No one likes surprises in manufacturing, and this is especially true during heat treatment. To avoid them, our attention is often focused on the type of material being supplied to us and the process/equipment variables we must control. What we don’t often consider is the condition of the incoming raw material. In this regard, the result of alloy segregation and banding after heat treatment can cause considerable angst between the heat treater and their customer. Let’s learn more.
Many people believe steels are classified only by their chemistry. In point of fact, the steelmaking process used; the casting process employed (ingot or continuous casting); the size/shape of semi-finished (blooms, billets or slabs) or finished products (plate, sheet or bars); and the properties they are required to have for the end-use application play a major role.
Steels are not chemically homogeneous; they do not have uniform chemistry throughout the entire cross-sectional area of their manufactured shapes. When steel is cast, the first material to solidify is the outer area adjacent to the mold walls (this surface zone is referred to as the chill zone). This results in a thin layer of equiaxed crystals with the same composition as the liquid metal. These crystals continue to grow inward as columnar grains in a dendritic shape parallel to the thermal gradient. Finally, solidification ends as the liquid temperature drops and neighboring grains impinge upon each other in the central zone of the as-cast shape (Fig. 1).
The solidification process causes macroscopic as well as microscopic partitioning of chemical elements as the liquid metal cools. Macroscopically, segregation occurs at the centerline of continuously cast products and at the centerline, top and bottom of ingots. Microscopically, segregation occurs between dendrites throughout the solidified section. Subsequent mechanical hot working (e.g., rolling) creates longitudinal bands of varying elemental chemistry.
Microsegregation is the difference in composition between the center of the dendritic stem (i.e. the dendritic core) and the region between the dendrite arms. The first liquid to solidify (in the center of the dendrite) will be alloy-rich if the alloy addition raises the melting temperature, whereas the alloy (i.e. solute-rich) area will be concentrated at the interdendritic regions if the alloy addition lowers the melting temperature.
For the heat treater, alloy segregation can produce differences in the hardenability of the steel. The difference in hardenability between alloy-rich and alloy-lean regions can manifest itself by creating harder and softer areas of martensite and mixed transformation products such as bainite.
Banding is caused by segregation of the alloying elements during solidification. Subsequent hot-working operations result in segregation aligned in the direction of working, which results in the banded appearance delineated in the microstructure. The distribution of microsegregation (in wrought steels) depends on how much working has been done to shape the part. The diffusion rates of the alloying elements in steel control the homogenization of the casting. For example, chromium and molybdenum homogenize readily, while nickel homogenizes very slowly. The alternating bands of varying alloy chemistry result in different microstructures, orientated parallel to the rolling direction of the material (Fig. 2).
Additional mechanical and/or thermal treatments to remove or reduce microsegregation add to the cost of manufacture and oftentimes are not considered economically feasible. For example, a high degree of homogenization can result from soaking the segregated steel at an elevated temperature (often above 1200°C or 2200°F) for a very long time (up to 100 hours or more in some cases).
The primary cause of banding is due to the segregation of substitutional alloying elements (e.g., manganese, chromium, molybdenum) during (dendritic) solidification. Cooling rate, austenitic grain size and austenitizing temperature also influence the severity of microstructural banding.
Banding occurs in all steels. While reheating of as-cast products and hot rolling tend to reduce chemical segregation, other factors (related to solid-state phase transformations and residual solidification) result in a greater or lower degree of banding in the microstructure of all finished steel products.
Effect of Banding on Heat Treatment
In general, hardness and microstructure will be heavily influenced by segregation and banding. Alloy-rich areas tend to transform to martensite or bainite, while alloy-lean areas show increasing amounts of pearlite and ferrite (due to slower cooling rates). For case-hardened parts, the hardness of the case will be impacted, particularly if higher concentrations of retained austenite or bainite are formed in the primarily martensite structure. In certain applications, retained austenite converts to untempered martensite in service (Fig. 3), resulting in hardness and property variation that can cause dimensional change or even component failure.
Tensile, yield and fatigue strength tend to be relatively unaffected by the presence or absence of microsegregation, although ductility and toughness properties are negatively impacted.
As an example, spotty hardness and poor mechanical properties can result from segregated or banded microstructures having significant amounts of coarse pearlite, coarse ferrite or ferrite clusters. In induction hardening, higher temperatures and longer heating times are required to fully austenitize these structures and may result in grain growth, formation of coarse martensite on quenching, surface oxidation/decarburization and increased chance of distortion.Thus, having a prior microstructure consisting of fine pearlite or a quench-and-tempered structure is the most highly desired for induction hardening.
Segregation and banding may or may not be detrimental to the final product, but the effects of these conditions should either be evaluated before the raw material is heat treated or negated (to the extent possible) by appropriate thermal treatments (e.g., annealing, normalizing).
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