MOULDING DEFECTS AND REMEDIES

MOULDING DEFECTS AND REMEDIES

BLOW HOLES OF ANGLES:
Nature of Defect: Smooth wall cavities at the angles of the casting
Remedies: The cause is insufficient feeding and rate of solidification. Influencing by moulding sand is possible reduce the moisture content and the organic addition of the sand improve the venting of the mould.


METAL PENETRATION:
Nature of Defect: Firmly adhering agglomerates of sand/metal at hot spots and in softly rammed areas.
Remedies: Reduce compatibility of the sand. Improve ramming of the moulds. Increase amount of carbonaceous additive and coke in the moulding sand.


SWELLING:
Nature of Defect: Irregular bulges, in most cases widely spread, sometimes with rough surface and/or penetration.
Remedies: Reduce compactability of the moulding sand ram moulds better and more evenly. Make sand less sensitive to over-and underrating by increasing the amount of bentonite.


SCABBING:
Nature of Defect: Mushroom type defect with sharp edges parallel to the casting surface.
Remedies: Increase bentonite level. Use carbonaceous additive with a wide softening range. Increase the amount of carbonaceous additive in the moulding sand, Reduce amount of new sand.


SLAG INCLUSION
Nature of Defect: Non-metallic irregular shaped inclusions, when occurring, Tend to be on upper casting surface.
Remedies: The Defects are caused by slag being present with the molten metal in the pouring ladle. If the formation is due to eroded sand, reduce the amount of inert fines and use a more active instrous carbon producer increase milling time of the moulding sand.


SAND INCLUSIONS
Nature of Defect: Irregularly formed sand inclusions just underneath the casting surface, associated with products from the metal slag reaction in other areas.
Remedies: keep the moulding sand sufficiently plastic by good sand preparation and sand cooling Adjust to optimum compactability.  Reduce the tendency of the mould to stick to the pattern by the use of parting agents.


SURFACE ROUGHNESS:
Nature of Defect: Rough casting surface finish due to coarse sand
Remedies: Use finer sand increase amount of lustrous carbon producing material in the moulding sand. Increase the amount of fines. E.g. coke in the sand.


PIN HOLES
Nature of Defect: Accumulation of small subsurface rounded holes defects appear after annealing and light machining.
Remedies: Normally the defect is of a metallurgical nature. The mould material may have a certain effect, In case of pin holes the moisture, the organic additions and the amount of nitrogen in the moulding sand should be reduced.


PENETRATION
Nature of Defect: Roughness caused by the molten metal penetrating the immediate sand grains of the mould surface, particularly in softly rammed areas.
Remedies: Use fines sand improve ramming of the moulds. Reduce compactability at the moulding sand. Increase amount at carbonaceous additive. Increase amount at coke in the moulding sand.


SURFACE BLOWHOLES:
Nature of defect: Cavities with smooth walls singularly or in groups.
Remedies: Low casting temperatures are often the cause. Benefits may be obtained by increasing mould permeability. Reduce finer or add coarser new sand. Reduce organic additions to the sands.


SCARRED SURFACE:
Nature of defect: Scarred casting surface around heat spot, particularly with S.G. cast iron.
Remedies: Reduce amount of impurities present in the sand that are destroying the oxide skin.


MICRO-CAVITIES:
Nature of defect: Porous structure found in the area at the casting which is last to solidify.
Remedies: Defect occurs with alloys having a wide solidification range. Little influencing by the moulding sand. Reduction of organic materials and an increase in the moisture level will improve thermal conductivity of the moulding sand.

Forging Defects

1) Incomplete Penetration: Due to light and rapper hammer blowing the penetration of deformation during forging may be less. To minimize incomplete penetration forging of large cross-section are made in forging press.                                                               

2) Surface Cracking: It can occur as a result of excessive working of the surface too low temperature or as a result of hot shortness. A high sulphur concentration in the furnace atmosphere can produce hot shortness.

3) Cold Shut or Fold: When the two surface of metal fold against each other without welded completely a discontinuity is produced, it is called cold shut. A common cause of fold shut is too small and die radius and low temperature. It happened in close die forging common case of it is very small die radius.

4) Internal Crack: Internal crack can develop during upsetting of the cylinder or round as a result of circumferential tensile stress changes of internal cracking in less in case of close die forging. In open die forging using of concave dies during upsetting can minimize bulging and minimize the probability of cracking.

Extrusion Defects

1)Internal Pipe: Because of homogeneous of a billet, center of the billet moves faster than periphery. As a result of depth metal zone extend along the outer surface of billet. After 2/3rd of billet extruded, outer surface of billet moves towards the center. Since, the surface of billet after contain of oxidized layer (skin), this type of flow results in internal oxide stringers. This defect is known as internal pipe. Section of the extruded metal, it will appear as a ring. High friction between billet and container increase the probability of this defect.

2)Axial Hole: While extrusion is carried out to the point at which the length of the billet remaining in the container is about 1/4th its diameter, the rapid radial flow into the die results in the creation of an axial hole/funnel in the back end of extrusion. This region must be discarded. The length of this defect can be reduced by inclining the face of ram of an angle to the ram axis.


3)Fir Tree Cracking: When ram speed is too high for the extrusion , surface of the billet becomes badly rough. Repeat ting transverse cracking is called fir tree cracking.

 It is also called chevron cracking. It can decure at low extrusion ratio. High frictional resistance at the tool and billet interface produces a sound product while center burst occur while the friction is low


5)Surface Crack and Lamination: The lubricant film must be complete and continuous between the billet and container wall during extrusion. Gapes in the film will cause to initiate shear zones which can developed into surface cracks. Lamination of oxide can be created in the same way. 

DEFECTS OF ROLL PRODUCT

1) Long Edges: In rolling of steels if the rolls deflect in concave way the edges of the sheet will be elongated to a greater extent in longitudinal direction than the center which cause the metal sheet has long edges.

2) Edges Buckles:  If the edges are free to move relative to the center, the center portion at the sheet are stretched in tension and edges are compressed in rolling direction this results in an widely edge.

3) Zipper Breaks: Long edge may also result in short cracks in the center which is called zipper breaks.

4) Center Buckles: This problem may be minimized by making the role parallel to the axis. When the center of the sheet is more elongated more than the edge. The center portion is in compression and edge intention. Such a sheet usually content center buckles. Mild shape problem may be corrected by stretch rolling. Stretch leveling the sheet in a roller leveler.

5) Edge Cracking: During rolling elements across the width have some tendency to explain laterally.  This tendency is higher at the edge and lower at the center. In this carrier and in homogeneity occurs in homogeneity in stress distribution occurs along the thickness which need to edge cracking.

6) Center Cracking: Under savvier condition of low uniform stress distribution center speed may occur.

7) Alligatoring: In heavy reduction center tends to expands more than the surface with this type of deformation grates speed occurs to the center than the surface, So the surface are placed in tension and center is in compression. If there is any metallurgical witness along the center line of the slag fracture will occurs along the thickness, this is called alligatoring. Edge cracking is minimize by employing vertical edge roles.   

Advantages of Open Die Forging

1. Higher cross section, and higher length jobs can be forged.
2. Dies are flat mostly, so no die-sinking is needed. Thus, die cost is low.
3. High rate of deformation at impact blow or high pressure.
4. Fibrous structure is obtained. Uniform properties than rolled product.
5. High strength, good ductility and good fatigue strength is obtained.
6. Refinement of coarse grain and structure of ingot.
7. Flaws, e.g., slag inclusions, porosity, etc., gets welded due to heavy working.

Difference between Hot Working and Cold Working

HOT WORKING

1. It is carried out above re-crystallization temperature.
2. Refined and equiaxed structure is obtained.
3. Surface finish is not good as Cold Working because of oxidation and decarburisation.
4. Blow holes , porosity get welded during hot working.
5. Probability of cracking is negligible.
6. Diffusion occurs due to high temperature.
7. It increases ductility and toughness.
8. Dimensional tolerance is high.
9. Oxidation and Decarburisation may occur.
10. High loss of metal due to scaling.
11. Isotropic properties.
12. Less power is required.
13. Severe embrittlement of reactive metals unless protected by protective atmospheres.

COLD WORKING

1. It is carried out below re-crystallization temperature.
2. Distorted grain structure is obtained.
3. Surface finish is good.
4. No chance of welding the blow holes and porosity.
5. Probability of cracking is high.
6. Diffusion does not occur due to low temperature.
7. It increases strength.
8. Dimensional tolerance is low.
9. Oxidation and Decarburisation does not occur.
10. Insignificant loss of metal.
11. An-isotropic properties.
12. More power is needed.
13. No embrittlement.

Transformation Temperature notations in Iron and Steel (Heating)

i. Acm – In hypereutectoid steel, the temperature at which the solution of Cementite in             Austenite is completed during heating.
ii. Ac1 – Temperature at which Austenite begins to form during heating (around 723°C).
iii. Ac2 – Temperature at which α-iron changes to non-magnetic β-iron (around 768°C).
iv. Ac3 -- Temperature at which transformation of α-iron to Austenite is completed.
v. Ac4 -- Temperature at which Austenite transforms to δ-ferrite during heating               (around 1492°C).

[A- arrest, c- heating]