Solidification and Co-Crystallization of High-Chromium White Iron Castings
High chromium white iron castings are an important component of engineering and construction applications, offering superior strength and durability. The addition of high chromium to traditional iron alloy provides the distinctive white iron castings with outstanding wear resistance, abrasive strength, and fatigue resistance. With a microstructure consisting of white iron matrix, ternary eutectic phases (FeCr2-Fe3C and FeCr-Fe3C), and micro-constituents (carbide, bainite and polygonal ferrite); these components are often found in equipment operating in harsh and abrasive environments, such as those encountered in mining and petrochemical processing.
Due to the high chromium content, white iron castings pose unique and complex solidification and co-crystallization behaviors. Both the complexity of this behavior and the subsequent microstructural evolution determine, to a great extent, the eventual mechanical properties of white iron castings. This article discusses the solidification process of high chromium white iron castings, the co-crystallization of the constituent phases, and the associated factors that influence the microstructural evolution and mechanical properties.
The solidification of high chromium white iron castings is a process that penetrates from the outside-in, starting with the conventional liquid-solid transformation. Upon solidification, the initially perfectly mixed alloy microconstituents such as austenite, carbide, and ferrite; undergo an intricate series of transformations. Due to the high Solideus-Liqueous interfacial energy, the carbon and chromium atoms segregate and form a semi-solid shell at the surface of the casting.
The number of interfacial free energy minimization processes increases with decreasing as the casting decreases in temperature — increasing the complexity of the solidification behavior. The application of suitable cooling conditions, such as slow-nose or tapered-shell molding techniques, down-cone molding, and spray-cooling, are among the techniques used to reduce the complexity and enhance the resultant mechanical properties.
The co-crystallization of high chromium white iron castings is complicated. The total contribution of the phases involved in this process heavily depends on the cooling rate; corresponding transformation is occurring as the cooling rate changes. As the temperature decreases, the carbides and austenite nucleate, precipitate, and form insulating shells. As these phases form, the alloy’s chromium content shifts and the ternary eutectic phases (Cr2Fe3C and Cr-Fe3C) facilitate co-crystallization.
At the allelomorphic temperature (~A3), austenite starts to transform mainly into bainite, polygonal ferrite, and pearlite. The elongated polygonal iron grains are caused by the presence of carbides and the movement of iron atoms at the growth front of polygonal ferrite. At lower temperatures, the polygonal ferrite occupies most of the uncarbonized austenite and hypoeutectic regions without forming a uniform ferrite, leaving a non-uniform microstructure primarily consisting of polygonal ferrite and bainite. Finally, at the martensitic temperature (~ Ms), martensite and delta-ferrite form lamellarly, replacing the polygonal ferrite and thus completing the solidification process.
The solidification and co-crystallization of high chromium white iron castings are interdependent. This interdependence has a great influence on the mechanical properties of white iron castings. The processes are influenced by various external and internal factors, including alloy composition, cooling rate, mold design, casting defects, and the microalloying elements, such as boron and nitrogen. The application of cooling techniques that provide uniform cooling, and the addition of austenitizing and quenching processes can greatly improve the mechanical properties of the components.
