Casting Defects
Figure 26 schematically shows various defects that are arise or occur during casting, especially sand casting processes, some of defects are indicated as follow.
Shrinkage
Shrinkage of molten metal as it solidifies is an important issue in casting. It can reduce the 5-10% volume of the cast. Gray cast iron expands upon solidification due to phase changes. Need to design pattern and mold to compensate of this amount of volume decreasing is important. Shrinkage defect can be reduced by good design the riser and pattern and molds.
Porosity
Porosity is a phenomenon that occurs in materials, especially castings, as change of state from liquid to solid during the manufacturing process. Casting porosity which either effects the surface finish or as a leak path for gases and liquids. The pouring temperature should be maintained properly to reduce porosity. Adequate flux of metal, controlling the amount of gas-producing and materials in the molding, core making and sand mixes can help in minimizing this defect.
Hot tear
Hot tears are internal or external discontinuities or crack on the casting surface, caused by rapid contraction directly after the metal solidified. They may be produced when the casting is poorly designed and sudden sectional changes occur. Incorrect pouring temperature and improper placement of gates and risers can also create hot tears. Method to prevent hot tears may improving the casting design, achieving directional solidification and even rate of cooling, selecting proper mold and materials to suit the cast metal, and controlling the mold hardness in relation to other components of sand.
Figure 26 Schematic of various casting defects..
Blowhole
Blowholes are smooth round holes that are clearly on the surface of the casting. To prevent blowholes, moisture content in sand must be well adjusted, sand of proper grain size should be used, ramming should not be too hard and venting should be adequate.
Shift
A shift results in a mismatch of the sections of a casting usually as a parting line. This defect can be prevented by ensuring proper place of the pattern for die parts, molding boxes, and checking of pattern locating pins before use.
Warped casting
Warping is an undesirable deformation in a casting which occurs during or after solidification. Wrap edge occurs in large and flat sections especially. Wrap edge may also be due to insufficient gating system that may not allow rapid pouring of metal or due to low green strength of the sand mold or inadequate tolerance in the pattern / mold cavity.
Metal Penetration and Rough Surfaces
This defect appears as rough external surface of the casting. It may be caused when the sand has too high permeability, large grain size, and low strength. Soft ramming may also cause metal penetration.
Fin
A thin projection of metal, this not part of casting, is called a fin. Fins occur at the parting of the mold or core sections. Molds and cores incorrectly assembled will cause the fin. Insufficient weighing of the molds or improper clamping of flasks may again produce the fin defect.
Cold Shut
A cold shut is a defect in which a discontinuity is formed due to the imperfect fusion of two streams of metal in the mold cavity. The reasons for cold shut may be too thin sections and wall thickness, improper gating system, damaged patterns, slow and discontinuity pouring, poor fluidity of metal caused by low pouring temperature, improper alloy composition, etc.
Inspections of Casting
Visual inspection
Visible defects that can be detected provide a means for discovering errors in the pattern equipment or in the molding and casting process. Visual inspection may not suitable of the detection of subsurface or internal defects.
Dimensional inspection
Dimensional inspection is one of the important discovering of casting. When precision casting is required, we make some samples for inspection the tolerance, shape, size and also measure the profile of the cast. This dimensional inspection of casting may be conducted by various methods:
• Standard measuring instruments to check the size of the cast.
• Contour gauges for the checking of profile, curves and shapes
• Coordinate measuring and Marking Machine
• Special fixtures
X-Ray Radiography
In all the foundries the flaw detection test are performed in the casting where the defects are not visible. This flaw detection test is usually performed for internal defects, surface defects etc. These tests are valuable not only in detecting but even in locating the casting defects present in the interior of the casting. Radiography is one of the important flaw detection for casting. The radiation used in radiography testing is a higher energy (shorter wavelength) version of the electromagnetic waves that we see as visible light. The radiation can come from an X-ray generator or a radiation source.
Magnetic particle inspection
This test is used to detect the location of cracks that extend to the surface of iron or steel castings, which are magnetic nature. The casting is first magnetized and then iron particles are sprinkled all over the path of the magnetic field. The particles are moved to the direction of the lines of force. A discontinuity in the casting causes the lines of the force to bypass the discontinuity and to concentrate around of the defect.
Fluorescent dye-penetration test
This method is very simple and applied for all cast metals. It is applying a thin penetration oil-base dye to the surface of the casting and allowing it to stand for some time so that the oil passes into the cracks by means of capillary action. The oil is then completely wiped and cleaned from the surface. To detect the defects, the casting is painted with a coat of powdered and then viewed under ultraviolet light. The oil being fluorescent in nature, can be easily detect under this light, and thus the defects are easily detected.
Ultrasonic Testing
Ultrasonic testing used for detecting internal voids in casting is based on the principle of reflection of high frequency sound waves. If the surface under test contains some defect, the high frequency sound waves when emitted through the section of the casting, will be reflected from the surface of defect and return in a shorter period of time. The advantage this method of testing over other methods is that the defect, even if in the interior, is not only detected and located accurately, but its dimension can also be quickly measured without in any damaging or destroying the casting.
Macroscopic examination
The macroscopic inspection is widely used as a test in steel production because it is provide effective means of determining internal defects in the metal. Macroscopic examination may detect one of the following conditions:
• Crystalline heterogeneity, depending on solidification
•Chemical heterogeneity, depending on the impurities present or localized segregation
• Mechanical heterogeneity, depending on strain introduced on the metal.
Microscopic Examination
Microscopic examination can enable the study of the microstructure of the metal alloy, composition, type and nature of any treatment given to it, and its mechanical properties. In the case of cast metals, as steels, cast iron, malleable iron, microstructure examination is essential to discovery of metallurgical structure and composition. Grain size and distribution, grain boundary area can be observed by this procedure. Distribution of non-metallic inclusion can also be found from this process of inspection.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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