MME 345, Lecture 37

Cast Iron Foundry Practices4. Grey iron foundry practices

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. Introduction

2. Moulding practices

3. Gating and feeding

4. Inoculation

1. Excellent fluidity

• Often expressed in terms of

Composition-factor, CF = %C + %Si /4 + %P /2

Fluidity (inch) = 14.9 CF + 0.05 T(°F) - 155

• Eutectic alloy has the maximum fluidity

• Hypereutectic alloys suffer kisking

2. Wide working temperature range (1200 - 1700°C)

• Permits easy handling

3. Skin freezing range alloys

• Low shrinkage characteristics

• High yield (60 – 70%).

Summary: Foundry Properties of Grey Irons

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The basic foundry operations are highly developed

• Tremendous amount of material and work involved in grey-iron-castings production

• Much of the engineering and automotion/mechanisation in foundry came through to

meet these developments

• To a greater extent, the transport industry was directly responsible for these

developments

• Grey iron foundry volume is twice the size of that all other cast metals combined

Introduction

Almost all casting processes, except die casting and plaster moulding,

may be employed

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Green sand moulding is the most commonly practiced method.

In most cases, grey iron foundries are modernised and mechanised.

Moulding Practices

Some features of moulding aggregates:

• 4-mesh base sand with AFS Fineness number ~60 commonly employed.

• Thermal stability, reusability, mouldability, ease of cleaning, and surface finish is the most

important features in moulding sand mixtures.

• Sand burn-on, sand fuse-on, reaction between sand and metal usually occurred. Sea coal,

coke, graphite, pitch etc. are mixed with sand to prevent or eliminate these defects. Their

effectiveness depends on producing reducing atmosphere in the mould or preventing

oxidising reactions.

• Sea coal (2-8%) is very commonly applied to improve surface finish.

Excessive amount can cause blow defects. Pulverised coke, pitch can also be used.

• Mould coats (graphite) are also applied to improve surface finish.

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Gating and Feeding

Regardless of the most favourable sand conditions and good moulding,

a bad gating practice can ruin the casting.

Certain metallurgical characteristics of grey iron require specialised practices

of gating design.

Fluidity

Important for thin sections with high surface area• misrun is difficult to avoid

• running system and the mould cavity need to be filled as soon as possible

Low fluidity resulted for• low C and Si level, low pouring temperature, hypereutectic alloy

To increase fluidity• increasing temperature rather than changing composition is preferred.

• for an increase of 0.25-0.30% C or 20°F temperature, fluidity increases by 1 in

Reduce transport• pouring of iron must be done with a minimum temperature drop

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Liquid volume shrinkage is about 0.5 % per 100 deg F

• Not problematical; can be compensated by liquid flow

Shrinkage characteristics

Solidification shrinkage

DV, % = 2 (% GC – 2.80 %) GC = graphitic carbon

• White iron (with 0% GC) have about 5.6% shrinkage

• Shrinkage in grey iron can be well compensated by controlling graphitisation

• A normal soft grey iron with composition 3.5% C and 2.2% Si with 0.4-0.6% CC and 2.9-3.1% GC

will virtually expand, rather than shrink

• For ordinary grey iron castings, no feeder is needed to feed solidification shrinkage

• But low C, low Si, less graphitisable castings need feeding

Thus, feeder in grey iron is often need not be used, unless some factors other than

solidification shrinkage requires their use.

• feeder may be needed if mould enlargement occurs at some time after pouring

• solidification under feeder pressure favours sound casting 7/24

Important parameters

• Adequate pouring rate

• Use of slag traps

• Provision of feeding, if required

t = Pouring time, sec

T = Average metal thickness, inch

A = Constant (0.95 for grey iron)

B = Constant (0.835 for grey iron)

W = Weight of casting, lb

t = A + WTB

Gating design

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Fig. 20.4 Series of gating designs

showing how an increase in runner and

sprue size permits them to feed castings

(a) Improper gating of this match-plate

group resulted in shrinkage defects;

(b) Use of a large sprue did not eliminate

shrinkage except in the two castings

directly under the sprue, one of which

shows an inclusion;

(c) Shrinkage was eliminated by use of a

larger runner and sprue

Often gating system can be

used as feeder for meeting up

the low liquid demand.

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Inoculation

To achieve desired mechanical properties in iron castings, the liquid iron must have

• the correct composition, and

• suitable nuclei to induce the correct graphite structure to form on solidification

The liquid iron must have a suitable ‘graphitisation potential’

• determined mainly by its CE value, and in particular, by the Si content

Even if iron of the correct chemical analysis is made in the melting furnace,

castings having the desired graphite structure will not be produced without the

addition of inoculants.

Inoculants are alloys added in small amounts to induce eutectic graphite nucleation.

• without the presence of suitable nuclei,

liquid iron will ‘undercool’ below the eutectic temperature

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Inoculation is the process of increasing the numbers of nucleating sites

• from which eutectic graphite can grow during the solidification of flake, nodular

and compacted graphite irons

Main aim of inoculation: minimize undercooling during eutectic solidification

• to make sure that the resultant cast microstructures are completely free from

eutectic carbides

Inoculation also plays a major part in controlling eutectic graphite morphology and

distribution, and hence, the levels of pearlite and ferrite in matrix structures.

In flake graphite (grey) irons, inoculation is used to:

• prevent eutectic carbides, especially in thin sections and at corners

• ensure a uniform distribution of fine type A graphite flakes;

prevent formation of Rosette graphite

• avoid the presence of undercooled graphite and the associated soft free ferrite

in the matrix

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graphite structure of (a) uninoculated grey cast iron (×100) and (b) inoculated grey cast iron (×100)

Uninoculated grey iron castings will contain:

• undercooled forms of graphite (type D/E); associated with this will be ferrite

• cementite in thin sections or close to edges and corners

Such iron is unlikely to meet tensile and hardness specifications and

will be difficult to machine.

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Inoculating materials

Common inoculating materials:

1. graphite base

2. ferrosilicon base

3. calcium silicide base

Graphite itself is a powerful inoculant but it is not effective on low sulphur irons

• these microconstituents are capable of forming microinclusions of complex oxides

or oxysulphides having suitable surface and crystallographic characteristics to

heterogeneously nucleate graphite.

Ferrosilicon (45% or 75% Si) is the most commonly used inoculant.

• pure ferrosilicon is not effective as an inoculant

• it is the presence of around 5% of carefully controlled levels of minor elements

(Ba, Zr, Sr, Bi, Al, Ca, Mn) that determine the effectiveness of the product

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Method of inoculation

There are many methods of inoculations based on time and type of addition.

These are generally classified into two groups:

1. Ladle additionthe inoculant is added either as the liquid iron enters the ladle or just afterwards

2. Late inoculationrefers to treatment after the metal has left the ladle, for example, as it enters the

mould (stream inoculation) or by using an insert in the mould (in-mould inoculation).

Inoculants reach maximum effectiveness immediately after treatment and

fade quickly over a period of 10–20 minutes.

It is therefore desirable to inoculate as late as possible before casting.

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schematic of inoculation methods

INOCULATION

METHODS / TECHNIQUES

Ladle

Inoculation

Gravity

Feeding

Air Assisted

Injection

Injection of

Cored Wire

Late / Post

Inoculation

In-Stream Inoculation

during pouring

into the mould

Wire Injection

during pouring

into the mould

In the Mould

Inoculation

Air Assisted

Injection of Inoculant

Gravity Feeding of

Inoculant

Inoculation in the

Sprue/Runner

Inoculation in the

Pouring Basin

Solid Insert Placed

into the Sprue Well

Crushed Inoculant

Placed into the

Reaction Chamber

Inoculant Placed

onto/into the Filter

Inoculant Block

Placed into the

Pouring Basin

Floating

Inoculant in the

Pouring Basin

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Ladle inoculation

Ladle inoculation should always be added to the metal stream when tapping from

furnace to ladle, or ladle to ladle.

Additions should begin when the ladle is one-quarter full and be completed when the

ladle is three-quarters full, so that the last metal merely mixes.

Never put inoculant into the bottom of the ladle and tap onto it.

The amount of inoculant needed is governed by several factors. The following rules

guide the use of inoculation:

• low carbon equivalent irons require greater amounts of inoculant

• for a given iron, the thinner the section of casting, the greater the inoculation required

• electric melted irons require more inoculation than cupola melted irons

• high steel scrap charges will require more inoculation

• higher treatment necessary if inoculated iron is held for more than a few minutes after inoculation

In general, additions of 0.1–0.5 wt.% of metal will be satisfactory for grey cast irons

Higher additions are needed for ductile (SG) irons

Over-inoculation causes shrinkage porosity problems16/24

ladle inoculation

gravity feeding

air-assisted injection

wire injection

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Late stream inoculation

For foundries with mechanised moulding, melting, and pouring lines,

conventional ladle inoculation is not possible.

Metal stream inoculation is intended for use in these conditions.

It is designed to add controlled amounts of inoculant to the liquid cast iron

just before it enters the mould.

The use of late stream inoculation techniques leads to the virtual elimination of fading.

This causes

• a substantial reduction in the amount of inoculant used

• the inoculant addition thereby produces a smaller change in iron composition leading

to improved metallurgical consistency

• the cost of inoculation is also lower

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Inoculant used in late stream inoculators must have a number of important features:

• It must be a powerful inoculant

• It must be finely divided to ensure free-flowing properties and rapid solution

• It must be very accurately graded, without superfine material which would blow away,

or large particles which jam the gate mechanism.

• It must dissolve rapidly and cleanly to avoid the presence of undissolved inoculant

particles in the castings

Stream inoculation is very efficient since fading is eliminated.

The normal addition rate for grey iron is from 0.03–0.20%, typically 0.1%, much

less than would be used for ladle inoculation.

For ductile iron, addition rates range from 0.06–0.3%, typically 0.2%.

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in stream inoculation

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Mould inoculation

There are several ways in which mould inoculation can be performed:

• powdered inoculant can be placed in the pouring bush; or

• it can be placed at the bottom of the sprue.

A more reliable method is to use sachets or precast slugs of inoculant in the

pouring bush or in the running system

• they gradually dissolve in the metal stream as the casting is poured,

giving uniform dissolution

• this ensures that inoculation takes place just before solidification of the iron

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in the mould inoculation

at the sprue wellin the runner

floating inoculation block

in the pouring basinanchored inoculant block

in the pouring basin

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Control Method

The wedge chill test is a simple and rapid method of assessing the degree of chill

reduction obtained by the use of inoculation in grey cast irons.

•The wedge is made in a mould

prepared from silicate or resin bonded

sand.

•After pouring, it must be allowed to

cool in the mould to a dull red heat

(600°C), after which it can be

quenched in water and fractured.

•The width at the point where clear

chill ceases, t, is measured and this

gives a good indication of the need for

inoculation and of the effectiveness of

an inoculation process.

In general, casting sections should be not less than three times the wedge reading

if chill at the edges and in thin sections is to be avoided.

the wedge chill test

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Next ClassMME 345, Lecture 38

Cast Iron Foundry Practices5. Metallurgy of nodular iron