Most vacuum furnaces currently active in the heat-treating world incorporate some form of, or combination of, graphite-felt insulation, with either a foil or board internal facing in the furnace hot-zone construction. The graphite felt used in high-temperature furnaces is either PAN-based or Rayon-based. PAN (polyacrylonitrile) graphite felt is most often used since it is approximately 20% less expensive than Rayon-based insulation.
Solar Manufacturing recently conducted testing of each material for energy conservation and cycle performance in order to determine the benefits of each felt when used in vacuum furnaces. This article presents our findings.
Testing was performed to evaluate the relative thermal efficiency of the two graphite-felt materials using one of our laboratory furnaces. This vertical furnace features all-graphite insulation and a workable hot zone measuring 12 inches deep x 18 inches high. Modifications were made to the furnace, including an upper extension, which provides sufficient space to vary the thickness of the upper hot-zone cover. This also allowed sufficient room for thermocouple placement.
Two upper-chamber covers were created, one with four 0.5-inch layers of PAN felt and one with four 0.5-inch layers of Rayon felt. A thermocouple was attached to both stainless steel chamber covers, simulating the radiating temperature on the outer support ring of a standard furnace (Fig. 2, point A).
Table 1 shows the relative temperatures recorded on each of the graphite covers when holding at elevated temperatures. The table illustrates the superior thermal efficiency of Rayon over PAN by approximately 6%. Our results would represent point A in Figure 2 radiating to the chamber-wall point B.
Although this highlights an improvement in temperature of the radiating surface in a small test arrangement, the significance of decreasing the temperature of the radiating surface A becomes apparent when using a typical production vacuum furnace with a hot zone size of 36 inches wide x 36 inches high x 48 inches deep and applying data from the Stefan-Boltzmann Law for Radiation (Equation 1).
The key is that the radiating surface temperature is raised to the fourth power. One can calculate the expected overall power loss at each hold temperature (Table 2) to compare power losses for each insulation package. Based on the test data, the Rayon insulation package is thermally more power-efficient than PAN by approximately 15%.
P = eA(T4 – Tc4) (1)
- P = Net radiated power
- e = Emissivity of the radiating surface (used 0.55 for SS)
- = Stefan constant – 5.6703x10-8 watt/m2K4
- A = The radiating surface area
- T = Radiating surface temperature (degrees Kelvin) of the support ring
- Tc = Surrounding surface temperature (degrees Kelvin) chamber wall
According to this data, Rayon felt also performs much better than PAN-based felt when running cycles with long holds at elevated temperatures.
Thermal efficiency is important. In vacuum processing, however, one must also consider the overall effect an insulation design has on the cycle time and residual contaminants that may affect the part purity. If a design requires an extensive pump-down that extends overall cycle times, this can be detrimental to the cost of running the process.
More importantly, if the insulation design releases unwanted residual gases during the heat-treat process, it may contaminate certain metals, resulting in unwanted surface contaminants. A review of the properties of PAN graphite felt to Rayon graphite felt suggests each insulation package might have a different impact on the pumping performance, owing to differences in density and ash content (Table 3).
For performance testing, we manufactured 10 13.5-inch-diameter pieces of both PAN and Rayon felt materials. We also replaced the standard lid of another laboratory furnace similar to the furnace shown in Figure 1 with a 10-layer configuration of each felt type to simulate the effective surface area of felt on a larger production furnace. Before the test, all felt samples were subjected to a bake-out process in deep vacuum up to a temperature of 2400˚F and held for one hour. The furnace-cooled samples were placed in separate sealed bags to minimize any effects due to humidity.
The tests were conducted in a furnace with an all-metal hot zone using the same extension piece, as shown in Figure 1. Utilizing an all-metal hot zone for these tests ensured that any contamination and pump-down issues were the result of the felt test cover pieces only and not from the use of felt in a standard all-graphite laboratory furnace. As mentioned previously, each cover stack of 10 pieces of PAN or Rayon graphite felt was supported on a molybdenum plate. The following vacuum heat-treat cycle was run for each test.
- Record ambient temperature, humidity and dew point before starting the cycle.
- Pump furnace to achieve 5 x 10-5 Torr.
- Ramp at 20˚F per hour to reach 2250˚F.
- Hold at 2250˚F for 30 minutes.
- Nitrogen static quench at -5 inches HG to 130˚F.
- Remove insulation lid and set in shop air for 60 minutes.
- Record ambient temperature, humidity and dew point.
- Place graphite cover back into the furnace.
- Pump down to 1 x 10-5 Torr and record time of pump-down.
- Backfill and unload furnace.
A complete record of the vacuum level reached and the overall time to achieve the vacuum recorded is shown in Figure 3. Although it is not apparent that the curves on the graph show a significant difference in pump-down time when calculating the pumping rate, one finds an approximate 20% improvement in vacuum performance for Rayon compared to PAN.
PAN/Rayon Revised Testing
When working with felt-insulated vacuum furnaces, moisture pickup can become a key source of oxygen contamination. For testing purposes, each felt stack was left at shop atmosphere for 10-12 hours before being placed in the furnace. As outlined below, the furnace cycle for each stack ran with a clean titanium test piece included in the furnace. 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 a clean all-metal hot zone.
The process cycle for these tests was as follows:
- Record temperature, humidity and dew point before loading furnace.
- Add furnace extension.
- Place titanium test piece in the furnace.
- Place the modified lid with 10 layers of PAN (or Rayon).
- Record which felt is in the run.
- Close furnace and initiate cycle.
- Pump to 5 x 10-5 Torr before initiating heating.
- Heat at 30˚F/minute to 1750˚F and then hold for one hour.
- Record vacuum/temperature performance throughout the cycle.
- Backfill with nitrogen and cool the furnace to opening temperature.
- Remove graphite cover and the titanium test piece, identifying it as the PAN or Rayon test.
When compared to the standard all-metal hot zone, the vacuum levels for each insulation package (Fig. 4) show less-effective vacuum levels, indicating the presence of more residual gas. We should note the humidity and dew point were considerably higher on the date of Rayon testing than the PAN test date. With this fact in evidence, the result in Figure 4 indicates the PAN insulation is much more sensitive to moisture pickup than Rayon-based felt.
The resultant alpha-case measurements (Table 4) 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 than the PAN test samples.
Advantages/Disadvantages of Working with These Felt Materials
Three things were immediately evident when working with the PAN graphite felt.
- The material frayed when cut, leaving ragged, uneven edges.
- The material was very dirty to handle.
- The material was not uniform in thickness, making it difficult to stack.
The Rayon graphite felt was the complete opposite.
- It was easy to cut to size and did not fray on the edges.
- It was much cleaner to handle.
- It was much smoother and did lay flat and stacked nicely.
Based on this, Rayon is the preferred felt from a manufacturing standpoint.
Solar Manufacturing has determined that Rayon felt will be the selected insulation on its future furnace designs based on the following conclusions.
- Rayon graphite felt, when properly used, can be as much as 20-30% more energy-efficient when compared to PAN graphite felt. This could be a definite advantage for those furnaces with process cycles that require long holds at elevated temperatures.
- PAN graphite felt has more ash content than Rayon felt, which could impact product surface results.
- The Rayon graphite felt demonstrated lower outgassing characteristics than the PAN felt, resulting in faster and better vacuum levels.
- The PAN graphite felt had a more drastic effect on a titanium sample relating to undesired alpha-case formation than that processed with the Rayon graphite felt.
- Rayon graphite felt is priced approximately 20% higher than PAN graphite felt, so economics are a consideration in a vacuum furnace hot-zone design.
- Solar Manufacturing has standardized on the Rayon felt for lower power usage and improved vacuum performance.
- Fradette, Real, “Methods of Improving Vacuum Furnace Insulation Efficiencies,” Industrial Heating, September 2013, https://www.industrialheating.com/articles/91268-methods-of-improving-vacuum-furnace-insulation-efficiencies
- Fradette, Real, Understanding Power Losses in a Vacuum furnace, Booklet Number 5, Solar manufacturing Inc., Solaratm.com, 2014
- Jones, William R., and Fradette, Real, A New, Innovative Approach to Designing a Vacuum Furnace Hot Zone, presented at FNA.2017