Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.7, pp.617-624, 2011
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617
Effect of Chromium on Magnetic Characteristics of Powder Processed
Fe-0.35wt%P Alloy
Deepika Sharma*, Kamlesh Chandra, Prabhu Shankar Misra
Metallurgical and Materials Engineering Department
Indian Institute of Technology, Roorkee 247667, India
*Corresponding Author: [e-mail]
ABSTRACT
The present investigation deals with hot powder preform forging technique for the development
of high density iron-phosphorus based alloys. These alloys are known for hot-shortness and are
therefore not considered suitable for high temperature working. To remove this problem proper
soaking at high temperature to eliminate iron- phosphide eutectic and bring entire phosphorus
into solution in iron was aimed. Attempting hot forging thereafter completely eliminates hot as
well as cold shortness and thereby helps to form these alloys into very thin sheets. It has also
been possible to eliminate the use of costly hydrogen atmosphere during sintering by use of
addition of carbon as a reducing agent to form CO gas within the compact by reacting with
oxygen of iron powder particles. The glassy ceramic coating applied over the compact serves as
a protective coating to avoid atmospheric oxygen attack over the compact held at high
temperature. Combined application of carbon and glassy ceramic coating has lead to economy
in P/M processing for soft magnetic applications. Fe- 0.07C- 0.3O- 0.35wt% P- 0.35wt% Cr
alloy so formed yielded coercivity as low as 0.42 Oe, resistivity as high as 21.8 µΩcm and total
loss as low as 0.170 W/Kg. Such a combination of properties may suit their application in
magnetic relays and transformer cores.
Key Words: Glassy Ceramic Coating; Preform Forging; Coercivity
1. INTRODUCTION
Iron-phosphorous alloy system for the production of magnetic materials is not known in wrought
processing route in spite of the fact that phosphorous as an alloying element has all favorable
characteristics to enhance magnetic properties of iron [1]. This is because of the fact that in

618 Deepika Sharma, Kamlesh Chandra, Prabhu Shankar Misra Vol.10, No.7
wrought route it is not possible to bring phosphorous in homogeneous solid solution with iron
due to its segregation tendency during solidification of the melt. Powder metallurgical
processing, due to its predominantly solid state processing approach, has been very successful in
exploiting potential of Fe-P alloy system for magnetic applications [2]. A number of products
such as magnetic cores of contactors, relays, magnetic brakes and electricity meters have been
produced out of this system [3].
The sintered parts mostly made of pure iron are now well established in the market for soft
magnetic applications. These components at high sintered density possess high saturation
magnetization but they exhibit low resistivity and high magnetic losses [4]. Increasing resistivity
reduces the eddy current losses [5] and alloying (P and Cr) primarily increases the resistivity [6].
Phosphorus activates sintering process in Fe-P alloys by the formation of low-melting eutectic
phase with iron [7]. It helps in carrying out alloy constituents into iron matrix which are
otherwise difficult to diffuse. It also increases induction and permeability [8] and decreases
coercivity of iron. Chromium increases the electrical resistivity of Fe-P based alloys and being a
ferrite stabilizer, it also improves magnetic properties of Fe-based alloys [6]. Chromium also
improves formability of the iron-based alloys significantly [9].
Conventional powder metallurgy processes enable compaction to be 93% of the theoretical
density [10]. Double pressing achieves higher density but at increased cost. Existence of prior
particle boundaries renders hot isostatic pressing unsuitable for magnetic applications. In view of
this, in the present investigation, densification is carried out by cost effective hot powder preform
forging technique. The process renders highest possible densification without resorting to
hydrogen as sintering atmosphere. It is essentially the process where shaping and consolidation
are deformation based. This causes redistribution of segregants if at all remained at the particle
surfaces (deformation can displace these from grain boundary and disintegrate them to fine
particles which easily dissolve inside the ferritic grains). Such a processing route provides
superior magnetic properties in comparison to existing sintering approach.
2. EXPERIMENTAL PROCEDURE
The present investigation utilizes atomized iron powder of M/S Hoganas India Ltd., Grade ASC
100.29. It has carbon ~ 0.01% and hydrogen loss value as 0.1%. Its particle size is -100 mesh.
Low-carbon ferro-chrome powder (C< 0.01 wt %) (size -200 mesh) was prepared by grinding the
ferro-chrome lumps. Iron-phosphide powder (C- 0.01 wt %) (size -100 mesh) was prepared by
reacting iron powder with ortho-phosphoric acid and a subsequent thermal treatment
(800
0
C/2h/H
2
) to yield Fe
3
P coating over iron powder.
The powders are suitably mixed with 0.3wt% of carbon (in the form of graphite; size -100 mesh)
to yield the following chemistry:

Vol.10, No.7 Effect of Chromium on Magnetic Characteristics 619
(1) Fe-0.3 wt% C-0.35 wt% P
(2) Fe-0.3 wt% C-0.35 wt% P-0.35 wt% Cr
The powder mix was filled in a rectangular die and the green compacts (preform) with 7mm
thickness and 25mm x 50mm size were formed. The oxidation resistant glassy coating [11] was
applied on the surface of preforms like a paint with a brush to serve as protective layer resisting
oxidation at high temperatures and protecting reducing gaseous atmosphere of CO produced
inside the compact at high temperature by reaction of C with O
2
(of iron powder). Coated
preforms were baked at 120
0
C for two hours. These performs were then transferred to a furnace
held at a temperature 1050
0
C and soaked there for one hour. The hot preforms were immediately
transferred to a forging die fitted in the press and were forged. Then the slabs were hot rolled at
temperature 900
0
C to form a sheet of 0.1mm thickness. Toroids were stamped from the sheet
using a die/punch arrangement. Glassy coating was applied on the toroids and these were again
annealed at 850
0
C for three hours to relieve residual stresses. The dimensions of toroids were
1mm thickness, 50mm outer diameter, 40mm inner diameter with 74 primary and 6 secondary
windings.
The samples prepared this way were characterized in terms of their density, microstructure,
electrical resistivity and magnetic properties. The content of carbon and oxygen was analyzed by
spectroscopic analyzer. Density was determined by Archimedes’ principal. Microstructure
(etched with 2% Nital) of the rolled and annealed sheets was analyzed by using image analyzer
to estimate the grain size and volume percentage of porosity in the alloys. Scanning electron
microscopy and X- Ray mapping confirmed the presence of ferrite phase and showed uniform
distribution of phosphorus and chromium in iron matrix. Electrical resistivity was measured by
four probe method. Magnetic properties were measured under d.c. mode.
3. RESULTS AND DISCUSSION
Incorporating carbon to the alloys has helped in number of ways, firstly as solid lubricant during
cold compacting; secondly as solid state reducing agent to take care of oxygen situated at iron
powder particles surfaces during high temperature processing. Thirdly pushing phosphorus into
ferrite grain as solute and thereby discourages it to precipitate as phosphide along ferrite grain
boundaries [12]. The complete chemistry of alloys developed (spectroscopic analysis) is given in
Table 1 (as first column) along with the measured values of density (forged, rolled and annealed
sheets), grain size and porosity percentage present in the sheets. It was observed that
densification has improved with the present processing technique.
The microstructure shows residual alignment of porosity due to rolling (Fig. 1(a), (b)). Pores are
elongated due to unidirectional compressive force [2]. Pore rounding and coagulation of smaller

620
Deepika Sharma, Kamlesh Chandra, Prabhu Shankar Misra
pores into bigger pores are observed in the microstructures. It may be due
phosphorous content in the alloy [13].
Table: 1 Density, porosity and grain size
Fig. 1: Microstructures of rolled and annealed alloys etched with 2% Nital
0.07C-0.3O-
0.35P alloy and (b) Fe
Phosphorus and chromium are both ferrite stabilizers and are added to iron below its solubility
limit; therefore only alpha phase is present (Fig. 2 (a), Fig. 3 (a)).
present investigation are free of any segregation of the alloying elements along the grain
boundaries. They get distributed uniformly in the entire structure (Fig. 2 (b)
(d)).
Deepika Sharma, Kamlesh Chandra, Prabhu Shankar Misra
pores into bigger pores are observed in the microstructures. It may be due
to the presence of
phosphorous content in the alloy [13].
Table: 1 Density, porosity and grain size
of the alloys.
Fig. 1: Microstructures of rolled and annealed alloys etched with 2% Nital
at 200X. (a) Fe
0.35P alloy and (b) Fe
-0.07C-0.3O-0.35P-0.35Cr alloy
Phosphorus and chromium are both ferrite stabilizers and are added to iron below its solubility
limit; therefore only alpha phase is present (Fig. 2 (a), Fig. 3 (a)).
The al
loys developed in the
present investigation are free of any segregation of the alloying elements along the grain
boundaries. They get distributed uniformly in the entire structure (Fig. 2 (b)
-
Vol.10, No.7
to the presence of
at 200X. (a) Fe
-
Phosphorus and chromium are both ferrite stabilizers and are added to iron below its solubility
loys developed in the
present investigation are free of any segregation of the alloying elements along the grain
-
(c), Fig. 3 (b) -

Vol.10, No.7 Effect of Chromium on Magnetic Characteristics 621
Fig. 2: (a) Compositional Image (Secondary Image), X- Ray Mapping showing uniform
distribution of alloying elements in Fe-0.07C-0.3O-0.35P alloy at 3000X. (b) Fe and (c) P
Fig. 3: (a) Compositional Image (Secondary Image), X- Ray Mapping showing uniform
distribution of alloying elements in Fe-0.07C-0.3O-0.35P-0.35Cr alloy at 3000X. (b) Fe, (c) P
and (d) Cr.

622
Deepika Sharma, Kamlesh Chandra, Prabhu Shankar Misra
The resistance of iron increases greatly due to alloying additions like P and Cr [6].
represents the electrical resistivity and d.c. magnetic properties of the alloys and comp
the commercially available soft magnetic material.
Table: 2 Comparison of electrical and magnetic properties of the alloys developed in present
investigation with commercially available soft magnetic material.
Coercivity of Fe-
based alloys falls as we add alloying elements such as P and Cr to Fe. The
higher the alloying content, lower is the coercivity value [8]. The superimposed hysteresis loops
of Fe-0.07C-0.3O-
0.35P alloy and Fe
The permeability increases greatly due to alloying additions like P and Cr [9]
permeability observed in Fe-
0.07C
low porosity percentage (1.53). Total magnetic loss is t
loss [5]. Combined addition of P and Cr has decreased the total magnetic loss of the alloys
developed. This may be due to the fact that the alloy has simultaneously high resistivity (21.8
µΩcm) and low coercivity
values (0.42 Oe). Saturation magnetization of iron increases when
phosphorous is added up to 0.8wt % and decreases thereafter [13]. In the present investigation
addition of 0.35wt% of Cr, lowers saturation magnetization at a rate corresponding to simple
dilution [9].
Deepika Sharma, Kamlesh Chandra, Prabhu Shankar Misra
The resistance of iron increases greatly due to alloying additions like P and Cr [6].
represents the electrical resistivity and d.c. magnetic properties of the alloys and comp
the commercially available soft magnetic material.
Table: 2 Comparison of electrical and magnetic properties of the alloys developed in present
investigation with commercially available soft magnetic material.
based alloys falls as we add alloying elements such as P and Cr to Fe. The
higher the alloying content, lower is the coercivity value [8]. The superimposed hysteresis loops
0.35P alloy and Fe
-0.07C-0.3O-0.35P-
0.35Cr alloy are shown
The permeability increases greatly due to alloying additions like P and Cr [9]
. Higher value of
0.07C
-0.3O-0.35P-
0.35Cr was due to large grain size (94 µm)
low porosity percentage (1.53). Total magnetic loss is t
he sum of eddy current loss and hysteresis
loss [5]. Combined addition of P and Cr has decreased the total magnetic loss of the alloys
developed. This may be due to the fact that the alloy has simultaneously high resistivity (21.8
values (0.42 Oe). Saturation magnetization of iron increases when
phosphorous is added up to 0.8wt % and decreases thereafter [13]. In the present investigation
addition of 0.35wt% of Cr, lowers saturation magnetization at a rate corresponding to simple
Vol.10, No.7
The resistance of iron increases greatly due to alloying additions like P and Cr [6].
Table 2
represents the electrical resistivity and d.c. magnetic properties of the alloys and comp
ares with
Table: 2 Comparison of electrical and magnetic properties of the alloys developed in present
based alloys falls as we add alloying elements such as P and Cr to Fe. The
higher the alloying content, lower is the coercivity value [8]. The superimposed hysteresis loops
0.35Cr alloy are shown
in Fig. 4.
. Higher value of
0.35Cr was due to large grain size (94 µm)
and
he sum of eddy current loss and hysteresis
loss [5]. Combined addition of P and Cr has decreased the total magnetic loss of the alloys
developed. This may be due to the fact that the alloy has simultaneously high resistivity (21.8
values (0.42 Oe). Saturation magnetization of iron increases when
phosphorous is added up to 0.8wt % and decreases thereafter [13]. In the present investigation
addition of 0.35wt% of Cr, lowers saturation magnetization at a rate corresponding to simple

Vol.10, No.7 Effect of Chromium on Magnetic Characteristics 623
Fig. 4: Superimposed hysteresis loops of Fe-0.07C-0.3O-0.35P alloy and Fe-0.07C-0.3O-0.35P-
0.35Cr alloy
Such a combination of properties is achieved by the use of hot powder preform forging technique
employed in the present investigation.
4. CONCLUSIONS
1) The forming technique does not require any binder. Thus the system remains
uncontaminated.
2) The use of ceramic protective coating eliminates the need of hydrogen protective
atmosphere during heating.
3) Combined application of glassy ceramic coating and use of graphite as a reducing agent has
lead to economy in P/M processing.
4) The technology developed in the present investigation showed very low coercivity and total
loss values.
ACKNOWLEDGEMENTS
The authors acknowledge the support provided by Defence Metallurgical Research Laboratory,
Hyderabad, India by way of collaboration.

624 Deepika Sharma, Kamlesh Chandra, Prabhu Shankar Misra Vol.10, No.7
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