Which type of discontinuity is identified as a finishing processing discontinuity?

The term "finishing processing discontinuity" typically refers to issues that arise during the final stages of manufacturing, rather than during the initial forming processes. A heat-treat crack occurs during the heat treatment phase, which is considered one of the finishing processes in metalworking. This type of discontinuity is often due to stresses that are introduced during the cooling or heating stages, potentially leading to cracks in the material as it transitions through different phases.

In contrast, internal forging bursts are related to the forging process itself and indicate a problem during the material shaping phase rather than a finishing issue. Casting defects arise during the casting process, where issues like insufficient filling or improper cooling can lead to various structural flaws, again not aligning with the definition of finishing processing discontinuities. Porosity, while it can be a defect found in finished products, generally originates from the manufacturing process itself, particularly in casting, and thus also does not fit the category of finishing processing discontinuities.

Thus, heat-treat cracks are specifically associated with the changes and stresses that materials undergo during heat treatment, marking them as the correct example of a finishing processing discontinuity.