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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 > 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

Solidificationshrinkage

shrinkage

RT TS TL TP

Sp

ecif

ic v

olu

me

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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”)

G24Mn5Al-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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