Austempered Ductile Iron
austempered ductile iron, the material
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Producing ADI

Customised Austempering furnaces used in ADI production

Customised Austempering furnaces used in ADI production

In all successful production Quality is both the goal and the key. On founding ADI Treatments Ltd we installed customised furnaces and acquired specialist Austempering know-how from our associates Applied Process Inc.

This has enabled us to hold tight control over plant and process and achieve our ISO 9001: 2000 Quality approval. We have continued to build our knowledge and experience, helped by our relations with Applied Process affiliates around the world. We share process and operational advances, improving our service to customers and enhancing the appeal of ADI in the wider market. In addition to ADI we apply Austempering to other forms of cast iron, and to certain steels.

Special furnace technology

Front view of furnace

ADI Treatments Ltd operates controlled atmosphere, batch austempering furnaces; each unit is capable of producing in excess of 100 tonnes per week. Individual payloads are up to 2.8 tonnes with maximum component dimensions 1m x 1m x 2m. The capacity is being extended to provide solutions for 1.825m diameter components from 2005.

A special feature enables us to accelerate the quenching speed by the measured addition of water, injected directly into the quench bath. In comparison with other furnace systems this allows “through austempering” of thicker cross-sections or the successful austempering of leaner alloyed materials at a given cross section. To attain maximum efficiency and capability, the furnaces are fully integrated into the operating system by computer control. Our unique facility can deliver competitive advantage in cost and performance.

Austempering compared to conventional hardening treatments

Austempering is an isothermal heat treatment applied to ferrous materials. It produces a microstructure that is stronger and tougher than the structures resulting typically from conventional heat treatment. Austempering calls for very precise control of process times and temperatures.

Traditional heat treaters heat the parts to “red heat”, a process called austenitisation, often in a controlled atmosphere to prevent scale. The parts are held at red heat for a time then quenched rapidly in a bath of oil or water, generally kept near room temperature. This produces the crystalline structure known as martensite, a hard but brittle phase. Parts quenched in this way must undergo a second, lower temperature heat treatment (“tempering”) to recover their ductility.

IT Diagram

Austempering starts similarly. The parts first undergo austenitisation in controlled atmosphere but they are then quenched in a bath of molten salt held at a specific temperature between 230°C and 400°C. At this stage the material completes an isothermal transformation. Because the quench temperature is above the martensite transformation temperature, a different structure results. In Austempered Ductile Iron and Austempered Grey Iron the resulting structure is Ausferrite; in steels it is Bainite.

In conventional heat treatment, as the part reaches the critical temperature the formation of martensite is instantaneous. The outside of the part may therefore be transformed while the inside remains red hot. This lack of uniformity results in distortion and microcracks that reduce the strength of the part.

By contrast, the austempering reaction that produces Ausferrite or Bainite takes place over many minutes or some hours. This results in uniform growth and a stronger, less disturbed, microstructure.

Austempering applied to ADI

ADI is produced by heat treating ductile cast iron.; sometimes small amounts of copper, nickel and molybdenum are added to improve hardenability. The heat treatment parameters are carefully selected for the application.

Typical Austempering Process Cycle

Castings are first austenitised to dissolve carbon, then quenched rapidly to avoid the formation of pearlite. Austempering brings about the nucleation and growth of acicular ferrite, accompanied by rejection of carbon, into the austenite. The resulting microstructure, known as “Ausferrite”, gives ADI special properties. Ausferrite exhibits twice the strength for a given level of ductility compared to the pearlitic, ferritic or martensitic structures formed by conventional heat treatments.

Because the carbon rich austenite phase is stable in ADI it enhances the bulk properties. Furthermore, while the austenite is thermodynamically stable, it can undergo a strain-induced transformation when locally stressed, producing islands of hard martensite that enhance wear properties. This behaviour contrasts with that of the metastable austenite retained in steels, which can undergo harmful transformation.

All heat treatments create dimensional change however because the process is isothermal components of the same incoming chemistry will grow the same way every time. Batch-to-batch and part-to-part, your components will have predictable growth from the Austempering process. That means you can design the dimensional change into your component and let it grow to its tolerance.

The final properties (grades) of the ADI are governed by the austempering temperature and duration. Based on customer input, our proprietary computer models can make accurate material and process selections and predict these properties. We can then assist the design engineer to find the optimum and most efficient solutions.

AGI, CADI and Austempered steels

The Austempering of Grey Iron forms AGI, a material with excellent damping and wear resistance. Current applications include cylinder liners and bearing collars.

Carbidic ADI (CADI) is a new material formed with a matrix of ausferrite and carbides to add strength and wear resistance to ductile iron parts. The result is a premium, engineered iron with longer life and greater wear resistance than Grade 5 ADI. Applications for CADI are to be found in off-highway vehicles, agricultural equipment, railroad car and track, construction and mining, general industrial, material handling, and ground engaging components.

Application of the Austempering process to steel provides the user with a tough, high-strength component that resists embrittlement. This dimensionally repeatable process is typically cost competitive with conventional quench and temper processes. Austempering is particularly appropriate for medium and high carbon stampings, forgings, castings, and full density powdered metal parts.

Carbo-Austempering™ develops a tough, high-carbon, bainitic case on high performance components. Carbo-Austempered™ parts have remarkable impact properties that are superior to either neutral hardened or carburised steels. Carbo-Austempering™ is routinely applied to steel gears, shafts, and power transmission parts that undergo periodic overloading in service.

ADI Treatments Limited, Doranda Way, West Bromwich, West Midlands, B71 4LE  Tel: +44 (0)121 525 0303