A Vacuum Gauge is a pressure measuring instrument that measures pressure in a vacuum (i.e., in a vessel operating at sub-atmospheric pressure). Depending on the type of vacuum system and the required operating vacuum level, different vacuum gauges are required, often in combination with one another, to accurately determine and/or control the vacuum level of the chamber at any given moment in time.

Vacuum gauges are basic measurement components for vacuum furnace systems and can be found either in vacuum lines or connected directly onto the vacuum chamber. For all intents and purposes, a vacuum gauge is an instrument for measuring pressures below that of atmospheric pressure. There are several types of vacuum gauges, each engineered for a specific function over a specific range of vacuum pressure (Fig. 1). Common types (Table 1) include; Mechanical gauges, Absolute pressure gauges, Thermocouples or Pirani gauges, Ionization gauges (hot & cold cathode) and McLeod gauges.

Vacuum valves are an essential component in any vacuum system. There are a variety of different types that fall into three main categories – operational valves, closing valves, and sealing valves. The selection of the proper valve type requires an understanding of the process application so that the proper design and/or functioning valve can be chosen for a given vacuum furnace operation.

When talking about vacuum valves the ones that most people think of are those installed in lines that connect a vacuum chamber to a vacuum pump: the roughing valve (aka main valve if a diffusion pump package is not installed), the poppet (aka main) valve, the foreline valve, and the holding valve.

What is a vacuum valve? A vacuum valve is a device placed in a feed or vent line on a vacuum furnace whose purpose is to isolate the vacuum chamber or direct the flow of gas into the vacuum vessel. These valves can be actuated manually, pneumatically, electropneumatically, electrically or electromagnetically. A series of pressure switches are also installed in most lines with vacuum valves (other than manual valves) to control the flow and changing the internal furnace pressure as demanded by the process being run.

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.

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.

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.