Journal of Minera ls & Materials Ch ar ac te ri zatio n & Engineeri ng, Vol. 8, No. 8, pp 611-620, 2009 Printed in the USA. All rights reserved
Effect of Ch romium on the Mechanical Properties of Powder-Processed
Fe-0.35 wt% P Alloys
Shefali Trivedi *, Yashwant Mehta, K. Chandra, P.S. Mishra
Indian Institute of Technology, Roorkee- 247667, Uttarakhand, India
* Corresponding Author:
The role of chromium in high density Fe–P binary, Fe–P–Cr ternary alloys is observed by
characterization in terms of microstructure, porosity content/densification, hardness and tensile
properties. The alloys were made using a hot powder forging technique. In this process mild
steel encapsulated powders were hot forged into slabs, hot rolled and annealed to relieve the
residual stresses. Densifications as high as 98.9% of theoretical density have been realized.
Microstructures of these alloys consist of single-phase ferrite only. Fe–0.35P, Fe–0.35P–2Cr
alloys showed very high ductility. As forged and hot rolled Fe–0.35P alloy showed very high
elongation and it improved further on annealing. It was observed in this present investigation
that, the alloying addition, such as Cr to Fe–P based alloys caused increase in strength
associated with the reduction in ductility. Alloys developed in the present investigation were
capable of hot/cold working to very thin gage of sheets and wires.
Key words: Phosphoric Iron, Mechanical properties, Powder Metallurgy, Forged, Ancient iron.
The role of chromium in temper embrittlement has been explained on the basis of the co-
segregation model proposed by Guttmann [1]. According to this model both chromium and
phosphorus in wrought route have a tendency to segregate to grain boundaries, and an attractive
energetic interaction occurs between them. Thus they attract each other at grain boundaries and
increase the segregation of the partner. In contrast, it has been reported that chromium has no
effect on the segregation of phosphorus in iron based alloys [2] and low alloy steel in which the
bulk concentration of chromium is varied [3]. The intrinsic effect of chromium has been
considered to be an increase in the embrittling potency of phosphorus and a reduction in the
grain boundary cohesion [3-5]. In contrast to the above, Kimura has shown that the addition of
chromium to a high purity Fe-0.25% P alloy decreased the ductile-brittle transition temperature
(DBTT) [6].
612 Shefali Trivedi, Yashwant Mehta, K. Chandra, P.S. Mishra Vol.8, No.8
Phosphorous helps carrying alloy constituents into iron matrix which are otherwise sluggish to
diffuse. P significantly improves ductility and strength of Fe–P based alloys [7]. Cr improves
formability of Fe–P based alloys [8]. It is therefore, realized that, the Fe–P based alloys,
containing alloying elements, such as Cr could be used for structural application because of their
higher strength than pure iron with reasonably good ductility. Since all these alloying elements
are ferrite stabilizers, ferrite phase will be stable even at high temperature when substantial
alloying is completed. Self-diffusion coefficient of iron [9] and inter-diffusion coefficient of the
alloying elements in ferrite is much higher than that in austenite. This diffusion helps in reducing
amount of pores in the P/M part. However, during alloying process some additional pores may
be created [9] (due to dissolution of elemental particles). However, if we follow the traditional
powder metallurgical process, such as compaction and sintering, for manufacturing high
phosphorous Fe–P based alloys, heavy volume shrinkage will be experienced [10].
There are several other densification processes available in the literature. Out of all densification
processes available, hot iso-static pressing (HIP) is the best as far as density and performance of
these P/M parts are concerned. However, the process is extremely costly. Therefore, some
pseudo HIP processing could be used for manufacturing these alloys to reduce the cost of
processing without sacrificing the benefit of HIP processing. However, HIP process does not
have scope of cleaning particle surfaces during processing. Furthermore, existence of prior
particle boundaries (PPB) renders them unsuitable because PPB’s are source of impurity
concentration resulting in inter-particle brittle failure. In view of this, in the present investigation,
the densification of the Fe–P based alloys were carried out by hot powder forging [8] technique.
Volume shrinkage associated with these alloys is also no more a consideration in hot powder
forging. Hot powder forging has another feature which is not available with compacting in a die
or HIP. It is essentially the process where shaping and consolidation are deformation based. This
causes redistribution of residual impurities (deformation can move them around but diffusion
moves them into the matrix) situated at the particle surfaces and results in improvement in the
properties of the final product [11].
For making iron–phosphorous, iron–phosphorous–chromium alloys by powder metallurgical
technique, iron powder (Fe-99.99 wt%, C-0.00 wt%) (100 mesh) was mixed with iron-
phosphide (C- 0.00 wt%) (100 mesh) with/without addition of low carbon ferro-chromium (C <
0.01 wt%) (200 mesh). Whereas, iron and low carbon ferro-chromium powders were of
commercial purity, iron-phosphide powder was prepared by mixing iron powders with ortho-
phosphoric acid and subsequent reducing heat treatment (800 C/2 h/H2). The reactions are as
Fe + H3PO4 = Fe3(PO4)2 ………………………..(1)
Fe3(PO4)2 +8H2 = Fe3P + 8H2O ………………..(2)
The powder blends were manually mixed to make different alloys. About 500 g of each blended
mixture was then poured in a mild steel capsule (as shown in Fig. 1). The encapsulated powders
Vol.8, No.8 Effect of Chromium on the Mechanical Properties 613
were heated in a tubular furnace at 1150 C for 45 min in dry hydrogen atmosphere in order to
remove the oxide layer from the surfaces of the powders. Heated capsules were then forged with
a 200T capacity friction screw press to make slabs using a flat/channel die. Two powder
metallurgical alloys were made in the present investigation. These are:
(1) Fe–0.35 wt% P alloy
(2) Fe–0.35 wt% P - 2 wt% Cr alloy.
The compositions of these alloys are based on the powder mixture. Alloys were made using
channel die where side wall restriction to metal flow was imposed. Fig. 2 schematically
illustrates the process of making slabs in hot powder forging technique. The slabs were then
homogenized at 1200 C for 2 h depending on the alloy composition to eliminate compositional
in homogeneity. All the alloying elements are present in the form of fine particles around pure
iron particles. This iron particle is 100% gamma-phase at the homogenizing temperature.
Phosphorous (in the form of ferro-phosphorous) combines with this gamma iron powder particle
and dissolves in it. As it dissolves, it gets converted into ferrite (Fig. 3, 4) and as ferrite phase
grows out of gamma phase, more and more phosphorous penetrates in it. This helps carry
chromium in ferrite phase.
The mild steel encapsulation was then removed by machining. The slabs, after removal of mild
steel skin, were hot rolled using flat roll and section roll at 900 C to make thin sheets and wires,
respectively. Rolling was carried out very slowly at 900 C with 0.1mm thickness/diameter
reduction per pass. The rolling was done using small laboratory scale rolling mill with 10 cm roll
diameter. The sheets and wires were then vacuum annealed at 950 C for 40 min to relieve the
residual stresses. All the samples prepared this way were characterized in terms of density,
microstructure, hardness, and tensile properties as detailed below. Homogenized slabs as well as
hot rolled and annealed sheets and wires were subjected to metallographic examinations. This
includes volume percentage of porosity and grain size measurements.
The microstructures were taken at the cross-section of the as forged and homogenized slabs as
well as rolled and annealed sheets and wires. Cross-section for sheet in this case is along short
transverse direction. Calculated volume percentage porosities matched with the
metallographically measured volume percentage porosities. Hardness of the hot rolled and
614 Shefali Trivedi, Yashwant Mehta, K. Chandra, P.S. Mishra Vol.8, No.8
annealed wires were measured with Vicker’s hardness tester using 10 kg load. For tensile testing,
samples were either punched out of sheet, or wires were directly tested using Hounsfield tensile
tester. The tensile testing were carried out at room temperature with a cross head speed 1
mm/min. Gauge length of the specimens was 20 mm. Gauge diameter of the tensile sample
(wires) was 1 mm.
Fig.2: Schematic diagram illustrating the production of slab by hot forging of encapsulated
powder mixture.
Vol.8, No.8 Effect of Chromium on the Mechanical Properties 615
Figure 3. Fe–P binary phase diagram (Kubaschewski 1982).
Fig. 4. High temperature gamma loop region of the Fe–P phase diagram (Kubaschewski 1982).
616 Shefali Trivedi, Yashwant Mehta, K. Chandra, P.S. Mishra Vol.8, No.8
Table 1 shows the chemical composition of the two alloys in weight percentage. Volume
percentage porosities were estimated from the measured density of the specimens. These
estimated volume percentage of porosities are recorded in Table 2. In order to verify the
correctness of the estimated volume percentage of porosity, the porosities were also measured
using quantitative metallographic. As forged and homogenized microstructures as well as rolled
and annealed microstructures with the experimentally measured volume percentage of porosity
were recorded and shown in Fig. 5. They are more or less matching with the theoretically
calculated volume percentage porosity. These observations are in agreement with the
theoretically calculated volume percentage porosity which also showed the similar trend. It was
found that volume percentage of porosity in grain interior is higher than grain boundaries. The
wires are made using section rolls. The cross-sections of the thin wires are shown in Fig. 6. The
cross-sections of the rolled wire showed the porosities which were elongated toward the rolling
direction, these are almost rounded porosity. To reveal grain boundaries sample were etched and
are shown in Fig. 6.
Table 1: Chemical composition of Phosphoric alloys (Weight percentage)
Sample P Cr Fe
(a) 0.35 - Balance
(b) 0.35 2 Balance
Table 2: Calculated volume percentage of porosities of the alloys
Sample N
Rolled a
Porosity in Rolled
annealed wir
calculated usi
measured dens
(vol %)
Porosity in Rolled &
annealed wires, calculate
using quantitative
metallographic technique
(vol %)
(a) 7.272 7.693 7.844 1.9 3.12
(b) 7.272 7.586 7.77 2.36 3.44
Fig. 5. Porosity distribution of the rolled and annealed wires in as polished and unetched
condition (magnification 200X).
Vol.8, No.8 Effect of Chromium on the Mechanical Properties 617
Fig. 6. Cross-section of hot rolled and annealed alloys (etched with 2% nital) revealing grain
structure. Residual alignment of porosity and flattening of pores are observed in all these rolled
and annealed alloys.
Powder metallurgical phosphoric irons 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. This has been confirmed by: Optical Microscope (Fig. 6), Surface
Morphology (SEM) and EDAX Pattern from different Spots (Fig. 7, 8), Composition Image
[Secondary Image] & Line scanning (Fig. 9). The hardness was found to increase with both, P &
Cr. Fig. 10 shows the hardness of different P/M alloys made in this investigation. Combined
effect of chromium and phosphorous causes more hardening of ferrite than that obtained due to
silicon in solid solution with iron [11]. However, porosity also affected the hardness of these
products. The alloy Fe–0.35P–2Cr with only 3.44 vol% porosity showed the hardness of 190
Hv/10 kg. The alloy Fe–0.35P with a porosity of 3.14 vol% showed hardness of 183.4 Hv/10kg.
Had there been similar porosity level of these two alloys, improvement in hardness due to P and
Cr alloying addition could have been realized. However, hardness improvements due to P and Cr
addition were realized truly in the case of two alloys. Fe–P based alloys containing 0.35 wt% P
showed moderate ductility. Tensile properties, such as yield strength, tensile strength, elongation
of these alloys are shown in Table 3.
Fig. 7. Surface Morphology (SEM) and EDAX Pattern from different Spots of sample (a).
(a)(b )
618 Shefali Trivedi, Yashwant Mehta, K. Chandra, P.S. Mishra Vol.8, No.8
Fig. 8. Surface Morphology (SEM) and EDAX Pattern from different Spots of sample (b).
Fig. 9. Composition Image [Secondary Image] & Line scanning of sample (a) and sample (b).
Vol.8, No.8 Effect of Chromium on the Mechanical Properties 619
Figure 10: Graphical Representation of the variation in Hardness of Alloys
Table 3: Mechanical Properties of Fe-P alloys
Sample Proof Stress(MPa)UTS(MPa) Total Elongation (%)
(a) 178 276 6.0
(b) 278.9 356 4.23
The following conclusion can be drawn from the present investigation:
i) Alloys developed in the present investigation have very good hot/cold workability.
ii) Alloys containing Cr showed higher strength (>350MPa) and higher resilience value with
moderate ductility under annealed conditions with scope for developing higher strengths by cold
iii) As forged and homogenized as well as rolled and annealed Fe–P based alloys developed in
the present investigation were characterized using metallographic technique. All the
microstructures show single-phase ferrite grains with porosities well distributed along the grain
boundaries as well as inside the grains.
iv) Chromium improved the strength without much reduction in ductility of sample (b) as
compared to sample (a)
v) Improvement in hardness levels due to the combined addition of Chromium was found.
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