For the purpose of study, two types of materials were taken - one, the Clear Chill Roll material and the other, the Double Poured/Spun Cast material. The techniques used included a back- scatter ultrasonic measurement using 6 MHz frequency transducer in the twin crystal mode. The results obtained from the measurement were compared, with those from direct optical metallographic observation.

The clear chill primarily comprises iron-base carbide and is free from flaky graphite; the mottle zone, on the other hand, contains flaky graphite that forms where the clear chill zone ends. The presence of graphite in the mottle is known to be the major source of backscatter signals. The grain boundary areas are also a minor source for these signals, these can however be effectively suppressed by manipulating the threshold. In order to calibrate the CRO reading, a standard sample with a known chill-depth (determined microscopically) was used. The results of the ultrasonic measurement was then compared with microscopic data, the agreement being within ± 1 mm for a typical chill-depth of 4-9 mm. The microstructure and hardness data on the samples were also generated for a complete characterization.

The shell thickness measurement was carried out on the Double Poured/Spun Cast materials in which the core was spheriodal graphite and the case contained a partially mottled microstructure with predominantly iron-base carbides along with small quantities of flaky graphite. In this case, the variation between the ultrasonic data and those from the microstructure measurement was of the order of ± 2 mm in a typical shell thickness of 50-60 mm. The general microstructural and hardness characterizations have also been done. In addition, ultrasonic velocity and attenuation measurements at various depths of samples (from the roll surface) were conducted and the results interpreted in terms of available theories.

In summary, the work conducted shows that ultrasonic back-scatter technique can be used effectively in order to characterize the complex microstructure of the rolling mill rolls.