Introduction

In the vacuum heat treating world, where critical components are often near-net-shape with minimal to zero stock removal, the surface aesthetics of the final product are critical to the end user. Across industries such as aerospace, medical devices, and power generation vacuum processing has become increasingly valued—not only for its precision, but also for its ability to eliminate downstream operations, ultimately saving cost and time.

Given these benefits, customers are frequently willing to pay a premium for “bright, clean work.” To achieve these pristine results, vacuum heat treaters insist that incoming parts must be “clean and oil-free.” However, what qualifies as “clean” in a manufacturing environment rarely meets the exacting standards required for vacuum thermal processing. As a result, many commercial heat treaters adopt secondary cleaning measures—not only to ensure part cleanliness but to protect their vacuum furnaces from contamination by machining oils, lubricants, Dykem, oxidation, or polishing compounds.

Pre-Heat Treatment Cleaning: Traditional Challenges

Before any vacuum heat treatment, components must be thoroughly cleaned to remove organic and inorganic contaminants. Common practices include solvent immersion/ drying, and vapor degreasing, all designed to eliminate residues that can volatilize and redeposit within the vacuum furnace, potentially compromising part quality and damaging the vacuum furnace hot zone and cold wall.

However, these cleaning agents are often:

• Flammable
• Toxic
• Environmentally regulated
• Costly to dispose of when spent

Given that commercial heat treaters process parts from thousands of upstream operations, each introducing its own set of contaminants, cross-contamination becomes a significant risk. Stainless steel foil wrapping is often used as a defensive measure, isolating parts from the furnace environment. While wrapping is often effective, this method is:

• Labor-intensive
• Expensive
• Hazardous – Even with the proper PPE, the foil edges are razor-sharp. Foil wrapping continues to be our number one health and safety concern for our valuable employees.

The MIM Furnace: A Catalyst for Innovation

Five years ago, 51���� of Western Pennsylvania was tasked with sintering pre-sintered metal injection molding (MIM) parts at 2200°F. The binders present in these parts volatilized during processing and heavily contaminated the vacuum furnace, resulting in extensive downtime and maintenance.

Instead of constructing a traditional cold trap to capture volatiles, CEO William Jones developed a more innovative solution: a “hot trap” designed to divert and capture contaminants before they could deposit inside the furnace. This proactive adaptation has proven to drastically improve part quality while eliminating the laborious and frequent cleaning of hot zones and cold walls.

After that MIM job ended, the underutilized furnace prompted experimentation. It was proven how well this adapted furnace performed on unwanted binders. We set out to test how this same system could be adapted to remove impurities from everyday production parts. After extensive trials using non-critical PH-grade stainless steel components, a fully integrated, vacuum-based cleaning and aging cycle was perfected. This development has since replaced traditional expensive pre-cleaning methods and dangerous foil wrapping, producing consistently clean and bright 17-4 PH aerospace components.

The Self-Cleaning Vacuum Furnace: How It Works

The key innovation lies in a dual roughing pump configuration:

Pumping System #1- Initial Pump-Down and Contaminant Removal:

• Components are loaded into the furnace unwrapped and uncleaned.
• Only Roughing Pump #1 is activated during the initial pump-down.
• A slow temperature ramp allows contaminants to vaporize and exit the hot zone through a heated port into Pump #1.
• Contaminants are safely trapped in the pump’s oil—the “hot trap”.

Pumping System #2 -Transition to Heat Treatment:

• After off gassing is complete, Pump #1 is isolated.
  Pump #2 system, which includes a roughing pump, booster, diffusion, and holding pump, takes over.
  The chamber is then brought to 1 x 10⁻⁵ Torr and the standard vacuum thermal cycle proceeds.

This two-stage pumping sequence cleans both the parts and the chamber prior to heat treatment—without ever opening the furnace door.

Results and Benefits

This newly developed vacuum furnace and process produces:

Cleaner Parts: Vacuum cleaning penetrates blind holes, threads, and keyways more effectively than traditional solvent or vapor methods.
Injury Reduction: The process eliminates the need for hazardous foil wrapping, significantly improving employee safety.
Environmental & Cost Advantages:

• Reduces or eliminates chemical solvent use.
• Cuts labor associated with pre-cleaning and wrapping.
• Reduces hazardous waste and disposal costs.

Furnace Maintenance Improvements:

• Hot zones and cold walls remain pristine—no weekly tear-downs.
• Pump #1 oil is changed biweekly, eliminating seizure concerns due to contaminated oil.

Conclusion: A Game-Changer for the Industry

Historically, part cleanliness in vacuum heat treating has been a persistent challenge—one often addressed through costly labor, chemicals, and stainless steel or titanium foil. 51����’ innovative dual-pump vacuum cleaning system, integrated seamlessly with a standard vacuum heat treatment cycle, redefines industry best practices.

This “self-cleaning furnace” concept not only delivers superior part finishes, but also enhances safety, reduces environmental impact, and cuts operating costs. In a world where precision, cleanliness, and sustainability matters more than ever, this advancement may very well represent the “Holy Grail” of clean vacuum processing.

Author: Bob Hill, President, 51���� of Western PA and Michigan

As published in Heat Treat Today Magazine:

The post NEW DEVELOPMENT: A Self-Cleaning Vacuum Furnace appeared first on 51����.

This paper explains reactions that can occur during a vacuum processing cycle and different methods of preventing these reactions. We will discuss how eutectic compositions can form while heat treating and how diffusion bonding can be a concern to heat treaters when two dissimilar materials are in close contact with each other in high vacuum at elevated temperatures.

To many people, the term “eutectic” is not well understood. The best way to think of a eutectic is a metallurgical meltdown. A eutectic reaction occurs when two components with different melting points and surfaces free of oxides come in contact with each other in the vacuum furnace. This can create an atomic diffusion. For some materials, when a specific atomic composition is reached, they will melt at a temperature much lower than the melting point of the individual metals. If that temperature is reached or exceeded during the heat treating cycle, melting will occur at the contact points. This is referred to as a eutectic melt.

The most common example of a eutectic reaction is with a tin/lead solder. Tin melts at about 450°F, while lead melts at about 620°F. When they are together as two components, the solder melts at 370°F. That is 80 degrees below where tin melts at and 250 degrees below where lead melts.

Another good example in vacuum heat treating is the titanium/nickel eutectic. In this scenario, titanium melts a little above 3000°F, whereas nickel melts at about 2650°F. If you heat treat them in a vacuum furnace, placing the titanium on a typical heat treat fixture such as a grid or stainless steel basket, the two materials would melt at around 1730°F. This could be a disaster if the technician does not understand the eutectic reaction.

We learned a good lesson on eutectic melting a couple years ago when a clean-up run was performed in one of our furnaces at 2400°F without prior removal of a work grid. The high nickel alloy cast fabricated grid was sitting on molybdenum support rods positioned on the furnace graphite support rails.

A form of a eutectic is diffusion bonding, a reaction that occurs well below the eutectic melting point and must be considered when two or more materials are in direct contact.

Diffusion bonding becomes more acute as the mass of the parts in contact increases. In vacuum brazing, a eutectic reaction can be a desirable formation when joining two pieces of metal using brazing filler metals. However, there are times when a eutectic reaction can occur with unintended and often damaging consequences.

Table 1 shows some alloy mating combinations and the eutectic melting points. ¹

Eutectic Barriers Used in Heat Treating

A eutectic barrier is the insertion of a material between the two mating metal surfaces that might be heading for a eutectic melt at elevated temperatures. This barrier material must be capable of withstanding the process’ maximum temperature to be effective.

The most effective barriers are:

• Refrasil cloth or Kaowool blanket
• Thin Ceramic plates
• Ceramic fixtures of different shapes
• Stop-off paints of different types

Refrasil is a clothlike material consisting of more than 96% silica (SiO2) that resists oxidation and most reactive melting. Most Refrasil textiles will not melt or vaporize until temperatures exceed 2650°F.

Kaowool is a good insulation and barrier when used in a blanket form.

Ceramic, high purity alumna can take the form of flat plates or individually designed fixtures to fit a particular application.

Stop-offs��are designed to protect metal surfaces from the flow of molten brazing filler metal, or to prevent metal surfaces from adhering or sticking to each other.

The following are illustrations of how barriers are used in actual applications.

1. Kaowool Barrier: Typical load of Titanium ingots on Kaowool with graphite fixturing. Processing temperature is 2350°F for 24 hours in high vacuum.

2. Refrasil Cloth Barrier: Separates the bars from the supporting graphite plate. This eliminates the possibility of a eutectic reaction when hardening the bars at 2125°F.

3. Ceramic Plate: Demonstrates the use of a thin ceramic plate to separate the fastening nuts from the graphite support plate. This process is for a sintering procedure to 2500°F.

4. Ceramic Plate: Typical load of nickel iron alloy, MuMetal, fixtured on Alumina sheet with graphite support. Processing temperature is 2150°F.

5. Ceramic Fixture: Using a special ceramic fixture to support the all-thread parts of high speed steel for a hardening cycle at 2100°F. The ribbed ceramic fixture separates the parts from the graphite support plate.

6. Stop-Off Barrier: Typical load of 420 stainless steel molds processed on graphite plate utilizing Wall Colmonoy Inc. white stop-off. Processing temperature is 1900°F.

7. Green Stop-Offs: Typical load of orthopedic implants fixtured with alloy 330 screens utilizing Wall Colmonoy stop-off fixtured in CFC grids. Processing temperature is 2175°F.

8. Other Barrier: Graphite plate supporting the load separated by a high temperature aluminum barrier developed by GMI.² This barrier would be similar to using the green stop-off material. This particular hardening process has an upper temperature of 2200°F.

These are just a few examples of how barriers are used to eliminate the possibility of eutectic, or sticking, reactions from forming.

Also, check out our detailed booklet on critical melting points.

Vacuum Heat Treating: Critical Melting Points Booklet

Written by:�� Real J. Fradette, Senior Technical Consultant, 51����, Inc., Roger A. Jones, FASM, CEO Emeritus, 51����, Inc. ����������������������

References:

1. Critical Melting Points and Reference Data for Vacuum Heat Treating Sept 2010, 51����, Inc.
2. Graphite Machine Inc., Topton, PA

The post Preventing Eutectic Reactions and Diffusion Bonding in Vacuum Processing appeared first on 51����.

With approximately 100 production vacuum furnaces in operation each day, the 51���� Group of Companies has amassed a vast body of knowledge and experience on the subject of vacuum furnace leaks, which we are pleased to make available to the rest of our industry. This article provides a detailed explanation of the various types of leaks that can occur, where they typically occur, and methods of locating and correcting these problems. Particular safety concerns relating to leak checking will also be discussed.

Production vacuum furnaces are designed to provide the ideal atmosphere for processing different materials, and to achieve excellent metallurgical results without surface contamination. However, when a vacuum leak occurs, the resulting integrity of the material being processed can be compromised.�� Therefore, it is most important that the furnace meet minimal leak rate standards for consistent treatment of any material.

All owners and operators of vacuum furnaces will eventually experience some type of leak affecting product quality or even damaging internal components of the vacuum furnace hot zone. ��Large leaks in a vacuum furnace are usually very obvious. They often result in the furnace not pumping down or the processed material showing clear signs of oxidation. Small leaks, however, often go undetected as the pumping system can offset the gas rate of the leak.�� As a result, the vacuum gauge might still show acceptable levels, misleading the system operators into continuing the process, which could lead to major scrap or damage.

What Is a Vacuum Leak in a Heat Treat Furnace?

Normal Leak

A normal leak can be identified as some type of opening, hole, or crack that allows air to be admitted or escape from a confined vessel.�� On a vacuum furnace, it is essential to prevent air from entering the chamber.�� With many components of the furnace requiring a seal, there are several areas at risk for leaks, such as threaded and brazed joints, improperly installed fittings, or O-rings. O-rings, particularly, should be examined regularly, as they can become cut or worn, flat or dirty, or lose elasticity, especially around doors and rotating or reciprocating assemblies. Because a vacuum furnace operates in deep or partial pressure vacuum during the heating phase of the cycle, and near atmospheric or positive pressure during the cooling phase of the cycle, proper sealing of all the above referenced components must be continually established.

Other Types Of Leaks

When a furnace fails to achieve an acceptable leak rate, based on a leak rate test over a one hour period, it does not necessarily mean there is a serious real leak occurring. The testing individual must determine if this is indeed a real leak or some other situation causing the failure.�� The problem could be caused by high gas loads, outgassing, back streaming of vacuum pump oil or a virtual leak occurring within the overall system.

Outgassing is defined as the release of vapor-pressure materials present in the vacuum system. Also, outgassing is often made obvious during the heating cycle by a large vacuum “spike” or rise in pressure during heating.

Sources for outgassing or virtual leaks could include:

1. Residual solvents or water remaining from cleaning, preventive maintenance, or water in the workload.
2. Leaks such as cooling water from vacuum chamber or heat exchanger.
3. Trapped volumes of gas due to poor vacuum construction practices.
4. Trapped spaces under non-vented hardware.
5. Gasses in spaces with poor conductance.
6. Porous material exposed to the atmosphere such as low density graphite.

Also, many sources of virtual leaks arise from an original manufacturing procedure or a repair on a chamber or system. ����As an example, small volumes can be trapped that have a connection to the vacuum side and yet, given the conductance restriction, cannot be pumped out easily or quickly. ��Some common examples of virtual leaks caused by poor design follow.

Our first example, shown as Figure 1, illustrates how a small volume of air or gas can be trapped at the bottom of the threaded hole.�� This small volume will eventually slowly leak out through the threaded area when the furnace reaches a certain vacuum level and will continue to slowly affect the vacuum performance of the furnace.

A second example of a possible virtual leak is shown in Figure 2.�� In this situation some trapped volume could occur if the O-ring groove is a dove tail design or if there are two O-rings and the space between them is not vented.

��

A third example of a virtual leak is related to improper or incorrect weld design. In this situation, a space could be left between members being butt welded. Should a small crack eventually appear in the weld area, leakage could occur and cause this undesired and difficult to find leak.�� This is illustrated in Figure 3.

��

It should be noted that most virtual leaks will not be detected in a normal leak checking cycle.�� The presence of a virtual leak will become apparent during the system pump down cycle when the ultimate pressures cannot be reached, i.e. hours, ��or it takes an excessive amount of time to reach blank off pressure.

In most cases, physical leaks and outgassing are present simultaneously where the leak is a flat pressure and the outgassing is constantly reducing.

Figure 4 highlights three examples of what can result during pump down. As is illustrated, a real leak reaches a limit while an outgassing occurance continues to achieve better vacuum over time.

��

Determining a Vacuum Heat Treat Furnace Leak Rate

It is essential to know that a vacuum furnace is leak tight.�� Measuring how tightly the vacuum chamber is maintained is accomplished by performing a “Leak-Up-Rate” test.

There are various ways to prepare a furnace for a leak-up-rate test.�� Our recommendation on normal production furnaces is as follows:

Heat the empty chamber to approximately 100^oF over the most recent highest process temperature for one (1) hour to thoroughly clean the furnace of product contaminants. Then nitrogen or argon gas cool the furnace to ambient temperature.�� Without opening the chamber, continually re-pump down the furnace to establish an ultimate vacuum level.�� This should continue for an extended period, usually, but no more than, two hours.�� After that time, the vacuum pumps are valved off and the system allowed to sit while the rise in vacuum level is measured.�� This leak rate is typically measured in microns/hour with certain standards established as acceptable for a given system. Normally a leak rate of 10 microns per hour is acceptable for most vacuum furnaces but may be lower or higher depending on the application or volume of the vacuum chamber.

A typical rate of rise curve could look similar to the one in Figure 5.�� If we break down this curve, we can conclude that the first 15 minutes represent a total of any real leak and outgassing occurring in the furnace.�� As things begin to stabilize, the rate of rise begins to straighten out slightly to represent the real leak that is occurring.�� If we analyze what occurred over the previous 30 minutes, we see a rise of approximately 6 microns, or extending this, 12 microns per hour. However, customarily we start from time zero, thus 10-20 microns per hour.

This would represent a marginally tight furnace with consideration given to possibly finding a potential leak, especially if critical product is being processed.

One should always remember that the results of observing a pump down cycle and then performing a leak-uptest are most often affected by the overall cleanliness of the furnace, and might not lead to an immediate detection of problems. Also, the rate of rise curve and other vacuum decay tests will not locate the leaks but will only indicate the relative magnitude of all leaks.

When pump down fails or an excessive leak rate is present, this suggests the need for a long high temperature bake-out on the hot zone emptied of work, fixtures, or work grids, to eliminate any suspected contaminants.�� However, this should be avoided as hot zone damage can occur in the bake-out if the true problem is a real leakage of air.

If the furnace fails the leak rate test, something is wrong regardless of prior leak testing methods. The leak simply has not been isolated or found.

Examples of Where Vacuum Furnace Leaks Might Occur

Based on the above and our experience, listed below are many examples of where leaks could occur.

1. Furnace Door O-ring Seal
2. High Vacuum Main Valve Seal
3. Gas System Flanges
4. Heat Exchanger Feedthroughs
5. Nasty Water Leaks in the Heat Exchanger (i.e., water to vacuum)
6. Internal Furnace Chamber Welds
7. Pneumatic Cylinder Feedthroughs
8. Power Feedthroughs
9. Heating Element Power Feedthroughs
10. Compression Seals as in thermocouple assemblies, wire feed-through glands, or thermocouple sheaths.
11. Welds and Brazed Joints
12. Shaft Seals on Vacuum Valves and particularly vacuum blower motors.
13. Flexible Connectors in Piping
14. Threaded Joints (Vacuum Plugs, Vacuum Gauge)
15. Gasket Seals on Feedthroughs, Manifolds, Sight Ports, Cylinder Glands
16. Other O-ring Seals
17. Nasty Water Jacket to Vacuum Leaks

Safety Concerns in Leak Checking a Vacuum Heat Treating Furnace

Quench Gas Concerns

Often, leak checking requires an individual to enter the open vacuum furnace chamber during the checking process.�� This is especially true on larger furnaces such as car bottom furnaces. Maintenance and leak checking of any furnace chamber internals should only be attempted when the chamber has been completely purged of any residual gas remaining from a prior cycle. Residual quench gases still in the chamber even when the door is opened can cause asphyxiation and death.

The hazard is most critical if argon was used in the prior quenching cycle.�� Since argon is heavier than air, it can remain in low-lying areas such as a quench tank or service pit for many hours. It has no discernible odour and there is usually no advance warning before unconsciousness occurs.

Diffusion Pump Heaters

On furnaces equipped with oil diffusion pumps, maintenance and leak checking should only be attempted with care.�� As illustrated in Figure 6, the diffusion pump operates by boiling oil to form a vapor.�� Heated by electric heating elements at the bottom of the pump, the oil reaches a temperature of approximately 5000^oF and higher, thus providing a very hot external surface until the pump has cooled.�� Any leak checking around this area of the furnace must be done with care. Never air release a hot diffusion pump to air, i.e. to check oil level, as it may explode and cause massive furnace damage.

��

Further Defining Leaks in Vacuum Heat Treat Furnaces

When a leak exists on a vacuum furnace system, the type of leak can be defined relative to the amount of flow occurring through the leak.

Turbulent Flow (Very rough vacuum)

Turbulent flow typically describes a gross leak situation. These leaks usually occur in unsoldered joints, open welds, open valves and plugs left off pipes.�� These types of leaks are usually the easiest to find.

Laminar Flow (Rough vacuum)

This term is commonly used to describe a small or fine leak.�� In this case, the leak is proportional to the differential pressure where doubling the pressure across the leak doubles the leak rate.

Molecular Flow (High vacuum)

This describes an extremely fine leak such as a virtual leak, or a membrane leak, as the result of a defective diaphragm in a valve or vacuum gauge.�� A leak like this occurs when the mean free path of the molecule is greater than the diameter or opening of the hole where the leak is escaping.�� In this situation, an increase in pressure has very little effect on the leak rate.

Also, gasses may penetrate through some solid materials such as elastomers (O-rings). These leaks are called permeation leaks. They are different from real leaks because the only way to prevent or stop them is to change to a less permeable material.�� These types of leaks usually occur in the 1×10^-6 Torr range or higher vacuum.

Methods of Leak Checking Vacuum Heat Treat Furnaces

There are three commonly used methods of detecting vacuum furnace leaks: (1) via sound emanating from the leak; (2) vacuum decay highlighted on the vacuum gauges when using a solvent to penetrate the leak; and (3) the use of a Helium Leak Detector (mass spectrometer).

Noisy Leaks

When a vacuum furnace demonstrates difficulty pumping down to expected rough vacuum levels, it typically indicates a fairly large leak.�� This could be as a result of some changes or modifications that have recently been made to the furnace. As the furnace is struggling to pump down, walk around the furnace and listen for any hissing or high-pitched whistling, an indication that air is penetrating a seal.�� As this may be a very small sound, it is important that surrounding noises are eliminated.

Since most furnaces cool under positive gas pressure, closing the furnace and backfilling to maximum backfill pressure will allow you to check for pressure decay and any sound coming from exiting gas.�� This will often show areas where proper seals have been broken or need tightening.

Some operators or leak checking personnel will often use a stethoscope when checking for leaks, as its ability to transmit low-volume sounds and eliminate external noise is exceptional. Often in gross leaks or some smaller leaks, a standard paint brush and soapy wash solution is helpful. Brush the solution around suspected leak areas. Big leaks will blow big bubbles at the leak while smaller leaks will form very fine foam.

Vacuum Gauge/Solvent Detection

For many years, operators and leak checking personnel have been using solvents to assist in finding vacuum leaks. Most solvents used today are either acetone (preferred) or alcohol.

The vacuum gauge of the vacuum furnace instrument panel is a very sensitive instrument. In the solvent method, the operator sprays or brushes the solvent around the suspected leaking area and watches for movement in the vacuum gauge.�� If a leak exists, the entering acetone will affect the gauge reading level since the gauge is calibrated for air, not acetone. The operator should allow sufficient time for the acetone to affect the gauge, normally 20 seconds is sufficient.

The solvent procedure is more sensitive when the furnace is at lower pressures (below 1 Torr) and is most commonly used to find gross leaks. This also allows the furnace to reach a vacuum range where the technician can then employ a helium leak detector to uncover smaller leaks.

If a gross leak is found, it can be temporarily sealed to continue leak checking using a sealant such as “duck seal,” Glyptal® red alkyd lacquer, or Tek-Seal clear vacuum sealant.�� However, the leak checker must realize that this is a temporary fix and must be permanently repaired prior to running any gas quenching cycle at positive pressures.

Helium Leak detecting

The preferred method of leak detection is with the use of a Helium Mass Spectrometer Leak Detector, a highly accurate instrument for locating leaks, especially in hard-to-reach areas. A mass spectrometer can detect extremely small amounts of helium. Helium is the tracer gas of choice because it is inert, nontoxic, relatively inexpensive (in small quantities), and not easily absorbed. Helium also easily flows through small leaks and has only a trace presence in air (usually 5 ppm).

The Spectrometer is connected into the vacuum system; Helium gas is sprayed on any area where a leak is suspected.�� The leak detector will sense the presence of Helium passing through the leak. Helium leak detectors usually work best at levels from the system’s ultimate vacuum to approximately 10 Torr. In some instances, it is necessary to “bag” or isolate a specific area on the furnace and inject helium into the contained space. The Spectrometer is sensitive enough to locate leaks even in moving or transition (vacuum to pressure) seals.

Figure 7 is the standard portable leak detector available through Varian Inc.

��

To connect a leak detector:

Connect the device between the blower and mechanical vacuum pump. You’re taking advantage of the high helium pumping speed of the high vacuum pump and blower, so response time is very good. Since the helium pumping speed for the mechanical pump is less than the blower, less helium is lost and sensitivity is good.

��

Typical Procedure For Leak Checking a Vacuum Furnace

A typical procedure for finding a leak in a vacuum furnace:

1. Before starting any testing, make sure the helium leak detector is calibrated (checked with a calibrated leak) and working properly. Make sure there are no leaks within the leak detector itself by spraying a small amount of flow into the cabinet. In addition, leak check the connection point from the leak detector to the furnace.
2. Connect the leak detector to the port on the roughing line with the ball valve closed.
3. Make sure water is turned on to the diffusion and roughing pumps.
4. Start the holding and roughing pumps.
5. Manually open the foreline to check for leaks prior to turning on the diffusion pump. Be sure to close the foreline valve following this test.
6. Slowly open the ball valve allowing the helium leak detector into the system.
7. Leak check all welds, threaded fittings, and flanges on the foreline and roughing system by spraying helium sparingly into these areas.
8. Repair any leaks resulting from this initial testing.
9. If any leaks were found and repaired, repeat steps E through H and leak check again.
10. If no more leaks are found, close the leak detector ball valve, and turn on the diffusion pump, allowing it to heat up for 45 minutes.
11. Start the process cycle and let the furnace pump down to optimum level.
12. If the furnace cannot cross over to high vacuum, open the ball valve slightly to the leak detector and leak check all flanges, threaded fittings, welds on the chamber, gas blower can, overhead piping, and vacuum switch copper lines. Fix any leaks found during this process.
13. Now go over the entire system by spraying helium at a high rate of flow.
14. If no more leaks are found, close the ball valve to the leak detector and remove it from the system.
15. The furnace should now be fully checked and should be ready to return to production.
    1. If no external helium leaks are found and the furnace still has a high leak rate as revealed by the leak rate test, there are several other possibilities: Gas leaking into the furnace via the gas backfill valve or partial pressure valves, i.e. nitrogen or argon. Take these valves apart and look for dirt or defective seals or diaphragms.
    2. A possible water-to-vacuum leak from the gas cooling heat exchanger. Take the heat exchanger out of the furnace and look for water deposits at brazed joints. Pressurize the heat exchanger with approximately 100 PSIG air pressure and soap bubble all brazed joints.�� If necessary, submerge the heat exchanger in a tank of water and look for air bubbles. A heat exchanger that has been submerged in water must undergo an oven bake-out prior to being re-installed into the furnace system.�� Do this at 300^oF for four (4) hours to boil out any residual water.
16. When all else fails, consider that water may be leaking from the chamber water jacket to the vacuum internal. It could also be from some other water jacket-to-vacuum enclosure such as the gas blower or heat exchanger enclosure. Remove all components of the hot zone and inspect for possible water deposits that could result from water leaks. If there are no apparent deposits observed, air pressurize the jackets to approximately 30 PSIG and soap bubble all welds and joints.�� Sometimes leaks can occur as cracks or even corrosion in the center of the chamber tank. All chamber leaks must be weld repaired and tested, with no resulting bubbles present.
17. Water leaking from a heat exchanger or vacuum chamber or other jacket into the vacuum will normally turn the roughing pump oil a milky color from a bad leak, or slightly discolored from a smaller leak. In a serious water leak, the vacuum chamber will not pump down below 1 Torr.�� If free water builds up in the bottom of the chamber or heat exchanger enclosure, over time the free water will freeze and form ice.�� Sometimes this will appear as frost on the air side of the chamber or heat exchanger tank.

Please remember that when using the helium leak detector, the time it takes for the leak detector to respond indicates the general area of a leak and further evaluation must be made to find the exact location.�� There is often the possibility of more than one leak within the same area.

If no leaks are found with the helium leak detector, this does not mean there are no leaks. Remember, the final test is the rate of rise test.

Conclusions on Vacuum Heat Treating Furnace Leaks and Detection Techniques

Leak detection is complicated and a detailed subject. All vacuum furnace users must eventually become experts in the “art” of leak checking or their operations will suffer difficult times.

All leak checking personnel should know and understand the following:

• The types of leaks that can occur, including the very difficult “virtual” leaks.
• How to properly perform a furnace leak rate check.
• Understand the safety aspects of leak checking.
• How to use the vacuum pumps and gauges while using a solvent solution to find simple leaks.
• Understand and have access to a helium leak detector for finding the more difficult vacuum leaks.
• How to properly repair or seal any vacuum leaks with consideration of both vacuum and positive pressure cooling cycles.
• Before starting a leak check, always review prior maintenance. Often a loose fitting like a replaced T/C improperly installed is the problem.�� Look for the obvious first before creating a more difficult situation.

Written By:

Real J. Fradette, Senior Technical Consultant, 51���� Inc.

Contributor:

References:

1. Varian, Inc, VS Series Leak Detectors for Vacuum Furnaces
2. Daniel Herring, Finding and Fixing Vacuum Leaks, Industrial Heating, 2011
3. Maintenance Procedures for A Vacuum Furnace, Jeff Pritchard, Steel Times International 2009
4. Meyer Tool & Manufacturing, Vacuum Chamber Design: What is a Virtual Leak?
5. Daniel Herring, Atmosphere Heat Treatment, Principles/Applications/Equipment, Volume 1 & Volume 2
6. JM Lafferty, Foundations of Vacuum Science & Technology
7. Basic Vacuum Practices, 3/E by Varian, Inc, reprinted by 51����
8. Figures 1-3,
9. Figure 6,

The post Vacuum Furnace Leaks and Detection Techniques appeared first on 51����.

Dew Point Considerations in Vacuum Heat Treating

Since the majority of commercial and captive heat treat facilities do not typically operate under controlled environments, the temperature and humidity swings can often be drastic. The environmental changes are especially substantial when comparing the seasonal averages, monthly averages, weekly averages (See Figure 1), or even day light to nighttime daily averages. Temperature, along with adsorption and de-sorption within the gas sampling system also affects the reliability of dew-point readings.

The precision required of any dew-point analyzer for determining water content within an open atmospheric furnace is much more forgiving. However, when analyzing very dry specialty gases that are used for partial pressures and quenching in vacuum processing, a greater precision is needed. The sensitivity of dew point measurement within very dry ranges is often meaningless. For example, the difference between -110°F and -95°F dew-point is only 2 ppm of water (See Figure 2). Therefore, the accuracy of a dew point instrument within that normal operating range for specialty gases is questionable.

Oxygen Analysis Considerations in Vacuum Heat Treating

The present capability to simultaneously compare and contrast dew point readings adjacent to a Trace Oxygen Analyzer on a daily basis has definitely proven the superiority of one measurement versus the other (See Figure 3). Having a consistent signal that is totally independent of temperature and humidity provides a more reliable measurement of the dryness of the gas species that is being analyzed.

Calibration

Often a dew point sensor can fail without warning. Typically, an erroneous signal would eventually read extremely dry (e.g. -120° to -150°F). This failure could degrade over a long period of time or overnight. Semiannual calibration of the sensor must be outsourced and maintaining an inventory of a backup instrument can be costly.

Unlike the dew point instruments, the Oxygen Trace Analyzer instrument can be calibrated right on the shop floor by procuring a cylinder of nitrogen gas with exact known oxygen content (See Picture 1).

Calibration of the Oxygen Trace Analyzer instrument using a higher value of Oxygen (30.3 ± 2ppm) versus the typical production testing range of 0 to 5 ppm was tested (See Figure 4). This was done so that the operator could observe the instruments upscale reaction during the weekly calibration schedule. This in-situ oxygen calibration process, which cannot be done with a dew point instrument, has already identified several failing dew-point cells.

Oxygen and Water in Vacuum Systems

It is well known by any vacuum practitioner that the primary residual gas that is prevalent during a pump down of a vacuum furnace is dominated by water vapor.��The water molecules from the atmosphere, bombards and clings to the inner surfaces of a vacuum system anytime a vacuum furnace door is opened.��The longer the vacuum door is left open the more saturation will occur.��Besides being detrimental to materials being heat treated, water vapor desorption or pumping time can be a major time-wasting problem in vacuum processing.

Once the vacuum furnace is completely pumped down and water vapor is minimized, the relative leak tightness of the vacuum furnace needs to be addressed.��A vacuum furnace with a high leak rate will always possess higher residual oxygen content.�� Therefore, depending upon the size of the furnace, leak rates should never exceed 10 microns per hour at a pressure of 70 microns or less.�� A leak tight system, one where the volume of oxygen leaking into the furnace is sufficiently low, is paramount for consistent and accurate heat treatments of any materials.

Oxygen Versus Water – What is More Forgiving

Thankfully, oxygen and water vapor is always found in the air around us.��However, the amount of water vapor will vary with temperature and atmospheric pressure.��The earth’s atmosphere has the maximum capacity to hold a maximum volume of 2% water, regardless of temperature and pressure.��In contrast, the earth’s oxygen content is 10 times that of water content (see Table 1). Additionally, the vacuum processing of critical components can only tolerate up to 2.5ppm of oxygen, compared to 10ppm of water vapor.��Therefore, water vapor is much more forgiving in vacuum processing than oxygen.

Conclusion with Dew Point Versus Oxygen Content in Vacuum Heat Treating

It is indisputable that water vapor (dew point) and oxygen are both problematic actors in vacuum processing. Currently all technical specifications are only addressing one of these measurements in our specialty gas usage, dew point. Given the natural inconsistencies of dew point readings, the lack of in process calibration of dew point instrumentation, and the abundance of oxygen in our atmosphere – are we really analyzing the correct elements?

For specification requirements, this author continues to record daily dew point readings for every specialty gas. However, for true verification of critical clean and dry inert gases being used in vacuum processing, this author exclusively assesses the oxygen content results derived from the Oxygen Trace Analyzer instrument.

Written by:

The post Dew Point Versus Oxygen Content in Vacuum Processing Part 2 appeared first on 51����.

Dew Point Versus Oxygen Content In Vacuum Processing Introduction

It is well known that accurate measurement of any heat treating atmosphere can have a significant effect on the quality and process yield of heat treated components.�� Traditionally, dew point analysis has always been the bellwether in determining our heat treating atmospheric conditions. This is because it was discovered very early on that moisture parameters can have a tremendous impact on carbon potential and thus on final properties.�� Since the main objective of any neutral furnace atmosphere is to prevent detrimental effects such as carburization, de-carburization, hydrogen embrittlement, oxidation, and soot formation, one must analyze much more than moisture content of a furnace atmosphere. Contending with such constituents as CO, CO₂, H₂, H₂O, N₂ and hydrocarbons (e.g. CH₄), a better more robust instrument was needed to analyze endothermic or exothermic atmospheres. Today, either Oxygen Probes or Three Gas Analyzers are the industry’s preferred analytical instruments to determine carbon potential within an atmospheric furnace.

Problems with Dew Point Content In Vacuum Processing

As the analysis and controls of atmospheric furnaces have evolved over the years, this author has questioned the antiquated method of measurement of purity of specialty gases within the vacuum processing arena. Why is dew point measurement still the bellwether of specialty gas providers, and more importantly within the vacuum heat treating industry? With the advent of such innovative manufacturing processes as Additive Manufactured components, the new metallurgy that comes with AM parts requires ultra-clean atmospheres. (See Picture 1) Is the dew point analyzer really the best instrument that we have in our toolbox?

Analyzing Dew Point in Vacuum Heat Treat Furnaces

Examine this picture of a rose (see Picture 2).�� The appearance of liquid water on this rose occurs only because of one simple fact:�� the temperature on the surfaces of the rose petals collecting the dew is below the dew point of the surrounding air.�� Likewise, the amount of water within specialty gas sampling lines in terms of the number of molecules or mass of water determines at what temperature the water vapor starts to go into liquid phase.

Therefore, an “exact” dew point measurement is most dependent upon the surrounding temperature. Note how the dew point values within the environs of a state of the art heat treatment plant process gas lines, i.e. N₂, AR, etc., vary from summer to winter (see Graph 1).�� The fact is today’s heat treatment plants are probably the worst environment to test for dew point. The majority of heat treating facilities in the world experience tremendous swings of ambient temperatures and relative humidity even within a 24 hour period of time. (See 51����’ Publication No. 3 in its Vacuum Furnace Reference Series entitled “Operating a Vacuum Furnace Under Humid Conditions,”).

Trace Oxygen Analyzers

A trace oxygen analyzer is a versatile microprocessor-based instrument used for detecting parts per million (ppm) levels of oxygen. Oxygen sensing instruments are typically sealed units and require reasonably regulated sample pressures (.2 to 2.4 slpm). The response time is dependent upon the flow rate, e.g. a low flow rate will result in a slower response to O₂. More importantly a trace oxygen analyzer results in a signal that is independent of temperature.

51���� engineers realized the advantages of trace oxygen instruments versus dew point instruments and decided to build a combination instrument that would employ both methods (See Picture 3). ��Solenoid controls automatically sample each of the four specialty gases (nitrogen, argon, helium and hydrogen) utilizing both instruments every six hours 24 hours a day. All dew point and trace oxygen results are recordable and traceable along with the plant’s ambient temperature and relative humidity. Alarm features are set for any values of dew point above -60° F and /or values of oxygen greater than 5 ppm in the process gas feed lines.

After one full year of side by side operation it was very clear to see the trace oxygen analyzer is the instrument of consistency (See Graph 2).

Specifications for Dew Point Measurements

So why has the vacuum heat treating community, when it comes to determining purity of their specialty inert gases, been slow to react when compared to the atmospheric heat treating community? It is this author’s opinion we are all being driven blindly by the specifications that govern us. Many specifications require only dew point measurements for gas purity.

More recently some specifications, such as Boeing Aircraft, acknowledge the Compressed Gas Association’s designations which include dew point AND ppm of oxygen for various gases (See Table 1). However, AMS 2769B paragraph 3.2.1.1 counteracts this allowance by addressing only dew point issues, more specifically the installation of sampling lines, the location of the dew point cell, and recording frequencies (See Table 2). ��This paragraph is often fertile ground for “findings” by any auditors who perform outside accreditation audits on heat treating facilities.

Dew Point Versus Oxygen Content In Vacuum Processing Conclusions

When vacuum heat treating metal alloys that oxidize readily in the presence of small concentrations of water vapor or oxygen, data suggests that dew point should not be the stand-alone gas purity analyzer.�� Dew point only measures the water vapor, not oxygen in the gas line.�� Adding an oxygen analyzer as an additional quality tool provides the heat treat shop greater assurance that the process gas entering the furnace is of the highest purity and meets the specifications of the customer.

Written by:

The post Dew Point Versus Oxygen Content In Vacuum Processing appeared first on 51����.

Author:

Originally Published: October 2015

A presentation by Solar’s Roger Jones given at the ASM’s Heat Treat 2015 trade show about transitioning to paperless maintenance logging.

Download Retiring Paper-based Maintenance Systems in Commercial Heat Treating Shops

The post Retiring Paper-based Maintenance Systems in Commercial Heat Treating Shops appeared first on 51����.

Vacuum furnaces require regularly scheduled, routine preventative maintenance for three main reasons. First, to ensure planned results are realized after the heat-treating process. Second, for longevity and reliability of the equipment. Third, to prevent catastrophic failure of components which could lead to safety concerns.

The following are the main maintenance concerns:

• Furnace chamber
• Leak tightness and leak prevention
• Hot zone functionality and integrity
• Bake-out and leak rate checking
• Power supply
• Vacuum pumps
• Water lines and supply
• Gas lines and supply
• Keeping accurate maintenance records

Following general guidelines in each of these areas will minimize the potential for problems in the future.

Vacuum Heat Treat Furnace Chamber

It is critical the chamber be kept as clean as possible. After each process cycle, the operator should wipe accessible areas of the furnace with a clean, lint-free rag to remove any debris or residue that has collected.�� If there is considerable build-up of contaminants, the furnace should be brushed or blown out with an air hose.

The chamber should also be examined externally for scorch marks known as “hot spots.” Hot spots indicate the water flowing in the water jacket is not properly cooling the vessel. This must be corrected before making additional runs in the furnace. Correct treatment of the cooling water is essential to prevent clogging or corrosion of the vacuum chamber, diffusion pump coils, power feed-throughs, VRT power transformers and main gas cooling heat exchanger.

Preventing Leaks in Vacuum Heat Treating Furnaces

Most leaks occur at joints (welded, brazed or soldered) and at seals (O-rings or gaskets) between flanges. The most frequent leak problem occurs on the O-ring seal for the front door. The O-ring is subject to flattening and, if this occurs, must be replaced. It also needs to be kept clean.�� After each cycle, and prior to closing the door upon load removal, the operator should wipe the O-ring with a clean, soft rag.

The door O-ring should be replaced at least quarterly. When replacing the O-ring on the door, clean the O-ring groove, being sure to remove all dirt, grease, or other debris.

Monthly, the door O-ring needs to be:

• Checked for cracks, gouges, and the retention of elasticity.
• Cleaned with a lint free cloth and have an application of a thin film of vacuum grease until there is a sheen on the surface. It is important to be careful not to apply too much grease.

Other O-rings on the system that are bolted between flanges only need to be changed if it is determined that a leak exists at the joint or the joint is disassembled for maintenance. These O-rings can be replaced using the same steps as the door O-ring. Any pipe thread fittings in vacuum or gas must be coated with a Teflon dope such as “Swak.” Teflon tape is not permitted as it will extrude and eventually leak.

Vacuum Heat Treat Furnace Hot Zone Functionality and Integrity

The hot zone, heating elements, insulators, and work support should be examined at least weekly for signs of failure or deterioration and repaired or replaced as necessary. Typical hot zone problems include:

• Signs of arcing due to loose heating element connections
• Broken or cracked elements
• Loose insulation
• Loose nozzles
• Damage to process thermocouples

Any loose connections should be immediately tightened or taken apart and repaired prior to any further cycles being run.

The hot zone should be cleaned on a regular schedule by performing a bake-out cycle.��However, if any of the following conditions exist between regularly scheduled bake-outs, an additional bake-out should be performed:

• The insulators have changed in color. This could be a result of metallization which will cause the insulators to short out the heating elements if they are not replaced. Heat element resistance to ground (chamber) should be greater than 10 ohms. A resistance reading less than 2 ohms may result in heating element arcing and failure.�� This test is only performed with all power turned off to the furnace.
• Signs of metallic build-up inside the gas nozzles.

Bake-Out and Leak Rate Checking

A bake-out cycle should be scheduled for the furnace on a regular basis based on the number of cycles and the types of materials being processed. It should also be performed when signs of contamination occur between regular bake-out runs. In addition, some users will consider a bake-out cycle prior to processing critical work to minimize possibility of contamination.

A typical bake-out cycle involves heating the furnace to 2400°F, holding for two hours, vacuum cooling to 1800°F and gas fan cooling to ambient temperature. This should result in a very clean internal for the start of the next process cycle.

The running of a bake-out cycle is the ideal time to perform a leak rate check of the furnace. The leak rate is the rise in vacuum level over a period of time, i.e., microns/hour.

For a leak rate check, the furnace should be pumped down to high vacuum, 3 x 10^-5 Torr on the ion gauge and the vacuum level reading on the vacuum TC gauge recorded. At that point, all vacuum valves are closed with the vacuum chamber isolated from the pumps.��After 15 minutes, the vacuum level on the vacuum TC gauge is recorded again.

Then, subtract the original vacuum reading from the 15-minute reading and multiply by 4. This will calculate the leak rate per hour of the vacuum chamber. For most applications, a leak rate of 20 microns/hour is acceptable.�� However, a very tight, vacuum degassed, furnace will achieve a rating of less than 5 microns/hour with many achieving a level of less than 1 micron/hour.

Power Supply Maintenance for Heat Treating Furnaces

Generally, power supplies require a minor amount of preventive maintenance. However, “best practices” for their upkeep include having users:

• Check that primary and secondary wiring and cables are tight and free from overheating, at least yearly.
• Inspect for proper air/water cooling of the supply. For water-cooled supplies, refer to the user’s manual for recommended water quality and flushing of the system. VRT power transformers must have their water-cooled hoses replaced about every five years.
• Inspect control relays and contactors for contact pitting or arcing, which could result in contact welding.
• Verify that power supply voltage is maintained within reasonable limits, ± 8%, to ensure against overloading or under-heating.
• Keep the power supply clean. Do not allow a buildup of dust, dirt, or moisture to collect anywhere inside the power supply enclosure.

Vacuum Pumps

The following maintenance items apply to all types of pumps:

• Check mounting bolts for tightness.
• Investigate for unusual noise or vibration if noticed.
• Check/tighten vibration couplings
• Install all guards before running.
• Check oil levels and for signs of contamination. Change oil if necessary.

1. Roughing Pump

Additional items to the above that should be checked on the roughing pump include:

• Check pulleys and belts to ensure proper tightness.
• Drain exhaust line filter daily and check for proper ventilation.
• Clean oil reservoir, valve deck and solenoid valve every six months.

The integrity of the roughing pump can be tested by running the pump with the roughing valve and foreline valve closed. The roughing pump should be able to pump the roughing line to 150 microns or less.

The oil on the roughing pump needs to be changed at regular intervals, typically monthly based on reasonable operation. Oil level should be mid-sight glass level with the pump running and a vacuum level of less than 1 Torr.

2. Holding Pump

The holding pump should be inspected with the same method noted for the rouging pump and its oil must be changed on a monthly basis. The correct oil level is mid-sight glass with pump running and vacuum level less than 1 Torr.

3. Booster pump

• Clean regularly.
• Check oil levels and look for signs of contamination. Change oil if necessary.
• Check and replace belts or spider as applicable.
• Re-time booster as needed.
• Ensure the pump housing is not distorted due to mounting or misaligned pipe connections.
• Change oil at least every six months.

4. Diffusion Pump

• Test for correct water flow from the water inlet at the top of the pump and the drains at the foreline.
• Ensure heating elements are tight and working within electrical parameters with equal amperage on all phase lines.
• Check oil level and look for signs of contamination. Drain and change oil every six months with the pump cold. On refilling, use the correct charge volume. Do not rely on the sight glasses.
• Clean pump when needed by dropping pump, removing jet assembly and thoroughly cleaning the inside prior to charging with new oil.

Caution:��Vacuum pump oils are specific to the Diffusion Pump, the Holding Pump, the Vacuum Booster Pump, and the Mechanical Roughing Pump. Do not interchange these vacuum pump oils and store them in a location marked for each specific pump.�� In addition, educate maintenance and operating personnel about their proper use.

Vacuum Heat Treating Furnace Water Lines and Supply

• Monthly – check ball indicators. The balls should move freely when there is proper flow. There should be no blockage or sediment buildup on any of the drain lines. Inspect the drain lines for acceptable temperatures. Outlet temperature should be less than 130°F when the hot zone is at temperature.
• As required – check and clean the pressure regulator and strainer.

Vacuum Heat Treating Furnace Gas Lines and Supply

• Yearly – check gas line from supply tank to furnace backfill valve and partial pressure valve by applying a soap solution with a brush. If a leak is present, the soap solution will bubble. If necessary, remove ball valve and clean.

Accurate Maintenance Records

It is essential that accurate maintenance records are kept to assure maximum performance from the vacuum furnace. Typical record keeping might look Chart 1 or Chart 2:

Chart 1

Chart 2

The maintenance of a vacuum furnace is an ongoing function that must be religiously performed and evaluated. Schedules should be adjusted based on careful observations and historical records. Following the aforementioned suggestions will reduce the risk of undesired outcomes, reduce furnace downtime and increase furnace life, and the likelihood of catastrophic failure of components will be reduced, enhancing worker safety.

Written By:

Real J. Fradette – Senior Technical Consultant for 51���� Inc.

The post Critical Areas In Vacuum Heat Treat Furnace Preventive Maintenance appeared first on 51����.

A Temperature Uniformity Survey ( TUS ) for a vacuum furnace to satisfy AMS 2750D must be performed using established procedures and methods that fully meet the requirements of the specification and allows for consistent and more accurate results of actual furnace capabilities.

51���� and Solar Manufacturing, with their extensive vacuum furnace experience and processing knowledge, have combined to create a standard procedure for TUS for all newly manufactured and current in-production vacuum furnaces. This procedure considers the many critical aspects of AMS 2750D that must be fully satisfied to produce acceptable processing results and the following outline could be applied to any vacuum furnace user to satisfy their TUS requirements.

Download Optimizing Procedures For Temperature Uniformity Surveying For Vacuum Heat Treating Furnaces Paper

The post Optimizing Procedures For Temperature Uniformity Surveying For Vacuum Heat Treating Furnaces appeared first on 51����.

Author:��

Originally Published: Industrial Heating, November 2008

Download Reconditioning Ceramic Insulators in Vacuum Furnaces Paper

The post Reconditioning Ceramic Insulators in Vacuum Furnaces appeared first on 51����.

Author:��Harry W. Antes, Consultant, 51����

Originally Published: Heat Treating Progress, November 2006

Download Heat Transfer and Insulation in Vacuum Furnaces Paper

The post Heat Transfer and Insulation in Vacuum Furnaces appeared first on 51����.