|Fig. 1. Annealing of nickel-based alloy jet-fighter afterburner assemblies|
Low-temperature vacuum heat treatment is one of The Doctor’s favorites, offering unique advantages over other types of low-temperature processing since component parts (Fig. 1) are placed in a controlled environment designed to minimize surface interactions. Let’s learn more.
Applications for this technology vary widely but generally fall into the following categories:
- Stress relief
Typical materials include:
- Alloy and high-carbon steels (including maraging grades)
- Beryllium copper and beryllium nickel
- 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.
Most processes run in the temperature range of 175-730°C (350-1350°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 vacuum 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 vacuum 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. If a change occurs, it is generally a positive one. In all cases, the surface condition of the parts being processed are said to be improved.
|Fig. 2. Annealing of beryllium-copper wire (Photograph courtesy of Surface Combustion, Inc.)|
Vacuum furnaces for low-temperature processing can be batch or continuous, stand-alone, integrated into continuous vacuum furnace systems or a separate “module” incorporated into a cellular system. For example, 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 contaminants 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% 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.
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 (Fig. 2) 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 cool-down rates (Figs. 3 & 4).
|Fig. 3. Typical heating-rate performance data|
Low-temperature vacuum processing of workloads is becoming increasingly more common due to 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.
|Fig. 4. Typical cooling-rate performance data|
Some of the key considerations for choosing low-temperature vacuum processing can be summarized as:
1. Vacuum heat treating is 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. Heating and cooling is uniform and fast 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. IH