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Calculation of carbon content of

austenite during heat treatment of

cast irons

Male, born in 1965, Ph.D, Professor, an expert with outstanding

contributions in Hubei Province. Research interests: forming

techniques and process control of metal materials, technologydevelopment and application of austempered ductile iron.

E-mail: gongwenbang@yahoo.com.cn

Received:2009-05-13; Accepted:2009-11-28

*Gong Wenbang

*Gong Wenbang, Chen Guodong and Xiang Gangyu

(Wuhan University of Science and Engineering, Wuhan 430073, Hubei, China)

Abstract: The austenitizing temperature controls the carbon content of the austenite which, in turn, inuences

the structure and properties of cast irons after subsequent cooling to room temperature. In this paper, for a

cast iron with known silicon content, a formula of calculating austenite carbon content at a certain austenitizing

temperature was developed. This relationship can be used to more accurately select carbon content of austenite

or austenitizing temperature to produce desired properties after subsequent cooling to room temperature.

Key words: cast iron heat treatment; austenitizing; austenite; carbon content of austenite

CLC number: TG143/151 Document code: A Article ID: 1672-6421(2010)01-030-03

Except for subcritical annealing and stress relief, allthe heat treatment processes of cast irons have anaustenitizing stage. The austenitizing temperature controls

the carbon content of the austenite which, in turn, inuences

the structure and properties of cast irons after subsequent

cooling to room temperature. For example, for obtaining fully

pearlitic structure, the SG iron normally is heated to a higher

austenitizing temperature to keep higher carbon content in the

austenite, which will be beneficial to austenite transforming

to full pearlitic structure; while for obtaining fully ferritic

structure, lower austenitizing temperature is applied to keep

lower carbon content in the austenite, which is benecial to

obtain fully ferritic structure. For ADI, high austenitizing

temperatures increase the carbon content of the austenite,

thus increasing its hardenability, but require a longer time

to transform to ausferrite. High austenitizing temperatures

generally produce high grade ADI with higher strength and

lower ductility. Lower austenitizing temperatures generally

obtain ADI with lower strength but higher ductility. For

obtaining desired ADI properties, especially for high ductility

ADI, close control is required for the silicon content, which

has a significant influence on the upper critical temperature

and the actual carbon content of the austenite[1-4]

.

Thus, for a given cast iron it is important to know the

exact relationship between carbon content of austenite and

austenitizing temperature. Knowing this, the right carbon

content of austenite or the right austenitizing temperature

can be selected, and desired properties can be obtained after

subsequent cooling to room temperature. Many elements, such

as Mn, P, S and other alloying elements, affect the critical

points on the Fe-C phase diagram, thus, inuence the carbon

content of austenite at austenitizing temperature. However, in

normal non-alloyed or low alloyed cast irons, the predominant

element influencing the critical points is silicon, since the

content of silicon is much higher than that of other elements

and silicon has more effectiveness compared with other

elements. Normally, the inuences of other alloying elements,

oxidation during heat treatment, heating and cooling rate etc.

on the critical points of Fe-graphite phase diagram are small

and can be negligible for production consideration[5]

.

In this paper, for a cast iron with known silicon content, a

formula of calculating carbon content of austenite at a certain

austenitizing temperature was developed. This relationship can

be used to more accurately select carbon content of austenite

or austenitizing temperature to produce desired properties after

subsequent cooling to room temperature.

1 The effect of silicon on the criticaltemperature and relationshipbetween austenitizing temperatureand the carbon content of austenite

Silicon has a signicant inuence on the critical temperature

and carbon content on the Fe-graphite phase diagram. Silicon

can decrease the critical carbon contentincrease the upper

critical temperature and make a A+L+G three phase region and

a A+F+G three phases region. Figure 1 shows the Fe-C phasediagram and Fe-Graphite-Si diagram with the effect of silicon:

the solid line is a pure Fe-C phase diagram and the dash lines

represent a Fe-Graphite-Si phase diagrams with x%Si. For an

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x% Si, the carbon content and temperature of critical points

have the following relationship[5-7]

:

Where Cax is the austenite carbon content at a certain

temperature Tx.

Following two equations can be obtained from equation (7):

Tx= TSx1 + x(Cax-CSx) (8)

orCax= CSx+ (Tx-TSx1)/x (9)

In which,

x= (TEx-TSx1)/(CEx-CSx) (10)

xcan be considered as a influencing coefficient of the

carbon on the temperature when the silicon content isx%.

Substitute the relationships (2) - (5)

x= (416-37.5x)/(1.4-0.202x) ( /%) (11)

Thus, for any cast iron withx%Si, using above relationships

(8)-(9) and x, Cax, the carbon content of austenite at a certain

temperature Tx, or the temperature Tx to obtain a specifiedaustenite content Cax, can be easily calculated.

2 Calculation of carbon content of theaustenite in the commonly used castiron during heat treatment

Different cast irons have different silicon content and different

applications, thus need different austenitizing temperature

or austenite carbon content in the austenitizing stage of heat

treatment. Figure 2 shows the calculated relationship among

the silicon content, the austenite carbon content and the

austenitizing temperature for different types of cast irons.The different boxes show the suitable ranges of austenitizing

temperature and austenite carbon content for different cast

irons. Table 1 lists the ranges of silicon content, austenitizing

temperature and calculated austenite carbon content for

different cast irons. Figure 2 and Table 1 can be a useful

reference for heat treatment of different cast irons.

It should note that the formula for calculation of the C ax,

the austenite carbon content at a certain temperature Tx, or

the temperature Tx to achieve a specified austenite carbon

content Cax are for fully austenitizing treatment, that means

a full austenite structure can be obtained if the austenitizing

temperature is held for long enough. For the partial austenite

heat treatment to obtain partial ferrite and partial austenite,

the temperature must below the upper critical temperature and

above the lower critical temperature. For obtaining a particular

ratio of ferrite and austenite, the temperature must be carefully

considered and selected between the upper critical temperature

TSx1 and above the lower critical temperature TSx2.

3 Conclusions

For a cast iron with a known silicon content, a formula

of calculating carbon content of austenite at a certain

austenitizing temperature was developed. This relationship canbe used to more accurately select carbon content of austenite,

or austenitizing temperature during austenitizing stage of

Fig. 1: The vertical sections of Fe-C binary phase diagram

and Fe-Graphite-Si ternary phase diagram

Solid lines show the Fe-C binary equilibrium diagram. Dash lines describe a

Fe-Graphite-Si ternary equilibrium phase diagram withx%Si. Thin solid lines

with the arrow represent the direction of silicon content increasing from 0%

to x% except the only vertical arrow shows temperature increasing from TS

(without Si) to TSx1 and TSx2. (withx%Si)

CCx= 4.26 0.317x (%) (1)

CEx= 2.08 0.217x (%) (2)

CSx = 0.68 0.015x (%) (3)

TEx= 1154 + 2.5x () (4)

TSx1 = 738 + 40x () (5)

TSx2 = 738 + 30x () (6)

Where:

CCx-eutectic carbon content;

CEx-maximum carbon content in austenite;

CSx-carbon content of austenite at upper eutectoid

temperature;TEx-lower eutectic temperature;

TSx1-upper eutectoid temperature and

TSx2-lower eutectoid temperature.

Thus, for pure Fe-C alloy when temperature changes

between TS and TE, the carbon content (or solubility) in

austenite varies along E-S; for cast irons with a certain silicon

content, when temperature changes between TEx and TSx1 the

carbon content of austenite varies along Ex-Sx line.

From the phase diagrams in Fig. 1, the lines E-S and Ex-

Sx can be considered as approximate straight-lines, for a cast

iron with x% Si, using the principle of geometric similarity,

following relationships can be derived:

(Tx-TSx1)/(TEx-TSx1)=(Cax-CSx)/(CEx-CSx) (7)

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Fig. 2: Relationship among austenitizing temperature, austenite carbon content and silicon content during heat treatment

of following cast irons: SGI- Spheroidal Graphite Iron, GCI- Gray Cast Iron, ADI-Austempered Ductile Iron, BMCI-Black heart Malleable Cast Iron, WMCI- White heart Malleable Cast Iron, PMCI- Pearlite Malleable Cast Iron.

Spheroidal graphite iron High-temp. graphitizing annealing 2.03.0 930960 0.8231.065

Spheroidal graphite iron Normalizing 2.03.0 910940 0.7711.006

Spheroidal graphite iron Quenching and tempering 2.03.0 890920 0.7190.948

Austempered ductile iron Austempering 2.32.7 850930 0.6510.949

Gray cast iron Quenching 1.02.5 850900 0.6761.051

Black heart malleable cast iron Qraphitization 1.22.0 910960 0.9191.205

Pearlitic malleable cast iron Qraphitization 1.02.0 930960 0.9771.241

White heart malleable cast iron Qraphitization 0.41.0 9501,000 1.2091.483

Table 1: The ranges of silicon content, austenitizing temperature and calculated austenite carbon content for different

cast irons

heat treatment to produce desired properties after subsequent

cooling to room temperature. The ranges of silicon content,

austenitizing temperature and austenite carbon content during

austenitizing for different cast irons are calculated, which can

be a useful reference for heat treatment of different cast irons.

References

[1] Liu Jincheng and Shi Shengli. The Microstructure and

Mechanical Properties of Austempered Ductile Iron (ADI).

In: Proceedings of the Fourth National Symposium on

Austempered Ductile Iron(ADI) Technology, Suzhou, China,

December 2006. (in Chinese)

[2] Matti Johansson. Austenitic-Bainitic Ductile Iron. Trans AFS,

The research is supported by the scientic and technological project of China Textile Industry Association.

1977(85):117122[3] ASTM Standard Specification for Austempered Ductile Iron

Castings, A897/A897M-03, ASTM 2003.

[4] Chen Chenjia. Development of Austempered Ductile Iron. In:

Prceedings of the International Academic Symposium on

Austempered Ductile Iron (ADI), Wuhan, China, September

2004. (in Chinese)

[5] Chinese Mechanical Engineering Society. Heat Treatment

Manual. Beijing: China Machine Press, November 2005. (in

Chinese)

[6] Chinese Mechanical Engineering Society. Foundry Handbook.

Beijing: China Machine Press, January 2006. (in Chinese)

[7] Wu Dehai. Ductile Iron. Beijing: China Water Power Press,

2006. (in Chinese)

Cast iron Heat treatmentSilicon content Austenitizing Calculated carbon content

(%) temperature () in the austenite (%)