1892 - Indira Gandhi Centre for Atomic Research

Trans. Indian Inst. Met.
Vol.57, No. 3, June 2004, pp. 271-276
TP 1892
RESIDUAL STRESS DISTRIBUTION IN
HARDFACED AUSTENITIC STAINLESS STEEL
SLEEVES
H.C.Dey 1 , A.K.Bhaduri 1 , S.Mahadevan 2 , G.K.Sharma 2 , T.Jayakumar 2 , V.Shankar 1 and B.P.S.Rao 3
1
Materials Joining Section, Materials Technology Division,
2
Non-destructive Evaluation Section, Division for PIE and NDT Development, and
3
Manufacturing Technology, PFBR Construction Group,
Indira Gandhi Centre for Atomic Research
Kalpakkam – 603102
(Received 21 January 2004 ; in revised form 2 April 2004)
ABSTRACT
The effect of thermal cycling between 473 and 823 K on the residual stress distribution of a nickel-base
hardfacing alloy deposited on an austenitic stainless steel sleeve was studied in the as-deposited and stress
relieved conditions. Thermal cycling was found to affect the peak value and gradient in compressive residual
stresses across the hardface deposits. Stress relieving heat treatment was found to have a marginal effect on
the residual stress distribution across the deposit subjected to thermal cycling.
1. INTRODUCTION
In sodium cooled fast breeder reactors, the high
operating temperature necessitates hardfacing of
austenitic stainless steel (SS) components to avoid
galling and also to reduce the wear loss of the base
materials. Nickel-base cobalt-free alloy Colmonoy-5
(or its equivalent Delero-50 or WT-50) has been
chosen as the hardfacing material for the Prototype
Fast Breeder Reactor (PFBR) components for
minimizing the dose rate to personnel during
handling, maintenance and decommissioning. 1 One
of the critical components, the grid plate (GP) sleeves
made of 316L(N) SS that holds the core
subassemblies, are hardfaced to prevent galling and
also to minimize wear caused by subassembly
insertion/removal and erosion due to high sodium
velocity at 670 K. The hardface deposit on the GP
sleeves must have good thermal shock resistance for
reliable operation during the reactor’s design life of
40 years, during which they would be subjected to
a large number of thermal cycles due to shut downs
and reactor scrams.
Hardface deposits on austenitic SS, made by arc
welding processes like plasma transferred arc welding
(PTAW), tungsten arc welding (TIG) etc., which
are carried out at relatively high temperatures,
generates residual stresses due to differential
shrinkage of the molten deposit and heated base
metal. These residual stresses are generated because
of process-induced thermal gradients and difference
in coefficients of thermal expansion between the
deposit and substrate material. However, the
magnitude and distribution of the residual stresses
vary depending on the preheat temperature, coating
thickness, heat input, deposition process, and the
geometry of the components. Preheating reduces
shrinkage stresses and eliminates risk of micro
cracking and distortions, 2 and in-plane residual
stresses change from tensile to compressive with
increase in preheating temperature. 3 In contrast, with
increase in coating thickness the in-plane residual
stresses change from compressive to tensile. 3–5
Residual stresses are known to affect the performance
of hardfaced components. Tensile residual stresses
perpendicular to the interface may cause delamination

TRANS. INDIAN INST. MET., VOL. 57, NO. 3, JUNE 2004
of the hardface coating, while locked-in residual
stresses may increase the risk of micro cracking
during thermal cycling in low-ductility coatings.
Further, high compressive residual stresses parallel
to the surface improves the tribological behaviour of
the deposit, 6 and a compressive pre-stress to the
deposit beneficially balances the tensile thermal stress
during high temperature application. 3 Also, the
magnitude and distribution of residual stresses
depends on the maximum temperature, holding time,
and heating/ cooling rate during thermal cycling.
The GP sleeves of PFBR are to be hardfaced at two
locations where it comes in contact with the core
subassembly (Fig. 1) – one on the top chamfered
portion and the other on the inner diameter 470 mm
from top. As thinner coatings have better thermal
shock resistance due to reduced accumulated residual
stresses, the low-dilution PTAW process was chosen.
To eliminate the risk of micro cracking and
delamination of the deposit, and to minimize the
magnitude of residual stress, an optimised PTAW
deposition procedure has been qualified. In this work,
the effect of thermal cycling during service on the
residual stress distribution of hardfaced austenitic
SS sleeves has been studied.
2. EXPERIMENTAL PROCEDURE
Hardfacing on the inside of the GP sleeve was
simulated by hardfacing a 316 SS sleeve (106 mm
OD x 80 mm ID x 45 mm height) on its inside
surface with nickel base WT-50 hardfacing alloy
powder (composition given in Table 1) using the
PTAW process (Fig. 2). After hardfacing, the sleeve
was cooled slowly in vermiculite powder, machined
to the required thickness of 1.5 mm, and examined
by liquid penetrant test (LPT).
The hardfaced sleeve was cut into two halves along
AB (Fig. 2). One half (with location C at the middle)
was stress relieved at 1123 K for 35 minutes, using
a heating rate of 150 K h –1 and holding time of 2.5
Fig. 1 : Drawing of grid plate sleeve, showing the two
hardfacing locations
min per mm of thickness, as per the specification. 7
The other half of the sleeve (with location D at the
middle) was retained in the as-deposited condition,
for comparison. Both the sleeve-halves were then
subjected to thermal cycling between 473 and 823 K
Table 1
CHEMICAL COMPOSITION OF WT-50 HARDFACING ALLOY POWDER USED
Elements C Si Cr Fe B Others Ni
Wt.-% 0.46 3.88 10.59 3.00 2.34 < 0.5 Balance
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DEY, et al., : RESIDUAL STRESS DISTRIBUTION IN HARDFACED AUSTENITIC STAINLESS STEEL
SLEEVES
for 20 cycles, with holding duration of 1 h at both
the temperatures and immediate transfer of samples
between furnaces maintained at the two temperatures.
The thermal cycling temperatures of 473 and 823 K
that were used corresponds to the minimum and
maximum temperature of liquid sodium that would
be encountered.
Residual stress measurements were carried out by
X-ray diffraction (XRD) technique on the substrate
and across the coating. For the austenitic SS substrate,
residual stress measurements were carried out using
Cr-K
radiation with the X-ray tube operating at
30kV and 7mA target current. The fcc austenite
structure gives a diffraction peak (2) at 148.5 o ,
corresponding to the (311) plane. The effect of peak
shift at various tilt-angles () was related to the
change in inter-planar (‘d’) spacing of the (311)
plane for residual stress measurements.
For nickel base hardfacing alloy, residual stress
measurement were carried out using Cu-K radiation
with the X-ray tube operating at 30kV and 7mA
target current. The scan range for the peak detection
(2) was between 140 o and 149 o . For both the halfsleeves,
in-plane residual stress measurements were
carried out in axial direction across the coating at
three locations (Fig. 3) in the as-deposited and stressrelieved
conditions, and after 5 and 20 thermal cycles.
For the austenitic SS substrate, residual stress
Fig. 2 : Nickel base WT-50 alloy hardfaced 316 stainless steel sleeve
Fig. 3 : One half of the hardfaced sleeve, with the arrow showing the direction of residual stress measurements
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TRANS. INDIAN INST. MET., VOL. 57, NO. 3, JUNE 2004
measurements were carried out in the as-deposited
condition only.
Optical metallographic examination of the as
deposited WT-50 hardface coating was carried out
to study the microstructure of the deposit and the
deposit/substrate interface. Microhardness
measurements were also carried out across deposit/
substrate interface at intervals of 0.2 mm at a load
of 200 g.
3. RESULTS AND DISCUSSION
The residual stress values measured in the radial
direction on the top surface of the austenitic SS
substrate in the as-deposited condition, corresponding
to the locations A, B, C and D in Fig.2, are given
in Table 2. The results show that the stress values
are very low as the deposit is significantly (~7 mm)
below the top surface.
The residual stresses measured across the hardface
deposit in axial directions on both half-sleeves (Fig. 3)
in the as deposited and stress relieved conditions,
and after thermal cycling are given in Figs.4 and 5.
The results show very high values of compressive
residual stress in the as deposited condition at all the
three locations across the deposit width. The
compressive residual stresses across the deposit are
caused by the low coefficient of thermal expansion
of the WT-50 deposit material (14–15 m m –1 K –1 )
as compared to 316 SS substrate material (17–
18 m m –1 K –1 ). During cooling after deposition, the
austenitic SS substrate will shrink more due to its
higher coefficient of thermal expansion. Therefore,
tensile residual stresses develop in the substrate and
balancing compressive residual stresses develop in
the deposit. The residual stress at the centre of the
deposit (location 2 in Fig. 3) is higher than those at
the periphery of the deposit (locations 1 and 3 in
Fig. 3), because the total restraint of the substrate
and deposit is higher at the centre than at the
periphery.
Fig. 4 : Residual stress profile across the coating in halfsleeve
‘C’ in as deposited, stress relieved and thermal
cycled conditions.
Fig. 5 : Residual stress profile across the coating in the halfsleeves
‘D’ in as deposited, and thermal cycled
conditions
Stress relieving heat treatment at 1123 K (half-sleeve
C) is found to significantly the compressive residual
stresses (110–200 MPa) reduce across the hardface
deposit at all locations (Fig. 4), as the tensile thermal
stresses generated during the stress-relieving heat
treatment offsets the compressive stresses present in
the as-deposited condition.
Table 2
RESIDUAL STRESS ON THE SURFACE OF THE AUSTENITIC SS SUBSTRATE MEASURED IN RADIAL DIRECTION
CORRESPONDING TO THE LOCATIONS SHOWN IN FIG.2.
Locations A B C D
Residual stress (MPa) –12 ± 23 15 ± 18 18 ± 26 –22 ± 20
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DEY, et al., : RESIDUAL STRESS DISTRIBUTION IN HARDFACED AUSTENITIC STAINLESS STEEL
SLEEVES
Thermal cycling reduces both the peak compressive
residual stress and the residual stress gradient across
the deposit (Figs. 4 and 5). Repeated expansion and
contraction during thermal cycling results in local
yielding thereby relaxing the prior residual stresses
and smoothening the residual stress distribution. It
is observed that after thermal cycling, there is an
increase in compressive residual stresses at peripheral
locations in the stress-relieved deposit. Differential
shrinkage between the coating and the substrate,
which depends on the cooling rate and the difference
in thermal expansion coefficients, increases the
compressive residual stress. On the other hand, local
yielding decreases the compressive residual stress.
The combined effect of these two factors results in
the observed changes in the residual stress in the
peripheral locations. However, these changes in the
residual stress distribution on thermal cycling did
not have any adverse effect on the integrity of the
deposit, as LPT, ultrasonic testing and radiography
of the hardfaced sleeves did not show any cracking
either in the deposit or at the deposit/substrate
interface.
Fig. 7 :
Micrograph of the WT- 50 hardface coating
The microstructure across the deposit/substrate
interface (Fig. 6) reveals a narrow dilution zone and
a planar interface. It also shows the presence of
dispersed precipitates of borides and carbides, 8 which
are responsible for the high hardness and wear
resistance of this hardfacing alloy. No appreciable
variation in the microstructural features is observed
in the deposit from the interfacial region to the interior
of the deposit. The microstructure of the hardface
deposit at higher magnification (Fig. 7) clearly reveals
Fig. 8 : Microhardness profile across 316 SS substrate/WT-
50 deposit interface in the as-deposited condition.
the dendritic structure with dispersed precipitates in
the inter-dendritic regions.
The microhardness profile across the substrate/deposit
interface (Fig. 8) shows that the hardness increases
appreciably from ~290 VHN at the interface to
~410 VHN in the deposit across a distance of
0.2 mm. The short distance over which the hardness
increases across the interface is indicative of the
narrow dilution zone in this PTAW deposit.
4. CONCLUSIONS
Fig. 6 : Micrograph across WT- 50/316 SS interface in asdeposited
condition
(1) Thermal cycling reduces the peak value and
gradient in compressive residual stress across
the hardface deposit in both the as-deposited
and stress-relieved conditions.
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TRANS. INDIAN INST. MET., VOL. 57, NO. 3, JUNE 2004
(2) Stress relieving heat treatment has a marginal
effect on the residual stress distribution across
the deposit subjected to thermal cycling.
(3) The integrity of the hardface deposit is not
affected on thermal cycling.
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