Standard Practice for Computed Tomographic (CT) Examination of Castings

Abstract

4.1 CT may be performed on an object when it is in the as-cast, intermediate, final machined condition, or as a flaw or failure detection tool. A CT examination can be used as a design tool to improve wax forms and moldings, establish process parameters, randomly check process control, perform final quality control (QC) examination for the acceptance or rejection of parts, and analyze failures and extend component lifetimes. 4.2 The most common applications of CT for castings are for the following: locating and characterizing discontinuities, such as porosity, inclusions, cracks, and shrink; measuring as-cast part dimensions for comparison with design dimensions; wall thickness quantification; and extracting dimensional measurements for reverse engineering. 4.3 The quality to which a CT image reproduces an object or a feature within an object is dictated largely by the competing influences of the specific geometry, spatial resolution, contrast sensitivity, and material of the object itself, and artifacts of the imaging system. Operating parameters strike an overall balance between image quality, examination time, and cost. 4.4 Artifacts are often the limiting factor in CT image quality. (See Guide E1441 for an in-depth discussion of artifacts.) Artifacts are reproducible features in an image that are not related to actual features in the object. Artifacts can be considered correlated noise because they form repeatable fixed patterns under given conditions yet carry no object information. For castings, it is imperative to recognize what is and is not an artifact since an artifact can obscure or masquerade as a discontinuity or affect dimensional measurements. 4.5 When processing castings in accordance to this document, Practice E3375 may be used to help in the development of an approved process.