Evaluation of residual stress gradients in ductile cast iron using critical refracted longitudinal (Lcr) wave technique

Abstract

Critically refracted longitudinal (LCR) waves have been investigated as a possible technique for the evaluation of the residual stress gradients present in ductile iron castings. Residual stresses are likely to develop in ductile cast iron during the cooling phase of a casting process. A LCR probe fabricated for previous research was used to make the velocity measurements. The samples investigated were of different modularity and two were annealed while one was not. The samples were milled at increments of approximately two millimeters and the LCR velocities were taken-at each increment. This provided data for the mapping of velocity versus depth which represents the residual stress gradient. To confirm the presence of the residual stress gradient, strain gages were attached to similar samples and the samples were saw cut with the intent to relieve residual stresses. The strain gages indicated that residual stresses were relieved by the sawcutting. The strain gage data was found to support the LCR velocity map to an extent. Both sets of data indicate compressive stresses on the surface. The data also allowed for an evaluation of the annealing procedure used. The results showed that even after full annealing remnant stresses are still present in the bars tested.