Defects Cast Steel
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Transcript of Defects Cast Steel
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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 higherClassification of size:
Macro inclusions > 20 m
Micro inclusions < 20 m
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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
coolingfeeding is necessaryif 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
Specificvolume
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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 witha 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)
G24Mn5Al-N-precipitations
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Defects caused by heat treatment
GS33NiCrMo
left: quenching and tempering not correctferrite, pearlite and bainite
lower ductility
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Defects caused by heat treatment
G24Mn5 (thick-walled casting)
quenching and tempering not completeferrite, 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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