Considerations for selecting vacuum furnace hot zone insulation ...

Over the years, there have been many variations of hot zone insulation designs for vacuum furnace hot zones and can include the following configurations and materials. This article will summarize these designs with performance results in vacuum and power efficiency:

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• a: Graphite foil facing backed by 2”-3” Kaowool insulation.

• b: Graphite foil facing backed by 2” PAN graphite felt.

• c: Graphite foil facing backed by 2” Rayon graphite felt.

• d: Standard Graphite board (2”) with foil facing.

• e: New GMI Graphite board (2”) with foil facing [1].

• f: All Metal (3 molybdenum, 2 stainless steel shields).

• g: All Metal (5 molybdenum shields).

How a vacuum furnace hot zone is insulated and selected is based on several factors including:

• 1: Desired ultimate vacuum level.

• 2: Maximum processing temperature.

• 3: Overall thermal efficiency and power losses.

• 4: Overall cycle time.

• 5: Minimal part surface contamination.

Analyzing the above factors individually, the following can be summarized:

1 Desired ultimate vacuum level

Many processes only require what might be called “soft vacuum” with an operating level in the mm Hg range or 10-3 to 10-4 Torr range. This can normally be achieved with any insulated hot-zone design. Hot-zone materials for (e) would easily meet this requirement.

Other processes require a super-clean environment with vacuum levels in the 10-5 Torr range or higher. These processes could be performed in the (b) through (e) hot zones.

Processes requiring a vacuum of 10-6 Torr or better normally require an all-metal hot zone design (f) and (g) to achieve the required vacuum levels and produce super-clean work.

2 Maximum processing temperature

Table 1 illustrates the relative maximum operating temperatures that are typically available with each hot-zone configuration.

Although some of the hot zones are often heated above the normal operating temperatures, these are the recommended normal processing temperatures.

3 Overall thermal efficiency and power losses

The efficiency of a particular hot zone is determined by the power losses being radiated from the outer support ring to the inner cold wall of the vacuum chamber. The outer support ring temperature varies based on the insulating materials. The ring temperature is most critical in that it is raised to the fourth power when calculating power losses. The relative outer ring holding temperatures for the different configurations are shown in Table 2.

Figure 1 illustrates several things. These include:

• a: Rayon graphite felt is more efficient than PAN graphite felt.

• b: The GMI board material [1] is more efficient than any other insulations or shield combinations. This board material also grows more efficient as the temperature rises.

• c: The all-metal hot zones are most inefficient.

Table 3 illustrates the projected relative power losses and wall density for the HFL- Furnace when holding at 2,000°F. The furnace has a working hot zone that measures 36” W x 36” H x 48” D.

With Table 3, we can conclude the following:

• a: The Rayon Felt hot zone is approximately 12.8% more efficient than the PAN hot zone.

• b: The GMI Board [1] hot zone is approximately 61.8% more efficient than the Rayon hot zone and approximately 66.7% more efficient than the PAN hot zone.

• c: The all-metal hot zone (f) is the least power efficient of all the various insulation configurations.

4 Overall Cycle Time

The overall cycle time for a furnace is based on several factors. These include:

• a: Vacuum pumping capacity.

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• b: Insulation moisture absorption characteristic of the insulation, most critical when the furnace door is open too long, especially on humid days.

• c: Efficiency of insulation to overcome recurring losses when heating the workload.

• d: Workload mass and thickness of parts being processed.

• e: Gas cooling system recirculating volume capability and cooling gas used based on process requirements.

• f: Type of insulation used in the hot zone.

5 Minimal part surface contamination

When working with felt-insulated vacuum furnaces, moisture, water, and pick-up can become key sources of oxygen contamination. For comparative testing, samples of PAN and Rayon felt stacks, top insulation for the hot zone only, were prepared for testing in a lab furnace. The furnace cycle for each stack ran with a clean titanium test piece in the furnace. This was compared to an all-metal shield test package. After each run, alpha-case analysis via metallurgical evaluation determined the level of contamination from moisture pickup for both PAN and Rayon insulation compared to the clean all-metal hot zone.

The vacuum levels for each insulation package shown in the following chart, when compared to the standard all-metal hot zone, show less effective vacuum levels, indicating the presence of more residual gas. It should be noted that the humidity and dew point was considerably higher on the date of Rayon testing than the PAN test date. With this fact in evidence, the results in Figure 2 indicate the PAN insulation is much more sensitive to moisture pick-up than Rayon-based felt.

The resultant alpha case measurements also support this conclusion considering the titanium sample used in the Rayon insulation testing produced a similar alpha case result to the all-metal hot zone, even though the Rayon felt stack suffered more severe humidity that the PAN test samples. (Comments based on a limited test.)

The results in Table 4 illustrate that the all-metal hot zone is superior and required for most super-clean applications.

6 Conclusions

As is illustrated in this presentation, there are considerable factors that must be evaluated when selecting the proper hot zone design for the application and intended use.

One remaining consideration, which was not included above, is the relative cost for each different configuration. Listing each hot zone from least expensive to most expensive, we have Table 5.

The comments in Table 5 should help future buyers in selecting the right vacuum furnace hot zone design for their specific application. 

References

1. www.graphitemachininginc.com

Obtaining precise process and metallurgical results is essential to any heat-treat application. Often, outcomes rely on the performance of a vacuum furnace. When a furnace fails, it leads to unexpected downtime or unplanned expenses. To prevent this costly setback, thermal processing departments should regularly evaluate the condition of their vacuum furnace equipment.

Whether you’re in the aerospace, automotive, energy, medical, tool & die, or any other industry, the goal is to keep your furnace running—and performing reliably—for as long as possible. To that end, Ipsen Customer Service offers free evaluations of any brand of vacuum heat-treating system.

Anyone charged with supervising, operating, or maintaining a vacuum furnace, who is concerned with uptime, should perform the following set of five checks, which closely follow Ipsen’s in-depth evaluation.

1. Hot Zone

Start with a visual inspection looking for broken or missing hardware, such as gas nozzles, element hangers, insulation retaining hardware, and the hearth assembly.

Next, examine the insulation and hot face integrity by checking these three areas:

• On a graphite hot zone, look for any signs the insulation is degrading and causing graphite dust.
• Check for areas where bare steel can be seen through the insulation.
• Where applicable, gently press on the carbon fiber composite (CFC) hot face to see if it is wearing thin.

Generally, these signs are indicative of a past or present air leak. Pitting or rough surfaces on graphite heating elements can also be a sign of potential air leaks. Next, visually inspect the ceramic insulators in the element hangers. If these appear cracked, broken or contaminated, check the resistance to ground for each bank of elements to make sure they are not shorting out.

2. Thermocouples

Once you determine the hot zone components are in functioning condition, it is vital to ensure the thermocouple (TC) readings are accurate, as this is critical to obtaining quality parts.

To check thermocouple accuracy, use a calibrated fluke multimeter to check that the work TC jack panel and control/overtemp TCs are reading correctly, then note the last time the control TC was replaced. It is important to keep good records. A resident system accuracy test (SAT) thermocouple can also give you peace of mind that the control and overtemp TCs are in good working condition and have not drifted. It can also reduce the frequency of costly calibrations or replacement of thermocouples.

Thermocouples used for SAT’s are required by the AMS specification to be extremely accurate (generally +/- 2 degrees F or 0.4% deviation for class 1 or class 2 furnaces). By using an SAT thermocouple, either during a single system accuracy test or as a resident thermocouple, you can get a reference temperature reading to give you a high degree of confidence your control and overtemp TCs are reading correctly.

Those who need to abide by AMSE need to re-calibrate (or replace) their control and overtemp TCs every 3 to 6 months depending on the type of thermocouple they are using. Those who maintain a resident SAT may be permitted to use their control and overtemp TCs longer.

For more information on thermocouples and SAT requirements for Nadcap accreditation, view these helpful articles on our website:

• Aerospace Aluminum Brazing and Compliance: Meeting AMS E and Nadcap Certifications
• Obtaining Nadcap Accreditation: Helping You Pass Your Audit 3.

3. Water Jacket

A vacuum furnace almost always require some type of water supply. Some have flow indicators on the water inlets that can give you a snapshot of whether the water-cooled assemblies are getting good flow. It is important to get a feeling for the integrity of the water jacket as proper water pressure and flow is critical for proper furnace operation.
To check, take a walk around the outside of the furnace, looking for stains or peeling paint on the vessel, which can indicate slow water leaks. Many times these will be at the weld joints and are a sign the vessel may be corroding.

Patches welded onto the outside of the chamber may indicate a past hard water buildup issue. These patches are created after cutting into the chamber to scrape out the buildup. This cut and scrape is done as an attempt to improve water flow through older plugged up jackets. If you see leaks or patches, consider additional investigating or testing to determine the integrity of the water jacket.

Next, check if the vessel is too hot. Most vacuum furnaces are meant to operate with an external wall temperature of 140 °F or below. Over time, a plugged vessel will cause the wall temperature to rise, causing unsafe conditions. This may be an indication that the sediment between the inner and outer wall is blocking flow of the cooling water. A catastrophic failure can occur if the water movement slows to a boiling point.

4. Pumping System

To evaluate the condition of the pumping system, start by determining if there is a preventative maintenance (PM) schedule and whether it has been followed regularly. Next, visually check the sight glass on the booster and roughing pumps to make sure oil levels are normal and the oil does not look “milky,” which can be a sign of excess moisture in the system. For the diffusion pump, determine the last time it was disassembled and cleaned, and if it was ever rebuilt. Then, make sure the diffusion pump oil cup for the cylinder is getting proper lubrication and check the oil level. Keeping up on preventative maintenance can drastically affect how often the pumps are sent out for major rebuild at the pump OEM or a third party.

5. Furnace Electronics

We now arrive at the last stop on our furnace inspection—the electronics, which are often sadly overlooked. Reaching and maintaining proper control and functionality of the equipment is paramount to achieving success in thermal processing.

Furnace controls are frequently subject to the adage, “If it isn’t broke, don’t fix it.” However, this can be a dangerous approach. Like smartphones, furnace electronics such as PLCs, recorders and OITs or controllers, can become obsolete quickly. Once this occurs, replacement parts become scarce and good service techs who know how to work on the antiquated electronics can be hard to find.

Meanwhile, the process of upgrading to new controls can have lengthy lead times, often stretching several months—bringing your operation to a standstill. That’s why proactive planning on this front is crucial, yet often overlooked.

Best Practices for a Smooth Furnace Operation

This quick exercise of reviewing your vacuum furnaces per these five recommendations can be an eye-opening experience. You may be relieved to find that all systems are in good working order and that regular maintenance has been followed. Or, you may discover hidden issues, which if not addressed promptly, could lead to degraded performance and costly downtime. For a more in-depth inspection, Ipsen’s free comprehensive furnace evaluation will give you an even better understanding of every aspect of your current equipment’s condition. The free evaluation report will allow you to plan maintenance activities and schedule out capital expenditures well in advance. Ultimately, this free service will ensure many more years of furnace operation with a high rate of uptime, planned scheduling for downtime, and greater accuracy, reliability and efficiency. Take advantage of Ipsen’s free in-depth evaluation of any brand of vacuum furnace. At Ipsen, we are committed to being your partner in success. During this free, comprehensive onsite inspection, an Ipsen Customer Service team member will check all major components of the furnace. The process typically takes under an hour, and Ipsen will provide a written health report with a suggested 18-month maintenance plan.

Let Ipsen Help You:

• Identify and correct problems before they cause costly program delays.
• Set up a regular preventive maintenance plan to avoid emergency equipment breakdowns.
• Improve the reliability, efficiency and overall capability of your furnace.

Ipsen has serviced the industry for over 70 years, so rest assured that we will be here to keep your furnace running for decades to come.

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