A Salt Bath Furnace is a device for the heating or cooling of work by immersing it in a liquid consisting of fused salts of the proper composition for the purpose intended. The salts may be heated by electrical or gas-fired means.
The operating principle of the electrode type salt bath furnace is to generate heat directly in the salt by utilizing its resistance to the passage of current. Salts, while insulators in the solid state, are excellent resistors in the fused state. Potential is applied to the molten salt with the use of heavy metal bars called electrodes that are connected to the secondary of special multiple voltage air cooled transformers. These electrodes are located and spaced in such a way to facilitate electrodynamic circulation whenever energized, providing a powerful pump to help aid temperature uniformity throughout the bath.
Salt Bath Furnaces have enormous potential and facilitate a variety of industrial applications used in the aerospace, medical, and manufacturing industries, from creating farming equipment and lawnmowers to treating high alloy stainless steels for airplanes, and dentist drills to keep our mouths clean. The salt bath process itself is relatively simple.
What Is the Heat Treatment Process?
Heat treatment encompasses a broad category of processes that affect metal workpieces to give them more desirable characteristics. This may include softening the surface for better formability or ductility, hardening the surface to make it resistant to damage, adding corrosion resistance, and toughening brittle workpieces.
The Basics of Heat Treating
While iron and steel are the most heat-treated materials, heat treatment is also beneficial for other materials such as aluminum, copper, magnesium, nickel, titanium and many of their associated alloys.
The basic process involves heating the metal to the correct temperature in a specific mixture of different salts for a sufficiently long period, and then cooling it with the right quenching processes to attain the desired properties. Each treatment process and metals themselves demand different salts in the bath, temperatures, temperature holding times, and quenching procedure.
Some heat treatments require temperatures of up to 2400°F or holding periods as long as 60 hours.
Why Salt Bath Heating?
In salt bath heating processes, users completely submerge workpieces in a bath of molten salt. The benefits of choosing this process over radiation or convection heating processes include:
- Controlled Atmosphere. By immersing the workpiece in a salt bath, you effectively remove its contact with the outside air eliminating scaling, oxidation, and decarburizing. The workpiece is similarly benefitted by the thin film of liquid salt protecting the metal from oxidation when transferring to the quench medium.
- Uniform Heating. The electrodes that heat the salt cause electrodynamic circulation in the bath providing a natural agitation. This constant, vigorous stirring action of Ajax furnaces assures extremely uniform and precise temperature throughout the bath regardless of size or temperature. Even temperature distribution also prevents hotspots that will shorten the life of pots and cause excessive salt breakdown.
- Faster Heating. In salt bath processes, the entire workpiece is uniformly surrounded by salts that hold a precise temperature forming a frozen ‘cocoon’ of salt, subjecting the work to an automatic preheat before melting. This modifies the workpiece’s temperature by conduction at a quick, but safe rate, reducing heat shock and distortion.]
- Thermal Efficiency. Internal heating of the bath does not have to be forced through pot walls allowing salt baths to recover quicker than fuel-fired furnaces. Salt also has a high specific heat and mass resulting in high heat storage capacity so that more work can be heated without a drop in temperature allowing for little recovery time between loads.
Typical Treatments Conducted in Salt Baths
Salt baths are compatible with a variety of metal treatment processes. These include:
- Neutral Hardening – Hardening of the work without oxidation or scaling and without addition or subtraction of carbon from the steel.
- High Speed Steel Tool Hardening – An exacting operation that requires up to 4 furnaces and temperatures of 2200-2400°F.
- Austempering – Quench process where the piece is left in the quench until its structure transforms into bainite, typically at 400-750°F. Used when hardness requirements are high but toughness, or ability to bend without breaking, is also required.
- Martempering – Quench process where the piece is removed from the quench before the bainite formation, temperatures need to remain above the temperature of martensite formation, between 400-480°F.
- Annealing – A process to increase the ductility and reduce the hardness of a metal. Due to their high heat retention, salt baths are better at anneals of low, medium, and high carbon wire and rod, also stainless steel and nickel chrome alloys.
- Brazing – Salt bath brazing, or the fusing of two pieces, works well because the flux that is used to hold the welds in place becomes soluble in the salt bath at a temperature just above the melting point of the weld, leaving nothing behind. A car radiator gets brazed this way to weld all the impossible-to-reach places inside.
- Nitinol (mem-wire) Treating – Nitinol (NiTi) is a nickel-titanium alloy with excellent shape memory and hyperelastic properties. The NiTi heat treatment process occurs over several cycles of heating and cooling within a fluidized bath.
- Cleaning – Thermal salt baths may be used to remove a variety of materials and contaminants from metal parts, including adhesives, paint, plastic, oil and grease, and much more.
- Solution Treating of Aluminum – Solution heat treatments significantly extend the age hardening time of aluminum components, ensuring workpieces can be manipulated for a much longer period of time.
Construction Features of a Salt Bath Furnace
A steel casing is the vessel where everything that makes up the furnace is built. Generally, a casing is a welded steel construction with reinforced corners, bottoms, and sides with heavy steel channels on the bottom for legs. A high heat (1650°-2400°F) furnace will typically have two types of insulation, firebrick, and high heat cement in many layers to keep the heat from entering the work area from the salt pot.
Ajax Electric makes brick and steel pots depending on the application and work the customer is trying to accomplish. High heat furnaces tend to be made with brick so that through the wall electrodes can be built inside of the pot wall itself. Steel pots, usually a high alloy stainless steel, cannot go as high in temperature, but are easily replaceable when the time comes to change out the old pot. Large slabs of corrosive-resistant stainless steel that heat the bath with current are called electrodes.
Ajax makes two types of electrodes, one designed to be built right into the pot walls (through-the-wall electrodes), and two, easily replaceable electrodes that enter the pot from the top of the bath (over-the-top electrodes). While the through-the-wall electrodes will not receive the normal wear from the oxygen and corrosive salt fumes escaping the bath elongating their life immensely, replacing them involves jackhammering out the old electrodes and re-building the pot, a time-consuming endeavor.
Miscellaneous equipment can be added to orders to facilitate better operation of the salt bath that can make work more consistent and not as costly. Covers will retain heat inside the bath when work is not being transported in or out so less energy is needed to get the salt back to the temperature needed for the next batch. Mixers will help keep the salt at a uniform temperature in furnaces that do not have electrodes providing electrodynamic circulation. Tools will help remove sludge and other particulates from the bath, scrape away the electrodes so they are operating at their fullest, and various other scoops and scrapers to keep the salt bath running efficiently. These auxiliary items can also be used in our other types of furnaces Ajax Electric Co. manufactures, TS (electric heaters) and DGT (gas-fired) furnaces.
Electrically Heated and Gas-Fired Salt-Bath Furnaces
Electrically heated (TS) models of salt baths can be heated by high-quality heating elements in ceramic tubes inside the bath, stainless-steel immersion heaters inside the bath, or even externally heated by heaters hugging around the metal pot.
Gas-fired (DGT) models use high-efficiency burners with overpressure operations and optimized flame management to control the temperature and prevent the entrance of false air. These models also have a lateral exhaust gas feed around the crucible with specialized insulation that is designed for open-flame environments.
A Salt Bath Furnace by its lonesome can be a relatively simple design, but besides the casing, some insulation, heating source, and perhaps some miscellaneous equipment like a cover and mixer, there are quite a few more parts to round out an entire Heat Treatment system.
Work, Salt, Transformers, and Control Equipment
Steel and aluminum workpieces are commonly treated in salt bath furnaces. These materials require the following specifications:
- Defined applications that stay within the bounds of the salt bath’s realm of operations.
- Salt temperatures between 1350-1850°F and temperature uniformity within 4°F of the preferred temperature in accordance with the current standards.
- Standard processes to measure both the salt bath’s control temperature, and over temperature to ensure that bath will not freeze or overheat.
Electrodes that heat Ajax Electric’s furnaces can not just be plugged into an electrical outlet. Very large air-cooled transformers are connected to the electrodes via large copper connectors each customized for every customer’s needs. Most transformers have tap switches that will change the amount of voltage being applied to the bath. As the electrodes wear down from use, the gap between them grows larger, meaning you will need more current to keep the bath up to temperature and to provide electrodynamic circulation, so you increase the tap on the tap switch to allow this. How does a customer know when to increase a tap? With monitoring equipment known as a Control Panel.
The setup may implement optical and acoustic alarm systems that can alert nearby operators when the bath temperature gets too high or low. Salt baths can reach critical temperatures if the salt bath temperature exceeds what the furnace can safely hold, or if the submerged workpieces elevate the bath temperature too high. The furnace’s temperature profile can be tracked and documented with precision equipment designed for this process. The control panel is there to make sure that the salt mixture remains at the temperatures needed; what temperature that is depends on the process and the types of salt used.
Different salts have different melting points, so manufacturers can choose a precise combination of salt types to achieve varying temperature ranges that best fit the cooling or heating processes they need. The specific salt required will depend on the process being conducted and the material of the workpiece. In general, a salt bath may contain a variety of carbonates, caustic sodas, chlorides, cyanide nitrates, nitrites, and other salts.
Different salt combinations cause three broad categories of effects:
- Neutral temperature changes. For neutral temperature changes that don’t change the surface of the workpiece, manufacturers need to use a combination of chlorides and nitrates — neutral salts.
- Surface treatments. To treat a surface, manufacturers use aluminizing, carburizing, cyaniding, and nitriding salts.
- Surface cleaning. There are even more salts that can clean a surface without treating it by removing organic compounds, paint and plastic, and scale.
Ajax Electric: Salt Bath Furnaces Experts for 7 Decades
Ajax Electric is a leading provider of salt bath solutions for a variety of thermal processes. We supply our customers with the materials they need for these processes and more:
- Annealing ferrous and non-ferrous metals
- Austempering steel parts
- Chemical strengthening of glass via ion exchange
- Descaling and removing mold
- Dip brazing of steel and aluminum alloys
- Glass removal (stripping)
- Hardening of high speed and tool steels
- Heat generation, storage and transfer for high-temperature stored energy applications
- Martempering of steel parts
- Neutral hardening
- Plastic curing
- Removal of plastics
- Rubber curing
- Shape setting of NiTi shape memory alloys
- Solution treatment of aluminum alloys
- Stress relieving
- Surface preparation of carbide tips for brazing
- Thermal diffusion of carbide-forming elements
Contact our team today to find the right solutions and reliable salt bath equipment to meet your facility’s manufacturing and heat treatment needs.