Unit - 6
Metal Casting
The costing process consists of charging a furnace, melting, pouring the metal into moulds, solidification, removal of casting, fettling and inspection.
Types of melting furnaces
The main types of furnaces used in foundries for melting of various varieties of ferrous and nonferrous metals and alloys are described below:
Pouring: Proper pouring of molten metal of correct temperature, plays an important in producing sound castings. In small works, it is done by hand. In case of large castings, and overhead crane is used to support and handle the receiving ladles. The ladle is lifted along with molten metal and password to desired destination. The pouring temperature of a metal should be kept as near to its melting point.
Defects: Large number of defects occur in land casting produced through various methods. The factors which are normally responsible to produce defects are as follows:
Various defects commonly occur in casting:
These winds may exist on the surface as depressions, called surface shrinkage, or within the casting, called internal shrinkage. Too much shrinkage may lead to crackles. Known as hot tears. This defect occurs on account of inadequate and improper gearing, risering and chilling.
4. Misron and cold shuts: When the molten metal fails to reach all the sections of the mould such that a certain part of it remains a field, resulting in an incomplete casting, the defect is known as misrun. When two streams of molten metal approach each other in the mould from opposite directions, fail to fuse together resulting in a discontinuous between them, it is known as a cold shut. both defects occurred due to lack of fluidity in the molten metal and faulty design.
5. Inclusions: Any separate nonmetallic foreign material present in the east metal, is called and inclusions. These inclusions may be in the form of oxides, slack, shirt, sand, and gas.
6. Hot tears: Also known as pulls or hot cracks. The main reasons of their occurrence are the low strength of metal after solidification, causing the metal to fail in coping up with excessively high stresses set up by solid shrinkage of the metal. These cracks maybe external or internal.
7. Cuts and unleashes: These defects occurred due to the corrosion of sand from the mould or core surfaces by the molten metal. The cavities formed on the mould and are surfaces due to this corrosion are filled by the molten metal and the some appear on the casting surface as an excess metal in the form of rigged spots. These spots are called scabs.
8. Metal Penetration: This defect occurs as a rough uneven external surface on the casting. It falls place when molten metal enters the spaces between grains and holds some of the sand tightly with it even after fettling.
9. Drop: This defect appears as an irregular the formation of the casting. it occurs on account of a portion of the sand breaking away from the mould and dropping into the molten metal.
10. Fusion: This defect appears as a rough Glassy surface over the casting, when the molten metal enters the mould cavity, it comes in contact with the sand and latter, under the action of excessive heat of the metal, melts and fused the casting surface.
11. Shot metal: This defect appears in the form of small metal shorts embedded in the casting, which exposed on the fractured surface of the latter. It happens when the molten metal is poured into the mould particularly when its temperature is relatively lower, it may splash.
12. Shift: A shift is a misalignment between two two metal surfaces, leaving a small clearance between them and changing their relative location. This may occur at the parting surface between the two parts of the mould, called a mould shift or at the core prints, called core shift.
13. Rat tails or buckles: When molten metal, poured into the mould, is at excessively high temperature, it causes the thin outer layer of moulding sand to expand appreciably when this layer fails to compress back and gets separated from the sand behind it, it remains over the casting and finally appears over it as an irregular line, called rat tails for a buckle.
14. Swells: The unintentional bulges found on the casting surfaces are known as swells. Search enlargements on the casting surface are caused by the liquid metal pressure which succeeds in pushing back the mould send at certain places.
15. Hard spots: Hard spots on surfaces are generally developed on iron castings rich in silicon content due to local chilling of those spots by moulding sand. Due to this chilling effect white cast iron is formed at these places, rendering them hard.
16. Run out: A run out occurs when the molten metal leaks out of the mould during pouring resulting in an incomplete casting. The main causes of this defect are defective molding boxes, which do not fit properly.
17. Crushes: A crush is a deformation of mould surface due to pressing or scrapping of the sand during setting of core or assembly of the mould boxes.
18. Warpage: It is an undesirable deformation in the casting which may occur during or after solidification. This deformation takes place due to informal stresses developed in the casting due to differential solidification and different sections.
Cleanings of castings:
After proper solidification of casting the moulds are broken to obtain the castings. After this cleaning processes are required:
Inspection of castings
Inspections of casting is done to defect the internal and external point broadly it can be classified into two methods
Destructive methods: It includes picking up of a few sample castings, cutting them into pieces, at the points where defects are suspected and then examining their sectioned surface to find out the internal defects. Following are drawbacks:
Due to these reasons DT are not much used and NDT methods are widely used
NDT
Sr No. | Defects | Possible causes | Remedies |
1 |
Blow holes | an excess moisture content in moulding sand. b rust and moisture on chills, chaplets and inserts used. c cores not sufficiently baked. d excessive use of organic binders.
e moulds not adequately vented.
f cores not adequately vented. g moulds rammed very hard.
| Control moisture content.
Use clean and rust-free chills, chaplets and metal inserts. Bake cores properly. Use organic binders with restraint.
Provide adequate venting in moulds and cores. Ram the moulds less hard.
|
2 | Porosity | a high pouring temperature b gas dissolved in metal charge. c less flux used
d molten metal not properly degassed. e slow solid ification of casting. f high moisture and low permeability in mould.
| a regulate pouring temperature. b control metal composition. c increase flux proportion.
d ensure effective degassing. e modify gating and risering. f reduce moisture and increased permeability of mould. |
3 | Shrinkage | a faulty gating and risering b improper chilling | an ensure proper directional solidification by modifying gating, risering and chilling. |
4 | Misrum and cold shuts | a lack of fluidity in molten metal. b faulty design c faulty gating | Adjust proper pouring temperature. Modified design Modify gating system. |
5 | Inclusions | a faulty gating
b faulty pouring
c inferior molding or course sand
d soft ramming of mould. e rough handling of mould and core. | Modify gating system.
Improve pouring to minimise turbulence.
Use a superior sand having more strength. Provide harder ramming. Take care in handling. |
6 | Hot tears or hot cracks or pulls | a lack of collapsibility in core. b lack of collapsibility in mould. c faulty design d hard ramming of mould | Improve core collapsibility. Improve mould collapsibility. Modified design Provide software ramming |
7 | Cuts and washes | a low strength of mould and core. b luck of binders in facing and core stand. c faulty gating | Improve mould and core strength. Add more binders to facing and core sand. Improve gating system |
8 | Metal Penetration | a large grain size sand used. b soft ramming of mould c moulding sand or core have low strength d moulding sand or core have high permeability e pouring temperature of metal to high. | Used sand having finer grain size. Provide harder ramming Increase the strength to required extent. Reduce permeability with the help of (a) and (b) Suitably adjust pouring temperature. |
9 | Drops | a Low green strength in moulding sand and core. b Too soft ramming
c inadequate reinforcement of sand projections and core | Modify sand composition for increase green strength. Provide harder ramming.
Provide inadequate reinforcement to sand projections and cope by using nails and gaggers etc. |
10 | Fusion | a low refractoriness in moulding sand. b faulty gating c too high pouring temperature of metal. d poor facing sand. | Improve refractoriness. Modified gating system Use lower pouring temperature. Improve quality of facing sand |
11 | Shot metal | a too low pouring temperature b excess sulphur content in metal. c faulty gating d high moisture content in moulding sand | Use higher pouring temperature. Reduce sulphur content. Modified gating system Reduce moisture content. |
12 | Shifts | a worn out or bent clamping pins. b mis alignment of two halves of pattern.
c improper support of core.
d improper location of core. e faulty core boxes
f insufficient strength of moulding sand and core. | Repair or replace the pins Repair or replace dowels causing misalignment. Provide adequate support to core.
Locate the core properly Repair or replace the core boxes.
Increase strength of moulding sand and core. |
13 | Rat tails or buckles | a continuous large flat surface on casting.
b Excessive mould hardness.
c lack of combustible additives in moulding sand. | Break continuity of large flat surface by providing grooves and depressions.
Reduce mould hardness.
Suitably add combustible additives to sand.
|
14 | Swells | a Too soft ramming of mould. b low strength of mould and core. c mould not properly supported. | Provide harder ramming. Increase strength of mould and core. Provide adequate support to mould. |
15 | Hard spots | a faulty metal composition.
b faulty casting design. | Suitably change the metal composition.
Modify the casting design. |
16 | Run outs | a faulty moulding
b defective molding boxes. | Improve moulding technique.
Change the defective molding boxes.
|
17 | Crushes | a defective core boxes producing oversized cores. b worn out core prints on patterns producing undersized seeds for cause in the mould. c careless assembly of cores in the mould. | Repair or replace core boxes.
Repair or replace core prints.
Take adequate care in setting of cores in the mould |
18 | War page | a continuous large flat surfaces on castings, indicating a poor design. b no directional solidification of casting. | Modify the casting design to break and the continuity of the large flat. Surfaces and facilitate proper directional solidification. |
References:
1. Hajra Choudhury S.K., Hajra Choudhury A.K. And Nirjhar Roy S.K., “Elements of Workshop Technology”, Vol. I 2008 And Vol. Ii 2010, Media Promoters and Publishers Private Limited, Mumbai.
2. Kalpakjian S. And Steven S. Schmid, “Manufacturing Engineering and Technology”, 4th Edition, Pearson Education India Edition, 2002.
3. Gowri P. Hariharan and A. Suresh Babu,” Manufacturing Technology – I” Pearson Education, 2008.
4. Roy A. Lindberg, “Processes and Materials Of Manufacture”, 4th Edition, Prentice Hall India, 1998.
5. Rao P.N., “Manufacturing Technology”, Vol. I And Vol. Ii, Tata Mcgrawhill House, 2017.
