MME 345, Lecture 39

Cast Iron Foundry Practices6. Process control in nodular iron making

Ref:

[1] Heine, Loper and Rosenthal. Principles of Metal Casting, Tata McGraw-Hill, 19670

[2] J R Brown (ed.). Foseco Ferrous Foundrymen’s Handbook, Butterworth-Heninemann, 2000

Topics to discuss today …

1. Metallurgical process control

2. Foundry process control

3. Heat treatment

4. Engineering properties

Metallurgical Process Control

Production of ductile iron is highly sensitive to process variation, requiring a

greater degree of control.

Effectiveness of magnesium treatment and inoculation is important.

The final CE value of iron is hypereutectic although the base iron composition

before Mg treatment and inoculation is often similar to grey iron and hypoeutectic.

The processes to be control are similar to those used for grey iron. However,

special attention must be given to the residual magnesium content after treatment.

spectrographic analysis of a chilled, graphite-free sample should be used

Test coupons (e.g., keel blocks or Y-blocks for making tensile test bars)

are routinely used to determine mechanical properties and to inspect graphite

shape and degree of spheroidisation.

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• Graphite shape and size and number

quality ductile iron is produced if the graphite is developed as spheroids with

a high degree of nodularity

a number of other types of graphite, including flake graphite, may be produced

if the process is not carried out properly

a high nodule count with appropriate size is also important

A number of metallurgical factors must be controlled in ductile iron production

to avoid structural imperfections.

These include:

• Carbide formation

occurrence of eutectic carbide must be prevented by maintaining high CE value

and good nodule count

• Dross formation

Mg functions first as desulphuriser and deoxidiser of base iron;

besides, Mg itself is highly oxidising

these creates dross, which adheres to the cope-surface of the casting

the situation worsen for high Mg addition, high pouring temperature, and turbulence

in the gating system and mould cavity4/26

Graphite Shape

ASTM A 247 - 67 Standard of Test Method for Evaluating the Microstructure of Graphite in Iron Castings

Up to 10% Type III, with the

remaining Type I or II has been

reported to have no noticeable

effect on properties.

Types IV and V are undesirable

and have significantly lower

mechanical properties.

I : (graphite nodules) II

III : vermicular graphite IV V : exploded graphite

Type I is the normal and usually desirable graphite form

in ductile iron, although the presence of Type II graphite

forms has little or no adverse effect on properties.

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Graphite Size

125 – 50 mm

212 – 25 mm

36 – 12 mm

43 – 6 mm

51.5 – 3 mm

6less than 1.5 mm

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nk = number of graphite particles corresponding to form number of graphite shape k.

Degree of Nodularity(As per JIS G5578)

A minimum of 75% degree of nodularity is required in order to be considered

the casting as ductile iron.

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Factors controlling graphite shape in ductile iron

• pouring temperature

• casting section size

• amount of effective Mg added

• post-inoculation

• base analysis of iron

Poorer graphite shapes are resulted for

• low pouring temperature

• heavy section size

• insufficient Mg addition

• lack of inoculation

• low CE in base analysis of iron

CE value higher than 4.6 often results graphite flotation and the development

of exploded graphite, Type V.

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combined effects of metallurgical factors on graphite nodules and properties

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Foundry Process Control

Most castings are made using green and dry sand moulds.

Moulding media are similar to those used for grey irons;

special consideration is given for moisture content, since Mg oxidises easily.

When a sufficient Mg is added, iron oxidises readily when temperature falls

below 1400 C and creates dross. So a pouring temperature of 1425 C or higher

is preferred to avoid dross formation.

Mg addition increases surface tension of iron, reducing wettability of the mould

by liquid iron, thereby reducing sand burn-in and metal penetration problems.

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Feeding is complex because of the method of solidification.

Wide freezing range, shrinkage differences between hypo- and hyper-eutectic

alloys and tendency of mould-wall movement are some of the points to

consider while designing feeding system.

Ineffective Mg treatment and inoculation may often result eutectic carbide or

vermicular graphite (Type III), which alters the solidification behaviour and

increases feeding requirements. Proper treatment is therefore necessary to

control solidification shrinkage.

Design of gating system should include measures for preventing turbulence

and entry of slag and dirt into the mould cavity.

While designing gating system, the following formula can be used to determine

the average pouring time

pouring time, s = 0.65 pouring weight, lb

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Heat Treatment of Ductile Iron

Have excellent response to heat treatment; behaviour is similar to those of steels.

Matrix structure can be all ferrite, ferrite and pearlite, all pearlite, martensite,

tempered martensite, banite, and can even contain carbide or austenite.

Common heat treatments:

1. Stress relieving

2. Annealing

3. Normalising and tempering

4. Quenching and tempering, austempering, martempering

5. Flame/induction surface hardening

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Austempered Ductile Iron

a grade of ductile iron in which heat treatment is utilized to produce a metastable

face-centered cubic matrix (i.e., austenite), which is stable at room temperature.

considerably higher strengths than the as-cast grades are obtained, yet retaining

excellent ductility.

microstructure of austempered

ductile iron (ADI)

the primary cause of this dramatic improvement in

mechanical properties stems from the presence of

the face centered cubic matrix together with a fine

scale dispersion of ferrite, commonly known as

ausferrite.

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Austempering is an isothermal heat treatment that, when applied to ferrous

materials, produces a structure that is stronger and tougher than comparable

structures produced with conventional heat treatments.

Conventional heat treaters heat the parts to about 900 C in a controlled atmosphere

and then quench them in a bath of oil or water that is near room temperature. This

produces a crystalline structure known as Martensite, a hard, brittle phase. The

parts are then tempered in another furnace at 175 to 600°C to decrease the

residual stress and “brittleness.”

Austempering starts the same way. The parts are heated to about the same

temperature in a controlled atmosphere (so they don't scale) but then are quenched

in a bath of molten salt at 230° to 400°C. This quench temperature is above the

martensite starting temperature. Therefore, a different structure (not martensite)

results. In Austempered Ductile Iron, the structure is Ausferrite (austenite and

ferrite), and in steel, it is Bainite.

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Ms

Mf

transformation starts

transformation ends

TRANSFORMED

AUSTENITE

MARTENSITEBAINITE (for steel)

AUSFERRITE (for ductile iron)

Will not be included in Exam !!!

Time – Temperature – Transformation (TTT) curve

Tem

per

atu

re

Time

AUSTENITE

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portion of the metastable Fe-C-2.4Si phase diagram

1. Casting is heated into the (austenite + graphite) field and held at Tg until the matrix

is fully austenitized. At this point the structure consists of austenite of 0.8 wt. % C

and graphite nodules.

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portion of the metastable Fe-C-2.4Si phase diagram

2. Cool rapidly to TA, the austempering temperature and hold, allowing the reaction:

g (0.8%C) a (0%C) + g (2.0%C)

[ausferrite]

This reaction usually requires

about 1 - 2 hours to complete.

In a piece of steel heat treated

the same way, the austenite

would transform to bainite, a

two phase mixture of ferrite

and iron carbide (Fe3C).

However, in cast irons, the

presence of silicon prevents

the formation of carbides and

the austenite phase retained

as a metastable phase.

If given enough time, the

austenite will eventually

transform to bainite.

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portion of the metastable Fe-C-2.4Si phase diagram

3. However, before that happens the casting will be cooled to room temperature,

a move which will retain the structure created at TA. The austempering reaction is

said to have “stabilized” the austenite.

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ausferrite

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Austempering Means Uniform Structure

During the process of quenching to Martensite, the Martensite reaction begins

immediately. It is a diffusion-less process. The result is that the outside of the part

may already be transformed while the inside is still red hot. It is this “non-uniform

phase transformation” that results in distortion and tiny micro cracks that lower

the strength of the part.

By contrast, the Austempering reaction that produces Ausferrite (or Bainite in steel)

is a diffusion-controlled process and takes place over many minutes or hours. This

results in uniform growth and a stronger (less disturbed) microstructure.

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The mechanical properties of ductile cast iron are significantly different than that

of gray iron because of the difference in shape between the graphites in these

cast irons.

In ductile irons the matrix is the continuous entity so that there are no easy crack

paths to propagate fracture.

As a result, ductile cast irons have significant ductility and toughness, properties

which place this unique material in competition with other ferrous materials such

as cast steels, forged steels, and even wrought steels.

As a result of the continuity of the matrix, the tensile properties of ductile cast iron

depend almost completely upon the microstructure of the matrix, a microstructure

which can be controlled by heat treatment.

Engineering Properties

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UTS v. % Elongation Data

summary of common as-cast and austempered grades of ductile iron22/26

composition – structure – properties of different grades of ductile iron

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Effect of Section

Section size effect on properties of casting is not as pronounced as in grey irons.

Thin sections are prone to carbide formation, and heavy sections may contain

undesirable nodule shapes.

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Other important properties to consider:

1. Machinability – superior to that of grey iron or steel

2. Corrosion resistance – equivalent to that of grey iron and better than steel

3. Wear resistance – equivalent to that of the best grades of grey iron and better than steel

4. Thermal shock resistance – good in ferritic ductile iron

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Next ClassMME 345, Lecture 40

Steel Foundry Practices