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Nine samples of 1095 steel were heat treated at varying temperatures and times. From the data collected, the activation energy required for carbon to diffuse from the surface of steel was obtained. It was found that the activation energy found in this experiment was between the activation energy of carbon diffusing through ferrite and the activation of carbon diffusing through austenite. Given that the trends of the decarburization layer varying with time and temperature follow intuitively, the value found for activation energy is reasonable.
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The Activation Energy of the Decarburization of 1095 Steel
Nash Anderson, Liz Brooks, Buddy Bump, Katie Burzynski, Jonathon Bracci, Santiago Caceres, Alex Stanely
California Polytechnic, Department of Materials Engineering, San Luis Obispo, CA 93410Received 22 April 2011
Abstract
Nine samples of 1095 steel were heat treated at varying temperatures and times. From the data
collected, the activation energy required for carbon to diffuse from the surface of steel was
obtained. It was found that the activation energy found in this experiment was between the
activation energy of carbon diffusing through ferrite and the activation of carbon diffusing
through austenite. Given that the trends of the decarburization layer varying with time and
temperature follow intuitively, the value found for activation energy is reasonable.
Background
A common problem with the heat treatment of steel is the tendency of carbon to diffuse out of
the steel’s surface. During heat treatment, the steel is exposed to elevated temperatures (between
800 and 1200˚C) in a furnace atmosphere containing oxygen. Carbon is removed from the steel
when the chemical potential of carbon in the atmosphere is lower than its chemical potential in
the heated steel, a process known as “decarburization”. This decarburization produces a layer of
ferrite on the steel’s surface due to the stability of ferrite at the low concentration of remaining
carbon. The ferrite initially forms at the austenite grain boundaries. As an increased amount of
ferrite is formed, an entirely ferrite layer appears at the surface of the sample. The Fe-C phase
diagram indicates that the surface will be rich in ferrite, while the undecarbed regions on the
sample’s interior will be pearlite (Figure 1). The intermediate region will have a mixture of the
two phases, which can be found by using the lever rule applied at the eutectoid temperature.
Measuring the diffusion distance of the decarburization layer will enable us to calculate the
activation energy required for carbon to diffuse through the steel sample. Decarburization could
be prevented by heat treating in an inert atmosphere, by using a stainless steel foil, or by painting
with with a protective coating.
Figure 1 The blue line represents the composition of 1095 steel sample on the iron-carbon phase diagram prior to decarburization. As decarburization increases, ferrite becomes the more stable phase on the surface of the steel due to reduced carbon content.
Testing Procedures
Nine samples of 1095 steel were cut to equal lengths and heat treated in a furnace at three
different temperatures. Each heat treatment had three different times associated with it (Table I).
Before each sample of 1095 steel was placed in the furnace, it was cleaned with a soap solution
and rinsed with ethanol to insure no outside contaminates would effect the decarburization
process. Each sample was heat treated one at a time to avoid opening the furnace door while
another sample was still being heat treated. Opening the furnace door changes the atmosphere
inside the furnace, which could affect decarburization.
The samples were placed in a ceramic boat inside the furnace for their designated heat treatment.
Post heat treatment, each sample was cut in half for mounting and polishing. A quickset acrylic
mounting process was used to mount each sample prior to polishing for metallography. Each
sample was polished to a 1 micron finish, and then etched with 50mL containing 2% Nital.
The etching procedure included:
1. Samples were viewed under optical microscope to confirm adequate polish.
2. Swabbing the sample with etchant using a q-tip for 20 seconds until sample became hazy,
rinsed with ethanol and dried.
3. Samples were viewed under optical microscope and a couple appeared to be over etched.
4. A light 1 micron polish was applied to these over etched samples.
Table I Time and temperature schedule.
Samples were viewed with an optical
microscope. The total and partial
decarburization layers were measured
using a calibrated computer software
measuring tool (Figure 2). The
decarburization layer and partial
decarburization layer were measured at ten
different spots to get an average for each.
Then they were added together to create an
average value for the full decarburization
layer.
Figure 2 Picture of
steel after heat
treatment at 100x
magnification.
Computer software was
used to obtain data for
partial (right) and full
decarburization (left)
layers.
Results
Once all of the
measurements of the decarburization layer were taken for each sample, they were averaged to
give us the total decarburization layer in each sample. For each of the three heat treatment
Temperature (C) Time (hours)
830 1
830 2
830 5
865 1
865 2
865 3
900 .5
900 1
900 3
temperatures, a plot was made of the decarburization layer measurements squared vs. time
(Figure 3). A linear trend line was fitted to the plots for each temperature. The equation of the
trend lines shows the slope to be equal to the diffusion coefficient (Equation 1). From the slopes
of each trend line, the diffusion coefficient for each heat treatment temperature could be
determined.
Equation 1
x = Decarburization layer
D = Diffusion coefficient
t = Time
The slopes for all three trend lines is positive, meaning that as time increases, the size of the
decarburization layer also increases. Furthermore, as temperature increases so does the slope of
the trend line, meaning that the diffusion coefficient increases with increasing temperature. The
Figure 3 Plot of time vs. decarburization layer squared used to find diffusion coefficients. All three heat treatment temperatures are included in the plot with a liner trend line fit to each one. Equations to each trend line are given and the slops are equal to diffusion coefficients for their respective temperature.
830 ºC heat treatment had the lowest diffusion coefficient, while the 900 ºC heat treatment had
the largest diffusion coefficient. Also, the experimental diffusion coefficients determined from
the plot reasonably match the tabulated diffusion coefficient values for carbon diffusing through
austenite, having the same order of magnitude of 10-12.
The next step was to find the activation energy for the diffusion of carbon through the steel
samples. This was achieved by plotting the natural log of the diffusion coefficients for each
temperature versus 1/RT (Figure 4). A linear trend line was fitted to the plot, with the slope of
the line being equal to the activation energy (Equation 2). The slope of the trend line is negative,
which is consistent with the equation used to model the line. Furthermore, the y-intercept is
equal to the natural log of the diffusion coefficient constant. The experimental value that was
attained for the activation energy of carbon though the steel sample was 92.3 kJ/mol. This
Figure 4 Plot of 1/RT verses Ln D for each of the three heat treatment temperatures used to find activation energy. Note the liner best-fit line with its equation. The slope of the line is equal to the activation energy.
experimental value falls between 80 kJ/mol and 148 kJ/mol, the activation energy of carbon in
ferrite and carbon in austenite respectively.
Equation 2
D = Diffusion coefficient
Q = Activation energy
T = Temperature
D0 = Diffusion coefficient constant
Discussion
Our experimental activation energy is lower
than the theoretical activation energy for the
diffusion of carbon through austenite. This
may be a consequence of initial
decarburization at the surface of the 1095
steel sample, during which the concentration
of carbon at the surface decreases. This
causes the alpha-ferrite phase to be more
stable at the surface than the austenite phase
due to the decreased carbon concentration.
Our experimental activation energy of the
decarburization process includes the
diffusion of carbon through both phases,
austenite and alpha-ferrite. Therefore, our
resulting experimental activation energy
(92.5 kJ/mol) is lower than the activation energy of the carbon diffusing through the austenite
phase, but greater than the activation energy of carbon diffusing through the alpha-ferrite phase.
Figure 5 Once decarburization has begun, the outer surface of the 1095 steel sample has a much lower carbon concentration than at t=0 and has transformed into alpha-ferrite. (free edge right side)
Another issue that arises during the heat treatment of steel is the surface oxidation. The rusting of
steel is an electrochemical reaction that occurs at high temperatures in an oxygen-rich
atmosphere. Various amounts of oxidation were created at different temperature and time
intervals. EDS was performed on the oxidation layer, confirming our assumption of oxide
formation. The oxidation layer collects oxygen from the furnace atmosphere, increasing
thickness of the steel sample. However, the “actual surface” of the steel is decreasing at the rate
of oxide formation, creating an increasingly lower thickness, and consequently deeper diffusion
distance. At the same time, the oxidation layer is hindering the rate of decarburization by
providing more material for carbon to travel through before entering the furnace atmosphere.
This oxide formation greatly alters our decarburization distance measurements, creating an
inaccurate experimental activation energy. We could not find a way to prevent this oxidation
while still producing decarburization.
Table II EDS Results on Oxidation Layer Composition
Element
(line)
Wt. % Error (+/-)
C (K) 4.52 0.77
O (K) 29.74 0.47
Fe (K) 65.75 2.00
Conclusion
Decarburization of 1095 steel was experimentally observed and characterized though a series of
heat treatments and optical images. The resulting data showed expected trends in diffusion
distance when related to time and temperature. By plotting experimentally determined values for
decarburization layers at different temperatures and time, the diffusion coefficients and
activation energies of carbon thorough our steel samples were estimated. The experimental
activation energy of carbon’s diffusion through steel was less than the theoretical value. This
information about the decarburization process in our steel samples will allow for adjustments to
the heat treatment in order to produce the desired amount of decarburization in the future.
Oxidation of steel due to oxygen present in the atmosphere proved to be a major complication in
the analysis of the decarburization process. This could have skewed our results and been the
cause of some of the deviation from our theoretical model of decarburization. Future experiments
should be conducted to better account for the effects of an oxidation layer on the diffusion of
carbon.
Sources
1. Decarburization of Steel Handout on MatE 370 Blackboard
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