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Casting Defect and solidification (1).pdf

  1. Casting Defects and Solidification ABP Prof. Aniket B.Pawar Mechanical Engineering Department RAJARAMBAPU INSTITUTE OF TECHNOLOGY, RAJARAMNAGAR Unit : 2
  2. 2 ➢ Types of fuel fired melting Furnaces- Cupola furnace, oil/gas fired furnaces, crucible furnaces, Electrical furnaces. ➢ Metal pouring equipment. ➢Introduction to permanent mould casting Process- Gravity and pressure die-casting, Centrifugal casting, Continuous casting. shell Molding, CO2 Molding, Investment casting ➢ Solidification control devices: chills, ceramics bricks, directional solidification . ➢ Pollution Control in foundries, Cleaning- fettling and inspection of casting. ➢ Defects in casting Content
  3. 3 CASTING DEFECTS ➢ A casting defect is an irregularity in the metal casting process that is undesired. ➢ It may sometimes be tolerated sometimes eliminated with proper moulding practice or repaired using methods such as welding, metallization etc. ➢ There are many types of defects which result from many different causes. Some of the remedies to certain defects may be the cause for another type of defect. ➢ Defects in castings occur due to various causes. Although it is quite difficult to establish a relationship between defects & causes casting defects are roughly broken down into five main categories : ➢ Gas defects, Solidification of metal, Sand problems, Moulding making problem.
  4. 4 CASTING DEFECTS ➢ Gas Defects • Blow hole • Porosity • Pin hole ➢ Solidification of metal • Miss Run • Cold Shunt • Hot Tears • Shrinkage Cavity ➢ Sand problems • Penetration • Swell • Fusion • Drop ➢ Moulding making problem - Shift
  5. 5 BLOW HOLES ➢ Balloon shaped gas cavities caused by release of mould gases during pouring are known as blow holes. ➢ Causes : • Ramming is too hard. • Cores are not sufficiently baked. • Excess moisture content. • Low sand permeability. • Excessive fineness of sand grains. • Rusted chills, chaplets & inserts. • Presence of gas producing ingredients. ➢ Remedies : • Baking of cores properly. • Control of moisture content in moulding sand. • Use of rust free chills, chaplets & inserts. • Provide adequate venting in mould and cores. • Ramming the mould less harder.
  6. 6 POROSITY ➢ Porosity is in the form of cavities caused due to gas entrapment during solidification. ➢ Causes : • High pouring temperature. • Gas dissolved in metal charge. • Less flux used. • Molten metal not properly degassed. • Slow solidification of casting. • High moisture and low permeability of mould. ➢ Remedies : • Regulate pouring temperature • Control metal composition. • Increase flux proportions. • Ensure effective degassing. • Modify gating and risering. • Reduce moisture and increase permeability of mould.
  7. 7 MISRUN & COLD SHUTS ➢ When the metal is unable to fill the mould cavity completely & thus leaves unfilled cavities it is called as misrun defect. ➢ When two metal streams meeting in the mould cavity do not fuse together properly causing discontinuity or weak spot inside casting it is called as cold shuts.
  8. 8 MISRUN & COLD SHUTS ➢Causes : • Low pouring temperature. • Faulty gating system design. • Too thin casting sections. • Slow and intermitted pouring. • Improper alloy composition. • Use of damaged pattern. • Lack of fluidity in molten metal. ➢ Remedies : • Smooth pouring with the help of monorail. • Properly transport mould during pouring. • Providing appropriate pouring temperature. • Modifying the gating system design.
  9. 9 SHRINKAGE CAVITY ➢ Shrinkage cavity is a void on the surface of the casting caused mainly due to uncontrolled and haphazard solidification of the metal. ➢ Shrinkage defects can be split into two different types: 1) External shrinkage 2) Closed shrinkage defects.
  10. 10 SHRINKAGE CAVITY ➢ Causes : • Inadequate and improper gating & risering system. • Too much high pouring temperature. • Improper chilling. ➢ Remedies : • Ensure proper directional solidification by modifying gating, risering & chilling system
  11. 11 HOT TEARS or HOT CRACKING ➢ Hot tears are ragged irregular internal or external cracks occurring immediately after the metal have solidified. ➢ Causes : • Lack of collapsibility of core & mould. • Hard ramming of mould. • Faulty casting design. ➢ Remedies : • Providing softer ramming. • Improve casting design. • Improve collapsibility of core & mould.
  12. 12 METAL PENETRATION ➢ Penetration occurs when the molten metal flows between the sand particles in the mould. ➢ Causes : • Low strength of moulding sand. • Large size of moulding sand. • High permeability of sand. • Soft ramming. ➢ Remedies : • Use of fine grain with low permeability. • Appropriate ramming.
  13. 13 SWELL ➢ A swell is an enlargement of mould cavity by localized metal pressure. ➢ Causes : • Insufficient or soft ramming. • Low strength mould & core. • Mould not being supported properly. ➢ Remedies : • Sand should be rammed evenly and properly. • Increase strength of mould & core.
  14. 14 Fusion and Drop
  15. 15 MISMATCH The casting that does not match at the parting line is known as Mismatch or Mould shift. ➢ Causes : • Worn out or bent clamping pins. • Misalignment of two halves of pattern. • Improper location & support of core. • Faulty core boxes. • Loose dowels. ➢Remedies : • Increase strength of mould & core. • Provide adequate support to core. • Proper alignment of two halves of the pattern. • Proper clamping of mould box. • Repair or replace dowels & pin causing mismatch.
  17. 17 Cleaning- fettling and inspection of casting ➢ Cleaning and inspection of castings are the processes through which products called castings are made neat and then subjected to examination before they are applied to particular use. ➢ Without cleaning and inspection of castings the castings can fail when used for certain purposes. ➢To avoid the failures, castings must be cleaned and inspected. ➢ Many buildings and constructions have failed today because proper inspections were not conducted on the metallic materials before their use.
  18. 18 Cleaning- fettling of casting The series of operations performed in cleaning shop may be grouped as follow: • Decoring • Removing of gates and risers. • Stripping off adhered sand from the castings. • Trimming burrs, rib. • Scale after heat treatment. • Straightening and painting of the castings if necessary. • Inspecting for acceptance after cleaning and cutting.
  19. 19 Inspection of casting Inspection in this context involves the operations that check the quality of castings and results to rejection or acceptance of the castings. Fundamental types of inspection are ➢ Visual inspection. ➢ Dimensional inspection ➢ Metallurgical inspection. ➢ Pressure test, Magnetic particle inspection, Radiographic test, Ultrasonic inspection etc. are used
  20. 20 ➢ The properties of the casting significantly depends on the solidification time cooling rate. ➢ Shrinkage of casting during cooling of solidified metal should not be restrained by the mould material otherwise internal stresses may develop and form cracks in casting. ➢ Proper care should be taken at the design stage of casting so that shrinkage can occur without casting defects. Solidification and Cooling
  21. 21 Solidification control devices ➢ A chills are the metal inserts used to increase the solidification rate. ➢ They are placed in critical regions of the mould. ➢ They are placed where the faster metal solidification is required for reducing shrinkage porosity. ➢ They are made up of same material as castings. ➢ It is used where it is impossible to provide riser for a part which is heavy. This may be due to complex shape of casting ➢ Metallic chills have the capacity of increasing the solidification rate as high as 14 times than sand moulds of same condition.
  22. 22 External Chills ➢ It is Rammed in mould sand walls. ➢ It is a good way of controlling cooling rates in critical regions of casting. ➢ Direct external chill is in level with mould cavity wall and thus comes in direct contact with the liquid metal. ➢ Indirect external chill is placed behind the mould cavity wall and is fully buried in sand. ➢ It does not come in direct contact with the liquid metal. ➢ Sometimes indirect chill is placed at an angle to casting surface. So its chilling effect decreases as the chill tapers away from casting ➢ Direct chills are more effective than indirect Chill. ➢ Normally made up of steel, cast iron and copper. ➢ Helps to eliminate porosity (micro and micro) at the portions of casting which is difficult to fed by riser.
  23. 23 Internal Chill ➢ Internal chills fuse into and become part of the casting. So these are made of the same metal which is to be cast. ➢ It is rarely used because structural homogeneity is less due to improper fusion which results in poor casting quality. ➢ Normally used in the area where it is used is to be removed after solidification. For example bosses to be drilled/bored. ➢ One end of these remain hanging in mould cavity and the other end is supported in sand.
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  25. Materials that • Withstand high temperatures and sudden changes • Withstand action of molten slag, glass, hot gases etc. • Withstand load at service conditions • Withstand abrasive forces • Conserve heat • Have low coefficient of thermal expansion • Will not contaminate the load What are Refractories 25
  26. Refractories Refractory lining of a furnace arc Refractory walls of a furnace interior with burner blocks (BEE India, 2005) 26