EXPERIMENT 6
HEAT TREATMENT OF STEEL
Purpose
The purposes of this experiment are to:
Investigate the processes
of heat treating of steel
Study hardness testing and
its limits
Examine microstructures of
steel in relation to hardness
Background
To understand heat treatment of steels
requires an ability to understand the Fe-C phase diagram shown in Figure
6-1. A steel
with a 0.78 wt%C is said to be a eutectoid
steel. A steel
with carbon content less than 0.78%C is hypoeutectoid
and greater than 0.78%C is hypereutectoid.
The region marked austenite is face-centered-cubic, and ferrite is
body-centered-cubic.
There are also regions which have two
phases. If one cools a
hypoeutectoid steel from a point in the
austenite region, reaching the A3 line ferrite
will form from the austenite. This
ferrite is called proeutectoid ferrite. When A1 is reached, a mixture of ferrite and
iron carbide (cementite) forms (pearlite)
from the remaining austenite.
Thus the microstructure of a hypoeuctectoid
steel upon cooling would contain proeutectoid ferrite
plus pearlite (a+ Fe3C).
The size, type, and distribution of
phases present can be altered by not waiting for thermodynamic
equilibrium. Steels are often cooled so
rapidly that metastable phases appear. One such phase is martensite,
which is a body- centered tetragonal phase and which forms only by very rapid
cooling.
Much of the information on
non-equilibrium distribution, size, and type of phases has come from
experiments. The results are presented
in a time-temperature-transformation diagram shown in Figure 6-2. As a sample is cooled the temperature will
decrease as shown in curve #1. At point
A pearlite (a mixture of ferrite and cementite) will start to form from austenite. At the time
and temperature associated with point B the austenite will have completely
transformed to pearlite. There are any number
of possible paths through the pearlite regions. Slower cooling causes coarse pearlite while fast cooling causes fine pearlite
to form.
Cooling can produce other phases. If a specimen were cooled at a rate
corresponding to curve #2 in Figure 6-3, martensite
instead of pearlite would begin to form at Ms temperature (point C), and the pearlite
would be completely transformed to martensite at
temperature Ms.
Martensite causes increased hardness in
steels.
Unfortunately, hardness in steels also
produces brittleness. The brittleness is
usually associated with low impact energy and low toughness. To restore some of the toughness and impact
properties it is frequently necessary to "temper" or "draw"
the steels. This is accomplished by
heating the steel up to a temperature between 500oF (260oC) and 1000oF (540oC). Tempering removes some of the internal
stresses and introduces recovery processes in the steel without a large
decrease in hardness or strength.
To obtain the desired mechanical
properties it is necessary to cool steel from the proper temperature at the
proper rates and temper them at the proper temperature and time. Isothermal transformation diagrams for SAE
1045 steel are shown in Figure 6-4.
Procedures
You are
provided with 6 specimens of SAE 1045 steel for your study. Measure the hardness of all specimens using
the RA scale.
1. Heat four specimens in one furnace at 1600
+ 25oF (870 + 15oC) for 1/2
hour.
2. Put the other 2 specimens in a separate
furnace at the same temperature for 1/2 hour.
3. Remove one specimen from the furnace with
2 specimens and cool it in air on a brick.
4. Turn off the furnace with the one
remaining specimen. Allow the sample to
remain in the furnace for one hour. The
air-cooled and furnace-cooled specimens can be cooled in water after one
hour. Why? (Answer this in your write up).
5. Remove the four specimens and quickly drop
them into water; the transfer should take less than one second. A little rehearsal could help. Be careful not to touch the specimens before
they are cooled in water.
6. Measure Rockwell hardness of the
air-cooled, furnace-cooled, and all of the quenched specimens before the next
step.
7. Temper 1 each of the quenched specimens
for 30 minutes at 600oF (315oC), 800oF (430oC), and 1000oF (540oC). After tempering, the specimens can be cooled
in water.
8. Measure hardness using the Brinell (3000 kg) and Rockwell A or C scales.
9. If available, examine and sketch the microstructures
of one quenched and tempered, one air-cooled, and one furnace-cooled specimen. The specimens are in the dessicator
jar near the
microscope. You may have to repolish and etch the specimens.
Data Analysis
1. Average all Brinell
impression diameters for each specimen.
2. Compute the Brinell
hardness numbers and compare with the numbers read from the conversion chart
for Rockwell A or C.
3. Plot curves with B.H.N. abscissas and
Rockwell numbers as ordinates.
4. Plot curve Rockwell A or C hardness vs.
tempering temperature (oC).
5. Compute the ultimate tensile strength (uts) of all specimens from the average B.H.N. for each
specimen using:
sult= 500 x B.H.N.
6. Answer all of the questions in the
procedure section of this experiment.
Glossary of Terms
Understanding
the following terms will aid in understanding this experiment.
Austenite.
Face-centered cubic (g)
phase of iron or steel.
Austenitizing. Temperature where
homogeneous austenite can form. Austenitizing is the first step in most of the heat
treatments for steel and cast irons.
Annealing (steel). A heat treatment used to produce a soft,
coarse pearlite in a steel
by austenitizing, then furnace cooling.
Bainite. A two-phase microconstituant,
containing a fine needle-like microstructure of ferrite and cementite, that forms in
steels that are isothermally transformed at relatively low temperatures.
Body-centered cubic.
Common atomic arrangement for metals consisting of
eight atoms sitting on the corners of a cube and a ninth atom at the cubes
center.
Cementite. The hard brittle intermetallic
compound Fe3C that when properly dispersed
provides the strengthening in steels.
Eutectoid.
A three-phase reaction in which one solid phase
transforms to two different solid phases.
Face-centered cubic.
Common atomic arrangement for metals consisting of eight atoms sitting
on the corners of a cube and six additional atoms sitting in the center of each face of the cube.
Ferrite. Ferrous alloy based
on the bcc structure of pure iron at room temperature.
Hypereutectoid.
Composition greater than that of the eutectoid.
Hypoeutectoid. Composition less than that
of the eutectoid.
Martensite. The metastable
iron-carbon solid solution phase with an acicular, or needle like,
microstructure produced by a diffusionless
transformation associated with the quenching of austenite.
Normalizing.
A simple heat treatment obtained by austenitizing
and air cooling to produce a fine pearlite structure.
Pearlite. A two-phase lamellar microconstituant,
containing ferrite and cementite, that forms in steels that are cooled in a normal fashion
or are isothermally transformed at relatively high temperatures.
Tempered martensite. The mixture of ferrite and cementite formed when martensite
is tempered.
Tempering.
A low-temperature heat treatment used to reduce the hardness of martensite by permitting the martensite
to begin to decompose to the equilibrium phases.
Write Up
Prepare a single memo report on the experiment, in
conjunction with experiment #7 (Hardenability of
Steels). The report should combine both
experiments in one report. Do not write
this up as a two part report.
(The hardness and hardenability concepts from
the experiments are related). Within
this report you should discuss the data referenced in the "Data
Analysis" appearing above as well as the following:
1. What is the purpose of quenching and
tempering steel?
2. Discuss the sources of error for the
various hardness testers, the relative ease with which they may be used, and
the comparative consistency of test results.
3. What factors probably contributed to the
scatter in the hardness data?
4. Which hardness test appears to be most
accurate?
5. Using the inverse lever law, estimate the
amount of carbide (Fe3C) present at 1338oF (733oC) (just below
the eutectoid temperature) for SAE 1045.
6. What are (or should be) the differences in
the microstructure for each heat treatment process and how do these differences
correlate with hardness?
7. Discuss errors in this experiment and
their sources.
References
Van Vlack, Elements of Materials Science and Engineering,
Chapter 5
Flinn and Trojan, Engineering
Materials and Their Applications, Chapter 6