Quality Heat Treatment Pty Ltd (QHT) has developed an alternate heat treatment process for metal components which replaces traditional molten salt baths for a wide range of applications. The company designs and supplies these fluidised bed heat treatment systems for heat treaters, and has supplied approximately 240 systems to clients in Australia and South East Asia. QHT also provides heat treatment services from its premises in Bayswater, Melbourne, using the fluidised bed technology, to approximately 400 customers.
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 workpiece when it is immersed in the bath can cause 'explosions' or eruptions.
The traditional benefit of salt baths was that they heated the workpiece quickly and uniformly by conduction. In addition, by use of a specific salt, the desired surface properties could be imparted to the workpiece.
Cleaner Production Initiative
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 costs and generally produces equivalent or 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.
The fluidised bed is heated indirectly by either electricity or gas. Quenching can also be carried out in the fluidised bed using air, nitrogen or helium. However, in practice, water and oil quench baths are most commonly used due to the expense of these gases.
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 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.