defects in cast steel

1. 1
Foundry-Institute
Seminar
Metallurgical defects of cast steel
Claudia Dommaschk
TU Bergakademie Freiberg
Foundry Institute, Germany

2. 2
Structure
• Gas cavities
• Oxide and slag inclusions, Nonmetallic
inclusions
• Shrinkage cavities
• Hot tear
• Primary grain boundary fracture
• Defects caused by heat treatment

3. 3
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 > 20 μm
Micro inclusions < 20 μm

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

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Prevention:
• use of clean charge
• optimization of gating and feeding system (lamellar flow)
• decrease of the dissolved oxygen
• decrease of the overheating temperature
Oxide and slag inclusions, nonmetallic inclusions

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Example: G42CrMo4
• nonmetallic inclusions arise by
reason of the reactions during the
melting process
Oxide and slag inclusions, nonmetallic inclusions

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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
shrinkage
RT TS TL TP
Specific
volume

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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 Carbon content on
the inclination of hot tears
Influence of Manganese and Sulphur
content on the inclination of hot tears
- Maximum of the hot tearing tendency
by ~0.4 % C
- Low tendency below 0.2 %
- Sulphur is very dangerous
- Manganese compensate

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Hot tear

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Hot tear
Partial annealed hot tear

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

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• Caused by Al-N-precipitations
• high content of Al and N and thick-walled castings
Primary grain boundary fracture (“Rock candy or shell fracture”)
G24Mn5
Al-N-precipitations

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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
• intercrystalline corrosion
• heat treatment not correct  precipitation of Cr-carbides on the grain
boundary  corrosion was possible
GX 5CrNiMo19-11-2

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