Transcript Defects Cast Steel
Foundry-Institute
Seminar Metallurgical defects of cast steel
Claudia Dommaschk TU Bergakademie Freiberg Foundry Institute, Germany
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Structure
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Gas cavities
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Oxide and slag inclusions, Nonmetallic inclusions
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Shrinkage cavities
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Hot tear
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Primary grain boundary fracture
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Defects caused by heat treatment
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Gas cavities Description and reasons:
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Cavities in castings, especially in the upper parts of the castings
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Formation during solidification because of degrease of gas solubility
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often in combination with oxide and slag inclusions
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formation of gas cavities depends on the concentration of oxygen, nitrogen and hydrogen
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the inner surface of the cavities is smooth
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Gas cavities
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Gas cavities Prevention:
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use of dry materials and ladles
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use of clean charge
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degasification of the melt
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look at the mould sands (permeability of gas, vent…)
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Oxide and slag inclusions, nonmetallic inclusions Description and reasons:
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Classification: endogenous and exogenous inclusions
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endogenous inclusions are caused by the reaction products during the melting process (especially during deoxidation)
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exogenous inclusion are caused by other materials in the melt (e.g. refractory lining)
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thin fluid slag can precipitate at the grain boundaries
danger of formation of hot tears is higher
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Classification of size: Macro inclusions Micro inclusions > 20 μm < 20 μm
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Oxide and slag inclusions, nonmetallic inclusions GX3CrNiMoN17-13-5 Slag inclusions GX2CrNiMo18-14-3
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Oxide and slag inclusions, nonmetallic inclusions Prevention:
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use of clean charge
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optimization of gating and feeding system (lamellar flow)
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decrease of the dissolved oxygen
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decrease of the overheating temperature
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Oxide and slag inclusions, nonmetallic inclusions Example: G42CrMo4
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nonmetallic inclusions arise by reason of the reactions during the melting process
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Shrinkage cavities Description and reasons:
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specific volume of melt is higher than the specific volume of solid
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contraction during solidification and cooling
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feeding is necessary – if the feeding is not optimal
formation of shrinkage cavities
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the shrinkage volume of cast steel is about 4-7 %
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the inner surface is rough RT T S T L Liquid shrinkage Solidification shrinkage shrinkage T P
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Shrinkage cavities GE 300 (GS 60)
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Shrinkage cavities Prevention:
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use of optimal feeding system (calculation and simulation)
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warranty of directional solidification
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use of exothermic feeder sleeve
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decrease of the pouring temperature
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Hot tear Description and reasons:
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hot tears are intercrystalline discontinuity
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cracks run along the grain boundaries
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the risk of cracks at alloys with a high freezing range is higher than with a small freezing range
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the reason are stresses during solidification because of hindered contraction (residual stress)
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the main reason for formation of hot tears are the geometry of casting
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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
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Maximum of the hot tearing tendency by ~0.4 % C - Low tendency below 0.2 % Influence of Manganese and Sulphur content on the inclination of hot tears
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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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Hot tear Prevention:
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design appropriate to casting, prevention of residual stresses, wide difference in the wall thickness and hot spots)
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prevention of hot sand effects
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Primary grain boundary fracture (“Rock candy or shell fracture”) G24Mn5
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Caused by Al-N-precipitations
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high content of Al and N and thick-walled castings Al-N-precipitations
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Defects caused by heat treatment GS33NiCrMo
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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)
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quenching and tempering not complete – ferrite, pearlite and bainite
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different structure and lesser properties
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Defects caused by heat treatment G30Mn5
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Decarburization of the surface area caused by heat treatment without protective atmosphere GS25
Chance of properties in the surface area
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Defects caused by heat treatment GX3CrNiMo20-18-7
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temperature of solution heat treatment to low and/or cooling rate not correct
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precipitation of delta-ferrite
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these components are brittle
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lower ductility
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Defects caused by heat treatment GX 120Mn13
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temperature of austenitizing to high
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coarse grain
bad mechanical properties
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Defects caused by heat treatment G105Cr4 = hypereutectoid cast steel
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hardening crack
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structure: coarse martensite and residual austenite
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reason: temperature of austenitizing and cooling rate to high
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Defects caused by heat treatment GX 5CrNiMo19-11-2
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intercrystalline corrosion heat treatment not correct
precipitation of Cr-carbides on the grain boundary
corrosion was possible
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