Edward Frank Ryntz

Transmission electron microscopy study of austempered nodular iron: Influence of silicon content, austenitizing time and austempering temperature

Abstract The structures produced by austempering nodular iron were evaluated by transmission electron microscopy to explain the influence of austempering temperature, austenitizing time and silicon content on the impact properties. No carbide precipitation was evident in a 2.88 wt. % Si nodular iron austenitized for 15 min at 950 °C and austempered for min at 370 °C. This iron had the highest impact strength. Austempering at 340 and 400 °C resulted in the formation of transition carbides in bainitic ferrite laths and a lower impact strength. In addition, both a lower silicon content and a longer austenitizing time promoted carbide formation in the bainitic ferrite during austempering at 370 °C.

A transmission electron microscopy study of austempered lower bainitic nodular cast iron

Abstract This study is the first transmission electron microscope examination of austempered nodular iron thin foils. An important part of this investigation was the development of a procedure for preparing thin-foil specimens of nodular iron. The procedure involves mechanically polishing specimens to about 0.13 mm thickness, followed by electropolishing to 0.03 mm, and ion milling until fine perforations are obtained. The microstructure of nodular iron austenitized at 915°C for 2 h and austempered at 240°C for 2 h consisted of alternating lath-like regions of ferrite and retained austenite with cementite finely distributed throughout the ferrite matrix. The cementite was believed to have precipitated from bainitic ferrite initially supersaturated with carbon. The crystallographic orientation relationship between the bcc ferrite and fcc austenite was determined to be the Bain relationship of [011]bcc … [001]fcc. No effects of silicon over the range 1.85–2.33 wt.% were found on the size and distribution of microconstituents present in the lower bainitic nodular iron.