Fig. 1. 4340 Material exhibiting heavy banding (light and dark longitudinally oriented striations)

Question:
It has been suggested to us that we make it standard practice to check all incoming material for "banding." We are told that it can affect the microstructure, especially core hardness and strength. What do you think?

Answer:
An excellent source for technical information on this subject is provided by David van Aken (Ref. 1).

Basically, segregation and banding of the alloys in a steel occurs during the steelmaking process. The more heavily banded a steel, the more difficult it is to control hardenability. The difference in hardenability between alloy-rich and alloy-lean regions manifests itself as a banded microstructure (Fig. 1).

In 8620, for example, this would result in layers of pearlite (hard) and ferrite (soft) in the core, lowering its overall hardness. The technical reason for this is that in wrought materials, the cast dendritic microstructure is partially broken down during rolling, creating a banded microstructures of ferrite and pearlite. This is typically a result of segregation of manganese during ingot solidification. The greater the degree of hot or cold working, the finer is the spacing between the bands. The segregation of manganese also produces a partitioning of the carbon (because manganese is an austenite stabilizer.) Thus, as the steel is cooled through the critical temperatures to form ferrite and pearlite, the manganese-lean region has a higher critical temperature for the formation of ferrite.

In the case area of carburized microstructures, the alloy-rich regions have greater amounts of retained austenite because the segregated alloy reduces both the martensite start and finish temperatures. In hardened steel, segregation can be so severe that the microstructure is not fully martensitic.

Van Aken points out that segregation and banding in steel has always existed, but in the past it was economically feasible to homogenize the steel in large soaking pits. This is no longer the practice in most steel mills as most steels are continuously cast, and soaking pits would not be considered because the operation adds cost (equipment, space, time and energy). As a result, it is always a good idea to normalize a steel prior to its use.

The microhardness traverse (surface to core) on this sample revealed a non-uniform hardness (44 HRC at the surface and subsurface regions, 38–40 HRC in the core). Retained austenite is also present.