Question

Explain the nature of defect in casting process and identify the defects through various NDT techniques.

Answer 1

Casting defects are undesired irregularities observed in the material manufactured from a metal casting process.

They are broadly classified into five categories depending on the cause of the deformation.

They are

1) Mold material defects

2) Pouring metal defects

3) Gas porosity defects

4) Shrinkage defects

5) Metallurgical defects

I've listed these defects in the order of occurrence of events in a casting process.

1)Mold material defects: These defects occur when the properties of mold material being use are not suited for the specific casting process. These include strength,Grain size,retained moisture etc;

The defects include local solidification,cracks causing burrs etc.

2) Pouring metal defects:These defects occur due to adoption of wrong pouring techniques of the molten metal into the mold. They include inclusions,cold shuts and mis-runs.

3) Gas porosity defects: It is the formation of bubbles within the casting after it has cooled. These occur when the mold material are not porous enough for the gases to escape out of the mould cavity. Depending on the size of hole they are names as Blisters,porosites or blowholes.

4) Shrinkage defects:They occur when the standard feed metal is not enough to compensate the shrinkage of the liquid metal inside the mould on thickening after cooling.pipes,caves and shrinkage porosity are some of its types .

5) Metallurgical defects:Hot tears and hot spots are two such defects. Hot tears or hot cracking occurs as the material cools due to the residual stresses formed when the metal was hot causing the material to fail. Hotspots are localised areas of elevated temperature , causing the metal here to cool at a quicker rate than the surroundings.

NON DESTRUCTIVE TESTING is the inspection of a material for potential defects without having to destroy it in some way such as by cutting.

Some of the NDT methods that can be used to identify the casting defects include :

VISUAL INSPECTION:some of the more obvious and evident defects such as surface roughness,cracks,shifts and omission of core can be identified by looking at the casting carefully and hitting it with mallet to listen for a change in the quality of tone from an ideal one.Another test called COIN TESTING is similar but involves hitting with a coin instead.

HYDROSTATIC PRESSURE TEST:To do this test,all holes which allow flow of water which are present in the design such as flanges or ports are blocked. Now the while casting is submersed under a soap solution for checking if any air bubbles emerge proving the present of a leakage due to defect.

MAGNETIC PARTICLE INSPECTION:In this method, magnetic particles are spread over the casting and a magnetic field is induced around it. Presence of any cracks or voids changes the field intensity abruptly causing the particles to border round around the defected area.

ULTRASONIC INSPECTION:In this method, an oscillator is use to send an ultrasonic signal which readily permeates a homogenous medium unless obstrcuted by a discontinuity,identifying the presence of a clustering or a void.This method is not suited for materials with high damping capacity such as cast iron which weaken the sound signal over a drastically distance.

DYE PENETRATION TESTING: This method which is used to detect invisible surface defects of non-magnetic materials required the material surface to be coated,brushes or dipped into a die of fluorescent nature .After cleaning away the surfuce and looking at the surface in the dark,surface defects are readily discerned.

RADIOGRAPHIC EXAMINATION:In this method, both X-rays and ?-rays are used. With ?-rays, more than one film can be exposed simultaneously; However, x-ray pictures are more distinct. Voids,cracks,pours and tears all can be detected The film appears darker at defected areas compared to the surrounding areas.However, It is an expensive method and is used only for sub- surface exploration.

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