decarb report (mate junior series)

8/3/2019 Decarb Report (Mate Junior Series)

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The Investigation of Decarburization In Heat

Treated 1095 Steel

Coherency Strain

Dr. Chen

Mate 370


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Abstract

The process of decarburization was taken into consideration in order to investigate the low

hardness values shown in 1095 steel samples. If allowed to occur, decarburization reduces the

carbon content within metal samples and may adversely affect its mechanical properties. A test

matrix was designed to investigate the role of temperature and duration of the heat treatment in

the process of decarburization. Heat treatments of 1095 steel samples were conducted at three

different temperatures (830, 865 and 900oC) and over three different times (1, 3 and 5 hours).

The resulting depth of the decarburization layer produced through each of the various heat

treatments was recorded and graphed.

Upon analyzing the resulting trends in the depth of the decarburization layer for the various heat

treatments conducted, it was observed that increasing the duration of the treatment increased

the depth of the layer regardless of the temperature at which the samples were treated. The

depth of the decarburized layer was greater for treatments held at 865oC than for those

conducted at 830oC for each of the three treatment durations. However, a clear, linear patternbetween the temperature of the heat treatment and the depth of the layer was not present.

Using the Arrhenius Equation, the experimental activation energy required for decarburization

was calculated and compared to a theoretical value. The experimental energy was found to be

substantially smaller than calculated theoretically.

Introduction

In the investigation of undesirably low surface hardness values in 1095 steel samples, potential

decarburization occurring during the heat treatment was suspected. Decarburization can

negatively impact steel's mechanical properties, causing the metal to exhibit decreased surface

hardness and strength. Because of this, it may be important to minimize the amount of

decarburization that may take place during the production process.

Decarburization occurs when there is a concentration gradient between the steel sample and

the furnace environment. Low carbon content in the furnace atmosphere causes its chemical

potential to be lower than that of the steel. Thus it becomes thermodynamically favorable for

the carbon to diffuse out of the steel. This effectively causes a phase change at the surface of

the steel sample where diffusion takes place, and the resulting phase, ferrite, has a lower

surface hardness than the pearlite that exists below the decarburized layer.

The opposite process, carburization, involves the use of a high carbon-containing source, likegaseous CO, to diffuse carbon into the surface of the steel from the surrounding atmosphere.

Carburization can improve surface hardness, wear resistance, and fatigue and tensile strengths

because the presence of excess carbon atoms increases lattice strain. A list of the material

properties induced by carburization is listed in Table I.


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Table I. Effects of carburization on various material properties.Work material properties Effects of carburizing

Mechanical Increased surface hardness

Increased wear resistance

Increased fatigue/tensile strengths

Physical Potential grain growth

Change in volume may occur

Chemical Increased surface carbon content

Background

The diffusion mechanism of decarburization of steel is the concentration gradient between

carbon in the surface of the steel and in the surrounding atmosphere. In steel, carbon is

trapped in the interstitial sites in the lattice, which causes strain. Diffusion proceeds in an

attempt for the material to reach a lower energy state. The type of diffusion that occurs in the

decarburization of steel is called inter-diffusion. This occurs on the surface of steel while it is

heat treated in a furnace. Since there is a higher concentration of carbon inside the steel, thecarbon moves from the steel to the metal's surroundings. The temperature, concentration

difference, and lattice size affect the rate of decarburization. As the temperature and the

concentration difference increase, the rate of diffusion also increases.

The amount of carbon dioxide in the furnace causes either decarburization or carburization to

occur in steel1. The equations listed in Figure 1 may be used to calculate the diffusion

coefficient D for the various times and temperatures of the each heat treatment.

Equation 1

Equation 2

Equation 3


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Figure 1. Equations used to calculate diffusion thickness and activation energy, where x = diffusion distance,

C = composition, D = diffusivity, Do = pre-exponential constant, Q = activation energy, T = temperature, and

t = time.

The calculated diffusion coefficient for each sample can be seen in the table below. The value

for Do was taken from Smithells Metals Reference Book. The activation energy Q is heldconstant throughout the calculations at 80 kJ/mol. These values were calculated using the

times and temperatures that were used in the actual experiment. It is apparent that with

increasing temperature the size of the decarburization layer also increases. The thickness of

the decarburization layer on each sample will increase as the time that the sample spends in the

furnace increases.

Table II. Diffusion coefficients and corresponding decarburization layer thickness.

Temp (Deg

C)

Diffusion

Coefficient D

Distance X

(microns)

830 2.25246E-12 90.04910229

830 2.25246E-12 155.9696203

830 2.25246E-12 201.355914

865 3.7002E-12 115.4154316

865 3.7002E-12 163.2220687

865 3.7002E-12 199.9053915

900 5.90106E-12 103.0626304

900 5.90106E-12 145.7525697

900 5.90106E-12 206.1252608

The times and temperatures used to calculate the diffusion coefficient and the thickness of the

decarburization layer were carefully selected with the intention of forming a readily measurable

region.

PROCEDURE

In order to properly measure decarburization, a procedure had to be developed that provided

three distinct temperatures and three distinct times in order to calculate the diffusion coefficient

and eventually the activation energy. By conducting the heat treatments at three different

temperatures, a plot could be generated and the diffusion coefficient extrapolated. The threetemperatures allowed for a plot of the data to be observed and the diffusion coefficient and

experimental activation energy calculated. It was determined that the samples would be heated

at the following temperatures and times:

Temperature: 830C; Time(hours):1,3,5

Temperature: 865C; Time(hours): 1,2,3

Temperature: 900C; Time(hours): half hour,1,3


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After each heat treatment, the samples were normalized to allow an appropriate microstructure

to form. Each sample was then cut in half in order to take proper metallographic observations.

The samples were placed in QuickSet and polished down to a 1 micrometer finish. Following

polishing, initial observations were made to ensure that the samples' microstructure was clearly

visible. 50 ml of 2% Nital was used to etch the samples. When etching the samples, they werefirst swabbed with etchant for 10 seconds, rinsed with ethanol and dried with the blower. The

samples were then examined under a microscope. If they appeared over etched, the samples

were polished lightly on 1um pad and reexamined. If this step failed to produce a distinguished

microstructure, the sample was etched a second time.

Directly following etching, the samples were photographed using the optical microscope, taking

3 photographs per sample. The STM E 1077 standards were followed to calculate the

decarburization layer. This involved adding the sums of the measured full decarburization layer

and partial decarburization layer. These measurements were then used to calculate the

diffusion coefficient and activation energy.

Results

After the samples were heat treated according to the defined experimental procedures,

metallography was performed on the samples to determine the width of the decarburization

layer. This etch-polish-etch cycle was performed on all of the mounted and polished samples,

and the microstructure of the decarburized samples could then be seen (Figure 2).

First, the samples were cut to expose their interior. Then, the cross sectioned samples were

mounted in acrylic. The samples were then polished according to accepted metallographic

procedures. Once the samples were polished they were etched using a 2% Nital solution. Thesamples were first swabbed with etchant and then 30 seconds later rinsed with deionized water.

Next the samples were polished again with a 1 m polish on a rotating wheel. The samples

were then etched again by swabbing with the 2% Nital solution and rinsed after 20 seconds.

(Should this go in Procedure?)

It was found that the fully decarburized region consisted of a low carbon ferrite phase, and the

partially decarburized region had both ferrite and cementite present, as it contained a

concentration of carbon similar to the eutectoid composition.


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Figure 2. Photomicrograph of mounted, polished and etched sample heat treated at 830C for 3 hours. The

decarburization layer can be clearly seen along the left edge of the sample, with the partially decarburized

region extending into the interior of the sample. The sample was etched with 2% Nital solution.

Photomicrographs were taken of each of the samples under an optical microscope, and the

decarburization layers were measured using image analysis software. The results are

summarized in Table III.

Table III. Summary of decarburization layer thicknesses for various heat treatments

Temperature (deg. C) x after 1h x after 3h x after 5h

830 0.0987 0.16 0.24

865 0.113 0.181 0.219

900 0.089 (0.5h) 0.144 0.237

The change in the decarburization layer thickness was representative of the change in the

concentration profiles of the different samples. These profiles proved to be a function of bothtemperature and time (Figure 3).


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Figure 3. A graph illustrating the change of the concentration profile of a sample with increasing time at a

given temperature.

As the duration of the heat treatment increased, the partially decarburized layer shifted further

away from the surface, as greater and greater amounts of carbon have had time to diffuse to the

surface and leave the sample. Although this trend was apparent, it was also observed that the

time required to advance the layer a unit of distance grew exponentially longer as carbon atoms

deeper within the samples had to diffuse further toward the surface. This phenomenon is

clearly observable from some of the collected data, but is not apparent in each sample.

Analysis

As mentioned earlier, many of the samples had a distinguishable, fully decarburized layer

followed by a slightly less distinguishable partially decarburized layer. The thicknesses of these

layers were measured and added together, ten measurements being taken and averaged for

each sample. The squares of the average thicknesses were plotted on a graph versus the time

the sample spent in the furnace, resulting in one group of three points for each temperature

(Figure 4).

t1

t2 t3t4 t5

C0

C (wt. %)

CS

x(m)


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Figure 4. Plot of measured thicknesses vs. time in furnace for each temperature. Trend-line slopes

represent D values in units of mm2

/h.

This plot clearly displays the trends relating time and temperature to diffusion distance. Longer

diffusion distances were observed in samples that were heat treated at each temperature for

longer periods of time, and similar diffusion distances were reached at shorter times for higher

temperatures. These trends match what would be expected based on both conceptual learning

and preliminary calculations. A trend-line was fitted to each group of points and the D values for

the three temperatures were read off of the slopes of these lines according to equation4.

These values were then converted into units of m2/s for ease of use in later calculations (Table

IV).

X2 = Dt (4)

Table IV. Calculated D values (m2/s) and the corresponding temperatures.

Temp (C) D (m2/s)830 2.972E-12865 4.417E-12900 5.222E-12

The final step in obtaining an experimental value for the activation energy of carbon

diffusing through an austenitic lattice was to plot the natural log of the obtained D values (ln(D))versus the value of (-1/RT) (Figure 5). From the Arrhenius equation (Equation 5) the activation

energy, Q, was calculated.

Ln(D) = Q(-1/RT) + Ln(D0) (5)

y = 0.0107xR = 0.9495

y = 0.0159x

R = 0.9827

y = 0.0188x

R = 0.9952

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 1 2 3 4 5 6

x2(

mm2)

time (h)

x2 vs t

830

865

900


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Figure 5. Plot of the natural log of obtained D values vs. (-1/RT) used to find Q. Q is

equivalent to the slope of the trend-line.

The value for Q obtained from this plot and thus the conducted experiment is 86.9Kj/mol. This

value is 41.2% less than the given theoretical value of 148Kj/mol. Some sources of error that

may have contributed to the discrepancy are:

Inaccurate measurement of layer thickness

Loss of thickness due to flakey oxide layer

Inappropriate modeling equations used to obtain values

Conclusion

Because of difficulty in determining the endpoint of the partially decarburized layer in somesamples, thickness measurements may have been incorrect and skewed the results. Since the

error was below the accepted value, it can be assumed that the measured layer thicknesses are

lower than they should have been.

The flakey oxide layer provided a perfect explanation to why the experimental measurements

were low, because some amount of the steel samples was likely lost from the surface when the

oxide layer fell off, the material thickness was lessened.

The model used to plot thickness versus time was derived from Ficks second law for infinite

systems, which modeled a composition value midway between the surface composition and

initial composition at the measured thickness. Since the thickness could not be accurately

determined, it would make more sense to develop a model from Ficks second law that could

relate to a more easily measurable thickness. This model could use a final composition value of

0.77wt %C and the thickness could be measured as the distance from the surface of the sample

to the point in the interior after which no more ferrite exists. The rationale behind this method

was that any free ferrite formed would be proeutectoid, and the composition at which pearlite

begins to form is the eutectoid composition (0.77wt%C). It would follow that the point in the

y = 86942x - 17.027R = 0.9556

-26.6

-26.5

-26.4

-26.3

-26.2

-26.1

-26

-25.9

-0.00011 -0.000108 -0.000106 -0.000104 -0.000102

ln(D)

-1/RT (mol/J)

ln(D) vs -1/RT

Series1


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sample at which free ferrite disappears would also be the point at which the composition would

equal that of the eutectoid. Following this method, a much more accurate length could be

measured and plotted according to a more appropriate model, yielding more accurate results.

Works Cited

2. Verhoeven, John D. "Steel Metallurgy for the non-Metallurgist." ASM International, 2007.

67-69.

Decarburization of Steel

R. Cornell and H. K. D. H. Bhadeshia

http://www.msm.cam.ac.uk/phase-trans/abstracts/M0.html

http://en.wikipedia.org/wiki/Carburizing

Carburization
http://www.msm.cam.ac.uk/phase-trans/abstracts/M0.htmlhttp://www.msm.cam.ac.uk/phase-trans/abstracts/M0.htmlhttp://en.wikipedia.org/wiki/Carburizinghttp://en.wikipedia.org/wiki/Carburizinghttp://www.msm.cam.ac.uk/phase-trans/abstracts/M0.html

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