CASTING DEFECTSCASTING DEFECTSBy: yaser elkelawyBy: yaser elkelawy

From: united company for From: united company for foundries. foundries.

General Origin of Defects in Casting

• 1. Casting Design.• 2. Technique of Manufacture

(Method).• 3. Application of Technique

(Workmanship).

Defects Grouped According to their Origin

1. Shaping faults arising in pouring.2. Inclusions and sand defects.3. Gas defects.4. Shrinkage defects.5. Contraction defects.6. Dimensional errors.7. Compositional errors and segregation.

Shaping faults arising in pouring.

• The liquid metal should satisfactorily fill the mould cavity and develop a smooth skin through intimate contact with the mould surface.

• The type of defects that might arise if this requirement is not met are:

Misrun; Shortrun; Cold laps; Cold shuts.

Shaping faults arising in pouring.

• Misrun or Shortrun: the metal solidifies prematurely and some limb or section of the casting is missing.

• Cold Laps: a less severe manifestation of misrun.

• Cold shuts: the discontinuity extends through the casting member.

Shaping faults arising in pouring.

These defects appear due to the following reasons:

Low metal temperature.Excessive chilling from the mould face.Too high moisture content in green sand.A method developed for dry sand used in

conjunction with green sand.Occurrence of rounded corners and edges and a

general lack of definition of sharp features and fine mould detail.

Shrinkage Defects• Shrinkage defects arise from failure to

compensate for liquid and solidification contraction so their occurrence is usually a symptom of inadequate gating and risering techniques.

• Types of shrinkage defects:o Major shrinkage cavity.o Discrete porosity:o Sinks and surface punctures.

Shrinkage Defects• Major Shrinkage Cavities Appear in those alloys of short freezing range i.e.

solidify by skin formation.• Discrete Porosity The longer freezing range alloys are subjected to

scattered porosity, susceptible alloys include: bronzes, gun metals, and numerous light alloys and phosphorous containing cast irons.

Chilling by inserting chills in the moulding material is used to combat surface porosity in these alloys.

Shrinkage Defects• Sinks and surface punctures: The solidified skins deform under

atmospheric pressure due to the occurrence of low pressure conditions within the casting.

This defect can be avoided by ensuring access of atmospheric pressure to the liquid metal in the feeder head.

Contraction Defects• Contraction occurs upon cooling from the solidus to

room temperature. Unlike the liquid and solidification shrinkages, which can be compensated by by influx of liquid, solid contraction affects all linear dimensions of the casting, hence standard pattern allowances are made.

• However, under practical cooling conditions, castings usually have thin and thick sections, so they do not contract freely and the metal develops cohesive strength to overcome significant resistance or hindrance to contraction which is offered by the mould or other parts of the casting itself which vary in their thickness.

Contraction Defects• Hot Tears: Hot tears or pulls are one form of this type of

defect often located at junctions where changes in section occur, as the thin part cools more rapidly then the thick part and when the thick parts cool and start to contract they cannot contract freely because the thin section has cooled enough to become hard and rigid so the material tears at the junction separating the thin part from the thick part.

Gas Defects • Defects of this type take the form

of internal blow holes, surface blows, airlocks, surface or subcutaneous pinholes or intergranular cavities.

Gas DefectsThey result from:• Entrapment of air during pouring.• Evolution of water vapour on contact

between liquid metal and moulding material.

• Precipitation during solidification as a result of chemical reaction or change in solubility.

Gas Defects• Gases enter liquid metal during melting

as temperature increases the solubility of gases in the liquid increases, and vice versa.

• The sources of gases are either: • mould gases, • Gases evolving from the metal upon

solidification.

Gas DefectsThe preventive measures are:Melting precautions:• preheating of charge materials to

evaporate surface moisture, also preheating any material added to the liquid bath during melting.

• Fast melting.• Using protective fluxes.

Follow Melting Precautions:

• Following proper melt treatment: oxidation-deoxidation reactions.

• Maintaining the metal temperature as low as possible.

• Drying and preheating all spouts, ladles, shanks, and furnace tools.

• Vacuum melting if justifiable.• Degasing molten metal: gas scavenging by

argon, nitrogen, or chlorine, or by vacuum degasing for non-ferrous metals.

Inclusions and Sand Defects

• Non-Metallic inclusions in castings may be considered in two Min groups:

• Indigenous arising from reactions within the melt (may not be regarded as defects).

• Exogenous resulting from entrainment of non-metallics during pouring (dross, slag, flux residues, or moulding material).

Inclusions and Sand Defects

• Inclusions are frequently concentrated at or immediately below the upper surfaces of castings, where they are revealed on machining or after scaling in heat treatment. It is advisable that the most important faces be placed downwards. Increased machining allowances are usually made on top surfaces of castings.

Inclusions and Sand Defects

The types of these defects are:• Erosion scabs: A rough projection on the

casting as sand is washed away.• Expansion defect: results from penetration

of liquid meatal behind the surface layer of the sand.

• Rat tail: Surface fissure or line defect.• Surface roughness and sand adherence.

Dimensional Errors• Such errors can occur in pattern

making, moulding, and casting, or fettling.

• Principal causes are misalignment of mould parts and cores, mould distortion, anomalous contraction and distortion in cooling.

Compositional Errors and Segregation

• The main causes for variation in composition from desired one are:

Melting losses. Incorrect furnace charge.• Careful melting practice is essential.• Segregation is unavoided (macro or micro) in

cast alloys.• Homogenisation treatment is necessary for

castings and it eliminates concentration gradients by diffusion.

SPHERICAL HOLESSPHERICAL HOLES

CO-gas Defects

Gas holes on top surface only, revealed by machining.

More severe type of the gas hole defects shown in previous figure with inclusion of slag particles.

Form: on the top surface, revealed during machining – sometimes discovered underneath a core.Causes:• Low pouring temperature• High Mn and S contents• Dirty ladles, lacks of skimming, poor refractory quality

SPHERICAL HOLESSPHERICAL HOLES

Hydrogen Pinholing

Small, spherical holes with shiny surface occurring just below casting surface

Shiny graphite film appearing on surface of defect type shown in previous figures.

Form: small spherical holes on all faces of the casting adjacent to the mould – with shiny surfaces.Causes:• Contamination of iron with Al. Form: - Scrap - Inoculates• Too high moisture content in sand• Too long runner system

SPHERICAL HOLESSPHERICAL HOLES

Nitrogen Defects

Occurrence of small spherical holes adjacent to core, often revealed immediately after knockout

Form: small spherical holes (sometimes more elongated) adjacent to coresCauses:• Nitrogen containing binders• Too much addition of resin• Too high N2-content in resin

ROUNDED HOLESROUNDED HOLES

Shrinkage Defects

Surface depression occurring at hot spot

Form: surface sink often with exuded bead of metal inside them or small depressions at hot spots with associated subsurface holeCauses:• too low mold rigidity causes yielding under ferostatic or solidification pressures• too high pouring temperature• too low carbon content

ROUNDED HOLESROUNDED HOLES

Gas Blowholes from Cores, Molds, or Inserts

Blowholes in plate casting

Causes:• Excessive moisture content of sand• Vents of cores are blocked• Cores are underbaked or with excessive binder content• Rusty or contaminated chills (denseners) or chaplets

Blowholes in cored casting resulting from metal entering core vent. The metal extracted from the core vent is also shown.

IRREGULAR HOLESIRREGULAR HOLES

Shrinkage DefectsMicrograph illustrating dendritic form of internal porosity

Form:• open metal, often interconnected areas• has a dendritic form• generally occurs in the heavier sections or hot spots such as positions adjacent to ingates or risersCauses:• lack of mold rigidity• high pouring temperature• over inoculation• high phosphorous content

IRREGULAR HOLESIRREGULAR HOLES

Gas Defects from Molten Iron

Form: discrete fissures rather than areas of interconnected open metal may be widely dispersed or more isolated

Causes:

• Too much gas content in molten iron• The more widely dispersed holes are due to N2

• The more isolated cavities are due to high H2 - content

Widely dispersed fissure defects

Larger, isolated fissure defects