macrosegregation

Macrosegregation is the large-scale, visible separation of alloying elements within a solidified casting. Unlike microsegregation, which occurs at the grain and dendritic scale, macrosegregation operates across whole sections or ingots, producing regions of significantly different chemical composition. It occurs when solidification is uneven, allowing liquid metal to flow and redistribute elements before the casting fully freezes.

The mechanism depends on density differences between solid and liquid phases. As dendrites form during cooling, they exclude solute atoms. This enriched liquid becomes either lighter or heavier than surrounding metal, depending on the alloy system. In steel ingots, the liquid iron-rich phase flows toward cooler regions, carrying dissolved elements like carbon and sulfur along with it. Large castings cool slowly from the outside in, creating thermal gradients that drive this convective redistribution over hours or days.

Common patterns and defects

Three main patterns emerge: normal segregation, where solute concentrates in the last-to-freeze center region; inverse segregation, where heavy elements float inward early; and channel segregation, where enriched liquid follows columnar grain boundaries to the ingot core. Severe macrosegregation produces hard brittle zones of high carbon or chromium content alongside soft, weak regions of depleted composition. These internal compositional cracks, sometimes called chevron cracks in the ingot center, can propagate during subsequent hot working.

The problem intensifies with casting size. A small laboratory ingot weighing kilograms shows minimal macrosegregation; a multi-ton industrial ingot shows severe banding. Thick sections cool slowly enough for gravity-driven liquid flow to segregate elements over centimeters of distance. Alloys with large solidification ranges, such as nickel-based superalloys or high-carbon steels, suffer most severely because the liquid remains mushy and mobile over a wider temperature interval.

Foundries and ingot makers combat macrosegregation through controlled cooling, electromagnetic stirring during casting, and hot-top casting practices that keep the ingot surface warm and slow. Vacuum induction melting followed by controlled solidification reduces but rarely eliminates it entirely. Subsequent homogenization heat treatment can reduce compositional gradients if time and temperature are adequate, though this is slow and energy-intensive for large ingots. Understanding where macrosegregation concentrates is critical for positioning ingots in the rolling mill: segregated core material is typically cropped away before hot working to specification.