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

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 pet Mn into a plate with 20- and 160-mum-thick segregated layers. Samples were austenitized at 850 degreesC, continuously cooled at 1 degreesC/s and 0.1 degreesC/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 mum bands cooled at 1 degreesC/s had martensite and bainite in high-Mn bands. In contrast, samples with 20 mum 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.