COMPARATIVE ANALYSIS OF CHARACTERISTICS OF STAINLESS STEEL CELLULAR
MATERIAL PREPARED THROUGH POWDER METALLURGY USING ACCICULAR AND
CRUSHED UREA AS SPACEHOLDER
SHAILENDRA JOSHI*
*Department of Mechanical Engineering, NPSEI, Pithoragarh, Uttarakhand, India.
ABSTRACT: Stainless steel foams are widely used in structural applications where light weight is
required along with high strength especially used in impact energy absorbtion applications
where damping capability is required such as in vehicles and buildings. The properties of
stainless steel foam, to a large extent depends on the pores arrangement which is decided by
the space holder utilized during manufacturing SS foam through powder metallurgy. In the
present paper we will discuss the pores distribution and true stress-strain curve of stainless
steel foam obtained using accicular and crushed urea as space holder. Depending upon their
structure and above characteristics, resulting applications are studied.
Keywords: foams, damping, arrangement, accicular, crushed
INTRODUCTION
Metal
foams
are
porous
structures
in
solid metal matrix where pores occupy a large
volume. They have various excellent properties
like greater compression strength,low density,
better stiffness, etc. which make it desirable for
structural & functional applications. The
commonly used metal foams are made up of
aluminium, titanium, tantalum and steel. Foams
microstructure comprises mainly solid and
voids. As a result of distinctness in various voids
structure in terms of dimension and shape, the
behavior of metal foams is more. As such
analysis of this
uncertainty in the
microstructure of metal foams is of much
importance.
Various methods had been developed for
manufacturing stainless steel foams e.g.
melt-gas injection, powder metallurgy,
investment casting [5], etc. Since powder
metallurgy is cost effective and flexible it
has been considered a suitable method for
stainless steel foam fabrication [5-7]. Also it
results in better uniform distribution of
pores in foams, more precise control of
process variables and pore size[8] & high
relative density. Steel foam is a material
that can create materials with a variety of
different morphologies that have desirable
mechanical properties. Metal foams have
an extended stress plateau in their
compressive stress-strain curves, which is
effective for absorbing energy at a constant
stress level.
Metal foams are affective in automotive,
railway,
aerospace
and
chemical
applications, where low weight, reduced
chemical pollution and high degree of
comfort and safety is desired [1]
EXPERIMENTAL WORK
Firstly, Stainless steel powder of
appropriate size as well as acccicular urea
are weighed as per to obtain porosity as
40, 50 & 60% respectively .Afterwards they
both are kept in a crucible apparatus in a
definite amount for obtaining proper
mixture . In same manner, the samples of
crushed urea particles are prepared
maintaining porosity 40, 50 & 60 %.
These mixtures are than pressed in a
hydraulic machine by putting inside a
cylindrical die of 10 mm diameter at a
pressure of 260 MPa for half minute
producing
green
compacts(cylindrical
shape) producing definite number of
samples. Then, preheating of green compacts
0
is carried inside the tubular furnace near 340 C,
for nearly two and half hour.
During sintering process the samples are heated
up to 11100C for a time interval of 115 minute
at a heating rate of 120C /min. Then the
samples are kept in the furnace for a time
period of 65 min. so that the samples volume
attains a constant temperature of 1100 °C. Then
the samples are kept in the furnace for cooling.
The pore morphology of both types of distinct
samples obtained through accicular and
crushed carbamide is studied on SEM.
Then, various properties of the samples are
characterized after performing compression
test on the specimen on universal testing
machine.
RESULTS AND DISCUSSION
Green compact samples compacted surface
are analyzed on SEM. The micro structural
image depicts the distribution of urea
particles in the matrix as shown in figure. In
the image the black area shows SS particles
& white regions represent particles of urea.
We can observe from the SEM images that
larger size pores results in case of green
stainless steel compacts containing
accicular urea whereas small size uniformly
distributed pores in case of that containing
crushed urea particles. This occurs due to
variation in the size of urea particles and
contributing mixing in crucible.
Due to the evaporation of urea particles,
whether accicular or crushed, during
preheating of green compact samples the
pores gets produced. The pores obtained are
larger in case of samples resulted from accicular
urea than that of crushed urea particles due to
deformation and distortions resulted by
increase in temperature and evaporation.
Pores dimension are more as compared to f
the dimensions of urea particles because on
getting sufficiently heated, the urea particles
evaporate after the mechanical bonds broken in
the foam structure which produced larger sized
voids.
Both types of samples were than sintered to
give strength and produce metallurgical bonds
in the stainless steel foam samples. The
sintering is done at a suitable temperature
between preheating temperature & melting
point.
The samples with higher percentage of urea
are irregular in shape because after
preheating of green compacts, the samples
with higher percentage of urea have higher
porosity making the pre-heated compact
much weaker. Due to less bonding strength
the samples having higher percentage of
urea did not sustain it’s shape after preheating. The irregularity in shape is more in
case of samples obtained through accicular
urea particles than that obtained from
crushed ones but the strength is increased
after sintering. The voids obtained are less in
dimensions than that of the pores obtained
after preheating due to the large driving force
producing more pore shrinkage during
sintering.
SEM Micrograph of green compact ss matrix
containing accicular urea
SEM Micrograph of preheated sample
containing acicular urea particles
SEM Micrograph of preheated sample
containing crushed urea particles
SEM Micrograph of the green compact ss
matrix comprising of crushed urea particles
Sintered stainless steel foam samples with
varying porosity processed using acicular
crystals of urea
accicular while in foam samples containing
crushed urea are approximately circular.
This is due to the shape of space holder
particles and metallurgical bonding in
stainless steel matrix during sintering.
Sintered Stainless steel foam samples with
varying porosity processed using fine urea
particles
During the compression test of stainless
steel foam samples of 40, 50 & 60%
porosity, when we use accicular urea as
space holder ,the plateau region results
when the porosity reaches above 50 vol %
while this regions is not distinct in case of
samples obtained from crushed urea as
space holder due to large size accicular
pores obtained in previous case. They have
high energy absorbtion capacity. Stainless
steel foam’s yield strength samples lowers
due to increasing porosity in later case. Also
the material is ductile and can be used in
lightweight structural applications.
SEM Micrograph of sintered foam sample
(using acicular urea space holder)
SEM image showing sintered foam sample
(using crushed urea space holder)
The voids resulted in sintered foams while
utilizing accicular urea as space holder are
Stress versus Strain diagram of ss foam
(acicular urea space holder)


Stress versus Strain diagram of ss foam
(crushed urea space holder)
CONCLUSION

Thus stainless steel foams can be
successfully processed using PM method.
Also the degree of porosity can be varied with

the quantity and nature of space holder. The
pores dimensions depend on the space holder
utilized, sintering temperature and preheating
&sintering period.
The foams having more dimensions pores
showed plateau regions at higher porosity while
those possessing circular pores doesn’t possess
distinct plateau region. The accicular foams
therefore have high impact energy absorbtion
characteristics & the later crushed particles
foams can be utilized in high strength structural
applications.
ACKNOWLEDGMENTS
The author thanks MANIT, Bhopal & CSIRAMPRI, Bhopal for providing support during
the study and research.
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