THE MINISTRY OF EDUCATION AND SCIENCE OF THE RUSSIAN FEDERATION

FEDERAL 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 7

HEAT TREATMENT OF CARBON STEEL GRADES:

ANNEALING, NORMALIZING, HARDENING

for students of all disciplines

Edited by prof. A.K. PRYGAEV

English translation assist. I.O. SELEZNEVA

Moscow – 2016

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Objective

1. To study effects of heating temperature and cooling rate on transformation

of austenite in carbon steel.

2. To familiarize with basic types and heat treatment process of steel.

Task

1. To conduct annealing, normalizing and hardening of samples of carbon

steel.

2. Measure hardness using the standard methods in heat-treated steel

samples.

3. To study effects of cooling rate on the structure and hardness of carbon

steel grades.

4. To study effects of carbon on the structure and hardness in carbon steels

grades under these kinds of heat treatment.

5. Conduct an individual research project assignment.

Basic information

Thermal processing of steel is a set of operations involving heating,

seasoning and cooling carried out to change the steel structure to improve

fabrication characteristics of blanks (during rolling, forging, and casting) and

to ensure obtaining the necessary operational properties of material in finished

products.

Shaping up the steel structure mainly deals with degradation of austenite

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during its cooling at different rates.

Steady state of austenite in steels occurs at temperatures above A3 (GSE

line, Fig. 1 in practical work No 6.

In the temperature range A3 - A1 (line GSE - PSK) cooling results in

formation of excess phases: ferrite in hypoeutectoid steels and the secondary

cementite in hypereutectoid steels. As a result, the dissolved carbon

concentration reaches 0.8% in residual austenite at a temperature of 727oC.

In equilibrium state at a temperature of 727oC austenite containing 0.8%

carbon degrades forming pearlite - an eutectoid mechanical ferrite and

cementite mixture.

Under continuous slow cooling at a rate of V1 (Fig. 1) the pearlite

transformation can begin and end at temperatures below 727oC. With

increasing the cooling rate (V3> V2> V1) the temperature of austenite

degradation decreases resulting in plates of ferrite-cementite mixture getting

smaller. As a result structures of pearlite, sorbitol and troostite are formed; they

have different sizes of ferrite and cementite structures (different dispersion

degree) and, therefore, they have different mechanical properties.

Large flaky ferrite-cementite mixture of perlite provides for the highest

values of impact hardness, of elongation percentage and contraction ratio

relative to sorbitol and troostite, whereas such properties as hardness, shear

strength tensile strength and yield strength are the lowest.

The disperse fine structure of troostite provides for better hardness

indicators and strength properties; the mechanical properties of sorbitol stay

between those of perlite and troostite.

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T,oC

800 (A)

A1

727

700

V1 (P)

600

500 V2 (C)

400 V3 (T)

300 M

n

200

(А М)

100

0

100

M

k

200 V5 (М+Аrest) Vcr V4 (Т+М+Аrest)

Lg τ

Fig. 1. Diagram of isothermal degradation of austenite with vectors showing different cooling rates

Under further increase of the cooling rate (V4) only part of austenite can

transit into ferrite-cementite mixture (troostite); and the remaining austenite

undergoes diffusionless transformation into supersaturated solid solution of

carbon in Feα, which is called martensite.

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The diffusionless martensit transformation is associated with changing

the face-centered cubic lattice of austenite into the body-centered cubic lattice

of α-iron maintaining the carbon concentration the same as in the original

structure.

The lattice of α-iron is capable of accommodating a limited amount of

carbon - no more than 0.02%. Therefore, an excessive amount of carbon from

the source austenite (max 2.14%) distorts the body-centered cubic lattice of α-

iron making it a body-centered tetragonal lattice. The value of ratio of periods

c/a - tetragonal parameter of martensite - increases with increasing the carbon

content (Fig. 2).

Under continuous cooling at a greater rate (V5 > V4) the diffusion

redistribution of carbon completely stops and only martensitic transformation

develops. In this situation part of austenite can remain untransformed.

Mon (the temperature of start of martensite transformation) and Mfin (the

temperature of the end of martensite transformation) are determined by the

content of carbon in steel; the greater the carbon concentration in austenite, the

lower the temperature Mon and Mfin (Fig. 3).

The amount of residual austenite depends on the carbon content in steel.

The amount of residual austenite is increased in high-carbon steel grades.

The rate vector Vk tangent to the step in C-shaped isothermal curve

(Fig. 1) characterizing the minimum rate of continuous cooling at which

diffusion degradation is completely suppressed is called the critical cooling

rate (the critical hardening rate).

Among all possible structures that can be obtained during continuous

austenite cooling martensite has the highest value of hardness and shear

strength.

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с

с

с/а =1 с/а >1

α-iron martensite

Fig. 2. Models of crystalline cells of α-iron and martensite

- iron; - carbon )

T,oC.

600

500

400

300

-200 A M

-100

20 Mn

0

-100 M + Ares Mk

-200 2.14 C,

0 0.4 0.8 1.2 1.6

Fig. 3. Effect of carbon content on position of

martensite points Mon and Mfin.

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However, the value of impact hardness, percentage extension relative

elongation, and contraction ratio of martensite are the lowest compared to other

structures.

The high hardness of martensite is determined by amount of dissolved

carbon: when the concentration of carbon increases in the lattice of α-iron the

value of hardness increases.

In machine engineering practice the following types of heat treatment

are used to obtain steel grades having different structures and desired set of

mechanical properties: annealing, normalizing, quenching and tempering.

Annealing is a kind of heat treatment comprising heating the steel above

temperature AC3 or AC1 followed by exposure of steel sample to these

temperatures and the subsequent slow cooling together with the furnace.

Slow cooling the steel during annealing promotes developing the

equilibrium phase transformations and formation of pearlite in eutectoid steel,

pearlite with excess ferrite or cementite in hypoeutectoid and hypereutectoid

steel, respectively.

After annealing the steel is characterized by high ductility at reduced

strength and hardness.

Normalization is the kind of heat treatment comprising heating the

steel some 30-50oC higher than the AC3, holding at this temperature and

subsequent cooling in air.

Phase recrystallization during heating and subsequent cooling in air

results in degradation of austenite at lower temperatures which improves the

dispersion of ferrite-cementite mixture resulting in formation of sorbitol.

In addition to sorbitol the structure of hypoeutectoid and hypereutectoid

steels will have respective excess ferrite and cementite present.

When sorbitol is formed in steel the strength and hardness of steel

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increase is compared to annealed samples. Therefore, normalization is widely

used to improve steel properties after casting, rolling and/or forging.

The normalization process has better engineering and economic

indicators; therefore it is practiced in low-carbon steel grades instead of

annealing.

In medium-carbon steel grades the normalization can be used instead of

hardening and tempering (combined process involving quenching followed by

a high temperature tempering).

For high carbon steels normalization prevents formation of cementite

fringe at the boundaries of pearlite grains observed in annealing at

temperatures between AC3 and AC1.

Hardening is a kind of heat treatment comprising heating steel some 30-

50oC above the AC1 and AC3 temperature for hypoeutectoid and hypereutectoid

(eutectoid) steels, respectively, holding samples at these temperatures, and

subsequent cooling at a rate equal to or higher than the critical cooling rate

(critical quenching rate). Water, oil, salt solution and alkali are used for

providing of required cooling rate.

As a result of quenching the hypoeutectoid and eutectoid steel grades

will have a martensite structure, and hypereutectoid steel will feature

martensite and excess cementite.

In addition to above structures hardened steel grades can have a certain

amount of residual austenite which is caused by a relative ability of the most of

cooling media to restrict the end of austenite-martensite transformation by

room temperatures.

After quenching steel with an optimum temperature has the highest

possible hardness, it has high strength and low indicators of ductility and

toughness.

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Methods of Work

Equipment, appliances, and tools

Equipment for different types of heat treatment of steel includes the

following: electric batch furnace having electromechanical temperature control

system; mobile dual quenching tanks with water and oil; claw, gloves, and

goggles.

Emery stone (disk), and sand paper are used for descaling and leveling

the end surfaces of the samples prior to hardness measurement.

Measurement of hardness using Brinell method is made using instrument

TSh-2; Rockwell hardness estimation technique involves instrument TK-2.

Sample Preparation

Cylindrical samples having a height of 12-15 mm, a diameter of 15-20

mm are used for testing. The end surfaces of the samples should be plane-

parallel featuring no cavities, fractures or other visible flaws.

Testing

The studies are conducted using the following grades of carbon steel:

Steel 20, Steel 40, Steel U7, Steel U10, and Steel U12.

As many as four samples are taken for each steel grade to be subject to

annealing, normalizing and quenching.

The temperature of heating samples is selected according to the type of

thermal processing and the carbon content in a steel grade. The holding time at

a given temperature is set within 5-10 minutes.

Cooling samples is made as follows: for annealing cooling is developing

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jointly with oven; for normalization cooling develops in air, quenching

involves immersion into oil or water.

Cooled samples are processed on both ends using emery stone (disk) to

remove scale and obtain plane-parallel surfaces.

The hardness in heat-treated steels is determined using two methods.

The Brinell method is used for steel after annealing and normalization.

The Rockwell method is used for hardened steels.

The hardness of a steel sample is measured in at least three points: the

distance between the two prints should be less than 1.5 mm for diamond cone

indentation and 4 mm for ball indentation.

Indicators of steel hardness according to Rockwell should be converted

into Brinell hardness values using standard conversion tables.

The test results should be an arithmetic average of three measurements.

The test results and calculations are recorded into Table 1.

The measurement data is used to develop two graphs:

- Graph of the hardness plotted vs. the carbon content for steels cooled at

different rates in coordinates: Brinell hardness (vertical axis) - the contents of

carbon (abscissa);

- Plot of the Brinell hardness as a function of the cooling rate for each steel

grade studied in coordinates: Brinell hardness (vertical axis) - the cooling rate

(x-axis).

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Table 1

Steel grade Method d Hardness

of cooling of impression HB HRC

Water

20 Oil

Air

Furnace

Water

40 Oil

Air

Furnace

Water

U7 Oil

Air

Furnace

Water

U10 Oil

Air

Furnace

Water

U12 Oil

Air

Furnace

The research project title and report format on assignment is provided by

supervisor.

The order of developing a report

The report should provide for the following:

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

2. Modes of process operations during annealing, normalizing and tempering.

3. Obtaining evaluation results on hardness (Table 1).

4. The graphs of the hardness plotted as a function of carbon content and hardness

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vs. steel cooling rate.

5. Summary of analysis of effect of heat treatments on the structure and

mechanical properties of carbon steels.

6. The results of research project should be compiled as ATTACHMENT

to report on laboratory work.

The laboratory work can be done by students after having a safety

briefing and orientation on effective industrial rules on conducting various

kinds of heat treatment of steel grades.

References

1. Solntsev Yu.P., Pryakhin E.I., Voytkun F. Material Science, Moscow,

MISA, 1999, 477 p. (in Russian)

2. Lakhtin Yu.M. Metallurgy and Heat Treatment of Metals. Metallurgy, 1993,

447 p. (in Russian)