Casting Defects And Their Inspection Methods

There are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the cast product. In this section we compile a list of the common defects that occur in casting and we indicate the inspection procedures to detect them

Casting Defects

Some defects are common to any and all casting processes. These defects are briefly describe in the following points

Misruns 

Misruns are the castings which solidify before completely filling the cavity. Typical cases includes

• Fluidity of molten metal is insufficient

• Pouring temperature is too low

• Pouring is done too slowly

• Cross section of the mold cavity is too thin

Cold Shuts 

Cold shuts occurs when two portions of the metals flow together but there is a lack of fusion between them due to premature freezing. Its causes are similar to those of a misrun

Cold Shuts

Cold shuts results from splattering during pouring, causing the formation of solid globules of metal that entrapped in the casting. Pouring procedures and gating system designs that avoid splatting can prevent this defects

Shrinkage Cavity

Shrinkage cavity is a depression in the surface or an internal void i  the casting caused by solidification shrinkage that restricts the amount of molten metal available in the last region to freeze. It often occurs near the top of the casting, in which case it is referred to as a pipe

Micro-porosity

Micro-porosity consists of a network of small void distributed throughout the casting caused by localized solidification shrinkage of the final molten metal in the dendritic structure. The defects is usually associated with alloy, because of the protracted manner in which freezer occurs in these metals

Hot Tearing

Hot tearing also called hot cracking occurs when the casting is restrained from contraction by a unyielding mold during the final stages of solidification or early stages of cooling after solidification. The defect is manifested as a separation of the metal at a point of high tensile stress caused by the metal inability to shrink naturally. In sand casting and other expendable mold processes, It is prevented by compounding the mold to be collapsible. In permanent mold process hot tearing is reduce by removing the part from the mold immediately after solidification

Defects Found Primarily In Sand Casting

Some defects are related to the use of sand mold and therefore they occur only in sand casting. To a lesser degree other expendable mold processes are also susceptible to these problems.

Sand blow

Sand wash is an irregularity in the surface of the casting that result from erosion of the sand during pouring and the contour of the erosion is formed in the surface of the final cast part.

SCABs

SCABs are rough areas on the surface of the casting due to encrusting of sand and melt. It is caused by portions of of the mold surface flaking off during solidification and becoming imbedded in the casting surface.

PENETRATION

PENETRATION to the surface defect  that  occurs when the fluidity of the liquid metal is high and it penetrates into the sand mold or sand core. Upon freezing, the casting surface consists of a mixture of sand grains and metal. Harder packing of the sand mold helps to alleviate this condition.

MOLD SHIFT

MOLD SHIFT refers to a detect caused by a sidewise displacement of the mold cope relative to the drag the result of which is a step in the cast product at the parting line.

CORE SHIFT 

CORE SHIFT is a similar to mold  shift , but it is the core that is displaced and the displaced and the displacement is usually vertical. Core shift and mold shift are caused by buoyancy of the molten  metal.

MOLD CRACK 

MOLD CRACK occur when more strength is insufficient and a crack develops into which liquid metal can seep to form a ‘fin’ on the final casting.

INSPECTION METHOD 

Foundry inspection producing include 

• visual inspection to detect obvious defect such as misruns , cold shuts, and serve surface flaws; 
• displacement measurements to ensure that tolerances have been met; and (3) metallurgical , chemical, physical and other tests concerned with the inherent quality of the cast metal 

Tests in category include ; 

• pressure testing- to locate leaks in the casting; 
• radiographic methods , magnetic practical tests, the use of fluorescent penetrants; and supersonic testing-to detect either surface or internal defect in the casting; the and 
• mechanical testing to determine properties such as tensile strength and hardness. If defects are discovered but are not too serious it is often possible to save the casting by welding, grinding, or other salvage methods to which the customers has agreed.