Passing a Nadcap audit is considered by most heat treaters to be both a significant company-wide accomplishment and a benchmark as to their commitment to quality. Nadcap accreditation not only allows the company to perform heat treating for the Aerospace/Defense industries but perhaps equally important announces to all of its customers and potential customers that […]
Vertical Oil Quench Vacuum Furnaces: A typical vertical oil quench vacuum furnace (Table 1) either utilizes a heating chamber mounted atop a movable gantry so that loads can be transferred in and out of the furnace and oil quenched in a separate oil tank or designs in which the main furnace chamber is stationary and separated by an isolation valve from a loading vestibule/oil quench tank.
Vacuum oil quenching offers the economic and environmental benefits for processing many critical-performance components such a landing gear bogie beams. Since vacuum furnaces are inherently leak tight, control of surface chemistry is assured, and problems with decarburization and high-temperature oxidation are avoided. Vacuum processing also allows producers of aircraft landing gear to finish machine critical surfaces on these components prior to heat treating. This, in turn, reduces final machining costs when the part is in the hardened condition. Further, quench-related distortion is minimized through load transfer to the quench tank via a high-speed elevator. Warm or hot straightening can be used when necessary to compensate for any dimensional changes.
Today, vacuum technology is utilized in every aspect of thermal processing in diverse industries. Processes are equally diverse, from brazing of aerospace components and hardening of stainless and tool steel dies to annealing of zirconium tubes and case hardening of power transmission components.
Vertical single chamber batch vacuum furnaces are quite common throughout the industry. These furnaces have many of the same basic attributes as their horizontal cousins, recognizing that in a vertical unit the load is placed on a bottom platform, which is then raised into the furnace. These vacuum furnaces are equipped with either gas (pressure) quenching or oil quenching, the latter taking place in a separate chamber.
Next Month: We will discuss VAC AERO’s Vertical Oil Quench Vacuum Furnaces and technologies.
Earlier we addressed some aspects of vacuum furnace safety in our article: Vacuum Furnace Safety Part One: Hazardous Materials (January 1, 2018) and before we talk about confined entry spaces and how this applies to vacuum furnaces, it is worth remembering what we talked about previously when we said:
“When problems arise, especially those related to safety, we want to know that we have isolated the root cause and instituted corrective action measures so as to avoid their reoccurrence. Worker safety and the safe operation of heat-treat equipment is both MANDATORY and NON-NEGOTIABLE, especially when operating and maintaining vacuum equipment where dangers of asphyxiation, electrocution, and explosion are as real as they are with any other type of thermal processing equipment. “It won’t happen to me” is not a phrase that belongs in the heat-treat shop and provides a false sense of security to all involved. There is no substitute for understanding the inherent dangers, taking the necessary steps and placing the right safeguards in place to prevent accidents from happening. Safety and safety issues are a serious matter and should be treated as such by all individuals within the company.”
We went on to talk about such topics as: (a) incidents involving gases; (b) incidents involving liquids and (c) incidents involving explosions. Now we need to address an important safety aspect that may apply when servicing vacuum equipment – confined spaces.
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.
Oil sealed rotary vane pumps (aka rotary vane pumps) are the primary pumps on most vacuum systems used in the heat treatment industry. They are also referred to as a “backing” pump when used in combination with a booster pump, or with both a booster and secondary (“high vacuum”) pump, typically a diffusion style. A rotary vane pump can also be used alone when high vacuum is not required and slower pump-down is acceptable.
Two-stage designs are available, which utilize two rotors in series internal to the pump. Single-stage designs can provide a vacuum of 3 x 10-2 Torr (4 x 10-2 mbar), while two-stage designs can achieve 3 x 10-3 Torr (4 x 10-3 mbar). Due to the prevalence of rotary vane pumps, it is important for designers and users of industrial vacuum equipment to have a good understanding of how these pumps function. This series of articles will cover pump principles of operation, pump designs, pump oils, single-stage versus two-stage pump designs, contamination and gas ballast (manual and automatic), common accessories, applications, troubleshooting and pump maintenance.
Principles of Operation
Of the various vacuum pump technologies, rotary vane pumps are considered wet, positive displacement pumps. They are often called “wet” pumps because the gas being pumped is exposed to oil used as a lubricant to help provide the seal. For this reason, the oil is carefully selected and specially designed for the application. Positive displacement indicates that the pump works by mechanically trapping a volume of gas and moving it through the pump, creating a low pressure on the inlet side.
Selecting the correct vacuum gauge or gauges is critical to the success of a heat treatment process. It is important to know how they work and what options are available so that the correct choice can be made. There are several important considerations when using a vacuum gauge. They include the method of operation, the gas composition (inert or reactive, corrosive), the gas sensitivity (calibration factor), and the process being performed in your system.
Given the wide range of pressures encountered when running processes in vacuum furnaces (a staggering 9 orders of magnitude), no one gauge is adequate over the entire range of possible vacuum levels. As with vacuum pumps, multiple gauges are necessary to properly cover the entire operating range with the needed precision and accuracy. Given that it is critical to monitor the vacuum pressure at various points in the process and perhaps multiple locations throughout the vacuum system, the correct selection of each gauge ensures that we achieve optimal results.