Evaluating duplex microstructures in polycrystalline steel with diffuse ultrasonic backscatter

The performance of metallic components is governed in large part by the microstructure of the base material from which the component is manufactured. In this presentation, diffuse ultrasonic backscatter techniques are discussed with respect to their use for monitoring the microstructure of polycrystalline steel as a result of the manufacturing process. To improve the mechanical properties, the surface of polycrystalline steel is quenched, a process which transforms the initial phase to a pearlite phase within grains. A diffuse ultrasonic backscatter model is developed that includes the duplex microstructure through the addition of an additional length scale in the two-point spatial correlation function. This function defines the probability that two randomly chosen points will fall into the same grain and/or same crystallite. The model clearly shows the dependence of the diffuse ultrasonic backscatter signal with respect to frequency, average grain size and lamellar spacing of the crystallites. Experimental results are used to show how the two length scales can be extracted from the measurements. The spatial variation of the microstructure with respect to depth from the quench surface is also examined. These diffuse ultrasonic techniques are shown to have the sensitivity to deduce the duplex microstructure throughout the sample.