We all have haunting memories of the one that got away. Truth be told, the Doctor is still in agony over not attending an excellent ASM Vacuum Maintenance Seminar held in Anaheim, Calif. a few years ago. Proper maintenance of all types of furnaces is important, but maintenance on vacuum furnaces is especially critical to their operation. Let’s learn more.

Common Problems and Solutions

Vacuum processing is designed to keep parts “bright and clean.” As such, discolored parts (Fig. 1) or contaminated work (Fig. 2)
is just not acceptable. Knowing where to look, what to maintain and how often to do it become key concerns. Discoloration can often be traced to air (oxygen), dirt (debris, oil, cleaning agents) or water vapor.

    Sources of air infiltration include dirty or damaged door seals, leaky or cracked O-rings and broken thermocouples. Wiping clean and re-greasing door O-rings prior to every run, cleaning out loose debris from the furnace interior, inspecting thermocouples and power feed-throughs, and performing weekly leak-up tests are simple, yet effective, routine preventive measures to detect and avert problems with air entering the furnace during processing.

    Sources of “dirt” include residues left on parts (oils, cleaning agents, metal fines), debris left in hot zones and contaminants deposited on cold vessel walls that can be carried back into the chamber over time by gas circulation and other means (Table 1). Being sure that parts are clean and dry, wiping down cold walls and the inside of the outer door, and routine cleaning of heat exchangers are simple and effective preventive measures.

    Sources of water vapor include moisture in the air, condensate (“sweating”) of water jackets and leaks in water-cooled components (e.g., element feed-throughs, heat exchangers, vacuum vessels). For example, a good rule of thumb is that raising the air temperature 10˚C (18˚F) doubles its moisture capacity. This means that air at 30˚C (86˚F) can hold eight times more water than air at 0˚C (32˚F). Closing the outer door between loads, maintaining the chamber water temperature higher than that of the ambient air, and using multiple pump-down and backfill cycles will help mitigate this issue.

    Water is generally considered the most problematic of the sources of contamination since water vapor will condense on initial pump down (due to the cooling effect associated with a sudden drop in pressure), creating a “fog” cloud and subsequently ice crystals. During initial heating, the ice will vaporize, and water vapor will be present in the area of the parts.

    The dew point of any gas introduced into the chamber during a furnace run must be on the level of -54˚C (-65˚F) or lower to ensure a low parts-per-million (ppm) of potential contaminant in the atmosphere. This is about 24 ppm of water vapor. In reality, gas should consistently run at or below -73˚C (-100˚F), which is about 1.5 ppm.

When to Perform Maintenance

The frequency of maintenance (i.e. interval between routine repairs) is highly dependent on such factors as the type and number of heat-treating processes performed; the skill of the operators; the equipment type and design; the quality of prior maintenance and type of spare parts used; and the quality of the water system and gas system from the supply to the equipment. Additionally, running clean parts and properly supporting parts in baskets or on grids often help reduce the frequency of maintenance.

    When performing maintenance it is important to understand why a particular task is necessary. Furthermore, the work should be signed off upon completion (which includes testing to ensure that the repair was successful).

How to Perform Maintenance

Each type of furnace is unique, but the following generic conditions should always be met before any repairs are undertaken:

1. 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.

2. The furnace should be cool, less than 50˚C (120˚F).

3. The furnace door(s) should be in the open position and secured so that they cannot be closed.

4. Disconnect all utilities, including gases, water and air. Lockout/tagout procedures should be followed.

5. Check that the furnace environment is safe and that adequate ventilation is in place and functioning properly.

6. Wear protective clothing, including safety glasses and safety shoes.

7. Be sure that all confined-entry procedures are thoroughly understood and followed without exception.

8. Use the buddy system.

9. Before entering the vacuum furnace, confirm that the oxygen level is safe for human exposure.

Preventive-Maintenance Checks

Setting up a planned preventive-maintenance program will minimize equipment downtime, ensure that proper spares are on hand for repairs and simplify the overall maintenance effort. As a minimum, the following checks should be performed at the specified interval.

Each Run

1. Inspect the front door O-ring for cleanliness and damage. Clean and re-grease as necessary.

2. Inspect hot-zone insulation and heating elements for signs of damage, deterioration and that connections are snug and secure.

3. Inspect the load thermocouple(s) for damage if appropriate.

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

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. Check for hot-zone deterioration (insulation and heating elements), including doors.

2. Check calibration of vacuum instruments.

3. Check all thermocouples (e.g., control, over-temperature, load).

4. Change vacuum pump oil.

5. Check belts for proper tension (e.g., mechanical pump).

6. Change all filter elements.

Semiannually

1. Replace or recalibrate all thermocouples.

2. Flush all cooling lines and clean all in-line filters, strainers, etc.

3. Clean and replace diffusion-pump oil if applicable.

4. Inspect all vacuum gauges.

5. Replace door gasket or O-ring seals.

6. Remove, clean and reinstall thermocouple vacuum gauges.

7. Test the pressure-relief valve per manufacturer’s instructions.

8. Make all necessary repairs to hot-zone components (including power feed-throughs).

9. Clean all mating flanges.

Annually

1. Drain and inspect the cooling water system (including temperature sensors).

2. Check all electrical connections. (Caution: Only a trained, licensed electrician should perform this activity).

3. Service all motors.

4. Clean the furnace heat exchanger if applicable.

5. Drain and filter quench oil if applicable.

6. Check the convection fan and/or oil agitators for proper operation if applicable.

7. Check, remove and inspect vacuum valves for proper operation, sealing and wear if applicable.

Final Thoughts

Vacuum maintenance should also 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! IH

References

1. Moyer, Michael, Keeping it Bright, ASM Vacuum Maintenance Seminar, October 2007, Anaheim, Ca.

2. Herring, Daniel H., Vacuum Heat Treatment, BNP Media, 2012.

3. Wall Colmonoy Corporation Data Sheet No. 7.3, 1969.