*Casting Processes and Modelling of Metallic Materials*

standards, 3D CT scans can be collapsed into equivalent 2D images, using a "thick slab" feature of CT scanning image analysis software [24].

The standard digital reference radiographs provide resolutions as low as 20 μm (pixel or voxel size) – however, it may not be possible to generate such high resolution production radiographs in practice due to equipment limitations, especially in the case of high density materials such as steels. Hence, a 100 μm resolution is seen as adequate for detection within the typical SDAS length scale (for ferrous metals).

Allowable porosity levels are often based on client specifications, but may also be part of quality standards associated with particular components. Such acceptance criteria, as in the case of high-performance valves for example, are shown in **Figure 16**.

The inspection process requires the area of the production radiograph to be prorated to an area of interest of the reference radiograph, as illustrated in **Figure 17**, and visually compared to establish the matching severity level.


This inspection process is considered to be subjective, and other objective methods based on image processing have been suggested [25, 26], a subject for

*Shrinkage Porosity in Steel Sand Castings: Formation, Classification and Inspection*

An overview of shrinkage porosity is given, with reference to steel sand castings,

1.Shrinkage micro-porosity arises during the solidification contraction of an alloy. Hence, the steel's phase transition and density properties in the mushy zone are important in understanding the likely mechanisms and expected

2.Linked to this, the lack of interdendritic feeding of the mushy zone as its solid fraction increases beyond a certain point, referred to as the coherency point, leads to interdendritic pore nucleation. In large freezing range alloys, the result is a sponge morphology, migrating to feathery linear morphologies as the

3.However, the cooling rate is a determining factor for secondary dendrite arm spacing, and hence level of shrinkage porosity in an alloy. This gives the foundry a degree of control in avoiding the more severe linear porosity.

4.At the intergranular level, entrapped liquid melt can lead to hot tearing, due to the weak resisting forces of the coherent dendritic structure, succumbing to inward pressure. Hence, hot tearing may show up as macro-porosity in

5.Shrinkage macro-porosity, on the other hand, is due to large entrapped liquid volume that cannot be adequately fed by the feeder system because of skin freezing. Whilst hotspots can easily be overcome through part and mould design modifications, narrow long sections may experience layered porosity due to declining pressurization of the mould as the skin layer thickens. Special attention should therefore be given to the design of the feeder system in such castings. This also goes for hotspots in the junction region between the casting and a feeder, which can lead to pipe shrinkage porosity extending into

6. It was shown that certain thermal criteria may be used in practice in

acceptance criteria for the various types of shrinkage porosity.

determining or reducing the level of shrinkage porosity, both micro-porosity

7.Finally, assessing the severity level of porosity in a casting is necessary in determining the acceptability of cast part quality. A range of standards have been issued, which provide the foundry with a benchmark in terms of

freezing range decreases (with decreasing carbon content).

from investigating the causes of shrinkage porosity to possible mitigation approaches, and finally the importance of understanding the technologies associated with the inspection and evaluation of shrinkage porosity severity levels. Further analysis of certain aspects of shrinkage porosity may be undertaken where an in-depth understanding is required. In summary, the following points

further development in the future.

*DOI: http://dx.doi.org/10.5772/intechopen.94392*

morphologies of shrinkage porosity.

**6. Summary**

are noted:

castings.

the part.

**151**

and macro-porosity.

#### **Figure 16.**

*Acceptance criteria for shrinkage porosity in steel valve castings of wall thickness* ≤ *50.8 mm (ASME B16.34), showing the critical regions on the right.*

#### **Figure 17.**

*Evaluating the porosity severity level by comparing the production radiograph with the reference radiographs of comparable resolution (e.g. 100 μm = 10 pixels/mm) from ASTM E2868, for a particular category of shrinkage porosity (e.g. CD - combined shrinkage porosity).*

*Shrinkage Porosity in Steel Sand Castings: Formation, Classification and Inspection DOI: http://dx.doi.org/10.5772/intechopen.94392*

This inspection process is considered to be subjective, and other objective methods based on image processing have been suggested [25, 26], a subject for further development in the future.
