1. NWS
Effect of Dilution on Microstructure and Hardness of a Nickel-base
Hardfacing Alloy Deposit
EFFECT OF DILUTION ON MICROSTRUCTURE
AND HARDNESS
OF A NICKEL-BASE HARDFACING ALLOY
DEPOSIT
C.R. Das, S.K. Albert, V. Ramasubbu, A.K. Bhaduri,
C. Sudha* and A.L.E. Terrance*
Materials Joining Section, Materials Technology Division, and
* Physical Metallurgy Section,
Indira Gandhi Centre for Atomic Research, Kalpakkam – 603 102
Abstract
Nickel-base Colmonoy-6 hardfacing alloy was deposited on an
austenitic stainless steel (SS) substrate using the gas tungsten arc welding
process. This deposit was characterised by hardness measurements,
microstructural examination and study of dilution using electron probe micro
analyser (EPMA). Identification of precipitates was carried out using X-ray
diffraction study of precipitates. The hardness of the first deposited layer was
significantly lower than that of the subsequent deposits. Macro-structural
examination revealed that the precipitate sizes are smaller near the interface
than those in the final deposit layers. The concentration profiles across the
interface using EPMA showed considerable dilution of the deposit by the
austenitic SS substrate material resulting in variation in hardness and
microstructure of the deposit with increasing distance from the
deposit/substrate interface. Also, stress relief heat treatment at 1123 K for 4 h
is found to have no significant effect on the properties of the deposit.
1. Introduction
In the proposed prototype fast breeder reactor (PFBR) austenitic stainless
steel (SS) of type 316L(N) is the main structural material with liquid sodium used
as the coolant for transferring the heat from the reactor to the steam generators to
produce steam. As sodium has very high affinity to oxygen, the liquid sodium
coolant acts as a reducing agent and removes the protective oxide film present on
the SS surfaces of the in-reactor components. Many of the in-reactor components,
which are in contact with each other, are subjected to relative motion during
reactor operation, and their exposure to high operating temperatures (typically
823 K) coupled with high contact stresses can result in self-welding of the clean
metallic mating surfaces when they are in contact for long duration. Further, the
relative movement of the mating surfaces can lead to galling, a form of high-
temperature wear, in which material transfer occurs from one mating surface to
another due to repeated self-welding and breaking at the contact points of the
mating surfaces. To avoid this problem of galling of SS surfaces in liquid sodium
environment, hardfacing of the mating surfaces has been widely used [1].
The high thermal conductivity of sodium causes almost immediate heat
transfer to the surfaces of components exposed to liquid sodium, unlike in cases
where water is used as the heat transfer medium. As a result, higher thermal
stresses are generated in components exposed to liquid sodium than in identical
component exposed to water under similar conditions. The thermal stresses

2. NWS
Effect of Dilution on Microstructure and Hardness of a Nickel-base
Hardfacing Alloy Deposit
generated in hardfaced components are further increased due to the difference
between the thermal expansion coefficients of the substrate material and
hardfacing alloy [2]. Thus it is quite possible that these thermal stresses can be
high enough to cause cracking or spalling at the substrate/deposit interface,
especially if there is lack of metallurgical bonding between the substrate and the
hardfacing alloy deposit. Radiation-induced damage can further aggravate the
integrity of the hardface coatings. Hence, weld deposition techniques like oxy-
acetylene, gas tungsten arc welding (GTAW) or plasma transferred arc welding
(PTAW), which ensure metallurgical bonding between the substrate material and
the hardface coating, are generally preferred for hardfacing of the components
used in liquid sodium cooled fast reactors.
The cobalt-base Stellite alloys are the most widely used hardfacing alloys
for high temperature applications [3]. However, in components hardfaced with Co-
base Stellites the induced radioactivity from Co60
isotope, formed due to (n,γ)
reaction in the reactor environment, can cause enhanced dose to personnel
during their handling, maintenance or decommissioning. Hence, for hardfacing of
many reactor components, Stellite alloys have been replaced with Co-free, nickel-
base Colmonoy alloys. In these Ni-base Colmonoys, chromium borides and
carbides contribute to their high hardness [4], however, their weldability is poor
compared to the Stellites. As the dilution from the substrate material is known to
influence the properties of the hardface deposits, especially when their final
thickness is low, a systematic investigation was carried out to study the effect of
dilution on the microstructure and hardness of a Ni-base Colmonoy-6 hardfacing
alloy deposited on 316L austenitic SS substrate material by the GTAW process.
2. Experiment
Five layers of Colmonoy-6 hardfacing alloy were deposited using 4 mm
diameter rods by GTAW process on a 316LN austenitic SS plate of dimensions
50 × 50 × 25 mm. The chemical composition of the Colmony-6 deposit is given in
Table 1, and the welding parameters used for the deposition are given in Table 2.
Immediately after hardfacing, the deposited plates were covered with vermicular
powder to ensure slow cooling and thus avoiding the formation of any cracks in
the deposit. Subsequently, the deposit was subjected to liquid dye penetrant test
(LPT). Specimens of size 10 × 10 × 33 mm, for metallographic examination and
hardness measurements were cut from the deposit by EDM wire cutting. A Leitz
MeF-4 optical microscope was used for microstructural examinations in the as-
polished condition, as most of the precipitates were visible without etching. The
hardness of the deposit was measured using a Vickers hardness tester at 10 kg
load. Hardness of the various precipitates and matrix was measured using a
Shimadzu HMV 2000 microhardness tester.
Table 1: Chemical composition of the Colmonoy-6 deposit
Element Ni Cr Fe Si B C Co
Weight % 73.5 13.56 4.75 4.25 2.5 0.6 0.24

3. NWS
Effect of Dilution on Microstructure and Hardness of a Nickel-base
Hardfacing Alloy Deposit
Table 2: Welding conditions used during weld deposition of Colmonoy-6
Welding process GTAW
Preheat temperature 673 K (400 °C)
Maximum interpass temperature 423–443 K (150–170 °C)
Filler rod diameter 4.0 mm
Shielding gas Argon
Shielding gas flow rate 10 litres/min
Arc voltage 18 V
Welding current 65 A
Welding speed 2–3 mm.s-1
One of the deposit specimens was heat treated at 1123 K for 4 h, to
simulate the stress relieving (SR) heat treatment after hardfacing as
recommended during the fabrication of some of the PFBR components. To study
the effect of this SR heat treatment, the hardness profile across this heat-treated
deposit was also determined.
A Cameca SX-50 electron probe micro analyser (EPMA) with wavelength
dispersive spectrometer was used for analysing the dilution of the Colmonoy-6
deposit from the 316L SS substrate. For this purpose, elemental line scans for Fe,
Ni, Cr, B and C were made across the substrate/deposit interface. The EPMA was
also employed to identify the borides and carbide present in the deposit from the
counts for B and C.
Next, the precipitates in the deposit were extracted using an
electrochemical extraction process in which a pure weld deposit, weighing around
3-5 g, was taken as the anode and platinum mesh was used as the cathode. The
electrolyte used was 10%HCl in methanol, and the extraction was carried out at a
potential of 1.5 V DC for a duration of 6-8 h. X-ray diffraction pattern of these
precipitates were taken using a Rigaku Miniflex X-ray diffractometer.
3. Results
3.1 Microstructure
The microstructures of the deposit, in the as-polished condition, at various
distances from the substrate/deposit interface (Figs. 1a-1d) show considerable
variation in the microstructure of the deposit with increasing distance from
substrate/deposit interface The volume fraction, size and distribution of the
precipitates are significantly different near the interface (Fig. 1a) compared to that
further away from the interface (Figs. 1c and 1d). Near the interface, the
microstructure comprises of a large number of small precipitates with needle-like
and blocky morphologies. As the distance from interface increases, large
precipitates begin to appear. At locations in the deposit sufficiently away from the
interface, many needle-like precipitates, having an aspect ratio considerably

4. NWS
Effect of Dilution on Microstructure and Hardness of a Nickel-base
Hardfacing Alloy Deposit
higher than those present near the interface, are observed (Fig. 1b) along with
many large blocky precipitates which are often observed in clusters (Fig.1d).
Fig. 1: Microstructure of the Colmonoy-6 deposit with increasing distance from
the 316LNsubstrate/deposit interface: (a) at the interface, (b) 0.3 mm from
the interface, (c) 3.6 mm from the interface, and (d) 6.6 mm from the
interface.
3.2 Hardness
The hardness profiles across the substrate/deposit interface in the as-
deposited and SR conditions (Fig. 2) show no significant difference indicating that
the SR heat treatment does not affect the deposit microstructure and hence its
hardness. Figure 2 also shows that there is a steps-wise hardness increase in the
deposit. In the as-deposited condition, the minimum hardness in the deposit of
~575 VHN is observed near the interface over a distance of ~2.5 mm. The
hardness of the deposit over the next ~2.5 mm is ~650 VHN and ~800 VHN over
the remaining thickness of the deposit. A very similar variation in hardness is
observed in the deposit after the SR heat treatment. Microhardness
measurements also showed that the hardness of the matrix is 500–550 VHN,
while those of needle-like and blocky precipitates are 1000– 1250 and 2300–
2800 VHN, respectively.
3.3 Dilution
Figure 3 shows the typical line profiles for Fe and Ni across the deposit/
substrate interface obtained by EPMA. In the Colmonoy-6 deposit, the Fe counts
over a distance of up to ~2.5 mm from the interface (~75 counts) are higher than
those for the rest of the deposit (< 50 counts), while the Ni counts near the
interface (~400 counts) are lower than those for the rest of the deposit (~500
(a)
50 µµµµm
(b)
50 µµµµm
50 µµµµm
(c)
(d)
50 µµµµm
Carbide
Boride
Eutectic

5. NWS
Effect of Dilution on Microstructure and Hardness of a Nickel-base
Hardfacing Alloy Deposit
counts). The line profiles of both Fe and Ni indicate that dilution from the austenitic
SS substrate alters the chemistry of the deposit very significantly up to a distance
of ~2.5 mm from the deposit/substrate interface.
Fig. 2: Hardness profiles across the 316LN SS/Colmonoy-6-deposit interface in
the as-deposited and stress-relieved (SR) conditions.
Fig. 3: Typical EPMA line profiles for (a) Fe and (b) Ni across the interface
between Colmonoy- 6 deposit (left) and 316LN SS substrate (right).
-4 -2 0 2 4 6 8
100
200
300
400
500
600
700
800
900
Endofdilution
Fusionline
316L(N)
stainless steel
Colmonoy-6
weld deposit
As weld
SR,850/4hr
Hardness(VHN)load=10kg
Distance across the interface
-7 -6 -5 -4 -3 -2 -1 0 1
0
50
100
150
200
250
300
Countsforiron
Distance across the interface (mm)
-7 -6 -5 -4 -3 -2 -1 0 1
0
100
200
300
400
500
600
CountsforNi
Distance across the interafce (mm)
(a
(b

6. NWS
Effect of Dilution on Microstructure and Hardness of a Nickel-base
Hardfacing Alloy Deposit
3.4 Identification of precipitates
Results of EPMA line scan across large needle-like and blocky precipitates
are shown in Figs. 4(a) and 4(b), respectively, in which different scales have been
used to plot for C or B in view of its markedly different count levels compared to
those for Ni, Cr and Fe. Figure 4 shows that both these precipitates are rich in Cr
and lean in Ni. The needle-like precipitates contain C but no B, and hence are
identified as a type of chromium carbide. Likewise, the blocky precipitates contain
B but little C, and hence are identified as a type of chromium boride.
Fig. 4: EPMA line scans across (a) large needle-like precipitates and (b) blocky
precipitates
0 2 4 6 8 10 12 14 16
0
100
200
300
400
500
600
CountsforC
Ni, Fe, Cr, B
CountsforFe,Ni,Cr,B
Distance across the precipitates (µm)
0
10
20
30
40
50
60
C
0 5 10 15 20
0
100
200
300
400
500
600
Fe, Ni, Cr, C
CountsforFe,Ni,Cr,C
Distance across the precipitates (µm)
0
10
20
30
CountsforB
B
(a)
(b)

7. NWS
Effect of Dilution on Microstructure and Hardness of a Nickel-base
Hardfacing Alloy Deposit
The X-ray diffraction pattern of the electrochemically extracted precipitates
(Fig. 5) show that the precipitates are mainly of the types Cr5B3, Cr7C3, Cr3B4,
CrB4 and CrB. Also, the diffraction pattern suggests that precipitates like
Fe1.1Cr0.9B0.9 may also be present.
Fig. 5: X-ray diffraction pattern of the electrochemically extracted precipitates
4. Discussion
The results from the EPMA analysis clearly show that the deposit up to a
distance of 2.5 mm from deposit/substrate interface contains higher Fe and lower
Ni than the rest of the deposit. This indicates that at least the first layer of the
deposit of 2–3 mm thickness is highly diluted by the substrate material. Both the
hardness and the microstructure of the deposit in this 2.5 mm diluted zone are
affected by the dilution from the substrate material. The hardness profile clearly
indicates that in this 2.5 mm diluted zone of the deposit the hardness is the lowest
with very little hardness variation. The EPMA analysis also confirms that the
microstructural variations observed in the deposit with increasing distance from
the deposit/substrate interface is clearly due to effects of dilution of the deposit by
the substrate material.
While the hardness of undiluted Colmonoy-6 deposit reported by the
manufacturer [5] is typically RC 56–RC (corresponding to 620–720 VHN), the
hardness of the first deposit layer is significantly lower at 570 VHN (or RC 53).
Beyond the 2.5 mm diluted zone in the deposit, the hardness increases and is
comparable to that reported for the undiluted deposit. Thus, it is clear that the
dilution, which is the highest in the first deposit layer, decreases in the subsequent
deposit layers resulting in consequent increase in hardness across the deposit as
the undiluted deposit layers is approached.
20 30 40 50 60 70 80 90
0
200
400
600
800
1000
1200
CrB
Cr3
B4
Fe1.1
Cr0.9
B0.9
CrB
Cr7
C3
CrB
CrB4
CrBCr5
C3
Cr7
C3
Cr5
B3
Intensity
2 theta

8. NWS
Effect of Dilution on Microstructure and Hardness of a Nickel-base
Hardfacing Alloy Deposit
As the Ni-base Colmonoy-6 hardfacing alloy has been chosen as a
replacement of the widely used Co-base Stellite-6 hardfacing alloy, it is pertinent
that the dilution effects in deposits of these two class of hardfacing alloys be
compared. In Stellite-6 hardface deposits, made using the same GTAW process,
the dilution and hardness variation were observed to be considerably lower [2,6]
than in Colmonoy-6 deposits. This can be attributed to the fact that influence on
the microstructure and properties due to compositional changes caused by
dilution in a Ni-base Colmonoy deposit is significantly more than in the Co-base
Stellite deposits. Further, as the melting point of a Ni-base Colmonoy alloy
(typically 1223–1338 K) is much lower than in a Co-base Stellite alloy (typically
1553–1663 K), the large differential in the melting temperatures between the
Colmonoy alloy and the substrate alloy (typically 1665–1717 K for austenitic SS)
adversely affects the degree dilution in the Ni-base Colmonoy hardfacing deposit.
In a Colmonoy-6 deposit, the borides and carbides, formed during post-
solidification cooling of the deposit, contribute to the high hardness of the deposit.
The EPMA analysis showed that the needle-like precipitates are chromium
carbides and the blocky precipitates are chromium borides, with the borides being
significantly harder than the carbides. Also, microstructural observations showed
that the size and volume fraction of the precipitates present in the deposit are
significantly influenced by deposit dilution by the substrate alloy.
For the various PFBR components, an 1123 K/4 h SR heat treatment has
been recommended after hardfacing to achieve final dimensional tolerances in the
components after machining. In the absence of this SR heat treatment, machining
may lead to stress relief and result in dimensional changes beyond the acceptable
tolerance limits. Further, this SR heat treatment would also reduce any risk of
cracking in the deposits during subsequent machining. The results of hardness
measurement show the SR heat treatment does not affect the deposit hardness.
5. Practical Implications
The present study has shown that dilution of the deposit and the
consequent changes in microstructure and hardness in the deposit are significant
more in the Ni-base Colmonoy alloy, compared to the Co-base Stellite alloy that it
replaces as a hardfacing material for in-reactor applications. Hence, due
consideration has to be given to the dilution effects while finalising the deposition
process and the post-machined deposit thickness. In the case of Stellites, a post-
machining GTAW deposit thickness of 1.5 mm is adequate to achieve the
hardness of its undiluted deposit. However, Colmonoy deposits of the same
thickness and made by the same deposition process would have a hardness
much lower than that of its undiluted deposit. As the hardness of undiluted
Colmonoy deposit is higher than that of the Stellite deposit at room temperature,
the hardness of the Colmonoy deposit even after dilution can be high enough for
most applications. However, with increase in temperature, the reduction in
hardness in Ni base hardfacing alloy is significantly higher than in Co base
hardfacing alloy [7]. Thus, at service temperatures, the hardness of the diluted
Colmonoy deposit can reduce to values lower than that of Stellites. Hence, it may
be required to either increase the thickness of the post-machined Colmonoy
deposit or opt for a low dilution processes for deposition of Colmonoy-6.

9. NWS
Effect of Dilution on Microstructure and Hardness of a Nickel-base
Hardfacing Alloy Deposit
6.0 Conclusions
1. The effect of dilution from the substrate material is significant in Colmonoy
deposits made by the GTAW process, and this leads to significant changes
in its microstructure and hardness up to a deposit thickness of ~2.5 mm.
2. The primary precipitates present in the Colmonoy-6 deposit are chromium
borides having blocky morphology and chromium carbides with needle-like
morphology. Dilution of the deposit by the substrate material affects the
size and volume fraction of these precipitates.
3. Stress relief heat treatment was found to have no adverse effect on the
microstructure and hardness of the Comonoy-6 deposit.
Acknowledgement
Shri L. Arumugam, Zonal WorkShop, H&CD, EDG, IGCAR who carried out
the Colmonoy weld deposition, is gratefully acknowledged. Authors would like to
thank Shri G. Kempulraj of H&CD, EDG, IGCAR for his help during this work.
Authors also thank Dr. S. K. Ray, Head, Material Technology Division, IGCAR for
his encouragement and support during this work.
Reference
1. J. Y. Chang, S. L. Schrock and R. N. Johnson, Microstructural Science, Vol. 6,
p.32.
2. R. A. Douty and H. Schwartzbart, Welding Jounal, pp. 406s-416s, August 1972
3. K. C. Antony, J. Metals 35 (1983) 52-60
4. Metals hand book, Ninth Edition, Vol. 6, ASM International, Ohio, USA1978.
5. Technical data sheet No. Ni-1.1B, Wall Colmonoy Corporation, MI 48071-
1650, :USA, 1990
6. S. K. Albert, I. Gowrisankar, V. Seetharaman and S. Venkadesan, Proc. of
National Welding Seminer, Bangalore, pp. A1-A7, 1987.
7. Tribaloy Product Catalogue, Deloro Stellite Lomited, Swindon, UK.