MME 345, Lecture 35

Cast Iron Foundry Practices2. Melting of cast irons in cupola

Ref:

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

[2] American Foundrymen’s Society, Cupola Handbook, 5th Edition.

Topics to discuss today …

1. Introduction

2. Cupola melting system

3. Cupola materials

4. Principles of cupola operations

5. Chemical principles of cast iron melting

1. Introduction

Common melting units for melting cast irons:

1. cupolas

2. open hearths

3. electric arc / induction furnaces

4. air / reverberatory furnaces

5. crucible furnaces

6. duplexing (e.g., melting in cupola, composition adjustment in air furnace)

Regardless of the type of furnace used

• the basic melting operation physically transforms solid into liquid

• composition of all materials charged into the furnace determine the composition

of slag/iron mixture

• control of major, minor, and trace elements in the charge influences the

properties of iron

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melting of cast irons in cupolas

cupola is a vertical, cylindrical, shaft-type furnace principally introduced

for re-melting pig iron especially for making grey iron castings

similar to the blast furnace but smaller and differs with respect to

the function served and the type of charge used

melting

rather than reduction

pig iron, iron and steel scraps

rather than iron ore

Advantages in cupola melting Disadvantages in cupola melting

• continuous melting

• low-cost melting

• easy control of composition

• adequate control of temperature

• obtaining low C (< 2.8%) is difficult

• loss of alloying elements

• difficulty in attaining high temperature

• difficulty in melting alloy cast irons

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2. The Cupola Melting System

The basic unit

1. the cupola

2. the blast delivery system

3. the charging system

4. the forehearth or duplexing furnace

5. the slag-handling system

6. the emission cleaning system

For increased energy recovery

7. recuperative blast preheat system

8. steam generation

9. plant heating, and

For water-cooled cupola and/or

wet-type emission cleaning and

slag-handling system

10. water systemstructure of the common cupola

A cupola itself is actually but one

component of a melting unit which

is called a cupola melting system.

It comprised of

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based upon lining used

1. conventional, refractory-lined

2. water-cooled liningless (water jacketed or external spraying)

3. water-cooled partially lined

4. combination water-cooled lined

types of cupola

based upon slag system produced

1. acid-slag cupola

2. basic-slag cupola (to provide low-S and/or high-C iron)

based upon energy conversion

1. hot-blast cupola (air temperature 375 – 425 C)

2. divided-blast cupola

3. cokeless cupola

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selection of a complete cupola melting system

1. Process requirements

(a) type of iron to be produced(b) chemistry(c) charge materials(d) typical charge makeup(e) spout metal temperature required(f) metal handling(g) slag handling(h) available utilities

3. Physical requirements

(a) space availability(b) access to equipment site(c) elevation(d) plan(e) equipment relation

2. Production requirements

(a) melting rate(b) metal demand(c) melting schedule

4. Equipment factors

(a) cupola(b) blast system(c) charging system(d) emission cleaning system(e) water system(f) controls and instrumentation

5. Miscellaneous factors

(a) metal transport from cupola without forehearth or duplexing furnace(b) external desulphurising(c) slag disposal(d) special attachments for collector, gas takeoff, top cap or stack burners(e) total weight of equipment to be supported by the cupola stack(f) tool and maintenance equipment(g) personnel safety equipment

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3. Cupola Materials

One of the most important and complex arts that must be mastered in

the foundry is that of assembling a good, economical melting charge.

Cupola melting is complex processing method occurring at high

temperatures in which metallic raw materials, the combustion materials,

the molten iron product, and the gas and slag by-products are all

intimately associated.

Factors considered in designing the charge makeup1. Size and number of cupolas

2. Hours of cupola operation each day

3. Iron-to-coke ratio

4. Physical condition and density of scrap

5. Maximum tonnage to be melted each hour

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Cupola Input Cupola Output

1.00 ton pig, scrap iron, steel 0.98 ton molten iron

0.15 ton coke 0.05 ton molten slag

0.03 ton flux 1.35 ton stack gases

1.20 ton air

2.38 ton total 2.38 ton total

1. Metallics• the source of iron

• foundry scrap, pig iron, steel,

ferroalloys.

2. Coke• the source of carbon

• the fuel to melt the iron.

3. Limestone• to flux the ash in the coke

and gangue materials in the ore

4. Other additions• to modify chemistry, structure and

properties of the iron produced

• ferroalloys, inoculants, nodulants, etc.

cupola charge materials

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metallic charge materials for cupola

Types of metallic charge in cupola

1. Pig iron (PI) / direct reduced iron (DRI)

2. Return scrap

3. Steel scrap

4. Bought scrap

5. Alloying additions

Pig Iron

• Pig iron is the original melting material for iron castings

• Until 1950, it was widely believed that, in order to achieve consistent, good quality grey

iron castings, it was necessary to retain PI as the dominant material in the charge.

Typical charge: PI – 40-50%, foundry returns – 25-30%, ferrous scrap – 20-30%

• After 1950, open hearth furnaces (large consumers of ferrous scrap) become obsolete.

Typical charge: PI – 7%, scrap – 93%.

• In practice, many large foundries use no PI at all. Only small plants use about 10-20% PI.

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Direct Reduced Iron

• Also been referred to as pre-reduced iron, metallised iron, and sponge iron.

• DRI is the product of a reduction process using carefully selected, superior quality raw

materials, especially high-quality iron ore.

• PI – product of a total reduction of iron ore

iron ore completely deoxidised to metallic iron state, melted and superheated to about 1595 C,

impurities removed as slag, significant percentage of C, Si, and Mn are absorbed, which are useful

to the foundrymen (even though some tramp elements S and P are introduced)

• DRI – product of the almost total reduction of iron ore

iron ore partially deoxidised in the solid state, leaving 5-10% FeO in the product, all impurities

remained disseminated through the product, and although it contains up to 0.15% C, no Si or Mn is

dissolved in the product. It also does not contain any tramp elements.

• Advantages of DRI – uniformity in composition, low tramp elements

• Disadvantages of DRI – low iron yield (due to the presence of gangue materials), wasteful

oxidation of Si and Mn of charge (by reacting with FeO of DRI), high coke consumption, reoxidise

and produce heat while in storage (by reacting with water and oxygen even at room temperature)12/35

Most important chemical factors determining scrap quality

1. Gross chemical analysis of the “big five”: C, Si, Mn, S, P

to produce grey iron, considering all other factors being the same,

a ton of cast iron scrap is worth considerably more than a ton of steel scrap

2. Residual or “tramp alloy” analysis: Cu, Ni, Cr, Mo, Sn, Al, Pb

tramp elements are frequently troublesome, especially for ductile irons

3. The melting yield of charge material

strongly determines the true value of the scrap;

low-yield charge often contains non-metallic materials which generate gas or slag, and

often associated with tramp elements

4. The by-product disposal effects on the environment

Foundry return / steel scrap / bought scrap

“It is lots cheaper to buy scrap than to make it.”

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1. The supplier: scrap dealers, brokers, and direct industrial sources

2. The consumers: melt shop managers, assisted by plant metallurgists

3. The purchasing agent: who coordinates the interests and capabilities of suppliers and consumers

The task to be undertaken to obtain the most satisfactory low-cost

melting charge and involves :

1. becoming familiar with all sources of scrap within a reasonable distance from

the foundry

2. learning how to select a few of the most suitable and most economical grades

of scrap from all available sources

3. developing purchasing shrewdness

4. obtaining reliable high-quality performance from all scrap suppliers.

foundry raw materials team

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4. Principles of Cupola Operations

Steps in Cupola Operation

1. Preparation of refractory lining, bottom, tap hole and slag hole

2. Lighting and burning the coke bed

3. Charging

4. Melting

(a) Starting air blast

(b) Re-charging

5. Tapping and slagging

6. Dropping the bottom

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Coke bed

• After preparation of cupola bottom, coke is charged up to and above the tuyeres.

• The height of coke above the tuyeres is defined as the coke bed.

• preparation, height, and burning-in of the coke bed are among the most critical

items in successful cupola operation.

• controls the liquid temperature and melting rate in the early stages of the melting.

• for a correct coke bed height, the time for first iron to appear at a correct temperature

range of 1510 – 1595 C at the tap hole after blowing begins is about 8 minutes.

• too low coke bed time <8 min, low melt temperature, high melt rate,

oxidation of iron, low CE value or increase chill depth

• too high coke bed time >10 -12 min, low melt temperature, low melt rate

Ideal coke bed height (in inch) = 10.5 air pressure in oz./in2 + FF = 6 (normal value) (low for low C content, high (up to 12-18) for high C content)

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combustion

• The cupola is blown with air to combust the coke and the air flow controls the melting

rate and metal temperature.

• The output of a cupola depends primarily on the diameter of the shaft of the furnace

and on the metal-to-coke ratio used in the charge.

relation of air and coke to combustion in the cupola

• the rate at which coke is charged

and air is delivered must be

properly balanced and this can

be judged from the composition

of stack gases.

• under proper operating

conditions: CO = 11-15%, CO2

= 12-14% in the stack gas.

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• For unbalanced coke and air supply, certain melting problem arises:

• excess coke wasted coke, low melt temperature, slow melting rate,

high C in iron

excessive refractory erosion

• excess air burned out coke bed, low melt temperature

oxidation of iron

higher loss of Si and Mn, low C in iron

• A useful measure of the efficiency of operation of a cupola is the „Specific Coke

Consumption‟ (SSC) which is

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melting rate, combustion and temperature

• the operational characteristics of cupola

are such that all factors are interrelated

• coke bed, coke charged, air supply,

melting rate, and melt temperature all

influence the ultimate operation

operating conditions of a 21.5 in ID cupola

higher melt temperature results, when

air blast is increased

coke ratio is decreased

melting rate increased, when

air blast is increased

coke ratio is increased

• since the balance of coke and air is reflected

in stack gases, analysis of stack gases may

also be utilised as a method of control

metal composition and properties

• if proper combustion conditions prevail during melting, control of chemical composition

and properties of iron is greatly facilitated.

• composition and property control depends on

1. charging metal charges of known analysis

2. known and consistent composition changes during melting

3. use of chill testing and inoculation

• composition of metal produced may be estimated by using “mixture calculations”:

1. empirically select a metal mixture (based on past experience)which would be expected to produce approximately the desired composition

2. calculate the gross chemical compositionon the basis of analysis of charge ingredients

3. determine net chemical compositionexpected after making corrections for changes in analysis anticipated during melting

4. adjust original mixture by trial-and-error calculationsuntil the net computed composition falls within the desired range

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Composition changes during cupola melting

Element Changes in analysis

Carbon

Silicon

Manganese

Phosphorous

Sulphur

Chromium

Nickel

Molybdenum

Copper

Pick up of about 10 – 20 % of original carbon charged

Loss of up to 10% of original silicon charged

Loss of up to 15 % of original manganese charged

No change

Gain in total of about 0.03 – 0.05 %

Loss of up to 10% of original chromium charged

No change

Loss of up to 5% of original molybdenum charged

No change

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example #1 of charge calculation

• Requirement of Grade 250 cast iron spout composition: TC = 3.2, Si = 1.7, Mn = 0.7, P = 0.10%

• Composition of charge materials are:

MaterialC

%

Si

%

Mn

%

P

%

Previous

Practice

Low P pig iron 3.0 3.0 1.0 0.10 25%

Grade 250 returns 3.2 1.7 0.7 0.10 35%

Low P iron scrap 3.2 2.2 0.8 0.15 15%

Steel scrap 0.1 0.1 0.3 0.03 25%

FeMn (late addition) 75.0 As required

FeSi (late addition) 70.0 As required

• Check the suitability of the previous charge make up to obtain Grade 250 cast iron and

determine the amount of FeMn and FeSi to be used.

• Si loss = 15% of the charge; Mn loss = 25% of the charge; P changes little.

• Total C% in spout = 2.4 + (Total C% in charge) / 2 – (Si% plus P% in spout) / 4 (Levi equation)

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Typical composition, % Material

used, %

Contribution to charge, %

C Si Mn P C Si Mn P

Low P pig iron 3.0 3.0 1.0 0.10 25 x 0.25 0.75 0.75 0.25 0.03

Grade 250 returns 3.2 1.7 0.7 0.10 35 x 0.35 1.12 0.60 0.25 0.04

Low P iron scrap 3.2 2.2 0.8 0.15 15 x 0.15 0.48 0.33 0.12 0.02

Steel scrap 0.1 0.1 0.3 0.03 25 x 0.25 0.03 0.03 0.08 0.01

TOTAL 2.38 1.71 0.70 0.10

Changes during melting 15% Si loss – 0.26

25% Mn loss – 0.18

Charge composition TOTAL 2.38 1.45 0.52 0.10

Late additions

at spout

FeSi to add = (1.70 –1.45) / 0.7 = 0.36 0.25

FeMn to add = (0.70 – 0.52) / 0.75 = 0.24 0.18

Final charge composition TOTAL 2.38 1.70 0.70 0.10

Expected spout

composition

TC = 2.40 + (2.38)/2 – (1.45 + 0.10) / 4 = 3.20%

Si = 1.70%

Mn = 0.70%

P = 0.10%

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example #2 of charge calculation

• Requirement of spout composition: C = 3.55, Si = 2.10, Mn = 0.75%

• Foundry returns and pig irons to be used are 45% and 10% of the charge, respectively

• Composition of charge materials are:

Material C Si Mn

Cast iron scrap 3.40 1.80 0.60

Steel 0.15 0.20 0.65

Pig 4.09 2.08 0.80

Foundry returns 3.55 2.20 0.75

Mn briquets 0 0 67.0

FeSi briquets 0 48.0 0

• Determine the charge make up for 1000 kg charge.

• Si loss = 10% of the charge; Mn loss = 15% of the charge; C gain = 20% of the charge

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Basis: 1000 kg charge

Charge should contains

Si = (1000 x 0.021) x (100/90) = 23.33 kg

Mn = (1000 x 0.0075) x (100/85) = 8.82 kg

C = (1000 x 0.0355) x (80/100) = 28.40 kg

Return used = 45% of 1000 kg = 450 kg

Si = 450 x 0.022 = 9.90 kg

Mn = 450 x 0.0075 = 3.38 kg

C = 450 x 0.355 = 15.98 kg

Pig used = 10% of 1000 kg = 100 kg

Si = 100 x 0.0208 = 2.08 kg

Mn = 100 x 0.008 = 0.80 kg

C = 100 x 0.0409 = 4.09 kg

Let the amount of steel and cast iron scraps

to be used are A and B kg, respectively.

Charge balance:

Total charge = return + pig + steel + cast

1000 = 450 + 100 + A + B

A = 450 – B (1)

Carbon balance:

Total C in charge = C in return + C in pig

+ C in steel + C in cast

28.40 = 15.98 + 4.09 + A (0.0015) + B (0.034)

A = 5553.33 – 22.67 B (2)

Using two above equations:

B = 235.50 236 kg

A = 214.50 215 kg

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Final charge

Return = 450 kg

Pig = 100 kg

Steel = 215 kg

Cast = 236 kg

FeSi briquets = 7.0 kg

Mn briquets = 2.0 kg

Other elements in cast:

Si = 236 x 0.018 = 4.25 kg

Mn = 236 x 0.006 = 1.42 kg

Other elements in steel:

Si = 215 x 0.018 = 3.87 kg

Mn = 215 x 0.006 = 1.29 kg

Total Si in charge = 9.90 + 2.08 + 4.25 + 3.87 = 20.10 kg

Total Si to be in charge = 23.33 kg

Si to be added as FeSi briquets = 23.33 – 20.10 = 3.23 kg

FeSi to be added = 3.23 x (100/48) = 6.73 kg 7.0 kg

Total Mn in charge = 3.38 + 0.80 + 1.42 + 1.29 = 6.89 kg

Total Mn to be in charge = 8.82 kg

Mn to be added as Mn briquets = 8.82 – 6.89 = 1.93 kg

Mn briquets to be added = 1.92 x (100/67) = 1.93 kg 2.0 kg

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chill control

chill depth vs. CE value relation

(coke ratio 7.5:1, blast rate 12.5 lb air/min)

chill test casting showing appearance of fractured surface

• chill testing is a procedure

for evaluating the graphitizing

tendency in the iron

• a test sample of melt is cast

in a core-sand mould in which

some sections are cooled

more rapidly than others

• the depth of chill or white cast

iron produced is measured

• factors influencing chill depth are:

1. composition

(low C/Si greater chill depth)

2. addition of inoculants (FeSi)

lowers chill depth

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carbon equivalent meter

• rapidly determines the

composition of grey cast

iron by measuring the arrest

points of the cooling curve

relation between carbon-equivalent phase diagram

and cooling curve as obtained using CE meter

correlation of liquidus and eutectic thermal

arrest points with carbon equivalent as

determined by chemical analysis

• more reliable than chill test

as chill depth is controlled

by many variables other

than composition

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5. Chemical Principles of Cast iron Melting

types of chemical reactions

1. Oxidation reactions

C + O2 (g) = CO2 (g)

2C + O2 (g) = 2CO (g)

Si + 2O = SiO2 (s)

Si + xFeO (slag, solid) = yFeO.SiO2 (slag) + 2Fe

Mn + FeO (slag, solid) = MnO (liquid) + Fe

2. Reduction reactions

SiO2 (solid, refractory, slag) + 2C = Si + 2CO (g)

MnO (liquid, slag) + C = Mn + CO (g)

Al2O3 (solid) + 3C = 2Al + 3CO (g)

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• marked changes in chemical reactions

occur over the temperature range of

room temperature to 1925 C inside the

cupola

effects of temperatures

• oxidizing reactions involving carbon

progress rapidly with increasing

temperature

• tendency of oxidation of Si and Mn

decreases with increasing temperature

• reduction of oxides of Si and Mn by

carbon occurs more readily as

temperature increases

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• C loss is high at higher

temperatures

• Si and Mn are lost primarily at low

temperatures

• A gain in Si and Mn occurs at high

temperatures

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effects of concentration

• all chemical reactions occurred inside cupola are concentration dependant

• type of refractory (acid or base), slag composition, gas atmospheres, and melt

composition are the important concentration factors

Example:

SiO2 (s) + 2C = Si + 2CO (g)

K =Si x (CO)2

SiO2 x (C)2

K value at various temperatures may be calculated

and then the equilibrium concentration curves may

be plotted for various temperatures

calculated equilibrium concentration of percentage carbon and

silicon for SiO2(s) + 2C = Si + 2CO(g) in molten iron-carbon-silicon

alloys contained in a silica crucible under 1 atm pressure of the CO.

Solid curves indicate temperatures at which silica reduction will

occur spontaneously if an excess of carbon is present.

3.5% C , 2.3% Si

@ 1300°C

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effects of iron oxide

• similar to air or carbon dioxide, iron oxide is also a source of oxygen

• presence of iron oxide (in slag, as rust or generated in any other way) will cause

Si and Mn loss even at high temperature, where these losses normally would not

occur because of the protective action of carbon

high temperature melting

• molten iron decarburises rapidly above about 1400 C

• no Si or Mn loss occurs (unless iron oxide is present)

• CO2, even at 100% concentration, will not cause Si loss

• SiO2 reduction and Si pickup take place

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Some useful source data for metallics

TABLE 1: Typical Analyses of Common Steel Scrap Grades

TABLE 2: Typical Analyses of Iron Castings.

TABLE 3: Specifications for Various Grades of Pig Iron.

TABLE 4: Typical properties of foundry coke.

TABLE 5: Different Sources of Silicon.

TABLE 6: Different Sources of Manganese.

TABLE 7: Different Sources of Chromium and Nickel

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Next ClassMME 345, Lecture 36

Cast Iron Foundry Practices3. Metallurgy of grey irons