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University of Chemical Technology and MetallurgyDepartment of Materials Science

Microstructure and Mechanical Properties of Austempered Ductile Cast Iron

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CHEMICAL COMPOSITION OF CAST IRON:Fe C Si Mn P S Mg

val. 3.5 2.5 0.25 0.038 0.015 0.05

Grey cast iron

Ductile cast ironAddition of cerium or magnesium to inducenodularisation of graphite

No addition of Mg or Ce

Tensile strength: 150-400 MPa

Tensile strength: 350-800 MPaElongation: 3-20 %

Elongation: 0 %

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Microstructure of Ductile irons

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1. Austenitising between 850 and 950oCtypically for 60 min.

2. Quenching into a salt or oil bath at a temperature in the range 450 – 250oCusually between 0.5 and 3 hours

3. Cooling to a room temperature

Austempered ductile cast iron (ADI)

A further improvement of ductile cast iron is obtained with anisothermal heat treatment named austempering

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Mechanical properties

STRENGTH : equal to or greater than steel

ELONGATION : maintain as cast elongation while double the strength of quenched and tempered ductile iron

TOUGHNESS : better than ductile iron and equal to or better than cast or forged steel

FATIGUE STRENGTH : equal to or better than forged steel.

DAMPING : 5 times greater than steel.

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ADI has excellent castability, it is possible to obtain near-ADI has excellent castability, it is possible to obtain near-net shape castings even of high complex parts. net shape castings even of high complex parts.

ADI is cheaper than steel forgingsADI is cheaper than steel forgings

ADI has a weight saving of 10% ADI has a weight saving of 10%

Economical advantages and applications

GearsAutomotive industry

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Processing window

The bainitic transformation in ductile iron can be described as two

stage reaction:

Stage I: Austenite decomposition to bainitic ferrite and carbon enriched austenite.

Stage II: Further austenite decomposition to ferrite and carbide.

Carbider

r

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Closed processing window

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Microstructure of ADI

Bainite Retained austenite Martensite Carbide Pearlite

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Element Cell boundary Close to graphite Mn 1.73 0.40 Si 1.75 2.45 Mo 0.60 0.07

Element Cell boundary Close to graphite Mn 0.81 0.57 Si 2.31 2.49 Mo 0.16 0.12

Fe-3.5C-2.5Si-0.55Mn-0.15Mo

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Homogenised at 1000oC for 3 days

Austempered at 350oC for 64 min

o

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Volume fraction of retained austenite and hardness

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Austempering temperature and carbon content in retained austenite

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= a + b (%C) + c (%Mn)

= a + b (%C) + c(%Mn) + d (%C x %Mn)

V= sin (%C) + tanh (%Mn)

Variables for modelling include:C, Mn, Si, Mo, Ni, Cu

V

sum

C x WcMn x WMn

TA MnC

V

V

INPUT

HIDDEN

OUTPUT

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Volume fraction of retained austenite to different temperatures

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Hardness /Vickers, HV/ to time of austempering, min

Other mechanical properties:

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Physical Model for Retained Austenite

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Babu etal. 1993

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Volume fraction of cementite I part

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Volume fraction of cementite II part

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Treatment, temperature and time

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SEM vision

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