NDT X-Ray Testing For Castings Explained

Updated , by John Dahlstrom

NDT X-Ray Testing For Castings Explained

Castings are made using various methods and out of different materials. Sand molded castings frequently have more irregularities on their surfaces than castings made using investment methods and produced with a metal mold. Having a good understanding of various casting defects and casting processes helps to make radiographic evaluation effective. The following are the major defects found in weldments and castings.

1. Gas porosity becomes visible as elongated or round soft shady spots. #

This takes place in clusters or individually or distributed throughout the entire casting. It is caused by gas that forms during the solidification process by the evaporation of volatile material or moisture from the surface of the mold. Entrapment of air, venting, or insufficient core baking in the casting's cope surface prior o complete solidification may also cause this. Gas porosity is a term that is used to describe the shady dark spots that appear on the radiograph, with diameters that are usually around 0-1 mm.

2. Gas holes appear in clusters, isolated, or as dark circular images. #

Gas trapped in molten metal is caused by gas holes. If molten metal solidifies inside the casting before all of the gases have escaped, it will result in gas becoming trapped and produce gas holes.

3. Micro-porosity/Micro-shrinkage/Shrinkage porosity #

In aluminum alloy casting, they have an overall mottled appearance, while in magnesium alloys, they have a spongy appearance or appear as dark streaks. They are very fine cavities, normally around the grain boundaries. The defect occurs in the casting when the overall shrinkage of metal is more than the shrinkage factor that is normally expected. The defect is caused by the molten metal being improperly fed and occurs when there is a higher than ideal pouring temperature.

4. Shrinkage appears as jagged darkened areas, filamentary, or dendrite. #

They are caused by metal contracting while the casting is solidifying. Usually, this type of defect occurs whenever there is a change in the casting's section thickness, and temperatures are not uniform at various thicknesses.

5. Cracks #

These occur as either cold cracks (also referred to as stress cracks) or hot tears. Hot tears look like rugged dark lines with various widths and many branches without any definite continuity. Hot tears form immediately after or during the solidification process. Cold state cracks generally look like a dark, sharp, and continuously single line across the entire length of the casting. This type of crack occurs when internal stresses occur in a thermal gradient.

6. Dross #

Dross has the appearance of an irregularly shaped, round, or dark image caused by slag filling the void up that is entrapped inside a casting. They might appear to be lighter when the inclusion density is more than the parent material's density. Oxides, sand, or slag can cause inclusions.

7. Cold shuts #

A cold shut has the appearance of a dark line with a smooth, definite outline and variable length. Cold shuts for whenever two molten metal streams flowing from various directions do not mix. Cold shuts are created due to the molten metal being poured at too low of a temperature, interrupted pouring, or slow pouring.

8. Segregation Faults #

Segregation appears as either darker or lighter patches on the radiograph that depends on the alloy's segregated constituents' density. During the casting and melting processes, certain alloy constituents may separate away from the alloy. The local constituent concentration results in density differences on the radiograph when the segregated portion's density is different from the casting alloy's density. Local segregation can occur, where a hot tear or shrinkage is filled with segregate. The terms that are used are sealed hot-tear and shrinkage segregation.

9. Misruns #

Misruns appear as darkened areas with variable dimensions and a smooth, distinct outline. Misruns happen when the molten metal fails to completely fill up a casting section and leave the region void. That can also occur due to pouring at a temperature that is too low or due to a lack of fluidity.

10. Diffraction #

Diffraction mottling has an appearance of a spurious image on a radiograph. It is not a casting defect. The mottling effect has a star-shaped appearance in aluminum alloys or austenitic steel. This visualization effect is caused by radiation diffraction by the metal's crystals. This mottling appearance disappears when the radiograph is taken at an angle of incidence that is slightly changed.

11. Diffused Chaplets #

Diffused chaplets are a type of small bar that has endplates that are used to maintain the part of the mold core. Normally the chaplets ruse with the casting. When they are not fused, they appear as smooth, dark lines on the radiograph that conform to the chaplet's shape. It is caused by the metal being poured at a temperature that is too low for the chaplet to fuse.