The advantages of processing in vacuum including some of the materials and common processes have already been discussed (Process Applications Run in Vacuum Furnaces – Part One). Here we look at custom heat treatment processes conducted in vacuum furnaces including vacuum applications in the laboratory, Research & Development department and for light industrial requirements as well as a look at the future of vacuum processing.
Custom Heat Treatment Processes
There are many types of highly specialized processes that can be run in vacuum furnaces, and most are highly application specific. Some of these include:
One of the applications not commonly considered in vacuum processing is that of chemical conversion. Sample material is loaded in non-reactive trays and placed inside the vacuum furnace or inside a retort (graphite or alloy). The material is then thermally processed under controlled temperatures and pressures to chemically convert a mixture of elemental materials into a compound. A typical chemical conversion process is run at 1370ºC (2500ºF) and requires up to several days for full transformation.
Creep and Compression Forming
Creep forming (aka hot sizing) is often used to flatten or form to a near-net shape and for correcting spring-back and/or inaccuracies in shape and dimensions of preformed parts. The part is suitably fixtured such that controlled pressure is applied to certain areas of the part during heating. This fixtured unit is then placed in a furnace and heated at temperatures and times sufficient to cause the metal to creep under its own weight until it conforms to the desired shape. Creep forming is done, for example, on titanium alloys, often in conjunction with compression forming.
Vacuum degassing is a term often used to describe improved cleanliness in the steelmaking process. However, it is also used to reduce the hydrogen levels in many alloys such as titanium, tantalum, and niobium to avoid concerns over hydrogen assisted cracking (aka hydrogen embrittlement). Hydrogen is imparted into titanium during ingot, rolling and forging operations and can also be diffused into titanium during pickling or other chemical processes. Newer aerospace specifications demand that the hydrogen levels be no greater than 70 ppm. Vacuum degassing usually performed between 535°C – 790°C (1000°F – 1450°F) depending on the alloy, can achieve hydrogen levels of less than 20 ppm.
Vacuum homogenization is performed primarily to eliminate or decrease chemical segregation by thermal diffusion in castings and ingots (prior to hot working). Homogenization typically has one or more purposes depending on the alloy, product and fabricating process involved. For example, one of the principal objectives is improved workability since the microstructure of most alloys in the as-cast condition is quite heterogeneous. In addition, a properly homogenized ingot will enhance the validity of ultrasonic testing conducted downstream in the manufacturing process for that material.
Hydriding and Dehydriding
The hydride/dehydride process is used during the manufacturing of transition metal powders such as tantalum, niobium, vanadium, and titanium. These particular metals have a unique characteristic: they undergo a reversible reaction during hydriding. Vacuum purification and activation of the metal is necessary before hydrogen is readily absorbed to form the metal hydride. Absorbed hydrogen leads to an expanded metal lattice, which promotes cracking and embrittlement. Once embrittled, the material can then be crushed into powder for further processing. The refined powder is then returned to the vacuum environment and the hydrogen removed from the material.
Nitriding / Nitrocarburizing
Gas nitriding/nitrocarburizing involves a vacuum purge followed by atmosphere processing. This is an alternative to traditional retort-type gas nitriding systems, and claimed benefits include reduced cycle times and fewer consumables.
Ion (plasma) Nitriding
Ion nitriding introduces nascent (elemental) nitrogen to the surface of a metal part for subsequent diffusion into the material. In actual practice, the injected gas is a mixture consisting of 25% N2 and 75% H2. In a low vacuum condition, high-voltage electrical energy is used to form plasma, through which nitrogen ions are accelerated to impinge on the workpiece. This ion bombardment heats the workpiece, cleans the surface, and provides active nitrogen.
A typical solution nitriding (a.k.a. high-temperature nitriding) process takes place in a vacuum furnace in the temperature range of 1050ºC – 1075ºC (1925ºF – 1970ºF). Nitrogen is introduced at pressures in the range of 1.5 – 1.9 bar for a typical period of 10 – 12 hours. The workload is subsequently rapidly vacuum quenched at pressures in the range of 10 bar.
Vacuum stress relief operations can be performed on a variety of materials, especially those where no surface oxidation is allowed. An example is the stress relief of nickel aluminum bronze alloy. The stress relief cycle consists of holding at 315°C (600°F) ± 9°C (15°F) for sixty (60) hours after a long, controlled ramp heat-up. After stress relief, a slow rate of cooling is used to prevent reintroduction of stresses into the parts.
Other Thermal Processes
Specialized areas in which vacuum equipment is being used include the following:
Various technologies fall under the umbrella of vacuum metallurgy. These processes are used for the production of substances of great purity, and for materials and alloys requiring extremely high-performance characteristics. Applications include aircraft turbine blades, ingots of tool steels for mining and milling cutters, superalloy ingots for aerospace and power turbines, alloys for the chemical industry, and high-purity metals for electronics and telecommunications. All of these processes use various methods for melting and solidifying materials.
Typical methods and equipment for this industry can be summarized as:
- Vacuum induction melting (VIM)
- Vacuum induction degassing and pouring (VIDP)
- Electroslag remelting (ESR) and various process variations (developed by ALD)
- Increased pressure (PESR)
- Inert gas atmosphere (IESR)
- Reduced pressure (VAC-ESR)
- Vacuum arc remelting (VAR)
- Vacuum precision casting (VPC)
- Vacuum inert-gas metal-powder technology (VIGMPT)
- Vacuum isothermal forging (VIF)
- Vacuum turbine-blade coating using EB/PVD coating applications
Processes such as melting, joining (brazing, welding) and casting are commonly performed by first creating a vacuum and then backfilling with an inert gas into the vacuum vessel. Products that are stronger, with thinner cross-sections and long life are possible due to avoidance of oxidation and contamination. Application in the fields of space, aerospace, atomic energy, automotive defense, lighting and medical have benefitted from processing in vacuum.
Vacuum Thermal Recycling
Vacuum thermal recycling is used for certain hazardous industrial and special waste products (e.g., mercury-containing waste, PCB-contaminated materials, and oil-laden grinding sludge) for the separation of products with the simultaneous recovery of base materials. In these instances, vacuum thermal recycling is a cost-effective alternative to other types of disposal operations.
Vacuum Reclamation of Metals
The principle of separating pure metal from mixed metallic particles by vacuum metallurgy is that the vapor pressures of various metals at the same temperature are different. As a result, the metal with a high vapor pressure and a low boiling point can be separated from the mixed metals through distillation or sublimation, and then it can be recycled through condensation under a certain condition.
Vacuum Freeze Drying
Freeze drying is common, especially in the food industry. The principle of freeze/sublimation drying is based on the fact that below its triple point, water only exists in the solid and the gaseous states. The freeze-drying process involves two distinct process steps. Step one is the freezing of the material below the triple-point temperature, and step two involves removing the ice crystals. Since the freezing process has a great influence on the quality of the finished product, this technology ensures that the product will be kept in the freshest possible condition.
Laboratory, Research & Development, Light Industrial
Laboratory and light industrial vacuum furnaces are suited for most common heat treatment processes where small-scale prototyping or product development are required. Melting and distillation processes in which reactivity or the special properties of the material, metal-based alloy or mixture require a complete or partial treatment under vacuum or protective gas. A major use of this equipment is for directionally solidified (DS) and single crystals (SX). In order to produce these components, high thermal gradients and constant-temperature solidification fronts are required.
Laboratory and light production applications include:
- Rare-earth metal production (yttrium, samarium, uranium aluminide)
- Optical coating materials
- Battery recycling
- Sintering of ceramics
- Distillation of metallic scrap and metals
- Ultra-pure materials for semiconductor devices and fiber optics
- Heat treatment of ceramics and glass
The Future of Vacuum Processing1
Industries with especially high potential for rapid growth (alphabetized listing) include:
- Additive manufacturing (medical, aerospace, energy)
- Biological (genomics, proteomics)
- Biotechnology (medical devices, regenerative medicine)
- Defense (anti-bioterrorism, security)
- Energy (fuel cells, fusion reactors)
- Environmental (emissions reduction, materials recycling)
- Material development (ceramics, composites, powder metallurgy)
- Nanotechnology (communications, biomedical, electronics)
These technologies are consistent with identified high-tech growth markets.
Vacuum processing and vacuum technology solutions are not a panacea, but they do represent the best way to ensure that the process is performed in a consistent and repeatable way. The challenges are in designing these furnaces, often requiring high temperature, high vacuum or pressure as well as being absolutely leak tight with leak rates in the 5 micron/hour range.
1. Sakhamuri, Nagarjun, “Vacuum Furnaces for Metallurgical Applications”, Hind High Vacuum Co, Pvt. Ltd.,
2. Zahn, Lu and Zhenming Xu, “Application of Vacuum Metallurgy to Separate Pure Metal from Mixed Metallic Particles of Crushed Waste Printed Circuit Board Scraps”, Environmental Science & Technology, American Chemical Society, 2008.
3. Jones Metal Products, “Why Vacuum Heat Treating Furnaces Are Important to Aerospace”, 2008.
4. Herring, Daniel H., Vacuum Heat Treatment, BNP Media, 2012.