Proceedings of the 7th Annual ISC Graduate Research Symposium
ISC-GRS 2013
April 24, 2013, Rolla, Missouri
CHARACTERIZATION OF NON-METALLIC INCLUSIONS AND MICRO-POROSITY IN
STEEL: FORGED INGOTS, CENTRIFUGAL AND NET SHAPE CASTINGS
Jianxun He
Department of Materials Science and Engineering,
Missouri University of Science and Technology
218 McNutt, 1400N. Bishop, Rolla, MO, USA 65401
Email: jhc7d@mail.mst.edu
ABSTRACT
Micro-porosity and non-metallic inclusions are in many cases
similar in size (micron/submicron size ranges) and appearance,
and both negatively affect the mechanical properties of cast
steel. However these defects typically have different origins,
and improving steel properties by eliminating or suppressing
their negative effects poses different technological challenges.
Therefore, differentiation of micro-porosity and non-metallic
inclusions is practically important.
In this article, practical aspects of sample preparation and
automated SEM/EDX analysis are discussed. The developed
experimental procedures are applied for analysis of different
industrial cast steel products including centrifugal steel casting,
investment casting and ingot casting products. The observed
results were compared for different steels. The possibilities of
healing micro-porosity by hot plastic deformation and hot
isostatic pressure treatment are also evaluated.
1. INTRODUCTION
Non-metallic inclusions and porosity are common defects in
steel. Their negative effects are evident in many areas of steel
production and product application. Non-metallic inclusions
affect steel’s castability and may cause problems such as nozzle
clogging during continuous casting [1, 2], reduce its ductility,
induce inferior surface defects, cracks and poor finish,
jeopardize steel’s mechanical properties and change steel’s
corrosion resistance when it is put into service. Porosity seems
to have some of the same effects as non-metallic inclusions,
acting as areas of stress concentration, surface defects, reducing
fatigue resistance, etc. In other aspects, porosity has different
effects from non-metallic inclusions, for instance, porosity can
lead to leaks in fluid-containing castings [3]. Moreover, nonmetallic inclusions may interact and combine with porosity,
which has been observed in many cases.
The origins of non-metallic inclusions and porosity are
different. Non-metallic inclusions can be divided into
exogenous inclusions, inclusions primarily derived from
external sources such as re-oxidation, slag entrainment and
lining erosion, and indigenous inclusions, inclusions formed
during solidification by the thermodynamic affinity of reactive
components with the remaining impurities in the melt. Porosity
sources in steel castings include air entrainment, blow holes
during filling, gas evolution from solute during solidification,
solidification shrinkage, mold or core blows, and volatile
binder [4].
Improving steel properties by eliminating or suppressing
negative effects of non-metallic inclusions and porosity is of
great importance and poses different technological challenges.
Today, techniques such as filtration of the melt, inert gas
stirring [5], flux and slag absorption [6], inclusion modification
[7], flow control [8, 9], dissolved oxygen control, protective
atmosphere, etc. are available to control non-metallic inclusions
[10, 11]. Porosity can usually be suppressed by degassing [12],
vacuum treatment [13], flow control [14], solidification control
[15], hot isostatic pressing [16-19], or hot plastic deformation
[20], to name a few. The ultimate goal is to reduce both nonmetallic inclusions and porosity as much as possible.
The current study focuses on the characterization of nonmetallic inclusions and micro-porosity in centrifugal and
investment castings and comparison to forged ingots on the
basis of previous work completed at Missouri S&T [21-23].
2. STEELS INCLUDED IN STUDY AND ANALYSIS
METHODOLOGY
Industrial steel samples were collected at different plants,
including castings and ingots. Steels used are shown in Table 1.
Table 1: steel types studied and processing.
Casting Process
Type of Steel
Centrifugal casting
Medium carbon low alloyed
cast steel
Forged ingot
316 L Stainless steel
Investment casting
High strength Eglin steel
Centrifugal casting: Medium carbon low alloy cast steel was
melted in an electric arc furnace followed by an argon oxygen
decarburization (AOD) treatment. Then the melt was
transported through a ladle to the centrifugal caster. In total
3077 kg of cast steel was melted and poured into the centrifugal
mold at a temperature of 1570 ºC. The ladle was argon stirred
for approximately 15 minutes before casting.
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The chemical composition measured in the AOD is shown in
Table 2. During the centrifugal casting the maximum rotation
speed was 860 rpm. The centrifugal force at the beginning of
the casting process was between 95-160 G. It was then reduced
at a constant rate to 30 G until solidification was completed.
The centrifugally cast tube is 6.7 m (22 feet) long, it’s inside
C
0.20
S
0.001
Si
0.16
C
0.02
Si
0.43
Mn
0.628
diameter (ID) is 0.336 m (14 inch) and its outside diameter
(OD) is 0.508 m (20 inch). The wall thickness of the tube is
0.076 m (3 inch). Centrifugal cast samples were taken from the
end of the tube which is opposite to the pouring side. Samples
from five different thickness locations were taken, which is
shown in Figure 1.
Table 2: Chemical composition in AOD, wt. %.
Cr
Mo
Ni
Cu
V
Ti
2.70
0.76
0.10
0.13
0.05
0.004
Table 3: Composition of 316L Stainless steel in wt. %.
Mn
S
P
Cr
Ni
Mo
1.50
0.004
0.022
16.31
10.28
2.10
OD
Al
0.062
Al
0.002
P
0.013
W
0.015
N
0.064
into round product with a reduction ratio of 2.04:1. Samples at
different heights and radii from the forged ingot were collected.
Width
Thickness
X=1.00
X=0.75
X=0.50
X=0.25
X=0.00
ID
Figure 1. Schematic representation of sampling of the
centrifugally cast tube.
Investment casting: Eglin steel (ES) was developed by the U.S.
Air Force as cost effective, ultra high-strength steel for military
applications [24]. ES was investment cast into test bars. The
chemical composition of the ES is shown in Table 4. The wax
pattern for the investment casting is shown in Figure 2. Each
test bar is 0.0254 m (1 inch) in diameter and 0.140 m (5.5 inch)
in length. The shell material is fused silica. Before pouring, the
shell was fired to 982 ºC for 1 h. The heat size was 68 kg.
Argon gas was introduced through a porous plug in the bottom
and under an insulation blanket on the top of the furnace. The
pouring temperature was 1566 ºC. The melt was poured into 4
hot test bar trees for each heat.
Forged ingot: 7168 kg of 316 stainless steel was bottom poured
into the ingot mold. The chemical composition of 316 L
stainless steel is shown in Table 3. The teeming temperature
was 1534ºC. Finally the as-cast square ingot was hot forged
Table 4: Composition of ES (wt %).
Fe
C
Cr Mn Mo Nb Ni Si Ti
V
W
92.984 0.265 2.6 0.65 0.42 0.01 1.0 1.0 0.006 0.065 1
Figure 2. Wax pattern of the investment cast ES test bars (left) and cast test bars (right)
2
Hot Isostatic Pressing (HIP): HIP was employed to treat the
investment cast ES bar at a participating plant in a high
temperature pressurized furnace under an inert argon gas
atmosphere. The HIP treatment was conducted at 1163 ºC for 4
h at 103.4 MPa. The test bars were cut at four different
locations along the longitudinal direction.
user-defined rules are divided into two categories: classification
and zero elements. Starting with the first rule in the list, ASPEX
categorizes each detected spot if the rule is true. If it is not, the
following rule in the list will be applied. Images with EDS
analysis can be obtained simultaneously during scanning [25].
The zero element rules are used to distinguish inclusions and
porosity from stains, which may cause error in the results. Zero
rules for the base elements such as Fe, Ni and Cr can reduce the
computational work of EDX spectra and make the scanning
process more efficient. For different steels, the rule definition of
inclusion and porosity is also different. An example of applied
rules is given in Table 5.
Automated SEM/EDX analysis: An ASPEX PICA 1020 was
used to simultaneously analyze the inclusions and porosity in
the steel samples. Information such as size, morphology and
composition can be obtained quickly using an automated SEM.
Rule files and vector files were applied for the analysis. The
Table 5: Example of classification rules of porosity and inclusions for centrifugal casting.
Process
Centrifugal
Casting
Classification
Vector File
Carbon
FeO Stains
Porosity
MnS
CaS
Ca-Mn-S
Other Sulfides
Al2O3
CaO
SiO2
MnO
Zero Elements
Cr=0
Fe=0
C=0
Rules
C, Na, Mg, Al, Si, S, K, Ca, Ti, Cr, Mn, Fe, Ni and O
If C>5
Fe>=30
Fe >=90 and O<5
Mn>30 and S>20
Ca>30 and S>20
Ca>10 and Mn>10 and S>20
S>20
Al>20 and Mn<20 and Ca<10 and Si<20 and S<20
Ca>5 and S<20 and Al<10 and Ca<10
Si>30 and Mn<20 and S<20 and Al<10 and Ca<10
Mn>20 and S<20 and Al<20 and Si<20
If Cr>=0.1
If Al>=2.5 or Mn>=2.5 or Ca>=2.5 or C>=2.5 or Si>=2.5
If Al>=2.5 or Mn>=2.5 or Ca>=2.5 or Si>=2.5
3. RESULTS AND DISCUSSION
Centrifugal casting: Micro-porosity and non-metallic
inclusions with diameters less than 10 microns were observed
in the samples from centrifugal casting. Two types of pore
shapes were observed. The first type included spherical pores
with attached inclusions on internal surface. Figure 3 shows an
example of a spherical micro-porosity with noticeable nonmetallic inclusions at the boundaries of the micro-pore. The
size of the micro-pore is around 5 μm. The second type was
irregular shape pores which are shown in Figure 4. The size of
the micro-pore is approximately 3 μm and different types of
non-metallic inclusions were associated to the micro-pore.
Figure 3. Centrifugally cast steel sample (X=0.5) showing inclusions around the boundaries of micro-porosity in a steel matrix [22].
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Average sizes of micro-porosity and non-metallic inclusions of
the centrifugally cast tube at different thickness locations are
given in Figure 5a. It appears that from inside diameter to
outside diameter, the average sizes of both micro-porosity and
non-metallic inclusions decrease, with the exception at the half
thickness.
In this study we intentionally preserved the ID and OD regions
to verify the distribution of micro-porosity and non-metallic
inclusions. However, practically speaking, the ID and OD
regions from centrifugal casting in industry are frequently
removed during final machining to remove the large regions of
inclusions and micro-porosity (see Figure 5b).
Figure 4. Centrifugally cast steel sample (X=0.5) exhibiting a ring of inclusions in the former boundary of a irregular shape micropore [22].
a)
b)
Figure 5. Average sizes (a) and areas (b) covered by micro-porosity and non-metallic inclusions in the centrifugally cast tube at
different thickness locations.
The micro-porosity size distribution of the as-cast and HIP
Investment cast ES: As-cast and HIPed samples were
treated ES samples is shown in Figure 7a. As-cast samples have
investigated. Metallographic images of as-cast and HIP treated
wide size ranges of porosity, while the HIP treated samples
ES samples are shown in Figure 6. Two groups of porosity with
exhibit a narrow size range between 0 to 5 μm. The largest
differing dimensions were observed in as-cast conditions: (i)
porosity nearly all disappeared in the HIP treated samples,
macro-porosity with a pore diameter above 10 μm (up to 100
which is in agreement with the observation in Figure 6. Nonμm) on the top of the as-cast test bars and (ii) interdendritic
metallic inclusion size distribution of the as-cast and HIP
micro-porosity with pore size below 10 μm in all casting
treated ES samples is shown in Figure 7b. Non-metallic
regions. After HIP treatment, the large macro-porosity
inclusions were not as affected by the HIP treatment as the
disappeared from the samples, while the micro-porosity still
porosity with large inclusions still evident. However, it does
remained.
appear that the inclusions are slightly smaller after HIPing and
the quantity of large size non-metallic inclusions decreased
slightly.
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a)
b)
b) Figure 6. Macro- and micro porosity observed in the as-cast ES (a) and small size of micro-porosity after
Metallographic images, samples were etched with Nital.
HIP treatment (b).
a)
b)
Figure 7. Histograms of the porosity and inclusion size distribution detected in ES for the as-cast and HIP treated samples: a) by
number and b) by covered area.
characteristic of the forged ingot in this study is that no porosity
Ingot and forged stainless steel: For the ingot casting, the
defects were detected through the ASPEX analysis. This can be
concentration density and size distribution of non-metallic
attributed to the hot forging process of the ingot. This result
inclusions is shown in Figure 8. It reveals a concentration of
suggests that even a relatively small reduction ratio (2:1) in hot
inclusions within the range of 0 to 5 μm in the ingot. Inclusions
forging is an effective method of reducing the major porosity in
with the diameter between 1 to 2 μm have the largest
cast products.
concentration density in the samples. Another notable
Figure 8. Histogram of non-metallic inclusion size distribution detected in the 316 L ingot. No micro-porosity was found.
5
Figure 9 compares the coverage area of non-metallic inclusions
and micro-porosity in each process. In terms of non-metallic
inclusions, the investment casting samples show the highest
amount among all the samples. On the contrary, samples from
half thickness of centrifugal casting have the lowest amount of
inclusions, which is not unexpected based on centrifugal forces
helping remove inclusions. As for micro-porosity, investment
casting as-cast samples contained the largest amount of micro-
porosity, 491 µm2 /mm2. HIP treatment effectively reduced the
amount of micro-porosity in the investment casting with little
effect on the amount of non-metallic inclusions. The centrifugal
casting samples had less porosity than the HIPed static cast
samples, indicating the effectiveness of centrifugal casting in
reducing porosity. However, the forged (2:1 reduction) cast
ingot, had effectively no micro-porosity proving the
effectiveness of hot deformation on micro-porosity removal.
Figure 9. Comparison of the amount of defects for different casting processes
µm2 /mm2 which stayed approximately the same at
286 µm2 /mm2 after HIP treatment. The results imply a
good control of the amount of non-metallic inclusions
by centrifugal casting.
4. CONCLUSIONS
An investigation of non-metallic inclusions and micro-porosity
was completed for investment castings (as-cast and hot isostatic
pressed), centrifugal casting, and forged cast ingots using
automated SEM/EDX. The following conclusions were
obtained:
(1) In terms of porosity, as-cast samples from the
investment casting samples exhibit the largest amount
of micro-porosity area coverage among the processes
investigated in this study, which is 491 µm2 /mm2.
HIPing was effective in making a major reduction in
micro-porosity in the investment castings as was
centrifugal casting. However, forging even at a low
reduction ratio (2:1) was the most effective technique
with very little micro-porosity detected after hot
forging.
(2) In terms of non-metallic inclusions, centrifugal casting
was the most effective in terms of the least area
coverage by inclusions, at 163 µm2 /mm2. Area
coverage for ingot cast is 258 µm2 /mm2. As-cast test
bars from investment casting has area coverage of 297
(3) HIP treatment is very effective in healing areas of
large porosity in castings. Porosity coverage of
investment cast samples reduced dramatically from
491 µm2 /mm2 to 56 µm2 /mm2 after HIP treatment.
However, HIP is not effective in eliminating microporosity in the size of 0-2 µm. The amount of microporosity between 0-2 µm actually increased indicating
the reduction but not necessarily elimination of microporosity through HIPing.
5. ACKNOWLEDGMENTS
The authors are grateful to the steel companies that provided
the cast samples and the previous students who had completed
their research related to the steels samples used in this paper:
Edith Martinez (centrifugal casting), Andrew O’Loughlin
(Eglin steel) and Jun Ge (forged ingot steel). The help from
Dr. Simon Lekakh, Mingzhi Xu and Kramer Pursell are also
deeply appreciated.
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