Page 330 - CITS - Welder - Trade Practical

WELDER - CITS

Job Sequence

- The major objective of radiographic testing of castings is the disclosure of defects that adversely affect the
strength of the product.

- Castings are a product form that often receive radiographic inspection since many of the defects produced by
the casting process are volumetric in nature, and are thus relatively easy to detect with this method.
- These discontinuities of course, are related to casting process deficiencies, which, if properly understood, can
lead to accurate accept-reject decisions as well as to suitable corrective measures. Since different types and
sizes of defects have different effects of the performance of the casting, it is important that the radiographer is
able to identify the type and size of the defects.
- ASTM E155, Standard for Radiographs of castings has been produced to help the radiographer make a better
assessment of the defects found in components.
- The castings used to produce the standard radiographs have been destructively analyzed to confirms the size
and type of discontinuities present.

- The following is a brief description of the most common discontinuity types included in existing reference
radiograph documents (in graded types or as single illustrations).
- Gas porosity or blow holes are caused by accumulated gas or air which is trapped by the metal. These
discontinuities are usually smooth-walled rounded cavities of a spherical, elongated or flattened shape.
- If the sprue is not high enough to provide the necessary heat transfer needed to force the gas or air out of the
mold, the gas or air will be trapped as the molten metal begins to solidify.
- Blows can also be caused by sand that is too fine, too wet, or by sand that has a low permeability so that gas
cannot escape.

- Too high a moisture content in the sand makes it difficult to carry the excessive volumes of water vapor away
from the casting.
- Another cause of blows can be attributed to using green ladles, rusty or damp chills and chaplets.

Wear protective clothing.

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CITS : CG & M - Welder - Exercise 96