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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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