Abstract
Austenitic stainless steel casts, welds and cladding exhibit a columnar grain structure which is elastically highly anisotropic. Consequently, the ultrasonic inspection of these materials is difficult due to the directional variability of the velocity and the attenuation of elastic waves. Beam skewing, bunching and spreading takes place. Thus, to improve the reliability of the inspection, one should know the columnar grain orientation within the material which is to be inspected. In this thesis, the use of surface acoustic waves in monitoring the spatial grain orientation in austenitic steels with a columnar grain structure is outlined. Surface acoustic waves used in the investigation were generated by using an ultrasonic goniometer that employs a simple reflection principle. Pole figures obtained from the austenitic steel casts, welds and cladding reveal that these materials possess strong fibre textures with the fibre/grain axis parallel to the columnar grain axis, but with somewhat isotropic behaviour around this axis. Macroscopically these materials thus possess a special orthotropic symmetry. SAW critical angles/velocities measured on the plane perpendicular to the grain axis show no directional dependence. On two other perpendicular planes, i.e. planes which include the columnar grain axis, SAW critical angles/velocities are directionally dependent in a manner which agrees closely with theoretical calculations based upon a special orthotropic model. Over some ranges of angles of rotation relative to the columnar grain axis, a pseudo-SAW mode was observed to accompany the real SAW mode. A correlation between the angle of grain tilt (gamma) (the angle of tilt of the fibre/grain axis from the free surface), and the SAW critical angle difference (Delta), (SAW critical angle along the Z'-axis - SAW critical angle along the X-axis), for an austenitic steel ingot has also been established and a calibration curve was obtained at 2. 5MHz. Similar correlation was also obtained for an austenitic steel cladding at 10.0MHz. With the help of a calibration curve, a rotated SAW critical angle/velocity curve would yield information about the angle of tilt of the fibre/grain axis from the free surface. The angle of deviation of the fibre/grain axis from a designated principal axis on the free surface (delta), may be estimated by noting the angle of displacement of the curve symmetry position. These are the angles that are needed in the construction of ultrasonic pole figures. The technique mentioned above has been successfully used in determining the preferred grain orientation in several samples of austenitic steel casts, weld and cladding. All the estimated grain orientations agree very closely with those indicated by visual examinations and X-ray pole figure measurements.