Maintenance of Vacuum Heat Treating Furnaces

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.

Having a good preventive maintenance program for your vacuum equipment ensures years of reliable operation. Most furnace manufacturers and third-party suppliers can provide training on their systems, offer troubleshooting advice and help design preventive maintenance programs, which always need to be customized to the individual company running the equipment. Often, however, help with process applications is outside the scope of their work, so if this is needed, it should be negotiated before the purchase

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 becomes far more reliable.

Vacuum furnaces are not unique. As with any piece of manufacturing equipment, proper maintenance at regular intervals is essential to extend service life and provide (relatively) trouble-free operation. The operating and maintenance manuals supplied with most furnaces provide detailed information on maintenance and troubleshooting. While these manuals should be read and understood before commencing furnace operation, this is seldom done. Instead, people refer to the manual only when a problem occurs. Reading the manual first, as a preemptive step, helps to more quickly maintain the furnace.

Typically, the mechanical components in a vacuum furnace require normal maintenance practices (e.g., lubrication, cleaning, proof of operation, etc.). However, where vacuum furnaces differ from other types of machine tools or heat treatment equipment is that their successful operation depends heavily on maintaining a leak free vacuum environment.

When performing any type of maintenance activity, the impact on the integrity of the vacuum system must be considered (it is not uncommon when an unacceptable leak rate is observed to troubleshoot the problem to the last place maintenance was performed). Leaks in areas such as seals, braze joints and the like, coupled with contamination of furnace internals, will adversely affect the ability to maintain proper operational vacuum levels and ultimately the quality of the end product produced. In addition, finding and correcting leaks is highly time-consuming and a painstaking process.

The location of tiny leaks often requires the use of helium leak detectors or even more sophisticated methods (e.g., residual gas analysis). However, keeping track of where leaks have previously occurred and especially replacing temporary sealing compounds (e.g., Glyptol®) used as stopgap measures is a critical part of a good preventive maintenance program and will minimize future problems.

Failure to execute a properly planned maintenance program is often a precursor to unanticipated equipment downtime, effecting production and which can have significant consequences (one of which is unpredictable and high costs of repair). By optimizing the performance and therefore the life of your vacuum furnace, the total cost of operation over time will be lower than taking a “wait until it breaks and fix it” approach.

To support internal plant maintenance working with an OEM or third-party supplier committed to field service and having the availability of original replacement parts is critical.

At a minimum, a good preventive maintenance program involves the proper care of the vacuum pumps on the system, replacement of O-rings (especially on doors and moving or rotating seals) every 6 – 9 months, daily leak-up checks, daily inspection of exposed flange sealing surfaces as well as inspection/cleaning of the furnace hot zone. In addition, regular inspection of the power feed-throughs (for arcing) and hot zone (for signs of deterioration due to attack by oxygen) helps to prevent downtown due to leaks and other issues. In addition, continuous monitoring of vacuum levels during processing can help to identify potential problems before they develop into major repairs.

Remember that the final product quality is a function of many factors, including such items as uniform temperature distribution throughout the hot zone, proper gas circulation (partial pressure or quench gas) and the cycle times and temperatures chosen. In addition, the anticipation of potential problems leading to extensive maintenance or downtime is beneficial. By way of example, use of a sacrificial layer of insulation material in the bottom 1/3 of the furnace hot zone during brazing can minimize downtime, as can the proper selection of the form of the braze alloy, that is, if brazing paste is used, its composition and outgassing characteristics must be fully understood. All of these steps will lead to achieving the best performance and ultimately the highest quality standards while achieving the required productivity even for the most challenging of vacuum applications.

Which Type of Maintenance Program is Right For Me?

Over the years, two distinct styles of maintenance practices have emerged in the heat treating industry, namely “planned preventative maintenance programs” and “repair as needed” strategies. While each approach has been found to require similar costs to perform, the trend today is toward planned preventative maintenance programs to better match the production demands of modern manufacturing.

A planned preventative maintenance program involves the following: Planning the activity, Executing the plan, Evaluating the results of the maintenance effort and Revising the plan to make it better going forward (the PEER system).

This includes the following activities:

• Identifying essential spare parts (via critical spare inventories)
• Monitoring of component usage times (via hour meters)
• Detailed record keeping
• Root cause determination when problems do occur
• A complete explanation of repairs (why, what, where, when and how)
• Establishing Mean Time Between Failure (MTBF) of critical components

For the success of any planned preventative maintenance program, it is critical to:

1. Understand the external constraints imposed on you with respect to such issues as:

a. Equipment usage
b. Budget

2. Understand the equipment being serviced, in other words:

a. How should it operate?
b. How is it working now?

3. Tailor the plan to meet realistic expectations including:

a. Identifying critical spares and have them in stock
b. Understanding which spares must come from an OEM provider and which spares can be purchased from third-party suppliers

4. Divide the furnace and work effort into manageable parts serviced by specific disciplines.

a. Focus on those components or assemblies (internal or external) that are critical to the functionality of the operation.
b. Do an exterior and interior review and observe how components interact.

5. Put the repair information into usable (i.e. searchable) and retrievable form.

a. Review needs with management
b. Get feedback through team meetings
c. Revise the plan as needed

6. Establish a mean time between failure for key components

a. Conduct cause and effect analyses
b. Determine the root cause of a failure (don’t just fix the obvious)

7. Be disciplined

a. Realize the benefits by having a carefully structured, rigorously adhered to program (this is not punishment but prevention)

8. Do the job right (or not at all)

a. Have the tools and supplies on hand to succeed.

Good electronic record keeping is an important element of today’s planned preventative maintenance programs. An operational logbook, as well as a detailed maintenance log, should be kept and every action noted. A key aspect of these logs is that they should be reviewed periodically to look for trends, identify problem areas, and used to gain a better understanding of the types of maintenance (and personnel skills) that are required.

Record Keeping

Having the correct spare parts, detailed construction drawings, and the right tools on hand is essential to minimizing downtime. Support by the plant engineering staff, the original equipment manufacturer (OEM) or a third party is often necessary. Remember, successful planned maintenance is a team effort.

But what happens if you buy a used furnace or “inherit” a piece of equipment from another location? First, try to gather as many facts as possible about its history. For example, find out when it was built, for whom, and for what application it was originally intended. Ask the OEM to look at his spare parts records to identify both the type and frequency of parts sales. This is a good first indicator of places to look for problems. Obtain a copy of the instruction manual and obtain as many detailed prints as possible (there is often a fee for such items).

Next, completely inspect the unit, inside and out. Look for the obvious, and ask yourself the reason for everything you see (and don’t see). Last, don’t rush the equipment into service. Measure and record such items as the speed of moving components, for example, doors or load transfer mechanisms and elevators. Once operational, record pump-down speed, leak rate values, current draws, incoming voltage and power gas usage, and install hour meters, energy monitors and other reporting devices. In other words, learn as much about the normal operation of the equipment as possible before putting it in service, so that you can monitor changes and quantify degradation that occurs over time.

Establishing a PM Plan

Divide and conquer. Begin by understanding the heat treat process(es) you will be asking the unit to perform and compare these to the design ratings/limitations of the equipment, items such as:

• Temperature Rating
  • Normal and maximum operating temperature
  • Cyclic operating conditions
  • Idling conditions
• Loading
  • Load size including volume or weight limitations
  • Load distribution and the necessity for load ballast
  • Maximum and minimum gross load weight as a function of temperature
• Atmosphere Requirements
  • Type and function of gas(es) – partial pressure and backfill
  • Gas flow rate, pressures, etc.
• Quench Requirements
  • Type of quenchant
  • The volume of quenchant – if a liquid (in relation to gross load weight)
  • Quenchant temperature
  • Flow characteristics of the quenchant around the part
• Special Requirements
  • Baskets & fixturing
  • Quench restrictions
  • Access & site ports
  • Water systems
• Design Specific Features
  • Special features
  • Support/ancillary items (heat exchangers, water circulating systems, etc.)

Now take the time to divide the equipment into logical “sections” so that the maintenance on each of these areas focuses on those components or assemblies that are critical to their functionality (and ultimately that of the entire machine). Then walk around the exterior and inspect the interior. Note: confined entry training/permits may be required. Observe how all components interact. This takes a surprisingly short amount of time and yields a significant amount of information. Next, understand the “external” constraints being placed on the equipment (usage, budget, etc.). These factors are important in tailoring your plan to meet the expectation. Identify critical spare parts and have them in stock. Understand which spare parts must come from the OEM manufacturer and which ones can be purchased through alternative suppliers.

Finally, put this information into a useable form (such as a spreadsheet), review with management, and implement your planned preventative maintenance program. Remember that feedback and refinements to the plan will occur constantly. Make sure that the reasons for the changes are captured in the documentation for later use, and make the system independent of changes that will inevitably occur in either the maintenance department, the heat treat department, or in management.

Establish a mean time between failure (MTBF) for critical components and be sure to conduct a “cause and effect” analysis whenever a part fails prematurely.

In order to realize the benefits of a planned preventative maintenance program, irrespective of the type of equipment being used, a carefully structured, well disciplined, and rigorously plan must be created, implemented and followed. It is amazing how, years later, many plants struggle to perform maintenance on older equipment because of poor record-keeping.

Finally, understand that the frequency of maintenance (i.e. the interval between routine repairs) is highly dependent on such factors as:

• The type and number of heat-treating processes performed;
• The skill level of the operators and maintenance personnel;
• Equipment design;
• Quality of prior maintenance;
• Type of spare parts used;
• Type of support structure.

Steps you can take include:

1. Intelligent equipment purchases – Taking the time to select the right equipment and features (standard and custom hardware and software) including consideration of future growth plans.
2. Commitment to a robust maintenance program – Using a planned preventative maintenance approach rather than a haphazard “fix it when it breaks” approach.
3. Having user-friendly HMI, instrumentation, and controls – Taking full advantage of today’s technology (including remote access capability, intelligent sensors and anticipatory controls (such as cloud-based reporting of abnormal operating conditions is almost mandatory today. This allows one to manage data through an intuitive, graphical interface so that changes to the system in real-time can be observed and corrective actions (including maintenance) initiated.
4. Safety – Safety is mandatory and cannot be compromised at any time. Whether this is following a set of company rules (e.g., confined entry regulations) or plain common sense and respect for the equipment and processes being run. There is a myriad of safety issues that must be considered when maintaining vacuum furnaces to avoid physical injury, electrocution, asphyxiation, burns and other injuries to yourself or others. Code requirements, such as NFPA standards must be followed by the OEM and in addition, there are several unique considerations specific to vacuum furnace equipment. For example, on furnaces equipped with oil diffusion pumps, maintenance should only be attempted after the pump has been allowed to cool to ambient temperature. The diffusion pump works by boiling oil to form a vapor. Heated by coils in the base of the pump, oil temperatures reach 240ºC (464ºF) and higher. At these temperatures, vapors from hydrocarbon-based oils can react explosively with air. As a result, the fill port in the pump should never be opened while the pump is operating. Silicone-based pump oils eliminate this explosive risk.

Maintenance of furnace chamber internals should only be conducted using approved confined space entry and electrical lockout procedures. Residual quench gases remaining in the tank even after the door is opened can cause asphyxiation. Particular care should be taken entering furnace chambers after argon has been used as a quench gas. Argon is heavier than air and can remain in low lying areas for some time. It has no discernable odor, and there is usually no advanced warning before unconsciousness occurs. Vacuum conditions in a furnace tank are even more lethal. Lockout procedures to prevent furnace operation must be in place before entering any furnace chamber.

What you should expect of your furnace OEM or Third-Party Supplier

In this day and age, one should expect support from their supplier partners in the form of:

1. Technical Support (24/7) – Internet and/or telephone support by personnel trained in supporting vacuum equipment. A prompt response and courteous reply followed by rapid resolution of the problem should be expected, especially if a critical issue is involved.
2. Diagnostic help (remote dial-in) – The ability of the OEM to access the vacuum furnace to aid in the troubleshooting process (this often requires the installation of a dedicated telephone line). Today, a number of cloud-based reporting systems are available so that the system can be monitored for abnormal operating conditions in real-time with alerts sent to both users and the OEM. Predictive analytics help determine an optimal preventative maintenance schedule as tasks are carried out only when needed.
3. Root cause analysis – Taking the time to determine the root cause of a failure and analyzing the various ways in which it can be corrected is essential to equipment uptime and ultimately high productivity. Temporary fixes to keep the equipment running in the short term may be necessary, but these should be instituted only with the understanding (and preset timetable) of shutdown for a permanent repair and/or replacement.
4. Availability of spare parts – Anticipation, as well as the availability of critical spare parts, is an important aspect of a properly planned preventative maintenance program. This should include both “standard” spare parts as well as specially designed components that may require extended lead times. Evaluation of which spares to keep in stock or on consignment at the OEM or third-party supplier is as critical as any aspect of the preventative maintenance plan.
5. Field support – Components fail and systems malfunction. This happens in the real world but dealing with an unanticipated breakdown of your vacuum furnace can bring operations to a standstill despite the best-laid plans. If the issue is significant enough to require service by an OEM or third-party provider, they must be available, have individuals with the right skill sets (mechanical/electrical/process) and be familiar with the style and type of vacuum furnace. This is essential to a quick resolution of the problem.
6. Periodic Service Visits – Setting up quarterly service visits by the OEM or third-party maintenance provider serves as a wellness check and helps prevent more serious problems from occurring before they become critical and produce unanticipated downtime. with a suitable maintenance program. Regular inspection, lubrication, testing, repairs, and fine-tuning adjustments to ensure safe and efficient operations.
7. Training – The OEM or third-party maintenance provider should be able to train your maintenance staff on routine service issues so that they can do the work themselves. In addition, they should be able to provide safety alerts and information on product upgrades, obsolescent and the like. Choosing a partner that will guarantee a level of professionalism and quality in the performance of their work is critical to success.

Heat Treat Checklist

Here is a list of questions that should be asked in order to determine if your heat treat or Heat Treater has the process under control and is operating equipment that is properly maintained.

• Is the overall operation in control?
  • Are written instructions, operating procedures and all rules & regulations being utilized on a daily basis or do they exist only for show?
  • Are all procedures understood by the workforce or only by management?
  • Are the heat treat practices being used effectively?
  • Is maintenance planned (predictive maintenance) or does it occur only when machinery breaks? Are the repairs patchwork fixes or permanent solutions to the problem, which caused them to occur?
  • What types of quality control checks being made on the furnace daily? Weekly? Monthly? Semi-annually? Annually?
• How effective is the pre-cleaning of parts?
  • Are the incoming parts clean?
  • How are they being cleaned?
  • How effective is the cleaning method?
  • How well is it controlled?
  • How often is it monitored?
  • Is a bath chemistry check performed?
  • How often are the washers monitored for proper concentration and pH?
  • How are the washers being cleaned?
  • Are oil skimmers in use and are they properly maintained?

• How often is the furnace inspected?
  • What method(s) is used?
  • What are the criteria for acceptance?
  • How effective are the inspections?
  • Are they frequent enough?
  • Does the process parameters remain steady state or do they fluctuate?
  • What method is being used to check the furnaces for leaks?
  • What type of thermocouple is being used?
  • Are they adequate for the temperature range being run?
  • When maintenance is performed on thermocouples are their insertion depths per equipment manufacturer’s recommendations?
  • Has the insertion depth of the thermocouples changed?
  • When is the last time a temperature uniformity survey (of the workload area) was performed?
• With respect to the quench system
  • How is the quench gas or oil being monitored?
  • How is the quench gas or oil being controlled?
  • Is the degree of agitation sufficient for the quenching operation being performed? How is the quench gas being introduced into the chamber and is there any signs of damage?
  • How often is the quench oil sent out for analysis?
  • How is the quench media checked for particulates?
  • Is the motion of the elevator (batch integral quench vacuum furnaces) smooth and quick (2 – 3 seconds being typical)?
  • How often is the quench tank serviced?
  • How much “drag out” (removal) of quench media occurs?
  • What is the transfer time of the workload to the quench?
  • What type of quench tank maintenance is performed and how often?
• Are trays, baskets, screens and other fixtures/racks being inspected?
  • What is the transfer time of the workload to the quench?
  • Is proper inspection and maintenance of tray sensors being performed? If they are cracked or sticking, trays will cause jam-ups or otherwise damage the interior of the furnace.

In Summary

Successful use of a vacuum furnace should be measured in terms of up-time productivity. Today, uptime should be in the range of 90% or greater and depends ultimately on a leak-free and well-functioning vacuum system. While there are numerous factors that influence uptime (e.g., planning, scheduling, identifying the root cause of a failure, having the tools and spare parts needed for the repair) fixing problems right the first time is critical to success. The goal of any maintenance performed on a vacuum furnace is to return the equipment to full operational service. Making sure that all problems are uncovered, the root cause of the problem has been correctly determined and that repairs don’t involve “band-aid” fixes is critical to achieving our goal.

In the article above we talked about establishing a sound maintenance strategy and once this has been determined, the real maintenance work can begin. Let’s talk about the specifics on what, when, how and why we maintain certain critical components on our vacuum furnaces.

General Maintenance

In the article above, we talked about establishing a sound maintenance strategy, implementing that strategy and working on critical components of the vacuum system to return the unit to full operational status. In the final part of this article on vacuum maintenance, we will continue to talk about critical maintenance areas (hot zone, water system, record-keeping, training and provide a preventative maintenance checklist showing the activities and frequency of common maintenance items).

Vacuum furnaces come in all shapes and sizes and have many common features and operational/maintenance needs. However, it is important to understand the particular needs of the vacuum furnaces in your shop to have an effective maintenance program. The following additional topics comprise the critical areas and systems that routinely require maintenance to ensure they operate correctly.

Vacuum maintenance includes and requires awareness of the following:

• Part (and fixture/basket) cleanliness is essential prior to running a load;
• Part placement and location (whether in baskets or on grids) must restrict movement into unwanted areas;
• Care/caution in loading and unloading the equipment to avoid physical damage;
• Quality of the backfill gas and backfill piping – from the supply to the equipment;
• Small parts in loads must be adequately constrained;
• Maintenance must be done with extreme care not to damage adjacent components or systems.

Vacuum Vessel Maintenance

The most common maintenance practice performed on the vacuum vessel is leak detection. For most furnaces, the front door “O” ring or door seal is the area most prone to leaks. Inspect and wipe clean this seal each time the door is opened after a furnace cycle. On “O” ring seals look for cracks, lack of elasticity, flat spots, dirt, and metallic fines. Wipe the door “O” ring and flange with a clean “lint-free” rag before closing the door. Reapply a thin coating of vacuum grease as necessary. Once a week, wipe the door “O” ring and mating flange with a clean rag soaked in denatured alcohol. Reapply just enough vacuum grease to produce a sheen on the “O” ring surface and check that a rubber-gloved finger glides freely along the surface (c.f. The Ubiquitous O-Ring, Industrial Heating, November 2009).

Figure 1 Key

1. “O” Ring seal care – after each run
2. Water system & hoses – visual inspection regularly
3. Cooling & Blower motor – inspect every 6 months
4. Heat exchanger – annual inspection
5. Vacuum gauge calibration – biannual
6. Pumping system – daily/weekly checks
7. Hot Zone inspection – daily checks
8. Hearth inspection – after each run

Figure 2 Key

1. Chamber door seal care – after each run
2. Water system and hoses – visual inspection daily
4. Hot-zone inspection – daily checks
5. Instrument calibration – per AMS 2750D
6. Hearth inspection – after each run

The most common maintenance items include:

• Dirty door seals (Fig. 3a)
• Damaged “O” rings (Fig. 3b)
• Broken thermocouples (Fig. 3c)
• Contaminated Hot Zones (Fig. 3d)
• Dirty Heat Exchangers (Fig. 3e)
• Dirt & Debris Buildup (Fig. 3f)
• Leaky fittings (Fig. 4a)
• Sludge buildup (Fig. 4b)
• Hot zone deposits (Fig. 4c)
• Hot Zone damage (Fig. 4d)
• Short to ground (Fig. 4e)
• Insulation damage (Fig. 4f)
• Dirty Pump Oil (Fig. 5a)
• Damaged/melted element feedthurs (Fig. 5b)
• General corrosion (Fig. 5c)
• Damaged element supports (Fig. 5d)
• Loose power connections (Fig. 5e)
• Collection of Foreign Material inside the furnace (Fig. 5f)

The frequency of maintenance (i.e. the interval between routine repairs) is highly dependent on such factors as:

• Type and number of heat-treating processes performed.
• The skill level of the operators and maintenance personnel.
• Equipment design.
• Quality of prior maintenance and type of spare parts used.
• Quality of the water system, gas system, etc.

When performing maintenance it is important to have a written plan defining the specific task to be performed, and the reason why a particular task is necessary (i.e. purpose of the task). A work order should be issued and the work signed off upon completion (which includes testing to ensure that the repair was successful).

The following conditions should be met before any repairs are undertaken:

• Power should be switched off for any repairs not directly involved with the electrical systems, controls or instrumentation. Lockout/Tagout procedures should be in place and checked by all personnel performing the maintenance.
• Disconnect all utilities including gases, water, and air. Lockout/Tagout procedures should be followed.
• The furnace should be cool, less than 50°C (120°F).
• The furnace door(s) should be in the open position and secured so that they cannot be closed.
• Check that the furnace environment is safe and if working inside the unit that adequate ventilation is in place and functioning properly. The oxygen level must be checked and confirmed to be safe for human exposure before entry into the unit by both the safety team and the personnel performing the maintenance functions.
• Wear protective clothing including safety glasses and safety shoes.
• Be sure that all Confined Entry procedures are thoroughly understood and followed without exception.
• Use the buddy system.

Safety

Safety is MANDATORY and cannot be compromised during any maintenance activity. Safety interlocks must never be bypassed and verification that all potentially hazardous energy sources have been isolated and disabled is a necessary first step in the maintenance process (reference NFPA 86 and NFPA 70).

Lockout/Tagout procedures are required to disable machines or equipment during maintenance to prevent injury as part of OSHA code (Regulation 1910.147). A vacuum furnace may have different places in which electrical power must be disconnected; a single main electrical disconnect (i.e. circuit breaker) for the entire furnace or power supplied from several electrical sources each with a separate disconnect device. The electrical drawings for the specific unit in question should be reviewed as well as physical inspections conducted in the event that undocumented changes have taken place.

In addition, a vacuum furnace may also have pneumatic or hydraulic systems including sources of compressed air, inert gases, and process (reactive) gases that must be isolated from the system.

Confined Entry Space restrictions also apply.

All safety interlocks present in normal equipment operation should be tested on a regular basis to ensure proper operation. These include:

• Over-temperature instrumentation (test monthly)
• Process interlocks (test semi-annually)
• Water interlocks (test semi-annually)
• Air interlocks (test semi-annually)

Hot Zone Maintenance

After ensuring proper ventilation and following all safety guidelines with respect to asphyxiation and confined space entry, the interior of the hot zone should be inspected after every load. The bottom of the hot zone should be cleaned of all debris and foreign matter and the heating elements and heating element connections inspected for damage and tightness.

Graphite heating elements can in some instances be patched and the damaged section replaced with a new element section. Molybdenum heating elements can be repaired although no more than three (3) repairs are recommended per element band.

Special procedures are required since molybdenum is brittle and molybdenum dioxide fumes should not be inhaled. Once a month, heating element resistance to ground should be checked with a volt/ohm meter. A good reading is between 90 – 100 ohms for most furnaces. As molybdenum elements, age or the element standoff metalizes, their resistance to ground drops. A failed reading would be 10 ohms or less.

Low resistance to ground is an indication of metalized ceramic insulators and an indicator for the need of replacement or cleaning otherwise heating elements will arc and fail. In some cases, the ceramic insulators can be removed and baked out in air to remove contaminants.

A hot zone bake-out cycle should be run every 200 hours or when deemed necessary by the performance of the equipment.

Several additional points regarding hot zone maintenance can be summarized as:

• Checking for insulation degradation
• Maintain proper tension on electrical connections (e.g. heating elements, power feedthrus)
• Measure resistance to ground
• Inspection of heating elements for wear and/or oxidation (e.g. thinning or a “sugar cube” appearance – indicating an attack by oxygen)
• Check that thermocouples and controls are functioning properly.

Vacuum Pumping System Maintenance

In most cases, the vacuum pumping system requires the most maintenance of any area on the vacuum furnace.

Vacuum Valve Maintenance

Vacuum valves require the least amount of routine maintenance of any component on a vacuum furnace due in large part to their design. In most cases, no lubrication or adjustments are required. However, maintenance is not only necessary but also critical at certain intervals in a valve’s operating life. For example, Butterfly type roughing and foreline valves should be removed from the vacuum line every two (2) years to inspect their rubber seats for cracking and dryness. When the valve is replaced between flanges in a vacuum line, the disc should be rotated to the open position before the flange bolts are tightened.

Poppet valve pistons and shafts should be lubricated monthly with vacuum grease through the fitting on the cylinder mounting block. Semi-annually the valve disc “O” ring should be cleaned and lightly lubricated. Annually, it should be replaced.

Mechanical Pump

Mechanical (wet) pumps rely on oil for proper operation and the correct oil type for the pump in question should be used. The oil level and the condition of the oil must be checked daily, usually at the beginning of the first shift of the day. The correct oil level for most pumps is at the center of the site glass or just below center when the pump is operating at high vacuum. Oil should be added when the pump is stopped, however, it can be added in some instances during operation if the vacuum level is below 1 Torr. Overfilling will cause a loss of efficiency as well as create the potential for backstreaming of oil into the hot zone.

The oil condition should be checked daily. Good oil is translucent and clean. Cloudy or milky oil indicates the presence of moisture. If this is observed the pump should be ballasted “offline” in accordance with manufacturers instructions. Dark or discolored oil indicates the presence of dirt, carbon (often in the form of soot) or other contaminants. If this is observed, the oil should be changed as needed or after 300 operating hours (whichever comes first). Every six (6) months or when the oil is excessively dirty, the oil reservoir should be drained and the oil reservoir cleaned with denatured alcohol and clean lint-free rags. At the same time, the exhaust valve springs (poppet valves) and all discs should be replaced.

Observe and check the oil temperature under normal operating conditions. Normal operating temperature is between 140°F and 160°F as indicated by the gauge on the side of the vacuum pump.

Monthly, check the drive belts for wear and adjust for proper tension. The pump manufacturer’s instructions should be followed regarding proper belt tension settings. A useful tip is that, with a new belt, at the belt midpoint (between the drive and motor pulleys) apply pressure (typically 5 – 7 pounds) to the belts. Record the resulting deflection and use this value for future adjustments. If the belt tension is too tight, damage can occur to shaft bearings. If the belts are too loose, slippage will occur causing excessive wear.

The gas ballast valve and spring should be checked and replaced, if necessary, every three (3) months.

Booster Pump

The booster pump does not normally require a great deal of maintenance. However, the oil level in all reservoirs should be checked weekly and drive belts checked monthly for wear and adjusted for proper tension, similar to the mechanical pump belts. Every 2000 hours of operation, the oil in the bearing and gear oil reservoirs must be changed.

Diffusion Pump

The diffusion pump should be taken down, jet assembly pulled and thoroughly cleaned as well as the inside of the pump and recharged with oil annually.

The oil level on the diffusion pump should be checked weekly. Oil must only be added to a cold (< 130°F oil temperature) pump and only to the cold mark on the sight glass. Never open the drain plug when the pump is hot (CAUTION: there is a risk of explosion). An oil change is recommended every six (6) months or immediately if discoloration or contamination is observed. One very common mistake found in the industry is to not use the correct diffusion pump oil.

Cooling water to the pump must be checked daily for adequate flow. If proper cooling is not provided to the diffusion pump the oil can fractionate (i.e. breakdown and form solid carbon deposits) damaging the pump, which will need to be factory repaired. The heating elements should be checked for tightness and proper operation at least once a year. A common cause of diffusion pump failure is heater burnout so the diffusion pump heater current should be checked regularly.

Holding Pump

On holding pumps, the oil level and condition of the oil are critical for efficient operation. The correct oil level is at the center of the site glass or just below center with the pump operating at high vacuum. Oil should be added only when the pump is stopped. The oil condition should be checked weekly and, similar to the mechanical pump, the holding pump should be ballasted or replaced if the oil is not translucent and clean.

Water System Maintenance

Most vacuum furnaces are cold wall designs with an annular spacing between an inner and outer shell. As such, proper conditioning of the water is important for effective cooling. Water should be treated for pH, hardness, bacteriological agents and (if appropriate) have rust inhibitors present to help minimize sediment and scale build-up, particularly in the bottom portions of the shell. A blockage of the vessel wall will result in a “hot” spot, so the shell should be periodically checked since most blockages occur slowly over time.

Several additional points regarding water system maintenance can be summarized as:

• Checking water quality;
• Cleaning/maintenance of the heat exchanger(s);
• Establishing corrosion protection;
• Maintaining coolant levels to various subsystems (e.g. pumps power feedthrus, vessel)

Record Keeping

Accurate record keeping is an often overlooked aspect of a successful maintenance program. It is at the heart of any efficient and effective maintenance plan. Record keeping should start on equipment installation and document any and all changes to the equipment over its lifetime. After the unit is put into service it is essential to create and maintain a performance log containing information such as:

• Blank-off pressures (esp. mechanical pumps)
• Pumpdown time to a given pressure.
• Ultimate vacuum and the time required to achieve it.
• Leak-up rate when the chamber is blanked off.
• Heating rate (empty, fully loaded) to the processing temperature.

This type of data is invaluable when evaluating a future problem or when trying to determine if the vacuum system has deteriorated.

Training

The value of training should never be underestimated. Over the years, the majority of vacuum furnace failures can be traced to the following causes:

• Inadequate training of operators;
• Lack or proper maintenance;
• Improper use of the equipment;
• Improper record keeping.

All operating personnel, supervisors, maintenance and quality control individuals should have a good understanding of what heat treating is, how vacuum technology differs from other types of heat treatment and how the equipment should be operated and maintained to ensure safety, efficiency and proper results. It is further recommended that annual retraining is conducted to maintain a high level of proficiency and effectiveness.

Preventative Maintenance Checks

Setting up a planned preventative maintenance program will minimize equipment downtime; ensure that proper spares are on hand for repairs, and simply the overall maintenance effort. As a minimum, the following checks should be performed:

Each Run

The following activities should be performed before each run:

1. Inspect the front door “O” ring for cleanliness and damage. Clean and regrease as necessary.
2. Inspect hot zone insulation and heating elements for signs of damage, deterioration and that are connections are snug and secure.
3. (If appropriate) Inspect the load thermocouple(s) for damage.

Daily

The following activities should be performed daily:

1. Inspect the exterior and interior of the vacuum furnace for indications of damage, discoloration, dripping fluids, and the presence of foreign material (e.g. dirt, grease, oil).
2. Check the water flow and temperature from each drain line.
3. Check the oil level on all pumps.
4. Ballast the vacuum pump (15 – 20 minutes minimum) before processing the first workload of the day.
5. During operation, inspect for hot spots, leaking fluids, excessive noise and/or vibration.

Weekly

The following activities should be performed weekly:

1. Perform a leak (rate-of-rise) test on the main vacuum vessel and pumping system. The furnace should be clean, dry, empty and outgassed before testing.
2. Check mechanical pump oil for contamination (e.g. dirt, particulates, water).
3. Check instruments for functionality.
4. Inspect the pumping system (pumps, valves, piping).
5. Visually inspect control and over-temperature thermocouples for damage.

Monthly

1. The following activities should be performed monthly:
2. Check for hot zone deterioration (insulation and heating elements) including doors.
3. Check the calibration of vacuum instruments.
4. Check all thermocouples (e.g. control, overtemperature, load)
5. Change vacuum pump oil.
6. Check belts for proper tension (e.g. the mechanical pump)
7. Change all filter elements.

Semi-Annually

1. Replace or recalibrate all thermocouples.
2. Flush all cooling lines and clean all in-line filters, strainers, etc.
3. (If applicable) Clean and replace diffusion pump oil.
4. Inspect all vacuum gauges.
5. Replace the door gasket or “O” ring seals.
6. Remove, clean and reinstall thermocouple vacuum gauges.
7. Test the pressure relief valve in accordance with manufacturers instructions.
8. Make all necessary repairs to hot zone components (including power feedthrus)
9. Clean all mating flanges.

Annually

1. Drain and inspect the cooling water system (including temperature sensors)
2. Check all electrical connections. (Caution: a licensed electrician trained in the procedure should only perform this activity).
3. Service all motors.
4. (If applicable) Clean the furnace heat exchanger.
5. (If applicable) Drain and filter quench oil.
6. (If applicable) Check the convection fan and/or oil agitators for proper operation.
7. (If applicable) Check, remove and inspect vacuum valves for proper operation, sealing and wear.

Summing Up…

Maintenance should be performed in such a manner as to return the equipment to full operational service. Never compromise; a job worth doing is worth doing right. This will ensure years of productive service from your vacuum furnace!

References

1. Herring, Daniel H., Vacuum Heat Treatment, BNP Media, 2012.
2. Pritchard, Jeff, “Maintenance Procedures for Vacuum Furnaces”, Vac-Aero International (www.vacaero.com), 2008.
3. “Maintenance of Vacuum Furnaces”, Vac-Aero International, 2018
4. “Technical Support and Maintenance Services”, FurnaCare
5. “The Harold – Blogging for the Heat Treatment Industry”, Ipsen
6. “The Vacuum Furnace Blog – Maintenance”, TAV Vacuum Furnaces