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

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


1
  • Casting defects are often very difficult to
    characterize due to the intrinsic complexity of
    the casting process.
  • All defects will fall into one or more of the
    established categories of casting defects.
  • The proper identification of a particular defect
    is the prerequisite to correcting and controlling
    the quality of castings.
  • The nature of a casting defect can only be
    determined by correctly categorizing the shape,
    appearance, location and dimensions of the
    defect.
  • Once appropriately classified, the possible
    causes can be examined and the corrective action
    can be taken.

2
  • 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.

3
  • Casting defects in a controlled process will
    generally fall into one of two categories.
  • Defects can be chronic, i.e., long-standing,
    which require a remedy through a process change
    usually via designed experiments
  • 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."

4
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.

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  • 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

7
Categories of casting defects
  1. Metallic Projections
  2. Cavities
  3. Discontinuities
  4. Defective Surface
  5. Incomplete Casting
  6. Incorrect Dimensions or Shape
  7. Inclusions or Structural Anomalies

8
DEFECTS IN CASTING
  1. Gas defects
  2. Shrinkage cavities
  3. Molding Material Defects
  4. Pouring Metal Defects
  5. Metallurgical defects

9
Gas Defects
  • Blow Holes
  • Open Blows
  • Air Inclusion
  • Pin Hole Porosity

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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

11
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)

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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.

16
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.

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

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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

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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

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Molding Material Defects
  • CUTS AND WASHES
  • Thickening 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

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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

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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

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  • 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).

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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.

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  • Remedial measures - Add more new sand- Reduce
    dust fractions in sand - Uniform compaction -
    Check the use of blacking
  • Reduce pouring temeprature

37
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

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Rattails
  • ManifestationThe defect involves a sand
    expansion defect, which can frequently occur in
    highly compacted parts of the mould.Typical
    characteristicsScratches appear on the surface
    of the casting, which can be partly arranged in
    parallel.Preferred defect locationsOn 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

40
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

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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

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Pouring Metal Defects
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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

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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.

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Spray bead in micro-section (etched)     
unetched                        
                     
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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).

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  • 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

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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

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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?)

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Dimensional variation
  • The casting has larger or smaller dimensions
    compared to the dimensionally checked initial
    pattern released by the customer and the
    drawing.
  • Typical characteristicsThis 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

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Dross
  • Irregularly shaped interruption in the
    materialTypical 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.

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  • 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

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Elephant skin
  • Irregularly scared and/or creased surface with
    net shaped notchesTypical characteristics
    Parchment like appearancePreferred defect
    locations Normally in the top part of
    thick-walled castings on horizontal surfaces. The
    defect can effectively always be identified in
    unmachined casting.

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  • 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

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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.

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  • 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

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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

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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
    locationsFillets 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

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Mould damage
  • ManifestationIrregular, 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 characteristicsEnlargement 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 locationsJoint face or
    core prints

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  • 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

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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

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

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Defect Analysis of castings
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