Changes
in Carbon Content and Microstructure of Carbonyl Iron Powders during Heating
Category: Student
Material: Carbonyl
Iron Powder (CIP)
Etchant: a
mixed solution of 4% Nital and 4% Picral
Special techniques: a.
Field Emission Scanning Electron Microscopy (FE-SEM)
b. Carbon
Analysis for combined dissolved carbon and non-dissolved carbon
c. X-ray
Diffraction Analysis
Carbonyl iron powder (CIP),
which is frequently used in the powder injection molding (PIM) process,
contains high carbon content and an onion-ring structure. During debinding and
sintering, the carbon content decreases and grain growth occurs, both of which
significantly influence the final mechanical properties of sintered PIM parts.
However, there is little direct experimental data reported on the evolution of
the carbon content and microstructure. Moreover, most carbon analyses reported
to date present the total carbon content of the specimen. The individual
amounts of the dissolved carbon and the non-dissolved carbon have rarely been
provided. Thus, the first objective of this study was to monitor the evolution
of the onion-ring structure and its correlation to the changes in the carbon
content. The second objective was to differentiate the amounts of the
non-dissolved carbon and the dissolved carbon, which are the main factors in
determining the mechanical properties.
Fig.
1a. The cross-section of as-received CIP, showing the onion-ring structure.
10kX
100kX
100kX
10kX
Fig. 1b. The cross-section of the CIP heated to 300℃, showing that
nano-scale Fe3C and Fe2C particles formed and
that the number of rings decreased.
10kX
100kX
Fig. 1c. The cross-section of the CIP heated to 600℃, showing that
micro-scale Fe3C and Fe2C particles formed and
that the onion-ring structure disappeared.
100kX
10kX
Fig. 1d. The cross-section of the CIP heated to 900℃, showing that
the particle size of the Fe3C and Fe2C decreased and that grain
growth occurred inside the particle.
0.043
0.036
0.007
D (900℃)
0.526
0.493
0.033
C (600℃)
0.734
0.560
0.174
B (300℃)
0.740
0.702
0.038
A (as-received)
Total
(wt%)
Non-Dissolved
(wt%)
Dissolved
(wt%)
Sample
Fig. 3. The XRD patterns of the residuals collected after CIP was
dissolved in the HCl solution, showing that Fe3C, Fe2C, and free carbon were present in the
powder.
The evolution of the onion-ring structure and the changes in the carbon
content in the carbonyl iron powder were investigated in this study. In the
as-received powder, the carbon was present mostly in the free carbon form,
with small amounts of Fe3C and Fe2C, as were measured using the self-designed
current settings in the C/S analyzer and X-ray diffraction analysis. As the
powder was heated in the N2-15%H2 atmosphere, decarburization occurred. Both
the amounts of Fe-C compound and free carbon decreased, and the onion-ring
structure disappeared at about 600℃. After heating to 900℃, only some
nano-scale Fe3C and Fe2C particles were present, and the grain growth
was obvious.
The iron powder obtained in
reaction [1] serves as a catalyst for reaction [2], in which carbon is formed
and deposited on the Fe surface. As the Fe surface is covered by the carbon
and the CO content diminishes, reaction [1] becomes intense and a fresh Fe
surface is produced again on top of the carbon surface. As this cycle
continues, the alternating carbon and Fe layers form an onion-ring structure,
as shown in Figure 1a, which is an SEM micrograph of the CIP-S-1641 powder
(ISP, Wayne, N.J.). This powder contains a high amount of carbon, usually
between 0.7 and 0.9wt%. But little information is available on whether the
carbon is present in the form of carbon soots, dissolved carbon, or Fe-C
compounds. As this powder is heated, the carbon reacts with the oxides on Fe
powder surfaces and/or with the residual oxygen, water vapor, and hydrogen in
the atmosphere. Thus, decarburization occurs and deteriorates the mechanical
properties of the sintered compact.
Table. 1. The contents of dissolved carbon and free carbon in the
carbonyl iron powder heated to 300, 600, and 900℃.
The CIP-S-1641 powder was heated
at 10℃/min
to 300, 600, and 900℃
in an atmosphere of N2-15%H2 and then furnace cooled in pure nitrogen. The
powder was then mounted using bakelite, ground, polished, and etched with a
mixed solution of 4% Nital and 4% Picral. To prevent the powders from being
pulled out of the bakelite during grinding, short grinding and polishing times
were used. Different etching times were also used in order to obtain optimum
microstructures, since the structures of the alternating layers changed after
heating. The etched parts were examined using a Field Emission Scanning
Electron Microscope.
To differentiate the dissolved
carbon and the non-dissolved carbon, the power input of the carbon analyzer
(Horiba, Kyoto, Japan) was adjusted so that the non-dissolved carbon could be
detected at low power inputs, while the dissolved carbon could be detected at
high power inputs, as shown in Figure 2. Three calibrations were performed
prior to the testing of the specimens. The first used a mixture of pure iron
powder and graphite powder with the known ratio as the standard. The second
calibration was performed using a mixture of pure iron powder and carbon soot,
which was collected from the un-burnt smoke that was deposited on a glass
plate placed above a candle flame. The third used a wrought low carbon steel.
In order to identify the white layer in the onion-ring structure, the
as-received powder and those that had been heated to different temperatures
were dissolved using HCl solution, and the collected residues were examined
using X-ray Diffraction Analysis.
The typical onion-ring structure, as shown in Figure 1a, consists of
alternating layers of pure iron and C-containing materials. The average
distance between the layers is about 100nm. The X-ray diffraction pattern, as
shown in Figure 3, indicates that the C-containing layer consists of Fe3C, Fe2C, and free
carbon. The iron chlorides resulted from the dissolution of the iron powder in
the HCl solution. As the temperature increased to 300℃, the alternating layers
became less smooth and particles were formed, as shown in Figure 1b. The
constituents remained the same. As the temperature further increased to 600℃, the ring
structure disappeared. The white rings broke into individual Fe3C and Fe2C particles and were dispersed in the Fe matrix,
as shown in Figure 1c. When the temperature increased to 900℃, decarburization
was severe, and only a small amount of nano-scale FexCy particles were present. With fewer
grain-boundary-pinning dispersoids, grain growth was obvious, as shown in
Figure 1d.
To differentiate the form of the carbon in the powder, the power input
of the C/S analyzer was adjusted. With the settings thus adjusted, the total
carbon content of the as-received CIP was measured at 0.740%. This included
0.702% non-dissolved carbon and 0.038% dissolved carbon, as shown in Figure 2
and Table 1. As the temperature increased, the total amount of carbon
decreased. However, the amount of dissolved carbon increased slightly to
0.174% at 300℃ and then decreased as the temperature further increased.
2q
as-received
300℃
600℃
900℃
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◆SiO2 ■Fe3C □Fe2C
▲FeCl2 ▽FeCl3 ●C
Fig. 2. The carbon analysis of the carbonyl iron powder heated to
300, 600, and 900℃, showing that the non-dissolved carbon is detected at a low power
input, while the dissolved carbon is detected at a high power input.