Austempering Treatment in HIP Improves Ausferritic Steels and Ductile Irons

Austempering heat treatments of steels and cast irons are usually performed using salt bath quenching followed by isothermal transformation of austenite to bainite or ausferrite.

High Pressure Gas Quenching (HPGQ) at 1-4 MPa gas pressures is increasingly used to replace oil quenching, but may also be used for austempering. However, to obtain sufficient heat transfer high gas speeds >25 m/s are required.

Hot Isostatic Pressing (HIP) is widely used for densifying castings and powder-based materials. Recent equipment developments enable Uniform Rapid Quenching (URQ) under 200 MPa pressure and 0.3 m/s speed, providing uniform cooling. Superplastic conditions during austenitization and initially during URQ reduce residual stresses and eliminate internal porosities in castings and PM materials.

Hardenability is increased by HIP pressure stabilization of close-packed austenite versus pearlite.

In contrast to isothermal austempering, nucleation and growth of acicular ferrite at different temperatures can reduce process time while improving microstructure and mechanical properties.