Salt Bath Processing
Traditional methods for heat treatment of metals, including hardening, carburising and nitrocarburising, utilise baths of molten salts such as nitrates, nitrites, carbonates, cyanides, chlorides and caustic salts. Cyanide salts are probably the most extensively used salts for heat treatment. The workpieces to be treated are immersed in the molten salt bath, with the time and temperature dependent upon the type of heat treatment required. Quenching (rapid cooling) of the metal being heat treated is often required, and is commonly achieved using either oil or water quench baths. Cleaning of the metal is required after oil quenching.
The use of cyanide salts generated both occupational health and safety, and environmental concerns. Disposal of used cyanide salt baths requires specialist waste management expertise and is expensive. Neutralisation of quench baths and washing water is required because of carryover of small amounts of the cyanide salts. Vapours from the salt baths may also require treatment using chemical scrubbing. Any moisture present on the work piece when it is immersed in the bath can cause 'explosions' or eruptions.
The More Environmental & Energy Efficient Method
A number of alternative heat treatment processes have been developed to replace the traditional molten salt bath method. The most common processes used are vacuum furnaces, atmosphere furnaces and fluidised bed furnaces. The fluidised bed has significantly lower capital, maintenance and energy costs and generally produces a better quality heat treatment than the atmospheric and vacuum processes.
The fluidised bed process utilises the suspension of particles of aluminium oxide in a gaseous stream, such that the particles behave in a manner similar to a liquid. A variety of gases, such as LPG, natural gas, ammonia and nitrogen, are used to impart the desired surface properties to the metal being treated (hydrocarbon gases for carburising, ammonia for nitriding and nitrogen for neutral hardening) and provide the fluidisation. The composition of the atmosphere within the furnace can be varied easily and quickly, according to the treatment required.
Clayton Thermal Fluidised Bed Furnaces are heated indirectly by electricity. Quenching can also be carried out in the fluidised bed using air & nitrogen. However, in some circumstances a rapid quench rate is required to achieve the desired mechanical properties, water and oil quench baths are most commonly used to perform this.
Advantages of the Process
The advantages of the system include elimination of costs associated with the cyanide salt bath (raw material and disposal), reduced energy consumption and decreased operating costs. Occupational, health and safety concerns associated with the use of molten cyanide salts are also eliminated. The main environmental & cost benefits associated with the replacement of salt baths by fluidised bed furnaces include:
- elimination of spent salt disposal;
- elimination of need to neutralise quench oil or water due to salt
carryover;
- elimination of need to chemically clean off gases or vapours from
the bath;
- improved quality through rapid and uniform heating, cooling and
temperature uniformity;
- lower installation costs;
- low capital costs;
- lower energy consumption; and
- improved working environment