By Dan Herring

Low temperature heat treatments (Fig. 1) that involve a vacuum purge at the onset of the cycle have become increasingly popular throughout the industry. These operations are conducted in vacuum furnaces and furnaces that employ a vacuum purge prior to the beginning of the heat process with parts placed inside a special vacuum tight vessel or in a retort. Processing being run using these methods take advantage of a highly controlled environment designed to minimize surface interactions.

Application Areas

Applications for this technology (Fig. 2) vary widely but generally fall into the following categories:

• Aging;
• Annealing;
• Oxidizing/Bluing;
• Stress relief;
• Tempering.

Figure 1 – Annealing of Nickel Based Alloy Jet Fighter Afterburner Assemblies – (Photograph Courtesy of Solomon Engineering, Inc.)	Figure 2 – Annealing of Copper Heat Sinks (Photograph Courtesy of Advanced Thermal Processing)

Typical materials include:

• Alloy and high carbon steels (including maraging grades);
• Beryllium copper and beryllium nickel;
• Brass;
• Copper;
• Inconel;
• Specialty alloys (Elgiloy®, NiSpan C, Nitralloy);
• Stainless steels, including precipitation hardening grades;
• Titanium alloys;
• Tool Steels.

Low temperature vacuum heat treatment is used by both captive and commercial heat treaters and spans such diverse markets as Aerospace, Automotive, Electronics, Optics, Housewares, Industrial Products, Tool & Die, Military/Defense and Farm Implement to name a few.

Table 1 – Field Data on Low Temperature Vacuum Furnace Uniformity [1] / Notes: a. Class 1 per ASM 2750 (Pyrometry)

Parts processed vary widely. Typical examples include:

• Bearing, gears, and springs
• Bolts, nuts, and screws
• Relays, switches and shielding components manufactured from stampings
• Tooling, tool holders, dies and taps

Most processes run in the temperature range of 175°C – 730°C (350°F – 1350°F). Special applications extend these ranges down to as low as 120°C (250°F) and up to as high as 925°C (1700°F), but this is unusual. Temperature uniformity (Table 1) in dedicated furnaces is considered excellent throughout the standard temperature ranges listed.

It is also worth noting that clean and/or bright work is most often associated with this type of processing. Since “clean” and “bright” are very subjective terms and difficult to define in a universal way, we tend instead to say that the part surface is not metallurgically damaged and if a change occurs, it is generally a positive one. In all cases, the surface condition of the parts being processed is said to be improved.

Vacuum Designs

Vacuum furnaces for low temperature processing can be batch (Fig. 3) or continuous, stand-alone, integrated into continuous vacuum furnace systems or a separate “module” incorporated into a cellular system.

The basic operation for a batch vacuum furnace is as follows. Mechanical vacuum pumps, optionally equipped with blowers, produce vacuum levels down to 1.3 x 10^-3 mbar (0.001 torr) with 6.7 x 10^-3 mbar (0.005 torr) common. This is normally achieved within 10 – 30 minutes of the start of cycle, depending on the size of the pumping systems and the nature of any contaminates present on the workload. The unit is then backfilled in the range of 66.7 x 101 mbar (500 torr) negative pressure to 0.10 bar (1.5 psig) positive pressure with an inert gas such as nitrogen, argon, or a mixture of nitrogen/hydrogen (3% hydrogen maximum) and heating begins. Double pumpdown cycles are often found to be advantageous to speed the overall cycle time. After reaching setpoint and soaking at temperature, the cooling cycle is initiated.

Figure 3 – Internal View of a Typical Vacuum Purged Tempering Furnace (Photograph Courtesy of CI Hayes)	Figure 4 – Oil Contaminated Vacuum Purged Tempering Furnace (Photograph Courtesy of CI Hayes)

Often parts are contaminated with oils from prior operations such as stamping, or they have moisture or alkaline cleaning compound residues present on them. By the very nature of vacuum processing, many of these surface contaminates are vaporized off and deposit on the furnace interior (Fig. 4) or redeposit on the work during cooling. The Heat Treater must be aware these foreign contaminates will pass through the vacuum system, often degrading its performance negating the benefits of vacuum purging unless proper maintenance is performed. While most low temperature vacuum processing units can tolerate a great deal of contamination, cleaning (and drying) the work thoroughly before processing is always recommended.

The materials of construction in the heating chamber are such that the furnace can be opened and unloaded at any required temperature. In most cases, however, surface condition is important and the workload must be cooled to at least 150°C (300°F) and more commonly to below 65°C (150°F) as measured by a thermocouple positioned in the workload itself. These units can be either gas fired or electrically heated. Fiber insulation is typical, often in the form of a “hard pack” or rigidized so as to withstand the high velocities produced by the convection fan. These design features translate into rapid heat-up and cooldown rates (Fig Nos. 5 – 6).

Figure 5 [1] Typical Heating Rate Performance Data	Figure 6 [1] Typical Cooling Rate Performance Data

Performance Capabilities / Requirements

Since low temperature processing in vacuum or vacuum purged vessels offers so many advantages, the question naturally arises, “What features are most desirable?” (Table 2). The answer often lies in knowing what the application requires and the performance benefits to be gained.

In Conclusion

Table 2 – Feature Comparison Chart  (10 = critical; 5 = important, often necessary; 1 = least desired)

Low temperature vacuum processing of workloads is becoming increasing more common driven by a need for improved surface quality, better process repeatability, control of process and equipment variability and the ability to predict quality results. Designs capable of meeting these needs are available and perform well.

Some of the key considerations for choosing low temperature vacuum processing can be summarized as:

1. Mandatory for parts that must be processed without surface damage (e.g. oxidation). Parts at all stages of the manufacturing process, not just finished surfaces benefit from this type of treatment.
2. Uniformity and speed of heating and cooling with minimum energy consumption.
3. High productivity requirements are best achieved by vacuum processing. The ability to heat slightly more rapidly and, especially, to cool much more rapidly in a positive pressure reduces cycle time. Gas fired equipment is especially beneficial in this regard.
4. Minimum atmosphere consumption. Once partial pressure or backfill gas is introduced, only small amounts of make-up gas are needed. Even in cases where repeated gas flushing is required, far less atmosphere is needed.
5. Process control is absolute including the ability to upload recipes and download process and equipment variables in real time. Planned preventative maintenance practices and a complete history are simple and straightforward.

References

Herring, D. H., Low Temperature Vacuum Heat Treating, ASM Heat Treating Conference, 1997.

Daniel H. Herring / Tel: (630) 834-3017) /E-mail: dherring@heat-treat-doctor.com

Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.