Microstructure and residual stress evaluation of ductile cast iron using the critically refracted longitudinal (Lcr) wave propagation technique

Abstract

Residual stress and microstructure evaluation of ductile cast iron using a nondestructive method (Critically Refracted Longitudinal Ultrasonic Wave Technique) was approached. Residual stresses, both good and bad graphite nodules, and different matrix phases such as ferrite, pearlite and martensite are developed in ductile iron castings for various reasons. The presence of residual stresses and bad graphite nodules can both have a serious effect on the mechanical properties of the iron. This in turn could lead to the castings having unsatisfactory properties for which they were designed. A critically refracted longitudinal wave probe was designed and fabricated for the use of ductile iron evaluation. A group of bars were investigated for the relationship between Lcr wave velocity and the matrix as well as the modularity. Each of these bars was then heat treated and investigated for a relationship between LCR wave velocity and the matrix and modularity once again. The bars were also investigated for velocity changes before and after heat treatment. Also, residual stress measurements ductile iron bars was attempted using the in four sand cast LCR wave propagation technique. Acoustoelastic constants were determined experimentally for as cast, fully annealed, normalized, and determined These acoustoelastic quenched and tempered specimens. These acoustoelastic constants were then used to calculate residual stresses in the different heat treated specimens.