Castings

Several factors effect the quality of metal castings.

Some of these factors are listed below:

• Coefficients of thermal conductivity
• Thermal expansion and contraction
• Chemistry
• Precision of molds and dies
• Shrinkage allowances
• Dryness of molds
• Casting design
• Method of pouring liquid metal
• Design of gates and risers

Imperfections in castings may not be of concern for many types of service. They are commonly referred to as casting defects since castings are not perfect. This is unfortunate as imperfections beyond engineering design specifications should be considered defects, while imperfections within engineering design specifications should not be considered defects.

Some casting imperfections may have no effect on the function or service life of castings.  Many imperfections are easily corrected by blast cleaning or grinding.  Other imperfections may be acceptable in some locations.

It is not uncommon for engineers to zone a casting drawing. Depending on the criticality of the location or zone the same imperfection would be judged acceptable in one location while unacceptable in another location.

Casting Discontinuities

Some common casting deficiencies are:

• Inclusions
• Porosity (blow holes, pinholes)
• Cold Cracking
• Hot Cracking
• Cold Shuts
• Surface irregularities
• Distortion
• Improper composition

Casting failures can be due to various causes.  Some castings fail due to design deficiencies, while other castings fail due to casting deficiencies.

Casting Failure Analysis

Casting failures can be due to various causes.  Improper loading or environment may contribute  to the cause of failure.  Casting imperfections may or may not contribute to the cause of failure.  Some imperfections may be commonly occurring discontinuities or anomalies that are normally expected to be present in castings.  Other imperfections are casting defects that result in failure of the casting.  Failure analysis can determine the cause of the casting failure and determine if a casting imperfections was the primary or contributing cause of failure.