1. Casting defects are often very difficult to characterize due to the intrinsic complexity of the casting process.

2. All defects will fall into one or more of the established categories of casting defects.

3. The proper identification of a particular defect is the prerequisite to correcting and controlling the quality of castings.

4. The nature of a casting defect can only be determined by correctly categorizing the shape, appearance, location and dimensions of the defect.

5. Once appropriately classified, the possible causes can be examined and the corrective action can be taken.

Casting has often been referred to as "a process that really knows how to keep a secret." Even in a controlled process, defects in the output can occur which defy rational explanation.

The complexity of the process is the result of integrating the varied disciplines of physics, thermodynamics and chemistry.

When these factors are combined with the problems associated with the high temperature alloys, the root cause of a casting defect can truly become a mystery.

Casting defects in a controlled process will generally fall into one of two categories.

1. Defects can be chronic, i.e., long-standing, which require a remedy through a process change usually via designed experiments

2. sporadic, which are sudden adverse changes in the normal capability of the process. Sporadic defects are probably the most troublesome for casters. In a controlled process defects do not just happen, they are caused. "If a defect occurs, measures must be adopted to eliminate its cause and prevent its repetition."

Defect Characterization Two distinct journeys must be taken to correct

sporadic defects. the diagnostic journey from symptom to cause the remedial journey from cause to remedy

There is a temptation to attempt to diagnose a defect by the possible causes; but, an incorrect diagnosis of the root cause can lead to an incomplete or incorrect remedy of the problem. It is important to correctly identify the defect symptoms prior to assigning the cause to the problem.

In general, a casting defect is defined as an observable and unplanned variation of a specification.

The identity of a particular casting defect is based upon the specific shape, appearance, location and dimension or profile of the anomaly.

The proper identification of a specific defect is the prerequisite to correcting and controlling the quality of castings.

A problem solving approach that is characterized by the unique "morphology of the defects is more logical than one based upon the causes since it requires no prior assumptions to be made

Categories of casting defects

1. Metallic Projections 2. Cavities 3. Discontinuities 4. Defective Surface 5. Incomplete Casting 6. Incorrect Dimensions or Shape7. Inclusions or Structural Anomalies

DEFECTS IN CASTING

1. Gas defects2. Shrinkage cavities3. Molding Material Defects4. Pouring Metal Defects5. Metallurgical defects

Gas Defects

Blow Holes Open Blows Air Inclusion Pin Hole Porosity

Blow Holes and Open Blows Cavities, mostly with smooth walls; bright or oxidised. In the case of large blowholes with corrugated walls too.

Always spherical form, even in large expansions (then always flat cavities). Frequently accompanied by slags and oxides

Typical characteristics: Rounded shapes, location always in the upper part of the castings and beneath cores or undercuts. Individual blowholes or large accumulations, then mostly with different sizes. Preferred defect locations: In parts of the casting in the top of the mould, often above larger cores. In thick parts, but also in thin walls. Beneath undercuts and large cores.

Apart from presence of moisture, they occur due to lower to venting and lower permeability of the mould

Blow Holes and Open Blows Remedial measures:

- Adequately dry cores- Larger proportion of new sand - Better gas permeability- Smaller proportion of binder- Check casting system - Reduce moisture content of the sand - Add a little oxide to the melt (e.g. rusty insert)

Air Inclusions The atmospheric and other gases absorbed

by the molten metal in the furnace or during the flow in the mold, when not allowed to escape would be trapped in the casting and weaken it.

It is due to high pouring temperature which increase the amount of gas absorbed

Or due to poor gating design such as straight sprues in unpressurized gating, abrupt bends and turbulence causing practices in the gating which increase the air aspiration and finally the low permeability of the mold itself.

Pin Hole Porosity Agglomeration of small, rounded cavities up to around 5

mm in size. Surface of the cavities mostly smooth and polished, occasionally with graphite. On contact with the atmosphere mostly clearly visibly pinholes in the casting skin; then the surface is often oxidised

Typical characteristics: All cavities have the same size and the long form is always oriented perpendicular to the surface, arranged at outer edges in the line bisecting the angles. Preferred defect locations: Outer edges and corners, as well as workpiece surfaces, defects always in the immediate vicinity of the surface. Often at mould parting lines or transitions to cores, then mostly arranged like strings of pearls/beads.

Remedial measures:- Optimise mould material    Water content  Nitrogen content   Residual moisture of the blacking- Nitrogen content in iron; set-point value < 80 ppm

Internal Shrinkage cavities Defect is almost always identifiable

during machining. Zones of visibly "loose" microstructure, occasionally sponge-like structure or an agglomeration of numerous small pores.

Remedial measures:- Check the gate and feeder system - Increase the degree of saturation - Reduce the casting temperature- Carry out a solidification simulation

Dispersed shrinkage Accumulation of small, crack-like cavities. Only

visible after machining. Cross-sections up to around 8 mm long and 1-2 mm wide, up

All cavities are the same size and always oriented perpendicular to the surface, arranged at the outer edges of the lines bisecting the angles. to 2 cm deep.

Remedial measures: - Minimise core sand intake - Reduce water content - Extend mixing time - Increase casting temperature

Molding Material Defects

CUTS AND WASHESThickening of the casting at one place.

Typical characteristics: Sand washout, combined with sand and /or slag inclusions because of low strength or molten metal flowing at high speedPreferred defect locations: Mostly close to the gate or on edges, which tend to heat

Cuts and Washes Remedial measures:

- Choose binder with good properties at high temperatures - Greater compaction of the mould sand - Avoid high flow speeds and long pouring times - Check use of blacking - Improve gate design to reduce turbulence ans increasing the size of the gates

Metal Penetration More or less thick sand adhesions on the

casting, which are held fast on the workpiece by iron penetrations. The iron between the grains cannot always be identified by the naked eye.

Widespread, frequently cushion-shaped form.

Can often not be removed by blasting. Penetrating adhesions in blackened moulds

and cores can often be cracked off the workpiece using a chisel

Typical characteristics: Metal between sand grains. Can often only be identified with a microscope.

Preferred defect locations: Thick parts, zones of inadequately compacted mould material and zones, in which cores or cods lie in the hottest parts. Preferably in the lower parts of the mould (higher casting pressure).

Fusion Manifestation:

Surface defect.Thin sand crusts, which firmly adhere to the casting.

Typical characteristics: Rough cast surface.Sand sintered on the casting, which can only be removed by grinding.

Preferred defect locations: Possible on all parts of the casting.Especially in thick-walled castings.Near to the gate.

Remedial measures:

- Add more new sand- Reduce dust fractions in sand - Uniform compaction - Check the use of blacking

Reduce pouring temeprature

Runout Manifestation:

During the casting, metal runs out of the box at the parting.

Typical characteristics: Casting is incomplete or fully missing

Preferred defect locations: The upper part of a casting is mostly missing

Remedial measures:

- Clamp the two halves of the box - Place weights on mould

Rattails Manifestation:

The defect involves a sand expansion defect, which can frequently occur in highly compacted parts of the mould.

Typical characteristics:Scratches appear on the surface of the casting, which can be partly arranged in parallel.

Preferred defect locations:On areas of the mould cavity covers but primarily on the bases. Can be mostly identified on the surface in the cast condition.Remedial measures:- Optimise mould material preparation with the aim of increasing the wet tensile strength - Fast mould filling- Reduce the casting temperature

Swell Manifestation:

Irregular, widespread thickening of the casting on the outside or inside.

Typical characteristics: Gradual, unwanted increase in wall thickness.

Preferred defect locations: Possible in all areas of the casting, however mostly in the lower part of the mould.

Remedial measures:

- Higher binder content- More uniform compaction- Reduce casting /sprue height

Washout/Erosion Manifestation:

Thickening of the casting at one place.

Typical characteristics: Sand washout, combined with sand and /or slag inclusions.

Preferred defect locations: Mostly close to the gate or on edges, which tend to heat up

Remedial measures: - Choose binder with good properties at high temperatures - Greater compaction of the mould sand - Avoid high flow speeds and long pouring times

Pouring Metal Defects

Mis Runs and Cold shuts Metal unable to fill the mold cavity completely Cold shut is caused when two metal streams

while meeting in the mold cavity do not fuse together properly, thus causing a discontinuity or weak spot.

Remedial measures:- Increase casting temperature - Increase casting speed and make gates larger

Cold shot inclusion/ spray bead Spray beads are pearl or bead shaped inclusions,

which are only loosely bonded with the metal. They are partly already visible at the surface, they are often not exposed until the machining.

The beads are created by turbulences during casting or by the effect of spray in the mould, metal particles prematurely solidify in drip or spherical form. These splashes oxidise very easily and the oxide skin prevents the spray beads from being re-incorporated by the subsequent melt flow.

Spray bead in micro-section (etched)      unetched:

                                             

Slag inclusion Irregular inclusions or cavities left behind by

inclusions, which have fallen out. Mostly on the surfaces in the top of the mould. Frequently grouped together with gas cavities, often with particles of mould material too. Tough slags often form lumps, the easily fusible slags float that upwards in the mould form skins, which at times can separate the whole casting wall. Occasional lustrous carbon on slag skins.

Glassy inclusion, several phases can be frequently be identified under the microscope. Skins wrinkled or creased, often bonded with a large number of mould material particles (scar like arrangement).

Preferred defect locations: As coarse slags at places which were in the upper part of the mould and under large area cores and core prints. Scar shaped finely distributed, frequently in the direction of flow on the surface of the work piece. As slag skin often running in a transverse direction to the casting wall. Occasionally relatively coarse slags inside castings too.

Remedial measures:

- Reduce elements which bond with oxygen - Properly dry ladles beforehand - Reduce pouring times - Increase mixing time

Hot Tears Material partition in liquid /solid state, which occurred

as the stresses, which occurred in the casting in the area of the elastic deformations, were larger than the strength of the material.

Stresses can occur in case of uneven cooling conditions (differences in wall thicknesses). Special design of the casting can prevent contraction. –shrinkage onto the core -Hot cracks primarily occur in steel castings.

preferred defect locations:- at sudden, steep transitions in wall thicknesses.- at wall thickness transitions with too small radiiRemedial measures:- Design suitable for stresses, balance out cross-sections with large wall thicknesses

Core damage Unintentional protruding core parts. Typical characteristics:

Parts of the core are displaced relative to each other.

Preferred defect locations: At parting lines of the cores.

Remedial measures:

Check core box guides - Check core boxes (is there a defect in the core box?)

Dimensional variation The casting has larger or smaller dimensions compared

to the dimensionally checked initial pattern released by the customer and the drawing.

Typical characteristics:This defect can conceal a range of casting defects, starting with pattern wear, so that it is not always possible to quickly narrow down the defect.

Remedial measures:- Check the pattern equipment and core boxes - Core print clearance- Check the mould making, mould box guides- Compact the mould

Dross Irregularly shaped interruption in the material

Typical characteristics: Dark scars, foamy dark surfaces, very finely distributed. Dross worsens the mechanical properties, Dross mainly consists of magnesium oxy-silicates and magnesium sulphides, and is a product of the reaction of magnesium with oxygen, sulphur and silicon.

Preferred defect locations: In the upper casting surface or beneath cores. Mostly in cast iron with nodular graphite, often only visible after the casting skin has been removed.

Remedial measures: Dross cannot be prevented, only reduced.

Melts:- High temperature - Low residual Mg contents- Clean melting of pure raw materials- Highly concentrated Mg master alloys or pure Mg- Gas purging- Refining slags (melt cleaner)

In the work pieces:- Draw off and restrain treatment slags (large pouring basin, optimum casting system)- Filters are very effective - Dry, closed moulds- HOT casting

Elephant skin Irregularly scared and/or creased

surface with net shaped notches

Typical characteristics: Parchment like appearance

Preferred defect locations: Normally in the top part of thick-walled castings on horizontal surfaces. The defect can effectively always be identified in unmachined casting.

Remedial measures:

- Use pure, base iron with low sulphur and oxygen contents - Use a teapot ladle- Adjust the degree of saturation of the melt according to the casting temperature - Possible extension of the dead-melting time after the magnesium treatment- Use refining slags- Use filter

Inclusion of foreign metal Manifestation:

Material not homogeneously formed.

Typical characteristics: Visible irregularities in the microstructure.

Preferred defect locations: On the inside of the casting, partly extending out to the surface too. during machining, the inclusions are worn down by the machining tool.

Remedial measures:

- Adequately high temperature- Clean melting of pure raw materials- Gas purging- Draw off treatment slags and hold back (large pouring basin,  optimum casting systems)- Mould fill rising, gently and fast - Filters are very effective - HOT casting

Mismatch in mold Manifestation:

Unintentional protruding casting parts.

Typical characteristics: Parts of the casting are displaced relative to each other.

Preferred defect location: On dividing planes.

Remedial measures:

- Check box guides, mould assembly and closure equipment- Check models

Fillet Manifestation:

Fillets in castings, especially in wall thickness transitions do not often occur according to the drawings, or they contain defects.

Typical characteristics:- Radii at the fillets are considerably smaller in the casting than in the patters. - The surface of the fillets is maculate.

Preferred defect locations:Fillets at wall thickness transitions with large agglomerations of mass. At these places the mould material is subjected to extreme thermal stresses .

Remedial measures:- Make radii larger than those given in the drawing. - Check addition of new sand

Mould damage Manifestation:

Irregular, widespread thickening of the outside or inside of the casting. After the pattern has been stripped, areas of the subsequent cast contour are broken off in the mould material area of the mould cavity.

Typical characteristics:Enlargement of the wall thicknesses or change in contour, the broken off areas of the mould material are then found in the casting as mould material inclusions, if the defect is caused when the mould is assembled and closed by pressing core prints or in the mould joint.

Preferred defect locations:Joint face or core prints

Remedial measures:-Re-open the mould after assembling and closing and check for pressure points and broken off edges.-Check pattern and core box, core print allowance

Rough casting surface / Roughness Manifestation:

Roughness must be assessed relative to the grain size of the casting selected. Under certain circumstances a workpiece cast in coarse sand with fully uniform surface must be assessed as being smooth, although it is rougher than a “rough area” on a workpiece cast in fine grained sand.

Preferred defect locations: Close to gate, parts deep in the mould, zones with inadequate compaction

Remedial measures:

- Use finer sand - Reduce water content - Increase compaction pressure - Reduce casting temperature

Defect Analysis of castings

Reasons for Casting Rejections

Casting Failed Pressurized

Leak Test (28%)

Visual Imperfection on

Machined Surface (52%)

Casting Breakage

During Machining

(16%)

Low Tool Life & Excessive

Machined Surface

Roughness (4%)

Defects Causing Surface Imperfection (relative frequency of each defect found)

Flow Lines (7.7%)

Cold Laps (46%)

Lubricant or Mold Coating Inclusions on Surface (16%)

Gas Porosity (85%)

Shrinkage Porosity (69%)

Oxide Inclusions

(38%)

Defects Causing Pressurized Leak Test Failure

(relative frequency of each defect found)

Gas Porosity (57.1%)

Shrinkage Porosity (71.4%)

Cold Laps (57.1%)

Mechanical Cracks (14.3%)

Defects Causing Casting Breakage at Machining (relative frequency of each defect found)

Gas Porosity (75%)

Cold Laps (75%)

Shrinkage Porosity (75%)

Oxide Inclusions (25%)

Mechanical Cracks (25%)

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