Quality Heat Treatment has developed a fluidised bed heat treatment technology which replaces the older molten salt technology. This eliminates concerns about disposal of cyanide salts and workplace safety issues that are associated with the molten salt technology. A payback period of approximately two years is expected on an average sized facility costing approximately $200,000.
- Mr Ray Reynoldson
- Managing Director
- Quality Heat Treatment Pty. Ltd.
- 1/18 Turbo Drive
- North Bayswater VIC 3153
- Ph: 61 3 9720 2744
- Fax: 61 3 9720 7690
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.
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.
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.
Any hydrocarbon gases (carburising) or ammonia (nitriding) introduced into the fluidised bed are combusted above the bed surface. The only discharges to atmosphere are products of combustion (carbon dioxide, water vapour and nitrogen oxide).
For a typical installation, involving an installation cost of $200,000, a pay-back period of two years can be expected.
The fluidised bed heat treatment process is now leading to new case hardening developments, especially for gears and similar components used in the mining industry.
A comparison between the operating costs for a salt bath and fluidised bed for the same capacity operation is given in the following figure.
Further development of the fluidised bed technology led QHT to realise that it had applicability for the removal of surface coatings from parts. The fluidised bed replaces more traditional methods such as salt baths (non-cyanide based), vacuum pyrolysis, chemical stripping (solvents, caustic solutions) and gas-tight ovens. The fluidised bed furnace uses a combustible mixture of air and gas to fluidise a bed of sand. This air/gas mixture is combusted within the bed to give an operating temperature of approximately 600 degrees Celsius. The parts to be stripped are then lowered into the furnace and the paint or plastic is pyrolised. Any waste gases from the pyrolysis are combusted at the bed surface.
The key consideration in the development of fluidised bed heat treatment was energy conservation, which arose out of the energy crisis of the early 1970s. The first fluidised bed furnace was built in the mid 1970s and was only moderately successful. Todays furnaces represent the fifth generation in the development of the technology.
The initial fluidised bed furnace only used nitrogen as the atmosphere for heat treatment, which limited the technologys initial application. However, the use of LPG, natural gas and ammonia has now allowed it to perform the same heat treatment processes as salt baths.
One minor barrier encountered is that the parts require cleaning after heat treatment to remove residual aluminium oxide particles that have adhered to the work piece.
Case study prepared: January 1997 by ACCP
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