Common consent. Castings must be sectioned to check metal-wall thickness. The area of dimensional inspection and accuracy involves the entire field of castings utilization by the machine shop.


METALLURGICAL INSPECTION: -

Metallurgical inspection includes chemical analysis, mechanical property tests, evaluation of casting soundness and product testing of special properties such as electrical conductivity, receptivity, magnetic effects, corrosion resistance, responds to heat treatment, strength in assemblies, surface conditions, cooling and surface treatment and others.


CASTING SOUNDNESS: -

Shrinkage cavities, blow holes, gas holes, porosity, hot tears, cracks, entrained slag, lapped or cold shuts surfaces etc. all will affect the casting soundness when they are present. In castings for aircraft, ordnance and other highly precision engineered applications absolute soundness and optimum properties are needed. These objectives will be met only when the casting inspection includes methods, which check the casting for soundness defects not visibly detectable.


PRESSURE TESTING: -

Pressure testing is used to locate leaks in a casting or to check the overall strength of a casting in resistance to bursting under hydraulic pressure. In this a fluid, oil or water introduced in to the casting. The casting is then subjected to a pressure, which is in excess of the maximum stress that the casting is supposed to enounter in service.





SECTIONING: -

Castings may be sawed up, and the sections examined for soundness. Macro etching may be used to locate suspected shrinkage, porosity or cracks.


RADIOGRAPHIC INSPECTION: -

X-ray or gamma rays are passed through the test piece, and the intensity of transmitted rays is recorded on a photographic film. Since most defects transmit the short wave length light than the sand metal does, the film is darkened more where the defects are in the line of the x-ray beam. The internal defects can be located in this way.



MAGNETIC PARTICLE INSPECTION: -


This method of inspection is used on magnetic ferrous castings for detecting invisible surface or slightly subsurface defects. The object is magnetized, and magnetic particles applied to the casting surface. Polar effects exist at the defects, which cause magnetic particles to be aligned around the defects.


SUPERSONIC TESTING: -

Sound waves audible frequency may be used to locate defects. In supersonic vibrations are initiated at on casting surface, they will be reflected from the opposite surface after a suitable time interval. Defect will also reflect the sound waves, but time required for reflection will be less than that of the opposite casting surface. The sound vibrations. The sound vibration and reflection measurements are made with equipment known as the “ supersonic reflectoscope”.



MECHANICAL PROPERTY TESTING: -

Castings and test bars must be tested to see that mechanical property specifications are met. Tensile, hardness, transverse, impact, fatigue and other properties are tested in accordance with standard procedures.



HEAT TREATMENT OT CASTING: -

Heat treatment of casting means, the controlled heating and cooling of the casting to secure the properties and conditions specially demanded.


Heat treatment of casting has two main purposes


Relief of stress development in cooling (of the casting) head removal, repair, welding or machining.

Development of structure sensitive properties by metallurgical changes.


STRESS RELIEF: -

Stress relief is often desirable when castings are liable to change dimensions to a harmful degree during machining or use. Stress if not relieved might later cause warpage or even failure of the casting.


Stress relief of casting may be carried out by mechanical means and thermal means.


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Mechanical means involve: -

Shot peening of the surface of casting to eliminate surface tensile stresses and introduce compressive stresses on the surface of the castings, this improves fatigue life of the castings.

Stresses in castings may be redistributed by stressing beyond the yield point. The use of autofrettage for pre-stressing large gun barrels is an example.


Large and complicated castings can be stress relieved by subjecting them to vibrations. It has been shown up to 25% of residual stresses in iron castings can be relieved by vibration.


Stress relief to some extend May also be achieved by ageing or weathering castings for prolonged periods before use.


Thermal means: -



Thermal means of relieving stresses in castings are much more common and effective than the mechanical means discussed above.


Stress relieving in gray iron castings: -


A typical stress- relief treatment consists in heating the casting slowly from about 800° F to 1100°F, holding one hour or more these and cooling slowly, at about 1100°F per hour to 400 to 600°F.