ausferritic

Ausferritic refers to a family of materials, typically ductile irons, that develop an ausferritic microstructure through austempering. This microstructure comprises acicular ferrite in a matrix containing stabilized retained austenite. It forms when austenitized iron is cooled to a low enough austempering temperature and held to allow bainitic transformation to proceed without forming pearlite or martensite.

Formation and microstructure

During austempering, a ductile iron is first austenitized and then quenched into a bath at a controlled temperature, generally well below the eutectoid temperature. Prolonged isothermal holding promotes the transformation to acicular ferrite with retained austenite. The retained austenite helps absorb energy and, in some cases, may transform to martensite under strain (the TRIP effect), potentially increasing strength in service. The resulting structure is typically a mix of finely distributed acicular ferrite and film-like austenite.

Properties and performance

Ausferritic materials often exhibit a combination of high strength and good toughness, along with favorable wear resistance. The retained austenite can provide ductility and impact resistance, while the ferritic phase contributes to strength. The microstructure generally offers good fatigue performance and dimensional stability compared with some conventional heat-treated steels.

Processing considerations

Alloying elements (such as nickel, chromium, molybdenum, copper, and niobium) and the austempering temperature and time influence the stability of retained austenite and the final properties. Proper control of the austenitizing and austempering steps is essential to achieve the desired ausferritic microstructure and performance in the final component.

Applications

Ausferritic ductile iron is used for components requiring a balance of strength, toughness, and wear resistance, including automotive parts such as crankshafts and gears, as well as wear parts and structural components in industrial equipment.