Austempering / Isothermal Heat Treatment

  • The process consists of heating to the hardening temperature, then cooling rapidly to a temperature above martensite start (Ms) temperature and held to produce bainite microstructure, then quenching the part from the optimum austentising temperature directly into a liquid salt bath at a temperature between 310°C to 375°C.  After complete transformation of the microstructure to bainite the part is removed and air cooled to room temperature.
  • Austempering is similar to Martempering but usually includes the use of higher isothermal quench temperatures and sometimes extended quench times.
  • Austempering usually produces lower hardness and better ductility than Martempering, although the latter can depend on final transformation temperature.
  • In contrast to Martempering, these processes are usually single stage with no final temper required.  However, additional final tempers are used with certain materials subject to secondary hardening effects, particularly some tool steels.
  • The Austemper process offers benefits over the more conventional oil quench and temper method of heat treating products that require optimum distortion control.
  • The main aim of Austempering is to develop specific microstructures and properties, whilst substantially reducing cracking and distortion risks.
  • Minimises treatment costs and times by using a single stage process with no need for a final temper.
  • In ADI, a relatively low grade SG iron can be treated to high strength and ductility.
  • Can result in lower residual stresses, superior surface finishes, reduced surface scaling and decarburisation and increased fatigue limits.
  • This superb combination of properties can often be achieved with treatment at near net product shape, with minimal final machining necessary for some items.
  • Higher ductility, toughness (resistance to shock) and strength for a given hardness.
  • Austempering can be applied to thin sections of certain medium or high-carbon steels or to alloy containing steels of thicker section.  The selection of the steel is based on the transformation characteristics.  Important considerations are the temperature at which martensite starts and the time required for transformation of austenite to bainite.
  • Ductile irons (ie. SG cast irons) are one family of materials which can often be very successfully Austempered – Standards exist to cover such materials including ASTM A897M and BS EN 1564.
  • In ductile irons a superb combination of properties can usually be achieved by the correct choice of base SG iron grade and the Austemper process.
  • This process is primarily used to harden medium to high carbon steels and ductile iron castings. Link to ADI 
  • Typical materials processed include SG cast iron, ASTM A897M and BS EN 1564, some tool steels, low alloy steels and carbon/low alloy spring steels.
  • Components up to 680mm in diameter and 1200mm long can be treated vertically using special jigging.
  • Weight up to 400Kg

James Menzies

Production

Leonard Powell

Technical Planner

David Wright

Production Director

Mark Barron

Business Development Manager

Keighley Laboratories Ltd
Registered Office:
Croft House
South Street
Keighley
West Yorkshire
BD21 1EG
Registered in England 164811
UK Tel: +44 (0)1535 664211
Email: info@keighleylabs.co.uk

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