Carbon Steels: Microstructure and Mechanical Properties
Reading Assignment: 9.14, chapter 10 in Callister
Objectives
•
Recognize the wide range of equilibrium microstructures and the effect of C
content on the mechanical properties.
• Recognize the main heat treatments for steels and the corresponding
mechanical properties.
Introduction
Steel is one of the most used engineering materials. It is used in the form of beams for
building support structures, train railroads, and reinforcing rods in concrete; in the
form of plates for ship construction; in the form of tubes for boilers in power
generating plants, car radiators, and oil and gas pipelines; in the form of sheet metal
for cars, washing machines, in the form of wire for elevator cables, and special steels
are used for cutting tools (hacksaw, blades, drill bits, knives) and for wear resistant
application such as ball bearings. There are two main reasons for the popular use of
steels: (1) steel is abundant in the earth’s crust in the form of Fe2O3 and require little
energy to convert it to Fe which makes its production inexpensive; and (2) it can be
made to exhibit a great variety of microstructures and thus a wide range of mechanical
properties. The microstructure that develops in carbon steels depends on both (1) the
carbon content and (2) thermal history or heat treatment.
Equilibrium Phases
To understand the microstructures that can be produced by heat treatment of steel, it is
necessary to consider the Fe-C phase diagram (Fig. 1). There are three equilibrium
phases in the phase diagram which can be obtained by very slow cooling rates to
allow equilibrium conditions to prevail. Each phase has particular characteristics,
some of which are listed in Table 1.
eutectoid
Figure 1: phase diagram for Fe-C showing the range of carbon steels.
1
Table 1 Characteristics of the Equilibrium Phases in Steel
phase
crystal
composition
strength
ductility
Structure
solid solution 0 to 2.1 wt% C
Low
High
Austenite (γ)
of C in FCC Fe
Ferrite (α)
Cementite
(Fe3C)
Solid solution
of C in BCC
Fe
Orthorhombic
compound
Intermediate
Intermediate
Extremely
hard
Extremely
Brittle
0 to 0.02 wt% C
6.7 wt% C
The amount of equilibrium phase changes that take place upon slow cooling from the
austenite region in the Fe-C phase diagram into the ferrite + cementite phase field
strongly depends on the carbon content. Depending on the carbon content, carbon
steels can be divided into three categories: eutectoid steels (contain exactly 0.76%C),
hypoeutectoid steels (%C < 0.76), and hypereutectoid steels (%C> 0.76). The
microstructure that develops when a eutectoid steel (0.76% C) is slowly cooled from
the austenite region to below 727 ºC consists of alternating layers of α and cementite.
This structure is called pearlite. For hypoeutectoid steels (%C < 0.76) the
microstructure consists of pearlite surrounded by pro-eutectoid α while
hypereutectoid steels (%C> 0.76) are composed of pearlite surrounded by cementite,
as illustrated in Figure 1. The equilibrium amounts of ferrite and cementite can be
calculated by the use the lever rule. The hardness of carbon steels increases with
increasing the carbon content due to increases in the hard phase, cementite. It should
be noted that slow cooling heat treatment is not important from practical point of
view. It is used here just to demonstrate the objectives of this experiment. Refer to
your textbook for further details.
Experimental Procedure Part#1
1. You are given 4 mounted specimens of carbon steels
i. Sp#1 with 0.4%C
Low carbon steel
ii. Sp#2 with 0.6%C (AISI 1040)
Medium carbon steel
iii. Sp#4 with 0.8%C (AISI 1080)
Eutectoid steel
iv. Sp#5 with 1.1%C
High carbon steel
2. Fully austenize the specimens by heating at 850 ºC for 20 minutes in a heat
treatment furnace. Austenitizing refers to heating the steel into the austenite
phase field so that all of the carbon is dissolved into solid solution γ.
3. Cool the specimens inside the furnace at very slow rate to get the equilibrium
structures (decrease the furnace temperature from 850 ºC to 720 ºC and then
quench in water to retain the microstructure).
4. Take five measurements of the hardness of each sample using the Rockwell
hardness scale B.
5. Etch the specimens using 2% Nital.
6. Observe the microstructure in the optical microscope at 100X.
7. Take photos for the microstructures.
2
8. Calculate the fractions of total ferrite and cementite for each specimen from the
phase diagram and the lever rule.
9. Calculate the fractions of the proeutectoid phase and pearlite for each alloy.
10. Prepare your data in a table form as shown below.
11. Plot the average hardness, total ferrite, and total cementite vs. C%.
12. Which phase is responsible for the increase in hardness?
C%
0.2
0.4
0.8
1.2
Total ferrite
Total cementite
Hardness
Non-equilibrium Heat treatments of Steels
From the previous experiment, you have looked at the effect of C on the strength for
different steels cooled from the austenizing temperature very slowly to reach
equilibrium conditions. In practice, however, it is not practical to cool at very slow
rate to get the equilibrium microstructure and real heat treatments almost always
involve the development of non-equilibrium microstructures. Note that the phase
diagram can not be used to predict the non-equilibrium microstructures. Heat
treatments of steels will be divided into two approaches: intermediate cooling or fast
cooling as explained below.
Intermediate cooling: when the steel is cooled at intermediate rates to room
temperature, C can diffuse relatively far and the spacing of the C rich phase Fe3C is
greater. The resulting pearlite is called coarse pearlite and the heat treatment is called
full anneal. This is can be done by shutting off the furnace while the specimen is kept
inside. When the steel is cooled at a faster rate (but still slower than quenching), the
transformation takes place at temperatures quite a bit below 727oC. At the lower
temperature C can diffuse only a short distance, and the spacing of the C rich phase
Fe3C is smaller. The resulting pearlite is called fine pearlite and the heat treatment is
called normalizing. This is can be done by taking the specimen from the furnace and
let it cool at room temperature. The range of lamellar spacings in steels vary from
about 1 µm to 0.1 µm.
Figure 2: (a) coarse pearlite resulting
from full anneal and (b) fine pearlite
from normalizing.
(a)
(b)
3
Fast cooling or quenching: the previous section provided an
indication that the rate of cooling has an effect on the
microstructure of steel. This is because the C atoms are in
solution in the FCC austenite phase. When the austenite is
slowly cooled and the crystal structure of Fe changes to
bcc, the phase diagram shows that most of the C atoms
must diffuse to form particles of Fe3C, leaving only a small
concentration of C atoms dissolved in the BCC Fe crystal.
Suppose that we cool the austenite very quickly - so fast
that the C atoms don't have time to diffuse before the steel
is cooled to such a low temperature that the C atoms are
frozen in position. The Fe atoms only have to shift their Figure 3: martensite
positions slightly to accomplish the transformation from
fcc austenite to bcc ferrite; therefore, fast cooling does not stop the austenite to ferrite
transformation from taking place. What it does stop is the diffusion of C atoms over
the distance necessary for them to form Fe3C particles. Thus, we have a different
situation than anything indicated on the Fe-C phase diagram: ferrite containing very
high %C in solid solution. The C trapped in solid solution in BCC Fe stretches the
lattice in only one direction, so that the unit cell no longer has 3 sides of equal length.
For this reason, the quenched steel containing extra C in BCC Fe is usually said to
have a body centered tetragonal (BCT) crystal structure. In an optical microscope, the
individual atoms are not resolved so this aspect of the structure is not visible. What
can be seen are the feathery (lath-shaped) crystals of Fe that contain more C in solid
solution than predicted by the phase diagram. Fig. 3 shows a microstructure of
quenched 1080 steel; this structure is know as martensite. The elongated lath shape of
the martensite crystals is a consequence of the way the transformation from austenite
(FCC) to ferrite (BCC) takes place. Martensite is the strongest and the most brittle
phase in steel.
Experimental Procedure Part#2
1. You are given 6 specimens of
eutectoid steels (0.8%C)
2. Fully austenize the specimens
by heating at 750 ºC for 20
minutes in a heat treatment
furnace.
3. Cool two specimens inside the
furnace to room temperature
(shut off the furnace). This is
called full anneal.
4. Cool two specimens at room
temperature. This is called
normalizing.
5. Quench two specimens in tap
water.
6. Take five measurements of the
hardness of each sample using
the Rockwell hardness scale
C.
Full anneal
Normalizing
Figure 4: continuous cooling curve for
eutectoid steel.
4
7. Etch the specimens using 2% Nital.
8. Observe the microstructure in the optical microscope at 100X.
9. Take photos for the microstructures.
10. Prepare your data in a table form as shown below.
11. Rank the heat treatments in a decreasing order of hardness.
The non-equilibrium microstructure can be predicted
transformation diagram as demonstrated in Figure 4.
Heat treatment
Slow cooling (Full anneal)
Intermediate (Normalizing)
Fast cooling (Quenching)
Structure
Coarse pearlite
Fine pearlite
martensite
Hardness
by
the
continuous
Ductility
5