In this work, a method has been proposed to measure surface/ subsurface average grain size in polycrystalline materials, using Rayleigh surface wave amplitude measurements. Following are the primary steps involved in the method:

1. sample preparation and heat treatment,
2. metallographic examination of the specimens,
3. determination of surface wave amplitudes
4. generation of calibration graph between the surface wave amplitudes and metallographically determined grain size,
5. determination of grain size in new specimens to validate the calibration graph and establishing the accuracy and reliability.

Specimens of AISI type 316 stainless steel were obtained from bar stock and given various heat treatments in order to obtain different grain sizes in the range 30µm to 170µm. Metallographic examination was carried out on each specimen and the average value of the grain size was obtained.

Rayleigh surface waves are generated when a longitudinal wave travelling in a liquid impinges on a solid surface at an angle of incidence equal to the Rayleigh angle r (with r = Sin-1 (Cp/Cs), where Cp is the velocity of the incident wave in the perspex delay line and Cs is the velocity of the surface wave in the material. Two 4 MHz contact type surface wave transducer along with a pulser/receiver model 10150PR were used to generate and receive the ultrasonic signals. Received signals were fed to a Tektronix digital oscilloscope and the wave amplitudes were measured on different specimens. For each specimen, several measurements were made and the average amplitude was used for the calibration. The calibration graph was drawn by plotting surface wave amplitudes against grain sizes obtained from metallography. It was seen that a good linear relationship exists between the average grain size and the wave amplitude.

Using the calibration graph, grain size measurements were made on new specimens which were having same chemical composition as that of the material used for making specimens with different grain sizes for generating the calibration graph. Results indicated that maximum inaccuracy in the determination of average grain size in these specimens is within 20%. This study demonstrates the usefulness of the measurement of Rayleigh surface wave amplitude for estimation of surface/subsurface grain size.