The microstructural evolution,crystallography, and thermal processing of ultrahigh carbon Fe-1.85 pct C melt-spun ribbon

The microstructural evolution, mechanisms of grain refinement, crystallography, and thermal processing of a rapidly solidified Fe-1.85 pct C alloy have been studied by transmission electron microscopy (TEM). Melt-spun ribbons quenched in liquid nitrogen consist of carbide-free highly twinned martensite plates between 0.5-and 2.0-µm long and 0.1-and 0.5 -µm thick, with approximately 40 pct retained austenite also present. Ribbons tempered at 600 °C for 10 seconds consist of ferrite of approximately the same grain size and both intragranular and intergranular cementite precipitates. The intragranular cementite particles are about 0.1 /um or less in size and exhibit a single variant of the Bagaryatskii orientation relationship with respect to a given ferrite grain; the intergranular particles are about 0.1 µm in thickness and can be as long as 0.5 µm due to growth and/or coalescence along ferrite grain boundaries. A heat-treatment cycle investigated with a view toward generating structures suited for superplastic consolidation of the rapidly solidified ribbons consists of quenching the ribbon in liquid nitrogen, tempering at 600 °C for 10 seconds, “upquenching” to 750 °C (austenitizing) for 10 seconds, and subsequently quenching again in liquid nitrogen. This treatment results in martensite grains highly misoriented with respect to one another and typically 0.5 µm or less in both length and thickness and cementite particles 0.4 µm or less in size. (Occasionally, longer martensite plates were observed; but they never exceeded 1 µm in length.) The microstructures produced here offer the potential for producing fine-grained ultrahigh carbon steels of very high strength without the brittleness associated with the formation of coarse carbide particles or the loss of strength due to graphite formation. This investigation has thus provided the basis for follow-on studies currently underway in ultrahigh carbon Fe-C-Cr and Fe-C-Cr-Si steels, with the intent of producing similar microstructures which will also exhibit enhanced high-temperature stability.