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