clear understanding of the equipment design as well as the extent of the repair is not taken into account. Not taking the time to determine the root cause of why a component failed can have disastrous bottom line consequences. Proper maintenance maximizes “up-time” productivity, and the utilization of planned preventative maintenance programs result not only in better equipment reliability but in improved process repeatability and control – essential to producing good parts with consistent metallurgical and mechanical properties. Once management understands, accepts and budgets for maintenance expenditures, the operation of all heat treating equipment and especially vacuum furnaces become far more reliable.
As in any discipline, understanding the underlying scientific principles has profound practical implications when properly understood. In this series of articles, we will review the first principles of vacuum technology and explain them using real-world illustrations. Most industrial vacuum systems can, in broad-based terms, be categorized in terms of low (i.e., “soft”), medium, high (i.e., “hard”) and ultra-high vacuum. These ranges are very useful in describing the various pressure, flow, and other phenomenon encountered, which leads to a better understanding of vacuum pump selection and operation, and system operational requirements at the different vacuum levels.
As shown by the difference in pressure from low to ultra-high vacuum, industrial vacuum systems must operate under an extremely wide range of pressure. In fact, the range is so large it is hard to actually comprehend. Consider a volume of gas at a pressure of 1000 mbar (atmospheric pressure) in a 1 meter by 1 meter by 1 meter container sealed so that no molecules can escape or enter. It is easy to understand that if the container is expanded in volume while still remaining sealed, the pressure will decrease (and a vacuum will be created) in direct proportion to the increase in volume (in accordance with Boyle’s law). If, for example, the container volume is doubled to 2 cubic meters, the pressure will decrease by half, to 500 mbar. When this relationship is expanded to the scale of industrial vacuum systems, the result is striking. If we take this same 1 cubic meter volume of gas and increase its volume sufficiently for the pressure to be reduced to 10-12 mbar (ultra-high vacuum), the container will be a staggering 99 km long x 99 km wide x 99 km high, or 200 times the volume of the grand canyon!
Vacuum furnaces are available in both batch and (perhaps less common) continuous styles with the vast majority of furnaces in use categorized as either horizontal or vertical 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.
Most of us are familiar with processing in the vacuum range up to around 1.33 x 10-3 Pa (1 x 10-5 torr) or slightly lower (Fig. 1). There are also lessons to be learned from understanding the demands of ultra-high vacuum applications (Fig. 2). Let’s explore what’s involved.
What is an Ultra-High Vacuum? Practical high vacuum levels (Table 1) range down to approximately 1.33 x 10-4 Pa (1 x 10-6 torr) while ultra-high vacuum (UHV) levels are in the vacuum range characterized by pressures of about 10-7 Pa (7.5 x 10-10 torr) and greater. These vacuum levels demand the use of special materials of construction and processing techniques such as preheating (i.e. bake-out) of the entire system for several hours prior to processing to remove water and other trace gases, which adsorb on the surfaces of the chamber.