A review is presented of banding in hypoeutectoid and hypereutectoid steels The data available on hypereutectoid steels are quite limited and therefore a study is presented on banding in a 52100 steel Similarities and differences are identified in the banding that occurs in commercial hypoeutectoid versus hypereutectoid steels The distinct surface patterns of Damascus steels, which are nearly pure hypereutectoid steels, have recently been shown to be due to carbide banding It is shown that the carbide banding in the 52100 steel occurs by a distinctly different mechanism than the carbide banding of the Damascus composition steels

A review of microsegregation induced banding phenomena in steels

The development of microstructural banding in low-alloy steel with Mn segregation has been investigated through the use of artificially segregated steel, interrupted cooling techniques, and optical microscopy. Mn segregation was simulated by hot roll bonding thin sheets of 5140 steel with 0.82 wt pct Mn and modified 5140M with 1.83 wt pct Mn into a plate with 20- and 160-µm-thick segregated layers. Samples were austenitized at 850 °C, continuously cooled at 1 °C/s and 0.1 °C/s, and quenched from progressively lower temperatures to observe the evolution of the microstructure. The segregated band thickness had a striking effect on microstructural development. Samples with 160 µm bands cooled at 1 °C/s had martensite and bainite in high-Mn bands. In contrast, samples with 20 µm bands cooled at the same rate had pearlite in high-Mn bands. The dramatic effect of band thickness on microstructural development was due to growth of a fully pearlitic band at the interface between segregated layers. The formation of interfacial pearlite is discussed relative to redistribution of carbon between adjacent high- and low-Mn bands during cooling.

Development of microstructural banding in low-alloy steel with simulated Mn segregation

Microstructural banding of a hot-rolled HY-100 steel plate was accentuated by cooling slowly from the austenite region, which resulted in alternating layers of soft, equiaxed ferrite, and hard “granular ferrite.” The segregation of substitutional alloying elements such as Ni and Cr was identified as the main cause for the microstructural banding. Such banding induces anisotropic flow behavior at large strains, with deformation constrained by “pancake-shaped” bands of the hard granular ferrite. Tensile tests of circumferentially notched HY-100 specimens were performed in order to explore the stress dependence of failure in the slow-cooled as well as the quenched and tempered conditions. The failure behavior of the slow-cooled, microstructurally banded material exhibited a pronounced susceptibility to a void-sheet mode of failure. However, the absence of carbides within the equiaxed ferrite delays void coalescence and material failure to higher strains than in a quenched and tempered microstructure, despite the increased susceptibility to shear localization.

The effect of microstructural banding on failure initiation of HY-100 steel

In industrial applications failures of mechanical parts made of carbon and alloyed steels may develop either during heat treatment steps or final finishing operations. Such failures have high impact costs for manufacturers, since heat treated steel products, in general, are high value products which increase in value with each step in the production process until the final life-cycle manufacturing steps are completed. This work highlights the selection of steels to avoid premature ruptures developing during either the heat treatment steps or finishing operations with emphasis on the role of banding in the failure process. Failure does not have to involve fracture but may simply imply a decrease in performance of surface treated components as consequence of surface properties, even in the presence of correct heat treatment parameters. The root causes for banding in steels, are described in literature, and banding has major effects on final product properties (and causal relationships). Therefore, the causes of banding are studied and classified. Conclusions suggest that microstructural defects such as (micro)segregation bands and other defects such as slag and oxides inclusions are developed in the early fabricating cycle steps and can cause premature failure of either semi-finished or finished products.

Failures Induced by Abnormal Banding in Steels

The initial microstructure of transformation-induced plasticity (TRIP) steel was found to be an important factor in determining the final microstructure and mechanical properties and their sensitivity to TRIP heat treatment. The presence of banded structure rendered inhomogeneous distribution of retained austenite and resulted in inferior and anisotropic mechanical properties.

The effect of primary thermo-mechanical treatment on TRIP steel microstructure and mechanical properties

The microstructure of a hot-rolled low-carbon plate steel has been examined using a combination of light microscopy and scanning electron microscopy. It has been found that in the hot-rolled condition, the microstructure consists of alternate bands of ferrite and pearlite, together with relatively large, irregular pearlite nodules. These large nodules were found to be comprised of pearlite, intragranularly nucleated ferrite (both Widmanstatten and idiomorphic), together with carbide-deficient and/or carbide-free regions. It is argued that the carbide-deficient and carbide-free regions form as a result of the premature initiation of the pearlite reaction, i.e. pearlite forms prior to the body of the austenite grains attaining the eutectoid composition. In order to model the formation of the banded structure, specimens were reaustenitized at 1050 °C for 10 min and furnace cooled. This heat-treatment cycle produced an austenite grain size which was less than the chemical banding wavelength. A model for the decomposition of austenite, under these conditions, is presented.

J. A. Eckert

Papers

Banding and the nature of large, irregular pearlite nodules in a hot-rolled low-alloy plate steel: A second report