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!

 

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. There are also lessons to be learned from understanding the demands of ultra-high vacuum applications. 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.

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. At these low pressures, the mean free path of a gas molecule is approximately 40 km (24.8 miles), so gas molecules will collide with the chamber walls more frequently than they collide with each other. Thus, almost all gas interactions, therefore, take place on various surfaces in the chamber.

Acquisition of a vacuum furnace represents a major capital equipment investment and one that creates a long-term relationship with your supplier partner. Thus the choice of what to buy and who to purchase it from requires careful planning and considerable up-front research. You need to know when and how to apply vacuum technology if it will be the most cost-effective solution for what you need to do, what questions to ask and what information to provide.

The process begins by understanding your specific needs and asking all the right questions. Is it more prudent to upgrade an older piece of equipment, purchase new or purchase used? Is it better to have one large furnace or two smaller ones? Is a batch solution best or is a continuous approach better?

Ask yourself what the equipment must do, what productivity must be achieved (now and in the future) and what type of specifications or compliance requirements (e.g. Nadcap, CQI-9) must be met. The type of material(s) being run, the skill of your workforce, the (internal and external) support available and the type of controls and/or quality records required are additional considerations. All of these will help define how much training and support will be needed from your supplier partner.

Low-temperature vacuum heat treatment offers unique advantages to a variety of industries including Aerospace, Automotive, Electronics, Household Appliances, Machine Tools and Tool and Die as well as Commercial Heat Treaters who must serve all of these customers. Low-temperature heat treatments that involve a vacuum purge at the onset of the cycle have become increasingly popular throughout the industry. These operations are conducted in vacuum furnaces and furnaces that employ a vacuum purge prior to the beginning of the heat process with parts placed inside a special vacuum tight vessel or in a retort. Processing being run using these methods take advantage of a highly controlled environment designed to minimize surface interactions.

Low-temperature processing can be batch or continuous, either as stand-alone units or “modules” incorporated into a continuous vacuum furnace system. The following is a basic description of the operation of a typical batch vacuum furnace. Once a workload has been positioned into the unit, an outer door is closed and the vacuum process can commence. A mechanical vacuum pump, optionally equipped with a blower, produces a vacuum level as low as 1.3 x 10^-3 mbar (1 x 10^-3torr), a common vacuum level being under 1.3 x 10^-1 mbar (1 x 10^-1 torr). This is normally achieved in 10 – 20 minutes depending on the size of the pumping system and the nature of any contamination present on the workload. In some instances, a double pump-down sequence is initiated once an initial vacuum level lower than 6.7 x 10^-1 mbar (5 x 10^-1 torr) is reached. Once the desired final vacuum level is reached, the unit is backfilled in the range of 667 mbar (500 torr) to 3.4 x 10^-2 bar (0.5 psig) positive pressure with an inert gas such as nitrogen, argon or nitrogen/hydrogen (3% maximum) and heating begins. After reaching setpoint and soaking at temperature, a cooling cycle is initiated, typically with hot gases circulated through an internal or external heat exchanger to accelerate the process.