This paper provides an overview of salt quench hardening and how it compares with oil quenching. It explains how salt quenching promotes hardenability, ductility, and strength as well as distortion control, heat extraction, and process reduction. It discusses equipment layout configurations, NFPA guidelines and safety practices, and salt quench processes for austempering, marquenching, and neutral hardening applications.

This work investigates the cooling performance of different salt solutions and quench bath parameters. The results show that increasing quenchant temperature can stabilize the vapor film, while the presence of various additives and the use of agitation can hasten its collapse. Ionic solutions containing NaCl, Na2SO4, NaOH, and NaNO2 were found to inhibit the vapor blanket at 35°C and improve cooling power. Adding salt-forming solutions promoted a more homogeneous cooling with high values of heat flux over most of the cooling cycle.

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 porosity in castings and PM materials. Hardenability is increased due to stabilization of the close-packed austenite. The inherent freedom provided by HIP to select optimum levels and rates for temperatures and pressures has been shown to improve mechanical properties and reduce process duration.

This presentation will discuss a flexible atmosphere heat treating system employing salt quench systems that are considerably faster, than existing technology utilizing internal quench systems. Topics include: improved temperature control, temperature uniformity/uniformity of the austenitizing process; improved automation and reliability of material handling transfers from the austenitizing furnace to the quench system; and increased flexibility, with regard to the quench system, to allow for the widest range of processes to be run in the system.