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.
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.
Passive Vacuum Gauges – Traditional vacuum gauges (aka vacuum sensors, transducers, and/or vacuum tubes) are considered passive gauges and including such sensors as thermocouple gauges, convection gauges, diaphragm gauges (for low vacuum measurement) and cold cathode gauges or hot ion gauges (for high vacuum measurement). All passive vacuum gauges require a vacuum controller or an active vacuum gauge (see below) in order to provide vacuum. Active Vacuum Gauges – Active gauges (aka digital vacuum gauges, smart gauges) combine the passive vacuum gauge and vacuum controller electronics in one device. These gauges often feature thermocouple gauges, convection gauges, and cold cathode gauge technology bundled together to provide a full range vacuum measurement from atmosphere to ultra-high vacuum applications. These gauges are especially attractive when frequent calibration and/or sensor replacement is needed as they can be easily disassembled, cleaned, and calibrated or replaced. In particular dirty applications, you may consider the use of an in-line filter before the gauge.
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.