Helium gas is more expensive than other vacuum-furnace cooling gases, but its benefits can offset its higher cost.

Large (24 ft deep) 50,000 lb capacity VFS vacuum furnace

Helium gas has been evaluated for use as a quench gas in vacuum heat treating in several studies over the years. For example, in the 1970s, Abar Corp. studied the effects of different cooling gases including helium at a specific pressure and flow rate on the cooling rate in a vacuum furnace in work that also studied the effects of varying nitrogen-gas quenching parameters (i.e., gas flow rate and gas pressure) on cooling rate. Results showed that the cooling rate for helium, as well as those for nitrogen and hydrogen gases are better than that of argon, and that the cooling rate of nitrogen increases with increasing gas flow rate and increasing gas pressure. Studies by Air Products in the early 1990s using its patented rapid gas quenching arrangement showed improvement in cooling rate using gas mixtures of helium and argon. Other studies showed that cooling rate improves incrementally with increasing gas-cooling pressure. Recent studies by Solar Atmospheres shows helium gas quenching provides significant improvements in performance with respect to cooling rate and energy costs, which can offset the higher cost of the gas.

Fig 1 Cooling rate comparison using patented rapid gas quenching arrangement. Source: Air Products patent (U.S. 5,173,124).

Results of previous studies

In 1972, Abar Corp. (Feasterville, Pa.) studied cooling rates using different gases and conditions in a Model HR36 vacuum furnace having a work zone 24 in. wide x 24 in. high x 36 in. deep. The workload consisted of 4 in. diameter x 36 in. long schedule 40 carbon-steel pipe weighing 570 lb. Work thermocouples were firmly bolted to the pipe load at various locations, and cooling rate results were measured. The cooling rate (quenching from a temperature of 2000 to 200F using nitrogen at a gas pressure of 5 psig) increased with increasing gas flow rate (which increases the hot-zone gas velocity) from 400 to 1600 cfm. However, the level in cooling rate improvement diminished with increasing flow rate. Similarly, at a gas flow rate of 1200 cfm, increasing the nitrogen quench-gas pressure from below atmospheric (20 in. Hg) to a positive pressure (15 psig) increased the cooling rate, also at a diminishing level of improvement with increasing gas pressure.

Comparing the cooling rates of different gases at a gas pressure of 5 psig and a flow rate of 1200 cfm, the study showed hydrogen, nitrogen and helium outperforming argon, with hydrogen having the highest cooling rate and nitrogen and helium having similar cooling rates. On a relative basis, the unit time to cool the 570 lb workload from a temperature of 2000 to 300F, using a basis of 1.0 for nitrogen, was 0.7 for hydrogen, 0.96 for helium and 1.36 for argon.

Fig 2 Cooling time versus gas pressure. Source Ref. 1.

In 1992, Air Products and Chemicals Inc.'s rapid gas quenching arrangement (U.S. Patent No. 5,173,124) showed an improvement in cooling rate using gas mixtures of helium and argon compared with cooling rates of nitrogen and helium (Fig. 1). Solar conducted tests using the blended gas at an operating pressure of 2 bar and did not note any improvement in cooling rate. However, it was anticipated that the blended gas would show improvement in cooling rate over pure helium at higher pressures to 10 bar. This conclusion was based on the results of studies by W.W. Hoke[1, 2] shown in Fig. 2 and by G.C. Carter[3], which showed that higher gas cooling pressure yields an improved cooling rate incrementally to 10 bar and higher, but with a diminished level of improvement.

Fig 3 Furnace and test load used to conduct cooling-rate studies at a 2-bar pressure

Helium gas cooling performance

Solar conducted a study in 2001 in a Vacuum Furnace Systems (VFS) Model HIQ-3836-7 furnace (Fig. 3) similar to the furnace used in the Abar work mentioned previously, but having a more advanced gas cooling design using a 100-hp gas blower, an improved low pressure-drop internal furnace, fin and tube heat exchanger[4]. The 571-lb workload consisted of 18-3 in. diameter _ 12 in. long steel bars having thermocouples in drilled holes at the front left, center, and right rear. This workload was considerably more massive that that used in the Abar study, with considerably less surface area, and was, therefore, far more difficult to cool.

Fig 4 Comparison of results for nitrogen and helium-argon blend cooling gases at 2-bar pressure in furnace shown in Fig. 3. Plotted results are average of three thermocouples in drilled holes at front left, center, and right rear workload locations.

Cooling rates shown in Fig. 4 are considerably improved compared with those obtained using older furnace technology due to improved gas cooling velocity within the furnace hot zone. The performance of the helium gas quench is better than that of nitrogen gas.

Solar also evaluated in 2001 the performance of helium gas cooling in its large 12-ft deep and 24-ft deep (see photo on opening page) VFS vacuum furnaces designed for cooling heavy titanium sheet coils or rolls. Cooling rates were compared using 100% argon and 100% helium gases at 1-bar and 2-bar pressure to cool different size workloads. Temperature measurements from three work thermocouples firmly embedded in the innermost core of the rolls were averaged and plotted.

Fig 5 Comparison of cooling rates for 12,000-lb titanium-coil workload

Helium gas reduces the cooling time for a 12,000-lb workload by nearly a factor of 3 with a reduction in horsepower, or electrical energy, by a factor of 5 (Fig. 5). Cooling an even heavier workload of 50,000 lb further demonstrates the benefit of using helium gas with an advantage of operating the gas blowers at overspeed (5000 rpm compared with a standard 3450 rpm motor) via a variable frequency drive arrangement (Fig. 6).

Fig 6 Comparison of cooling rates for 50,000-lb titanium-coil workload

Although the cost of helium gas is approximately 4 times the cost of argon, the reduction in electric power cost alone per cycle pays for the cost of helium gas. The improvement in production time is a greater cost saving freeing the furnaces for higher revenue generation.

Using helium cooling gas, high-speed gas blower fans and operating at an over-pressure to 10 bar opens the way for more standard size production vacuum furnaces to provide cooling rates necessary to process alloy steels such as AISI types 4140 and 4340, as well as O1 and O2 oil-hardening tool steel grades in production lots, avoiding the use of oil quench and protective atmosphere furnaces.