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Foundry-Institute
Seminar Metallurgical defects of cast
steel Claudia Dommaschk TU Bergakademie
Freiberg Foundry Institute, Germany
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• Structure
• Gas cavities
• Oxide and slag inclusions, Nonmetallic
• inclusions
• Shrinkage cavities
• Hot tear
• Primary grain boundary fracture
• Defects caused by heat treatment

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Gas cavities

• Description and reasons
• Cavities in castings, especially in the upper
  parts of the castings
• Formation during solidification because of
  degrease of gas solubility
• often in combination with oxide and slag
  inclusions
• formation of gas cavities depends on the
  concentration of oxygen,
• nitrogen and hydrogen
• the inner surface of the cavities is smooth

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Gas cavities
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Gas cavities

• Prevention
• use of dry materials and ladles
• use of clean charge
• degasification of the melt
• look at the mould sands (permeability of gas,
  vent)

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Oxide and slag inclusions, nonmetallic inclusions

• Description and reasons
• Classification endogenous and exogenous
  inclusions
• endogenous inclusions are caused by the reaction
  products during the
• melting process (especially during deoxidation)
• exogenous inclusion are caused by other
  materials in the melt
• (e.g. refractory lining)
• thin fluid slag can precipitate at the grain
  boundaries ? danger of
• formation of hot tears is higher
• Classification of size
• Macro inclusions gt 20 µm
• Micro inclusions lt 20 µm

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Oxide and slag inclusions, nonmetallic inclusions
Slag inclusions GX3CrNiMoN17-13-5
GX2CrNiMo18-14-3
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Oxide and slag inclusions, nonmetallic inclusions

• Prevention
• use of clean charge
• optimization of gating and feeding system
  (lamellar flow)
• decrease of the dissolved oxygen
• decrease of the overheating temperature

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Oxide and slag inclusions, nonmetallic inclusions

• Example G42CrMo4
• nonmetallic inclusions arise by
• reason of the reactions during the
• melting process

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Shrinkage cavities

• Description and reasons
• specific volume of melt is higher than
• the specific volume of solid ?
• contraction during solidification and
• cooling
• feeding is necessary if the feeding
• is not optimal ? formation of
• shrinkage cavities
• the shrinkage volume of cast steel is
• about 4-7
• the inner surface is rough

Liquid shrinkage
Solidification shrinkage
Specific volume
shrinkage
RT TS TL TP

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Shrinkage cavities
GE 300 (GS 60)
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Shrinkage cavities

• Prevention
• use of optimal feeding system (calculation and
  simulation)
• warranty of directional solidification
• use of exothermic feeder sleeve
• decrease of the pouring temperature

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Hot tear

• Description and reasons
• hot tears are intercrystalline discontinuity
• cracks run along the grain boundaries
• the risk of cracks at alloys with a high
  freezing range is higher than with
• a small freezing range
• the reason are stresses during solidification
  because of hindered
• contraction (residual stress)
• the main reason for formation of hot tears are
  the geometry of casting
• if melt can flow into the crack - partial or
  completely annealed hot tears
• are possible

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Hot tear
Influence of Manganese and Sulphur content on the
inclination of hot tears
Influence of Carbon content on the inclination of
hot tears

• Sulphur is very dangerous
• - Manganese compensate

• Maximum of the hot tearing tendency
• by 0.4 C
• - Low tendency below 0.2

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Hot tear
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Hot tear
Partial annealed hot tear
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Hot tear

• Prevention
• design appropriate to casting, prevention of
  residual stresses, wide
• difference in the wall thickness and hot spots)
• prevention of hot sand effects

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Primary grain boundary fracture (Rock candy or
shell fracture)
Al-N-precipitations
G24Mn5

• Caused by Al-N-precipitations
• high content of Al and N and thick-walled
  castings

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Defects caused by heat treatment

• GS33NiCrMo
• left quenching and tempering not correct
  ferrite, pearlite and bainite ?
• lower ductility

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Defects caused by heat treatment

• G24Mn5 (thick-walled casting)
• quenching and tempering not complete ferrite,
  pearlite and bainite
• different structure and lesser properties

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Defects caused by heat treatment

• G30Mn5 GS25
• Decarburization of the surface area caused by
  heat treatment without
• protective atmosphere ? Chance of properties
  in the surface area

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Defects caused by heat treatment

• GX3CrNiMo20-18-7
• temperature of solution heat
• treatment to low and/or cooling
• rate not correct ?
• precipitation of delta-ferrite ?
• these components are brittle ?
• lower ductility

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Defects caused by heat treatment

• GX 120Mn13
• temperature of austenitizing to high ?
• coarse grain ? bad mechanical properties

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Defects caused by heat treatment

• G105Cr4 hypereutectoid cast steel
• hardening crack
• structure coarse martensite and
• residual austenite
• reason temperature of austenitizing
• and cooling rate to high

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Defects caused by heat treatment
GX 5CrNiMo19-11-2

• intercrystalline corrosion
• heat treatment not correct ? precipitation of
  Cr-carbides on the grain
• boundary ? corrosion was possible

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Defects caused by heat treatment

• G71Si7 spring steel
• Black shortness
• Softening anneal not correct ?
• precipitation of graphite

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