Critical Temperature Range in Standard and Ni-bearing Spheroidal Graphite Cast Irons
Describing the conditions for reaustenitization of spheroidal graphite cast irons is of interest for their heat-treatment after casting, e.g. for manufacturing austempered ductile irons. Differential thermal analysis has been used to characterize the direct eutectoid transformation and the reverse transformation, i.e. the reaustenitization. This has been applied to a standard and a Ni-bearing alloy, with a ferritic matrix for the former, both a ferritic and a pearlitic matrix for the latter. The results are discussed in relation with the stable and metastable three phase fields. While earlier description of the direct eutectoid transformation is confirmed, the one for reverse eutectoid has been found more complex and is amended.
Mechanical properties and rolling-sliding wear performance of dual phase austempered ductile iron as potential metro wheel material
Abstract Dual phase austempered ductile iron (dual phase ADI) was produced through intercritical austempering with various partially austenitizing temperatures. Partial austenitization within the intercritical interval has a profound influence on the amount and morphology of microstructure in dual phase ADI. The influence of partially austenitizing temperatures on the mechanical properties and rolling-sliding wear performance of dual phase ADI were also investigated. The results show steady increases in the tensile strength and yield strength with increasing partially austenitizing temperature, while the opposite behavior is observed for elongation. The optimum combination including ultimate tensile strength of 746 MPa, impact toughness of 125 J and elongation of 14% is obtained in the dual phase ADI partially austenitized at 810 °C, which is composed of 20% proeutectoid ferrite and 80% ausferrite constituent. Wear behavior could be understood from the manner the hardened layer fragmented and detached. The main wear mechanism under air cooling condition is delamination due to subsurface deformation. Wear rate decreases as matrix hardness increases. According to the results of mechanical and wear tests, dual phase ADI partially austenitized at 810 °C demonstrates superior wear performance as well as relatively reasonable mechanical properties, suggesting the potential of this material for application in metro wheels.
Free ferrite in pearlitic ductile iron—morphology and its influence on mechanical properties
The present research focuses on the study of the precipitation of free ferrite in ductile iron during cooling through the intercritical temperature range. The main objectives are to recognize the morphology of the free ferrite precipitated in pearlitic ductile iron alloyed with Cu and Mn, and to evaluate the mechanical properties resulting from different microstructures. The microstructures investigated were generated by normalising heat treatments from regular and low austenitising temperatures. The results indicate that different free ferrite morphologies are produced depending on the thermal cycles carried out. As-cast samples present bull's eye morphology, while austenite grain boundary precipitation dominates in recrystallised structures. Low austenitising temperature normalising causes, in the alloys studied, the formation of a fine network of free ferrite precipitated within the pearlitic matrix at the recrystallised austenite grain boundaries. Low austenitising temperature normalising markedly imp...