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Heat Treatment
Engineering Materials
Module 4
Heat Treatment
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4.1 Equilibrium Diagrams
4.1.1 Exploration: Equilibrium Diagram
There are various types of steel alloys and their specific carbon content
differentiates all. A graph, or Equilibrium Diagram, represents the varying states of
steel as related to its iron and carbon content percentages. These states represent
liquid, solid, and states where both forms exist. Let’s see if you can mark where the
different steel alloys belong on the iron-carbon equilibrium diagram.
Student Exercise: Read Equilibrium Diagram
1. Obtain four (4) samples each of low carbon (example-1020), medium carbon
(example-1040), and high carbon (example-1095) steel.
2. On the following iron-carbon equilibrium diagram draw a vertical line
representing the actual steel alloys obtained in step 1.
Figure 4.1.1
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Questions: Equilibrium Diagram Exercise
1. At what point would the medium carbon steel begin to melt? _____________
______________________________________________________________
______________________________________________________________
2. At what point would that same medium carbon steel be 100% liquid? _______
______________________________________________________________
______________________________________________________________
3. How does that melting point of medium carbon steel compare to the melting
point of 100% carbon? ___________________________________________
______________________________________________________________
______________________________________________________________
4.1.2 Dialog: Equilibrium Diagrams
Since ancient times heat treatment has played an important role in society. A
highly secretive process was developed for producing superior steel for Damascus
Swords during the era of Alexander the Great. Today, that process is no more than
legend, but undoubtedly involved heating, quenching, and tempering. We will look
at each of these processes in detail.
The wide range of products produced today from microwaves, electronics,
automobiles, aeronautics, to the space industry would not be possible without the
modern methods of material processing including heat treatment.
In order to understand the basics of heat treatment, we must first investigate
the process of changing temperature, pressure, and alloying percentages. As these
variables are changed, we will study the associated changes in the physical
properties of metals. These changing variables are represented on equilibrium
diagrams.
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Equilibrium diagrams are shown below in Figure 4.1.2 and 4.1.3 for Copper-Nickel.
Figure 4.1.2
Figure 4.1.3
This shows the reaction to the changing alloy percentages versus
temperature. A solid solution phase is shown on the bottom of the diagram with a
pure liquid phase shown on the top portion of the diagram. In between there is a
curved region or two-phase region of both liquid and solid that exists. On the left side
of the vertical axis there is 100% copper that has a single melting point, on the right
side, 100% nickel with a single melting point. In between, a copper-nickel alloy will
go through a two-phase transition from a solid to a liquid.
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A following iron-carbon equilibrium diagram graphically depicts the addition of
carbon as an alloying element for iron. From 0 to 2% carbon materials are called
steels and greater than 2% carbon, the materials are called cast irons. We will study
the effects of these changing variables in detail in this module.
4.1.3 Dialog: Understanding the Iron-Carbon Equilibrium Diagram
Looking at a portion of the iron-carbon equilibrium diagram shown below that
show the steel portion of the curve, we can see the effects of the heating of the alloy
versus alloying percentage. At low temperature, iron is in a body centered cubic
(bcc) crystal structure called ferrite. Ferrite or  iron can hold little carbon atoms
interstitially and is very weak.
As the percentage of iron increases in the steel alloy, the iron and carbon
combine to form cementite, a chemical compound called iron carbide (Fe 3 C).
Cementite is a very hard and brittle structure with magnetic properties.
Pearlite is a layered or lamellar structure of ferrite and cementite. When
observed under a microscope, the white layers are the ferrite and the black ridges
the cementite.
At elevated temperatures, steel makes an allotropic conversion from body
centered cubic (bcc) ferrite to a face centered cubic (fcc) austenite. Austentite is not
magnetic and when slowly cooled will reverse its crystal structure to bcc ferrite.
Austentite or  iron has a high solubility for iron and can hold approximately 2%
carbon interstitially.
Knowing the percent of carbon in the steel you can determine the structure of
the steel at any given temperature. As steel is heated from the ferrite and cementite
region of the diagram you first reach the lower transformation curve labeled A1,
which begins the transformation from BCC-ferrite to FCC-austenite. Upon reaching
the upper transformation curve, labeled A# and Acm, there is complete
transformation to austenite.
The primary reason for changing the carbon content in steel is to achieve the
maximum degree of hardness. The carbon content in low carbon steel (0.05 to
0.30% carbon) is insufficient to produce substantial hardness changes in the steel.
Medium carbon steels (0.30 to 0.80% carbon) are very responsive to the heattreating process. High carbon steel (  0.80% carbon) is also very responsive to heat
treatment, but really has too much carbon in the crystal structure. The maximum
hardness value is reached as the carbon content in steel is increased to
approximately 0.60%.
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Heat Treatment
Figure 4.1.4
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4.1.4 Exploration: Heat Treatment of Various Types of Steel
Let’s look at the effect of heating and cooling on various types of steel. The
steel samples will vary in the percentages of carbon.
Student Exercise: Heating and Cooling
1. Obtain at least four (4) samples each of low carbon (ex:1020), medium
carbon (ex:1040), and high carbon(ex:1095) steel.
2. Test each type of steel for its Rockwell (B or C) Hardness and record the
value. Mark/stamp each specimen so that it can be identified after heat
treatment.
3. Load all twelve (12) specimens in a heat-treatment furnace and heat to 1250
deg F. Hold at 1250 F for 20 minutes. Quench one (1) of each type of steel
in cold water, perform a Rockwell Hardness test and record the value.
4. Repeat this process at 1400 F, 1550 F, and 1700 F.
DATA SHEET
Heat Treating Module
Engineering Materials
Exploration 4.1.2 Heat Treatment of Various Types of Steel
Hardness Values - Steel Specimens
Temp - F
Low Carbon
Medium Carbon
Room
1250
1400
1550
1700
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High Carbon
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Questions: Heating and Cooling
1. Were there any changes in hardness values due to the various
temperatures?__________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
2. Did the different types of steel react differently to the various temperatures?
______________________________________________________________
______________________________________________________________
______________________________________________________________
3. Which type of steel drastically changed hardness values first?____________
______________________________________________________________
______________________________________________________________
4. In your opinion, do these results agree with the curve associated with the
iron-carbon equilibrium diagram? ___________________________________
______________________________________________________________
______________________________________________________________
4.1.5 Exploration: A Heat Treating Recipe
Using your knowledge gained from the use of the equilibrium diagram, and
the two (2) previous explorations, recommend a heat-treating temperature for the
following two (2) types of steel.
1. Recommend a heat-treating temperature for 1060 steel using the iron-carbon
equilibrium diagram to assure that the material will get into the austenite area
of the diagram. _________________________________________________
______________________________________________________________
______________________________________________________________
2. Recommend a heat-treating temperature for 1030 steel using the iron-carbon
equilibrium diagram to assure that the material will get into the austenite area
of the diagram. _________________________________________________
______________________________________________________________
______________________________________________________________
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3. Discuss what expectation you would have in terms of hardness for the heattreated and quenched metal. ______________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
4.2 Quenching and Tempering
4.2.1 Exploration: Investigating Quenching Mediums
Quenching is the rapid cooling of heated steel in a quenching medium for the
purposes of hardening.
Steel is heated into the austenite region and then quenched. The steel is
quickly plunged into the quenching medium to rapidly cool the metal. The
quenching medium may need to be agitated to allow for uniform cooling of the
steel. The most common quenching mediums are air, oil, water, and brine.
After heating and quenching, steel is generally hard and brittle. Quenched
austenite is transformed into martensite. If left in this state, steel is subject to
cracking.
Reheating, or tempering, causes martensite to break down. The steel is
provided a degree of stress reduction and an increase in ductility.
In part 1 of this module, we looked at the effect of heat treatment on various
percentages of carbon steel. In this exploration, we will vary the cooling rate of
steel by changing the quenching mediums.
1. Obtain at least four (4) samples of medium carbon (1040) steel.
2. Test each sample of steel for its Rockwell (B or C) Hardness and record the
value. Mark/stamp each specimen so that it can be identified after heat
treatment.
3. Load all four (4) samples in a heat treatment furnace and heat to 1600F (for
medium carbon steel). Hold at this temperature for one hour.
4. Remove the first sample and quickly quench the specimen in water at room
temperature. Note: be sure to quickly close the furnace door after removing
this and each sample to prevent the premature cooling of the other samples.
5. Remove the second sample and quickly quench the specimen in a
commercial grade of motor oil.
6. Remove the third sample and allow it to air cool. Either place it in open air on
a ceramic tray of place it in front of a fan.
7. Allow the fourth sample to oven cool.
8. Once all samples have cooled, perform a Rockwell Hardness test and record
the values for each.
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DATA SHEET
Heat Treating Module
Engineering Materials
Exploration 4.2.1 Investigating Quenching Mediums
Hardness Values - Steel Specimens
Temp - F
Air
Water
Oil
Oven
1600
Questions:
1. Were there any changes in hardness values due to the various quenching
mediums? Explain ________________________________________________
________________________________________________________________
________________________________________________________________
2. Which cooling medium resulted in the greatest gain in hardness of the samples?
________________________________________________________________
________________________________________________________________
3. Which cooling medium resulted in the least gain in hardness of the samples?
___________________________________________________________________
___________________________________________________________________
4. Rank the order of most gain to least gain of the quenching mediums?
_______________________________________________________________________
______________________________________________________________________
________________________________________________________________
Note: This exploration may be performed with other types of steel. For low
temperature steel, ex: 1018 steel, heat samples to 1700 F. When using high
carbon steel, ex: 1095 steel, heat samples to 1500 F. Low carbon steel will be
less effective than either medium or high carbon steel.
4.2.2 Dialog: Quenching
The heat-treating process is continued after heating the metal it is suddenly
plunged into a cooling medium until it is cool. Agitation may be required to assure
rapid and uniform cooling.
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Quenching mediums are commonly liquids. Some examples of non-liquid
quenching mediums are sand and air. The most common quenching mediums
are water, brine, oil, and air.
Water is the most common quenching medium. It provides a good quench
due to its high heat of vaporization. Water has the advantage of being commonly
available and is cheap. On of the major disadvantages of water is that vapor
bubbles form next to the metal and produce soft spots due to uneven cooling.
One method of minimizing this effect is to provide agitation during the quenching.
Water can cause internal stresses, cracking and distortion. Water is a good
quenching medium for low carbon steel.
Brine quenching is a modification of water as a quenching medium. Salt is
added to water to nucleate the bubbles that limit the effect of water. Brine cools
the metal faster than water. However, the results of brine quenching are similar to
the results of water quenching. Brine will increase corrosion problems unless the
salt is completely removed from the metal.
Oil is used as a quenching medium when a slower, gentler cooling rate is
required. Oil is used for parts having thin cross sections of sharp edges. Oil has
reduced internal stress, cracking and distortion. However the steel will not be
quite as strong or as hard as water or brine quenched steel. Oil is most effective
as a quenching medium when slightly heated (100 – 150 F). Heating the oil
reduces the oil viscosity promoting better cooking. High flash point oils should be
used for quenching.
Air quenching is the least drastic to the metal that is being cooled. The
heated metal is placed on a screen and air is blown over the metal. Compared to
the other quenching mediums, air is the least likely to cause internal stress,
distortion, or cracking. The resulting strength and hardness are not as high as the
faster quenching mediums are. Air quenching is best used on high carbon steels
that have the most potential for quench cracking, distortion, or internal stresses.
The addition of alloying elements such as chromium will improve the hardening of
steel using air quenching. The following figure shows the potential cooling rates
of steel using water, brine, oil, and air quenching. There is a curve that shows the
normalizing cooling rate for steel.
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Steel Cooling Rates
1800
1600
Temperature (F)
1400
1200
Normalizing (Still Air)
Air Quench
1000
Oil Quench
800
Water Quench
Brine Quench
600
400
200
0
0
5
10
15
20
25
30
Quench Time (Sec)
4.2.3 Application: Select A Quenching Medium
Using your knowledge gained from the use of the exploration and dialog
section that have developed the approach to quenching. Choose the quenching
medium for the following 2 types of steel.
1. Recommend a quenching medium for 1060 steel that will maximize the
strength and hardness of the steel but minimize the potential internal
stress, distortion, of cracking. ___________________________________
___________________________________________________________
___________________________________________________________
2. Recommend a quenching medium for 1030 steel that will maximize the
strength and hardness of the steel but minimize the potential internal
stress, distortion, or cracking. ___________________________________
___________________________________________________________
___________________________________________________________
3. Discuss what expectation you would have in terms of hardness for the
heat-treated and quenched metal. _______________________________
___________________________________________________________
___________________________________________________________
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4.2.4 Dialog: Tempering
The as quenched state of steel consists of a material that is hard and brittle
and has relatively high level of internal stress. If left in this condition the material
will have a tendency to crack. Tempering is the immediate reheating of the steel
after quenching. The steel is reheated to a temperature below the lower
transformation temperature. The overall effect on the material is increased
toughness, internal stress relaxation and an increase in ductility.
During quenching the austenite in the steel was transformed to martensite.
Martensite is hard and strong but very brittle. Martensite has little uses as an
engineering material due to this brittleness. The crystal structure of the austenite
transform from its face-centered cubic structure to a body-centered cubic type
structure. However, due to the quick quenching, the carbon atoms held
interstitially in the austenite are not diffused form the structure. The bodycentered structure is elongated, forming a body-centered tetragonal structure.
When martensite is investigated under the microscope, it appears fine and
needlelike.
Tempering requires a temperature between 300F and approximately1275F.
The upper limit of temperature should be just below the lower transformation
temperature (approximately 1333F). A lower tempering temperature (i.e. 400F)
produces high strength and hardness. High temperature (i.e. 1100F) produces
increased toughness and ductility.
Time at the tempering temperature is important to the material properties.
The material should soak at the tempering temperature for approximately one (1)
hour. This time may be adjusted upward for thicker material cross sections.
The effect of tempering on engineering material properties is decreased
hardness, strength, and brittleness. However, tempering causes an increase in
toughness and ductility.
4.2.5 Application: Quenching and Tempering
Objective: To gain an understanding of the methods and effects of heat treatment
on the
properties of ferrous materials.
Specimens: Four (4) Steel – 1040 hot rolled steel impact specimens
Apparatus: Two (2) heat –treating ovens
Gloves and tongs
Rams Rockford-Model 10A Rockwell Hardness Tester
Water
Model SI-1 Universal Impact Testing Machine
Dial Calipers
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Reference: Modern Materials and Manufacturing Process; Bruce, Tomovic, Neely
and Kibbe, Prentice Hall, Ch4, pp. 75-89, 2nd ed., 1998.
Background Information:
Steel is a highly useful alloy of iron containing approximately 1% or less
carbon. The use of heat treatment can produce a variety of effects on properties
such as ductility, strength and hardness.
The amount of carbon in steel has an effect on the response of the steel to
heat treatment. Low carbon steel (.2% and below) cannot be hardened greatly
hardened, medium carbon steel (.4%) can be hardened with a rapid quench. High
carbon steel (.8%) to 1%) can be easily hardened.
With control of temperature, time and rates of processing, iron alloys can be
engineered to meet many engineering needs. Carbon is an allotropic element,
which means that it will undergo a crystal lattice structure change above its critical
temperature.
Critical temperature is the temperature at which iron changes its crystal lattice
structure and will hold some carbon atoms interstitially. For most steel, the critical
temperature is above 1450F.
Quenching is a rapid cooling process, which lowers the material’s
temperature form the critical temperature quickly enough to freeze the carbon in the
crystalline structure.
Drawing is a re-heating process following quenching performed to increase
ductility while hardness decreases. Usually the material is heated to a
predetermined temperature and allowed to air cool.
Annealing is a heating and cooling cycle designed to reduce hardness,
improve ductility, refine grain size and improve toughness. Annealing involves
heating the steel above the critical temperature and allowing slow cooling.
Procedure:
Heat treat 3 of the 4 impact specimens as indicated below. Make an impact
test of the specimens following the procedure for impact testing and record the
results.
Specimen #1:
Specimen #2:
Specimen #3:
Specimen #4:
Heat to 1600F and water quench, test as is.
Heat to 1600F, water quench, draw at 400F.
Heat to 1600F, water quench, draw at 600F.
Heat to 1600F, water quench, draw at 1100F.
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Using the Rams Rockwell Hardness Tester, determine the hardness using the
broken impact specimen as a specimen. Draw a file firmly across the specimen
and observe its behavior.
DATA SHEET
Heat Treating Module
Engineering Materials
Application 4.2.3 Quenching and Tempering
Sample #
Type of Specimen
Rockwell Hardness
Absorbed Energy (ftlbs)
Type of Surface
at break
1
2
3
4
5
4.3
Metallography
4.3.1 Exploration: Microscopic Evaluation of Heat-Treated Steel
Objective: To observe and interpret the microscopic effects of the heat
treatment of steel.
Specimens: Various carbon steel specimens. A low carbon steel, medium
carbon steel and a high carbon steel specimen is recommended.
Apparatus:
1. Metal cut-off saw
2. Specimen mounting press
3. Grinder/Polisher
4. Handimet II Roll Grinder
5. Rockwell Hardness Tester
6. Metallurgical microscopes
Procedure:
1. Rockwell Hardness testing: Measure and record the Rockwell
“C” hardness of each of the specimens using the Rams
Rockford-Model 10A, Rockwell Hardness tester.
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2. Section the sample: Sectioning (cutting) is necessary to expose
the internal areas of the specimen for observation. Use the
metallurgical cut-off saw with abrasive blades and coolant flow.
Avoid overheating of the specimen.
3. Mounting the specimen: This step provides a mounting of the
metal specimen in plastic using the Simplimet Mounting Press.
This mounting provides a safe and efficient means of handling
small and irregular samples. Compression mounting resins
cure at 3000 to 4200 psi pressure and 150C (302F)
temperature.
a. Spray the mold cavity and the mold closure thoroughly with
silicone mold release spray.
b. Place the specimen on the ram face of the mounting press.
c. Lower the ram ¼ of the way and pour in enough resin to
cover the sample.
d. Lower the ram to its furthest point and add enough phenolic
powder to complete the mount.
e. Replace the mold closure and tighten in a clockwise
direction. Place the thermometer in the top of the mold
closure to monitor the temperature for the curing process.
f. Turn on the heater coil to heat the resin.
g. Apply the pressure of 4200 psi for the 1 ¼” mold and allow
curing for 7 minutes. No further operation is needed until the
end of the curing period.
h. After the recommended curing time, release the pressure for
an instant, closing the valve quickly. Using gloves, carefully
unscrew the closure until the threads disengage the mold.
Operate the pump handle to elevate the ram raising the
closure out of the mold cavity. Carefully remove the hot
closure and the mold specimen.
4. Rough Grinding: This is a corrective technique needed to
remove gross surface irregularities. Additionally, sharp edges
produced by the molding process are removed by grinding.
Coarse grinding employs grinding using a 120 or 180 grit
abrasive disk on the grinder/polder.
5. Fine Grinding: This step progressively removes coarse
scratches and deformations utilizing a series of decreasing grit
size (240, 320, 400, & 600) abrasive papers. Between steps in
the grinding process, the specimen should be thoroughly
washed to prevent abrasive particles from being carried to finer
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grit papers. Fine grinding should be performed on the Handimet
2 Roll Grinder using a continuous water flow for lubrication. A
valve located on the front of the grinder controls water flow.
Carbimet Silicon papers are waterproof and may be used in
either a wet or dry configuration. (We will grind the specimens
wet.)
To perform fine grinding, manually draw the specimen back to
front (one direction only) across the paper. Each step should
take approximately two (2) minutes. After proper washing and
drying, move to the next grid paper. Rotate the specimen 90
degrees for this next step. When the visible scratches from the
previous step have been removed, continue grinding additional
6-8 strokes.
6. Rough Polishing: The rough polishing step has the greatest
potential for success or failure of the entire process. This step
should completely remove the remaining scratches from the fine
grinding step.
Use METADI II Diamond compound, which is tough and
removes material faster and cleaner than other abrasives. Use
a nylon polishing cloth on the Leco grinder/polisher. Apply the
METADI II diamond abrasive using the applicator syringes and
distribute uniformly over the polishing cloth. A liquid extender is
then applied, to aid in the distribution of diamond particles and
provide lubrication. Rough polishing should take approximately
two (2) minutes.
7. Final Polishing: The final polishing step should provide a
scratch free surface. The material removal at this step is
extremely low. This step is just a final polish, it will not correct
errors committed in the previous steps.
Final polishing should also be accomplished using the Leco
Grinder/Polisher with a microcloth on the 8” wheel. Moisten the
cloth with deionized water. Use micropolish B, alumina
suspension and apply in a circular bead. For best results, do
not overwet the cloth. Rotate the sample in a direction opposite
the wheel rotation. Wash the sample in alcohol immediately or
the polished surface will oxidize.
8. Etching: Use an etchant of 98 ml ethyl alcohol and 2 ml Nitric
acid for a time of 45 seconds. Perform this step using an
approved hood in the chemistry lab. Placing the completed
specimens under a stream of water should stop the etching
action. Clean the specimen with alcohol and use a hair dryer to
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dry the sample. Do not touch the polished/etched specimen.
This will alter the metal surface condition.
9. Microscopic Examination: Determine the grain size of the
specimen and graphite type and distribution.
4.3.2 Dialog: Metallography
Metallography is the science of interpreting the microstructure of engineering
materials. Sample preparation involves sectioning, mounting, grinding, and
polishing. The sample is then investigated under a microscope for crystal
structure and grain size.
Steel is an alloy having important mechanical properties. Heat treatment is
one of the most common and widely used manufacturing processes used to
alter the mechanical properties of steel.
The size of grain affects the mechanical properties of metals. During heat
treatment the process including soak time, temperature, and alloying elements
all affect grain growth and size. A small grain is preferable to a large grain
structure. Small grain metals have greater hardness and tensile strength and
distort less during the quenching process. In steels, large grain materials have
increased hardenability and are better for the cold working process. Grain size
is determined by the count of grains per square inch under 100x microscopic
magnification. Steel is considered fine-grained if it is between 5 and 8 and
coarse-grained if it is between 1 and 5. Acceptable grain structure has
approximately 70% of the grain within a given range.
The metallurgical microscope is used to determine the structure of metals.
Grain size and the effect of heat treatment may be determined by microscopic
evaluation. Approximate carbon content may be evaluated by the pearlite,
which appears as dark areas in the steel.
The specimen is sectioned using a metal cut-off saw. In the process of
cutting, overheating of the specimen should be avoided. Overheating may
subject the material to additional tempering, thus modifying the steel. Grinding
is the gross removal of surface metal using abrasive materials. The next step is
polishing of the surface, which is the slight cutting of this surface. Etching of the
specimen surface aids in the visual examination of the surface.
4.3.3 Application: Metallography of Heat-Treated Specimens
Objective: To gain a further understanding of the methods and effects of
heat treatment on the properties of ferrous materials.
Specimens: Four (4) Steel – 1040 hot rolled steel specimens
Apparatus:
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1. Three (3) heat treating ovens
2. Gloves, goggles, and tongs
3. Rams Rockford - Model 10A Rockwell Hardness Tester
4. Water
5. Metal cut-off saw
6. Specimen mounting press – Simplimet II
7. Ecomet 3 – Automet 2 – Grinder/Polisher
8. Neomet Microscope
9. Pixera Digital Camera for use with Neomet Microscope
10. Heat Gun
11. Nital Etchant
12. Various consumables for Metallography Process
Procedure:
1. Heat Treatment as follows:
a. Specimen #1 – leave as is
b. Specimen #2 – heat to 1600F and water quench
c. Specimen #3 – heat to 1600F, water quench, draw at
400F
d. Specimen #4 – heat to 1600F, water quench, draw at
800F
2. Rockwell Hardness Testing: Measure and record the Rockwell
Hardness ‘C’ hardness of each of the specimens using the
Rams Rockford – Model 10A, Rockwell Hardness tester.
3. Section the sample: Sectioning (cutting) is necessary to expose
the internal areas of the specimen for observation. Use the
metallurgical cut-off saw with abrasive blades and coolant flow.
Avoid overheating of the specimen.
4. Mounting the specimen: This step provides a mounting of the
metal specimen in plastic using the Simplimet II Mounting
Press. This mounting provides a safe and efficient means of
handling small and irregular samples. Compression mounting
resins cure at 3000 to 4200 psi pressure and 150C (302F)
temperature.
a. Spray the mold cavity and the mold closure thoroughly with
silicone mold release spray.
b. Place the specimen on the ram face of the mounting press.
c. Lower the ram ¼ of the way and pour in enough resin to
cover the sample.
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d. Lower the ram to its furthest point and add enough phenolic
powder to complete the mount.
e. Replace the mold closure and tighten in a clockwise
direction. Place the thermometer in the top of the mold
closure to monitor the temperature for the curing process.
f. Turn on the heater coil to heat the resin.
g. Apply the pressure of 2000 psi, quickly release and reapply
a pressure of 4200 psi for the 1 ¼” mold and allow curing for
7 minutes. No further operation is needed until the end of
the curing period.
h. After the recommended curing time, release the pressure for
an instant, closing the valve quickly. Using gloves, carefully
unscrew the closure until the threads disengage the mold.
Operate the pump handle to elevate the ram raising the
closure out of the mold cavity. Carefully remove the hot
closure and the mold specimen.
Using the Ecomet 3 and Automet 2 Power head, perform steps 5-8.
5. Place three (3) mounted specimens in the three (3)-specimen
barrel holder using the plate to align the specimens properly.
Use a dummy mount if three (3) specimens are not available.
6. Planar Grinding Stage: Cover the bimetallic plate wit release
agent, attach a 180-grit Buehler-Met II disk and carefully place
on the magnetic disk on the Ecomet 3. Set the speed, force
per sample, relative rotation and time accordingly. Once the
controls are set, attach the barrel specimen holder to the
Automet 2 power head and move the head so that it is located
to the outside of the disk and lock into place. Pressing the
green buttons on each side of the power head will lower the
barrel holder down onto the plate and start the grinding
sequence. Note: Only use 180-grit disk for one grinding
operation.
7. Sample integrity stage: This stage will involve two (2) steps.
a. Using Ultrapad surface and Metdi Supreme Diamond
Suspension, 9  m set the controls accordingly and follow
the procedure similar to step 6. The surface should be
charged prior to operation and should be maintained wet
during the entire stage (application approximately every 30
seconds). Note: These cloths may be used over and over
again by placing in a protective plastic bag and sealing.
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b. Using Trident, Nylon or Texmet surface and Metadi Supreme
diamond suspension, 3  m set the controls accordingly,
follow the procedure to step 6. The surface should be
charged prior to operation and should be maintained wet
during the entire stage (application approximately every 30
seconds). Note: These cloths may be used over and over
again by placing in a protective plastic bag and sealing.
8. Final Polishing: Using the Chemet or Microcloth surface and
Masterprep Alumina Suspension, follow the procedure similar
to step 7 above.
9. Etching: Use an etchant of 98 ml ethyl alcohol and 2 ml Nitric
acid for a time of 45 seconds. Perform this step using an
approved hood in the chemistry lab. Placing the completed
specimens under a stream of water should stop the etching
action. Clean the specimen with alcohol and use a heat gun to
dry the sample. Do not touch the polished/etched specimen.
This will alter the metal surface condition.
10. Microscopic Examination: Determine the grain size of the
specimen and graphite type and distribution.
4.4 Transformation Diagrams
4.4.1 Exploration: Isothermal Transformation Diagrams
Using figure 4.4-1, Isothermal Transformation diagram for a generic ironcarbon alloy, answer the following questions:
1. What variable missing on the iron-carbon equilibrium diagram is included
on the isothermal transformation diagram?_________________________
___________________________________________________________
___________________________________________________________
2. What type of microstructure exits above the upper transformation
temperature line?_____________________________________________
___________________________________________________________
___________________________________________________________
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3. What type of microstructure exits below the lower transformation
temperature line, above the Ms temperature line, and to the left of the Ccurve?_____________________________________________________
___________________________________________________________
___________________________________________________________
4. What type of microstructure exits to the right of the C-curve?___________
___________________________________________________________
___________________________________________________________
___________________________________________________________
5. What does the dashed line between the C-curve and the right C-curve
represent?__________________________________________________
___________________________________________________________
___________________________________________________________
6. If a sample is quickly quenched to 1100F and held at a constant
temperature of 1100F, how long does it take for the austenite to pearlite
reaction to go to 50% completion? How long to 100% completion?
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
Insert Isothermal Transformation Diagram for Generic Iron-Carbon Alloy
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Figure 4.4.1
4.4.2 Dialog: Transformation Diagrams
Equilibrium diagrams were used in part one to show the carbon content and
temperature on the phase changes of iron-carbon alloys. However, this diagram
accurately depicts the effect of temperature and carbon content only for slow
cooling. This slow cooling results in a microstructure of ferrite, cementite, or
pearlite. Fast cooling or quenching results in martensite. The iron-carbon
equilibrium does not distinguish between different cooling rates. The missing
variable is time. Another type of diagram is required to depict the effects of
varying cooling rates.
The isothermal transformation (I-T) diagram was developed to include the
variable of time and the transformation of austenite. Another name for the I-T
diagram is a time-temperature-transformation (T-T-T) diagram. Plotting
temperature versus time creates these diagrams. Each diagram is created for a
unique alloy of carbon and iron.
The upper and lower transformation temperatures to austenite are first shown
as horizontal lines on the upper portion of the diagram. To create the
transformation information many like samples are heated above the upper
transformation temperature to achieve 100% austenite. The samples are then
quickly transferred to a heated molten salt bath and held at this temperature
(isothermal condition). At incremental times, the samples are removed and
quenched in a iced brine. These samples are the investigated using
metallography techniques to determine the microstructure of the samples.
Microstructure, time, and temperature are recorded for each sample. This
procedure is repeated with decreasing temperature to determine the
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transformation information. An I-T diagram is the prepared for this particular
iron-carbon alloy.
Transformation at upper temperatures is the combination of ferrite and
cementite, forming fine and coarse pearlite. Below the nose of the
transformation curve, the transformation will be to a mixture of ferrite and
bainite. At lower temperatures and at times missing the nose of the curve, the
transformation process will be from austenite to martensite. These structures
will be defined in the following paragraphs. I-T diagrams are useful in
determining the type of microstructure and information on the process of
transformation.
Martensite
Martensite transformation occurs with rapidly quenched steel alloys with the
transformation occurring nearly instantaneously at or near room temperature.
This rapid quenching prevents carbon transformation with the carbon atoms
trapped in iron microstructure. The carbon atoms that are trapped interstitially in
the microstructure elongate the body-centered-cubic microstructure. This
microstructure is called body-centered-tetragonal (BCT). Martensite appears as
a needle-like microstructure under the microscope. Martensite is a very hard,
strong and brittle structure that has little engineering application. The structure
is susceptible to cracking and retained internal residual stress. Martensite is
very difficult to machine due to its high hardness values. Martensite is in a nonequilibrium condition with this trapped carbon transforming if reheated or
tempered. Martensite is not included on the iron-carbon equilibrium diagram but
is included on the I-T diagram. Martensite transformation is shown with a
horizontal line indicating Ms of martensite start, M50 – 50% transformation, and
M90 – 90% transformation to martensite.
Bainite
Bainite is a microstructure of transformation that exists between the fine
pearlite and the martensite region of the I-T diagram. Named for E.C. Bain who
discovered the existence of this type of microstructure. Bainite is not as hard or
strong as martensite and has good ductility and toughness. The range of
hardness of bainite is 50-55 Rockwell C. An electron microscope is needed to
investigate the microstructure of bainite.
Example I-T Diagram
For an example of the I-T diagram see Figure 4.4.1, a diagram for generic
iron-carbon alloy. The upper dashed line indicates the lower transformation
temperature. Above this line, the carbon steel transforms to a face-centered
cubic (FCC) microstructure with some carbon atoms held interstitially in the FCC
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microstructure. As the steel is cooled, there are two (2) C-shaped curves. The
left hand C-curve is the beginning of the transformation from austenite to coarse
pearlite, fine pearlite, or bainite. The right hand C-curve represents the end of
this transformation process. The Ms horizontal line shown on the bottom of the
diagram indicated the start of the transformation to martensite.
4.4.3 Application: Isothermal Transformation Diagram
Problem: Using the isothermal transformation diagram for a generic iron-carbon
alloy (Fig. 4.4.1), specify the final microstructure for a specimen that is
subjected to the following heat treatment:
1. The specimen is heated to 1700F and held for 1 hour to achieve a 100%
austenite microstructure.
2. Rapidly cool the specimen to 1100F.
3. Hold at 1100F for 100 seconds at isothermal conditions.
4. Quench the specimen at room temperature.
Solution: The following figure (Fig. 4.4.2) graphically depicts the above conditions.
The initial quench to 1100F is rapid enough to prevent any transformation.
At 1100F the austenite completely transforms to pearlite. Since the
specimen is completely transformed prior to passing through the martensite
region, the final microstructure is 1005 pearlite.
Insert Isothermal Transformation Diagram for Generic Iron-Carbon Alloy
Figure 4.4.2 dialog example problem
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4.4.4 Application: Using the Isothermal Transformation Diagram
Using the isothermal transformation diagram for a generic iron-carbon alloy
(Fig. 4.4.1), specify the final microstructure for specimens that are subjected to
the following heat treatments:
The specimen is heated to 1700F and held for 1 hour to achieve a 100%
austenite microstructure.
1. Rapidly cool to 1100F, hold for 10 seconds, quench to 600F, and hold
for 10,000 seconds, and quench to room temperature.
2. Quench to 500F, hold for 10 seconds, and quench to room temperature.
3. Quench to 1250F, hold for 100 seconds, rapidly cool to 800F, hold for
1000 seconds, and quench to room temperature.
4.4.5 Dialog: Tempering Martensite
The as quenched martensite is not a suitable material for many if any
engineering applications. The material is extremely hard and brittle and is often
weakened by internal stresses.
Tempering will relieve internal stress, and increase both ductility and
toughness. While tempered martensite will possess increased ductility and
toughness it will lose hardness and strength.
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The tempering range of martensite is between 400-1200F. The unstable
body-centered-tetragonal (BCT) microstructure of martensite will transform to a
stable mix of cementite in a matrix of ferrite.
Spheriodite
Tempering martensite of bainite to a temperature just below the lower
transformation temperature (approximately 1300F) and holding for a period of 18 to
24 hours creates spherodite. The cementite structure is changed to sphere shaped
particles in a ferrite matrix. Spherodite transformation is not shown on the I-T
diagram.
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