THE MINISTRY OF EDUCATION AND SCIENCE OF THE RUSSIAN FEDERATIONFEDERAL STATE BUDGETARY EDUCATIONAL INSTITUTION OF HIGHER EDUCATION

«GUBKIN RUSSIAN STATE UNIVERSITY OF OIL AND GAS (NATIONAL RESEARCH UNIVERSITY)»

DEPARTMENT OF PHYSICAL METALLURGY AND NON-METAL MATERIALS

S.P. GRIGORIEV, V.P. EROSHKIN, А.P. EFREMOV,B.M. KAZAKOV, G.А. TROFIMOVA

LABORATORY WORK NO 4

BUILDING PHASE DIAGRAM FOR IRON-CARBON ALLOYS AND MICROSTRUCTURE ANALYSIS OF CARBON STEEL IN

EQUILIBRIUM STATE

for students of all disciplines

Edited by prof. A.K. PRYGAEVEnglish translation assist. I.O. SELEZNEVA

Moscow – 2016

Objective

1. Preview the phase diagram of iron-carbon alloys and study the nature of transformations in carbon steels at gradual cooling.

2. Study the microstructure of carbon steels in equilibrium state.

3. Study the effect of carbon content on mechanical properties of gradually cooling steel.

Task

1. Develop the phase diagram for system Fe-Fe3C.

2. Develop a cooling curve for an alloy having carbon concentration provided by supervisor.

3. Study standard thin sections of steel using microscope to determine the phase composition, structure and the approximate carbon content. Sketch the microstructure of steels investigated.

Basic information

It is important to know that the iron-carbon alloys have the major component iron

existing in two allotropic modifications: as a volume centered cube Feα) and as a

face-centered cube). The cooling curve of pure iron (Fig. 1) shows that Feα exists in

two temperature ranges: below 911°C and between 1392 and 1539°C. When the

temperature 1392°C is reached during cooling, Feα undergoes an allotropic

transformation resulting in the crystal lattice changing at a constant temperature from

a body-centered cube to a face-centered cubic lattice of Feγ. The second allotropic

transition during cooling occurs at a temperature of 911°C, when Fe γ (face-centered

cubic lattice) is transformed into a body-centered cubic lattice of Feα.

Iron undergoes a magnetic transformation at a temperature of 768°C called the

Curie temperature: below 768°C iron becomes magnetic. Magnetic transformation is 2

a special kind of transformation; it has several features that distinguish it from

allotropic transformations.

Iron and carbon form solid inclusion solutions or chemical compounds.

Depending on the carbon content the iron-carbon alloys are divided into two

classes: steels and cast irons.

Steels are alloys containing up to 2.14% carbon. Cast irons have the carbon

content in amount between 2.14 and 6.67%.

Depending on the content and structure carbon steels are broken down into:

- Technical iron - alloys containing up to 0.02% carbon.

- Hypoeutectoid steel - alloys containing from 0.02 to 0.8% carbon

- Eutectoid steel - alloys containing 0.8% of carbon

- Hypereutectoid steel - alloys containing from 0.8 to 2.14% carbon.

3

Т, oC.

L → Feα1539

Feα → Feγ1392

Feγ

Fe → Feα911 γ

768 Curie point

Feα

TIME

Fig. 1 Cooling curve of pure iron

4

Primary and secondary crystallization of steel

The phase diagram "iron-cementite" (Fig. 2) is useful to study transformations

in iron-carbon alloys during gradual cooling and to examine microstructures in

equilibrium state; the basics of phase diagrams were developed by D.K. Chernov in

1886.

Similar to other two component phase diagrams the phase diagram “iron-

cementite” is developed in coordinates "temperature-carbon concentration (%)". The

maximum concentration of carbon in the phase diagram is 6.67%, which corresponds

to 100% cementite.

The primary crystallization is a transition of metal from liquid to solid state,

i.e. the process of forming solid crystals directly from the molten liquid.

For carbon steels this process begins during cooling when the temperature

drops to a value corresponding to the ABC line; the process ends at HJE line. When

the primary crystallization is over and the temperature reaches relevant HJE line

regardless of the carbon content, the steel acquires a polyhedral structure of austenite

which under further gradual cooling maintains the structure until line GS is reached

in hypoeutectoid steels or SE line is reached in hypereutectoid steels.

Unlike the primary crystallization the process of forming crystals from the

solid phase is called secondary crystallization.

The essence of the secondary crystallization of carbon steel deals with

decomposition of austenite when steel is cooled resulting in formation of new phases

of ferrite and cementite.

The secondary crystallization in hypoeutectoid steels starts with formation of

ferrite when the temperature attains the proper level during cooling to correspond to

GS line. The phase diagram shows that

5

5

Т,oСA L + F1539

BF

J1392

N F + A

L + A

EA

G A + Cn911Austenite

+A + F S Cementite

F (SECONDARY)P

F + C P E F + CFERRITE R PEARLITE

+ L +PEARLIT

E AND CementiteT

0 0.8 2.14

0 10 20 30

L

CF + C 1

FLED A + CEBU Ledebourite +R CementiteANDT

KF + C

Ledebourite + Cementite

L4.32 6.67, %C

70 8090

Fig. 2. Phase diagram plotted for iron - cementite system

6

The temperature of the onset of secondary crystallization is not constant. In

hypoeutectoid steels it decreases with increasing carbon concentration.

In GSP area the structure consists of two phases: austenite and ferrite. When

the temperature decreases from GS line to PS line the amount of ferrite gradually

increases and amount of austenite decreases; the carbon concentration increases in

remaining austenite along the GS line towards point S reaching 0.8% at 727°C (PS

line).

When hypereutectoid steels are cooled from austenite along ES line, the

secondary cementite starts forming. At further cooling between lines ES and SK the

steel structure consists of austenite and secondary cementite whose quantity

continuously increases. When cooled austenite loses, carbon reaching the eutectoid

composition (0.8% C) at a temperature of 727°C (SK line).

Thus at 727°C austenite contains 0.8% C in hypoeutectoid, eutectoid and

hypereutectoid steels; it decomposes into two phases of ferrite and cementite at a

constant temperature:

A 0.8%C → (Fer 0.02%C + Cem 6.67% C),

and the structure of resulting mechanical mixture is called perlite.

Structure of carbon steel in equilibrium state

According to the phase diagram, alloys containing up to 0.01% carbon are

single-phase alloys having a structure of pure ferrite. When the carbon content

increases from 0.01% to 0.02% the structure of alloy consists of ferrite and tertiary

cementite formed from ferrite along PQ line. Due to trace amount of the tertiary

cementite it is usually not observed in the structure.

The structure of hypoeutectoid steel containing more than 0.02% carbon

7

consists of ferrite and pearlite. When the concentration of carbon increases the

amount of perlite grows, and the amount of ferrite decreases.

The microstructure of hypoeutectoid steels allows accurately determining the

carbon content, assuming that all carbon is associated with perlite. It is necessary to

determine what part of the viewing field area on a thin section is occupied by pearlite

to determine the content of carbon; then this area should be multiplied by 0.8. For

example, if 40% of the area occupied by perlite, the carbon content in steel 40:

100 x 0.8 = 0.32%.

The eutectoid steel has structure of perlite, i.e. it is a mechanical mixture of the

two phases:

of ferrite and cementite in which cementite particles are evenly distributed in

the bulk of ferrite. Depending on the shape of cementite formation there are lamellar

pearlite and granular perlite distinguished.

Hypereutectoid steel structure consists of pearlite and secondary cementite. As

the carbon content increases in steel the amount of secondary cementite also

increases; in hypereutectoid steel the secondary cementite is formed primarily as a

thin fringe at the boundaries of pearlite grains.

During conventional etching using four percent solution of nitric acid in

alcohol the cementite network has the same light color as the ferrite network in

hypoeutectoid steels. To be sure that inclusions are made of cementite the thin section

should be polished anew with subsequent etching using a special solution of sodium

picrate which tints cementite whereas ferrite color remains the same.

Mechanical properties of slowly cooled steels

Increasing carbon concentration increases pearlite in the structure of

hypoeutectoid steels and the secondary cementite in the structure of hypereutectoid

steels.

8

Therefore, when the carbon content is increasing in the structure of

gradually cooled steels the amount of cementite is increasing and the amount of

ferrite is decreasing. These changes in the steel structure result in changing its

mechanical properties. Ferrite has low strength (σ ≈ 250 MPa) low hardness (HB ≈

80) and high ductility (δ ≈ 50%). Cementite has a high hardness HB ≈ 800), and

virtually it has no plasticity.

Fig. 3 shows the mechanical properties of hot-rolled steel for which the final

formation of the structure, and hence the properties is determined by a relatively slow

cooling after hot rolling; the profiles are plotted as a function of carbon concentration.

The values of mechanical properties are averaged and can vary within 10% accuracy

range depending on conditions of cooling after rolling, on impurity content and other

indicators.

These curves show that increasing the carbon content in slowly cooled steel

results in increasing hardness and strength whereas ductility (the percentage of

elongation and contraction ratio) decreases.

Lowering tensile strength with increasing carbon content higher than 1% is

associated with developing a brittle network of the secondary cementite at the

boundaries of pearlite grains in the steel structure.

The order of developing the report

The report should provide for the following:

1. The purpose of the task and assignment to implement it.

2. Plotted cooling curve for steel showing given concentration of carbon.

3. Graphs plotted for hypoeutectoid, eutectoid and hyper eutectoid carbon steel

with analysis.

9

HB

σ, MN/m2

300 1200

δ1000

200 800

600

100 400

200

00.4

δ, ϕ, %

60

HB

50

σв40

30

20

10

φ

0.8 1.2

Carbon concentration,Fig.3. Effect of carbon concentration on mechanical properties of steel.

10

References

1. Solntsev Y.P., Pryakhin E.I., Voytkun F. Material Science, Moscow, 1999, 477 p. (in Russian).

2. Lakhtin Y.M. Metallurgy and Metal Hot Processing, Moscow,

Metallurgy, 1993, 447 p. (in Russian).

11