Some key points about ductile iron castings

With the rapid advancement of modern science and technology and the continuous expansion of industrial production equipment, the demand for thick-walled ductile iron castings in heavy-duty machinery is steadily rising. For instance, the ductile iron transmission gears used in large-scale ductile iron machines can reach diameters of 9 Rice, the gear rim thickness is nearly 200 mm Even when using separate casting, the quality of a single-cast blank still exceeds 30 Ton. 2800 The largest ductile iron casting produced in the ton injection molding machine reaches a wall thickness of 245 mm , quality exceeds 35 Ton. The container walls used for storing and transporting nuclear fuel are thicker than 400 Millimeters, with a mass of 70–120 Tons. These large, thick-walled castings have high quality requirements, and their metal structure, mechanical properties, and internal defects must undergo non-destructive testing and spheroidization rate inspection.

More than 50 Millimeter-thick ductile iron castings exhibit slow cooling and solidification rates. Their as-cast microstructure, mechanical properties, and casting processes also differ significantly from those of conventional castings. To produce thick-walled ductile iron components that meet stringent quality requirements, the casting industry has recently conducted extensive research into the solidification process and the evolving microstructural and property characteristics of these thick-walled parts. Moving forward, we will analyze and investigate the structure, performance, and manufacturing processes of these thick-walled ductile iron castings.

The most distinctive feature of the casting process for thick-walled ductile iron castings is that the molten iron, after inoculation and spheroidization, solidifies and cools at an extremely slow rate. Some castings take dozens, even hundreds, of hours to complete the solidification process. During this extended period, the microstructure and mechanical properties of the castings undergo significant changes, setting them apart from conventional ductile iron castings. In terms of microstructure, the following key characteristics can be observed.

( 1 ) A ferrite-dominated matrix structure has the opportunity to fully diffuse and migrate toward the graphite surface or form secondary graphite when the cooling rate is very low. As a result, the carbon content in the matrix decreases, while the amount of ferrite surrounding the spherical graphite significantly increases, along with ferrite grains distributed either in a network-like or blocky manner. Consequently, the volume fraction of ferrite rises, leading to the formation of a ferrite-based matrix.

( 2 In thick sections, the number of graphite spheres decreases, while their diameter increases. During the slow cooling and solidification process, the supercooling degree of the molten iron remains relatively low. Within a certain range, reducing the supercooling degree of the molten iron leads to an increase in the critical size of effective graphite nuclei and a decline in the probability of forming these nuclei. Consequently, as the thickness of the casting increases, the number of graphite spheres diminishes, the inter-sphere spacing widens, and the average diameter of the spheres grows larger.

( 3 In thick-section, non-homogeneous ductile iron castings with thick walls tend to experience slow cooling and prolonged solidification times, making them prone to spheroidal graphite decay and inoculant decay. The core and hot spots of the thick section often generate graphite flakes, while other areas frequently exhibit various types of abnormal graphite formations.