There is a constant need throughout the industry to produce the highest quality parts to the most stringent product specifications. Both long-established and new materials are being employed to meet the needs of lighter, stronger, smaller and more efficient designs, and the use of vacuum technology in manufacturing is of paramount importance in achieving these goals.
The strategy being adopted by manufacturing to meet these needs relies heavily on vacuum processes and equipment through:
- Process development – New materials, new products, and new applications demand absolute cycle repeatability, flexibility, and control, and as such, designers are specifying vacuum processing over other heat treatment methods.
- Process substitution – Older process technologies and the equipment associated with them are being replaced by vacuum equipment. The justification lies in reduced unit cost achieved by lowering the overall cost of manufacturing and/or through material and efficiency savings.
- Process replacement – Product-performance demands are forcing designers to look toward vacuum processing and its ability to offer a superior product for the same or very similar cost.
Typical processes performed in vacuum furnaces, by industry, include but aren’t limited to:
- Aerospace – engine, landing gear, etc.
- Age Hardening
- Solution treating
- Automotive – passenger and commercial vehicles
- Industrial products – appliance, tool & die, etc.
- Commercial heat treating – outsourcing
- Solution treating
- Stress relief
- Tempering (bright)
A few of the many types of materials being processed in vacuum include:
- Brass*, bronze, beryllium copper (e.g. marine industry)
- Carbon and alloy steels (e.g., automotive industry)
- Ceramics (e.g., nuclear, biomedical, astronautics industries)
- Iron and steel castings (e.g., oil & gas industry)
- Stainless Steels (e.g., food industry)
- Tool Steels (e.g. tool & die industry)
- Nickel Alloys – Inconel, specialty grades (e.g., chemical, electronics industries)
- Powder metallurgy grades – aluminum, steel, stainless steel, tool steels, tungsten carbide (e.g., automotive and tool industries)
- Copper and copper alloys (e.g. consumer electronics industry)
- Superalloys – iron, nickel and cobalt-based (e.g., aerospace industry)
- Tantalum (e.g., electronics industry)
- Titanium and titanium alloys (e.g. aerospace, medical industries)
- Zirconium alloys (e.g., nuclear industry)
* Note: Special precautions are needed to prevent dezincification of these alloys
Common Vacuum Heat Treatment Processes4
Some of the most common heat treatment processes run in a vacuum are outlined below along with a brief explanation of the advantages that vacuum treating provides.
Annealing treatments are undertaken primarily to soften a material, to relieve internal stresses and/or to modify the grain structure. These operations are carried out by heating to the required temperature followed by soaking at this temperature for sufficient time to allow the material to stabilize followed by slow cooling (except for solution annealing) at a slow and often times controlled rate. The choice of vacuum annealing is primarily influenced by the cleanliness and high quality of surface finish that can be obtained relatively easily compared to other controlled atmosphere heat treatment operations.
Vacuum brazing represents one of the growing uses for vacuum furnaces. Tight process (time/temperature) control and the possibility of a metallurgical clean joint at the onset of brazing are attractive attributes. The transportation industry (e.g. automotive and aerospace), in particular, has provided the impetus for increasing demand for vacuum furnace brazing,. In addition, the gain in popularity of lightweight, high-strength materials has also contributed to the popularity of vacuum brazing.
Vacuum furnace designs used for the brazing process can be either horizontal or vertical in design and offer the following technical advantages:
- The process permits brazing of complex, dense assemblies with blind passages that would be almost impossible to braze and adequately clean using atmospheric flux brazing techniques.
- Vacuum furnaces using diffusion pumps to achieve vacuum levels of 10-4 to 10-5 Torr remove essentially all gases that could inhibit the flow of brazing alloy, prevent the development of tenacious oxide films, and promote the wetting and flow of the braze alloy over the vacuum conditioned surfaces.
- Properly processed parts are unloaded in a clean and bright condition often avoiding additional processing.
- A wide variety of materials ranging from aluminum, cast irons, stainless steel, steels, titanium alloys, nickel alloys, and cobalt-base superalloys are brazed successfully in vacuum furnaces without the use of any flux.
Carburizing/carbonitriding are specialized case hardening processes performed on gears and many other industrial parts. The process has gained popularity in vacuum furnaces given its ability to precisely control surface hardness and case depth. Quenching is done in either oil or high-pressure gas (up to 25 bar nitrogen being a practical limit).
The case depth on a component surface is a function of the rate of carbon absorption at the surface and the diffusion of carbon away from the surface and into the material. Once a high concentration of carbon has developed on the surface, during what is commonly called the “boost stage”, the process normally introduces a “diffuse stage” whereby the surface carbon concentration is reduced by diffusion into the interior. The result is a reduction of the carbon concentration at the surface while increasing the depth of carbon absorption.
In the carburization process, residual compressive stresses result from the delayed transformation and volume expansion of the carbon-enriched surface. This induces the desirable residual compressive stress through the case-hardened layer.
Hardening of steel and other alloys is advantageous in vacuum furnaces where surface oxidation in the form of intergranular oxidation/intergranular attack (IGO/IGA) and surface contamination needs to be avoided. The time/temperature recipes can include controlled ramp rates, single or multiple preheats (common in tool steel hardening, for example) and variable soak times based either on workload thermocouples or time. Quenching can be done in either oil or high-pressure gas (up to 25 bar nitrogen being a practical limit) and isothermal holds are possible as well.
Sintering, Sinter Compaction, MIM and Additive Manufacturing
Vacuum sintering, compaction (CIP, HIP). MIM and secondary heat treatment operations are performed on both conventional powder metal (PM) as well as particulate (CIM, PIM, MIM) materials. In an industry dominated by atmosphere processing, increased interest in controlled vacuum sintering arises from factors such as:
- The purity of the vacuum environment and its effect on part microstructure.
- The use of sub-atmospheric (partial) pressure to improve the efficiency of the sintering reactions especially with highly alloyed materials that require elevated sintering temperatures.
- The use of reducing gasses, at temperatures typically less than 500°C (930°F), aid in binder removal, reduce porosity and minimize oxide formation.
- The ability of the vacuum process to reduce pore size and improve pore size distribution.
- The higher furnace temperature capabilities that permit faster sintering reactions carried out much closer to the melting point and with alloys of higher melting point interstitial elements or liquid phase metal sintering itself.
- The ability to modify, in-situ, carbon balance and affect carbon additions or reductions.
- The use of over-press sintering (HIP) to densify the materials, close type porosity without formation of alloy rich areas.
- Single-cycle debind, sinter & HIP with rapid cooling for rapid floor-to-floor times.
The limitation on the application of sintering in vacuum furnaces is binder removal and the vapor pressure of the metals being processed at the chosen sintering temperature. If the vapor pressure is comparable with the working pressure in the vacuum furnace, there will be considerable loss of metal by vaporization unless a sufficiently high partial pressure of inert gas is used. In certain situations, care must be taken as the partial pressure gas selected may react with the surface of the part creating an undesirable surface layer or condition.
Solution Treating and Aging5
Solution treatment is the heating of an alloy to a suitable temperature, holding it at that temperature long enough to cause one or more constituents to enter into a solid solution and then cooling it rapidly enough to hold these constituents in solution. Subsequent aging (i.e., precipitation) heat treatment allows the controlled precipitation of these constituents either naturally (at room temperature) or artificially (at higher temperatures).
Many aerospace applications requiring high tensile strength, high fatigue strength, and good stress-rupture properties use a solution treatment and a one or two-step aging treatment. Inconel® 718 is one example of a material that is usually used in the solution treated and aged condition. The exact temperatures, times, and cooling rates depend on the application and desired mechanical properties.
Another example involves solid-solution strengthened alloys (e.g., Hastelloy® X, Inconel® 625 and HA® 230) in which work hardening from manufacturing operations limits the ability to further process the material. As such, in-process solution treating (i.e., stress relief) reduces this condition and allows additional processing. In addition, manufacturing processes, such as brazing, welding or coating can have an undesirable impact on material properties which may be reversed through solution treating prior to further downstream processing. Finally, various manufacturing processes may result in the premature start of the final precipitation age hardening process, which can be reversed through re-solution treating prior to further processing.
Age hardening results in the development of final material properties. Typically, this step is performed at or near the end of the manufacturing process, as the heat treat process results in a significant increase in material hardness, and there is a predictable amount of size change that occurs (shrinkage) that must be accounted for.
Where surface finish is critical and “clean and bright” parts are desired to avoid any post heat treat processing, many heat treaters, especially commercial shops, now employ vacuum furnaces for tempering and stress relief. These units typically operate in the temperature range of 130°C – 675°C (275°F – 1250°F), which is below the temperature at which radiant energy is an efficient method for heating. As such, heating by convection is utilized; the furnace is normally evacuated to below 0.10 mbar (0.075 Torr), then backfilled with an inert gas such as nitrogen, argon or even 97% nitrogen / 3% hydrogen mixtures to a pressure slightly above atmospheric, typically in the range of 0.5 – 2 bar. A fan in the furnace recirculates this atmosphere, and parts are heated by both convection and conduction. Temperature uniformity in the range of ± 5°C (± 10°F) is common with tighter uniformities possible.
Next time: Part Two will look at custom heat treatment processes conducted in vacuum furnaces along with vacuum applications in laboratory, Research & Development, and light industrial equipment. The future of vacuum processing will also be discussed.
- Sakhamuri, Nagarjun, “Vacuum Furnaces for Metallurgical Applications”, Hind Hihg Vacuum Co, Pvt. Ltd.,
- 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.
- Jones Metal Products, “Why Vacuum Heat Treating Furnaces Are Important to Aerospace”, 2008.
- Herring, Daniel H., Vacuum Heat Treatment, BNP Media, 2012.
- Bodycote (www.bodycote.com)