1892 - Indira Gandhi Centre for Atomic Research
1892 - Indira Gandhi Centre for Atomic Research
1892 - Indira Gandhi Centre for Atomic Research
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Trans. Indian Inst. Met.<br />
Vol.57, No. 3, June 2004, pp. 271-276<br />
TP <strong>1892</strong><br />
RESIDUAL STRESS DISTRIBUTION IN<br />
HARDFACED AUSTENITIC STAINLESS STEEL<br />
SLEEVES<br />
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<br />
1<br />
Materials Joining Section, Materials Technology Division,<br />
2<br />
Non-destructive Evaluation Section, Division <strong>for</strong> PIE and NDT Development, and<br />
3<br />
Manufacturing Technology, PFBR Construction Group,<br />
<strong>Indira</strong> <strong>Gandhi</strong> <strong>Centre</strong> <strong>for</strong> <strong>Atomic</strong> <strong>Research</strong><br />
Kalpakkam – 603102<br />
(Received 21 January 2004 ; in revised <strong>for</strong>m 2 April 2004)<br />
ABSTRACT<br />
The effect of thermal cycling between 473 and 823 K on the residual stress distribution of a nickel-base<br />
hardfacing alloy deposited on an austenitic stainless steel sleeve was studied in the as-deposited and stress<br />
relieved conditions. Thermal cycling was found to affect the peak value and gradient in compressive residual<br />
stresses across the hardface deposits. Stress relieving heat treatment was found to have a marginal effect on<br />
the residual stress distribution across the deposit subjected to thermal cycling.<br />
1. INTRODUCTION<br />
In sodium cooled fast breeder reactors, the high<br />
operating temperature necessitates hardfacing of<br />
austenitic stainless steel (SS) components to avoid<br />
galling and also to reduce the wear loss of the base<br />
materials. Nickel-base cobalt-free alloy Colmonoy-5<br />
(or its equivalent Delero-50 or WT-50) has been<br />
chosen as the hardfacing material <strong>for</strong> the Prototype<br />
Fast Breeder Reactor (PFBR) components <strong>for</strong><br />
minimizing the dose rate to personnel during<br />
handling, maintenance and decommissioning. 1 One<br />
of the critical components, the grid plate (GP) sleeves<br />
made of 316L(N) SS that holds the core<br />
subassemblies, are hardfaced to prevent galling and<br />
also to minimize wear caused by subassembly<br />
insertion/removal and erosion due to high sodium<br />
velocity at 670 K. The hardface deposit on the GP<br />
sleeves must have good thermal shock resistance <strong>for</strong><br />
reliable operation during the reactor’s design life of<br />
40 years, during which they would be subjected to<br />
a large number of thermal cycles due to shut downs<br />
and reactor scrams.<br />
Hardface deposits on austenitic SS, made by arc<br />
welding processes like plasma transferred arc welding<br />
(PTAW), tungsten arc welding (TIG) etc., which<br />
are carried out at relatively high temperatures,<br />
generates residual stresses due to differential<br />
shrinkage of the molten deposit and heated base<br />
metal. These residual stresses are generated because<br />
of process-induced thermal gradients and difference<br />
in coefficients of thermal expansion between the<br />
deposit and substrate material. However, the<br />
magnitude and distribution of the residual stresses<br />
vary depending on the preheat temperature, coating<br />
thickness, heat input, deposition process, and the<br />
geometry of the components. Preheating reduces<br />
shrinkage stresses and eliminates risk of micro<br />
cracking and distortions, 2 and in-plane residual<br />
stresses change from tensile to compressive with<br />
increase in preheating temperature. 3 In contrast, with<br />
increase in coating thickness the in-plane residual<br />
stresses change from compressive to tensile. 3–5<br />
Residual stresses are known to affect the per<strong>for</strong>mance<br />
of hardfaced components. Tensile residual stresses<br />
perpendicular to the interface may cause delamination
TRANS. INDIAN INST. MET., VOL. 57, NO. 3, JUNE 2004<br />
of the hardface coating, while locked-in residual<br />
stresses may increase the risk of micro cracking<br />
during thermal cycling in low-ductility coatings.<br />
Further, high compressive residual stresses parallel<br />
to the surface improves the tribological behaviour of<br />
the deposit, 6 and a compressive pre-stress to the<br />
deposit beneficially balances the tensile thermal stress<br />
during high temperature application. 3 Also, the<br />
magnitude and distribution of residual stresses<br />
depends on the maximum temperature, holding time,<br />
and heating/ cooling rate during thermal cycling.<br />
The GP sleeves of PFBR are to be hardfaced at two<br />
locations where it comes in contact with the core<br />
subassembly (Fig. 1) – one on the top chamfered<br />
portion and the other on the inner diameter 470 mm<br />
from top. As thinner coatings have better thermal<br />
shock resistance due to reduced accumulated residual<br />
stresses, the low-dilution PTAW process was chosen.<br />
To eliminate the risk of micro cracking and<br />
delamination of the deposit, and to minimize the<br />
magnitude of residual stress, an optimised PTAW<br />
deposition procedure has been qualified. In this work,<br />
the effect of thermal cycling during service on the<br />
residual stress distribution of hardfaced austenitic<br />
SS sleeves has been studied.<br />
2. EXPERIMENTAL PROCEDURE<br />
Hardfacing on the inside of the GP sleeve was<br />
simulated by hardfacing a 316 SS sleeve (106 mm<br />
OD x 80 mm ID x 45 mm height) on its inside<br />
surface with nickel base WT-50 hardfacing alloy<br />
powder (composition given in Table 1) using the<br />
PTAW process (Fig. 2). After hardfacing, the sleeve<br />
was cooled slowly in vermiculite powder, machined<br />
to the required thickness of 1.5 mm, and examined<br />
by liquid penetrant test (LPT).<br />
The hardfaced sleeve was cut into two halves along<br />
AB (Fig. 2). One half (with location C at the middle)<br />
was stress relieved at 1123 K <strong>for</strong> 35 minutes, using<br />
a heating rate of 150 K h –1 and holding time of 2.5<br />
Fig. 1 : Drawing of grid plate sleeve, showing the two<br />
hardfacing locations<br />
min per mm of thickness, as per the specification. 7<br />
The other half of the sleeve (with location D at the<br />
middle) was retained in the as-deposited condition,<br />
<strong>for</strong> comparison. Both the sleeve-halves were then<br />
subjected to thermal cycling between 473 and 823 K<br />
Table 1<br />
CHEMICAL COMPOSITION OF WT-50 HARDFACING ALLOY POWDER USED<br />
Elements C Si Cr Fe B Others Ni<br />
Wt.-% 0.46 3.88 10.59 3.00 2.34 < 0.5 Balance<br />
272
DEY, et al., : RESIDUAL STRESS DISTRIBUTION IN HARDFACED AUSTENITIC STAINLESS STEEL<br />
SLEEVES<br />
<strong>for</strong> 20 cycles, with holding duration of 1 h at both<br />
the temperatures and immediate transfer of samples<br />
between furnaces maintained at the two temperatures.<br />
The thermal cycling temperatures of 473 and 823 K<br />
that were used corresponds to the minimum and<br />
maximum temperature of liquid sodium that would<br />
be encountered.<br />
Residual stress measurements were carried out by<br />
X-ray diffraction (XRD) technique on the substrate<br />
and across the coating. For the austenitic SS substrate,<br />
residual stress measurements were carried out using<br />
Cr-K <br />
radiation with the X-ray tube operating at<br />
30kV and 7mA target current. The fcc austenite<br />
structure gives a diffraction peak (2) at 148.5 o ,<br />
corresponding to the (311) plane. The effect of peak<br />
shift at various tilt-angles () was related to the<br />
change in inter-planar (‘d’) spacing of the (311)<br />
plane <strong>for</strong> residual stress measurements.<br />
For nickel base hardfacing alloy, residual stress<br />
measurement were carried out using Cu-K radiation<br />
with the X-ray tube operating at 30kV and 7mA<br />
target current. The scan range <strong>for</strong> the peak detection<br />
(2) was between 140 o and 149 o . For both the halfsleeves,<br />
in-plane residual stress measurements were<br />
carried out in axial direction across the coating at<br />
three locations (Fig. 3) in the as-deposited and stressrelieved<br />
conditions, and after 5 and 20 thermal cycles.<br />
For the austenitic SS substrate, residual stress<br />
Fig. 2 : Nickel base WT-50 alloy hardfaced 316 stainless steel sleeve<br />
Fig. 3 : One half of the hardfaced sleeve, with the arrow showing the direction of residual stress measurements<br />
273
TRANS. INDIAN INST. MET., VOL. 57, NO. 3, JUNE 2004<br />
measurements were carried out in the as-deposited<br />
condition only.<br />
Optical metallographic examination of the as<br />
deposited WT-50 hardface coating was carried out<br />
to study the microstructure of the deposit and the<br />
deposit/substrate interface. Microhardness<br />
measurements were also carried out across deposit/<br />
substrate interface at intervals of 0.2 mm at a load<br />
of 200 g.<br />
3. RESULTS AND DISCUSSION<br />
The residual stress values measured in the radial<br />
direction on the top surface of the austenitic SS<br />
substrate in the as-deposited condition, corresponding<br />
to the locations A, B, C and D in Fig.2, are given<br />
in Table 2. The results show that the stress values<br />
are very low as the deposit is significantly (~7 mm)<br />
below the top surface.<br />
The residual stresses measured across the hardface<br />
deposit in axial directions on both half-sleeves (Fig. 3)<br />
in the as deposited and stress relieved conditions,<br />
and after thermal cycling are given in Figs.4 and 5.<br />
The results show very high values of compressive<br />
residual stress in the as deposited condition at all the<br />
three locations across the deposit width. The<br />
compressive residual stresses across the deposit are<br />
caused by the low coefficient of thermal expansion<br />
of the WT-50 deposit material (14–15 m m –1 K –1 )<br />
as compared to 316 SS substrate material (17–<br />
18 m m –1 K –1 ). During cooling after deposition, the<br />
austenitic SS substrate will shrink more due to its<br />
higher coefficient of thermal expansion. There<strong>for</strong>e,<br />
tensile residual stresses develop in the substrate and<br />
balancing compressive residual stresses develop in<br />
the deposit. The residual stress at the centre of the<br />
deposit (location 2 in Fig. 3) is higher than those at<br />
the periphery of the deposit (locations 1 and 3 in<br />
Fig. 3), because the total restraint of the substrate<br />
and deposit is higher at the centre than at the<br />
periphery.<br />
Fig. 4 : Residual stress profile across the coating in halfsleeve<br />
‘C’ in as deposited, stress relieved and thermal<br />
cycled conditions.<br />
Fig. 5 : Residual stress profile across the coating in the halfsleeves<br />
‘D’ in as deposited, and thermal cycled<br />
conditions<br />
Stress relieving heat treatment at 1123 K (half-sleeve<br />
C) is found to significantly the compressive residual<br />
stresses (110–200 MPa) reduce across the hardface<br />
deposit at all locations (Fig. 4), as the tensile thermal<br />
stresses generated during the stress-relieving heat<br />
treatment offsets the compressive stresses present in<br />
the as-deposited condition.<br />
Table 2<br />
RESIDUAL STRESS ON THE SURFACE OF THE AUSTENITIC SS SUBSTRATE MEASURED IN RADIAL DIRECTION<br />
CORRESPONDING TO THE LOCATIONS SHOWN IN FIG.2.<br />
Locations A B C D<br />
Residual stress (MPa) –12 ± 23 15 ± 18 18 ± 26 –22 ± 20<br />
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DEY, et al., : RESIDUAL STRESS DISTRIBUTION IN HARDFACED AUSTENITIC STAINLESS STEEL<br />
SLEEVES<br />
Thermal cycling reduces both the peak compressive<br />
residual stress and the residual stress gradient across<br />
the deposit (Figs. 4 and 5). Repeated expansion and<br />
contraction during thermal cycling results in local<br />
yielding thereby relaxing the prior residual stresses<br />
and smoothening the residual stress distribution. It<br />
is observed that after thermal cycling, there is an<br />
increase in compressive residual stresses at peripheral<br />
locations in the stress-relieved deposit. Differential<br />
shrinkage between the coating and the substrate,<br />
which depends on the cooling rate and the difference<br />
in thermal expansion coefficients, increases the<br />
compressive residual stress. On the other hand, local<br />
yielding decreases the compressive residual stress.<br />
The combined effect of these two factors results in<br />
the observed changes in the residual stress in the<br />
peripheral locations. However, these changes in the<br />
residual stress distribution on thermal cycling did<br />
not have any adverse effect on the integrity of the<br />
deposit, as LPT, ultrasonic testing and radiography<br />
of the hardfaced sleeves did not show any cracking<br />
either in the deposit or at the deposit/substrate<br />
interface.<br />
Fig. 7 :<br />
Micrograph of the WT- 50 hardface coating<br />
The microstructure across the deposit/substrate<br />
interface (Fig. 6) reveals a narrow dilution zone and<br />
a planar interface. It also shows the presence of<br />
dispersed precipitates of borides and carbides, 8 which<br />
are responsible <strong>for</strong> the high hardness and wear<br />
resistance of this hardfacing alloy. No appreciable<br />
variation in the microstructural features is observed<br />
in the deposit from the interfacial region to the interior<br />
of the deposit. The microstructure of the hardface<br />
deposit at higher magnification (Fig. 7) clearly reveals<br />
Fig. 8 : Microhardness profile across 316 SS substrate/WT-<br />
50 deposit interface in the as-deposited condition.<br />
the dendritic structure with dispersed precipitates in<br />
the inter-dendritic regions.<br />
The microhardness profile across the substrate/deposit<br />
interface (Fig. 8) shows that the hardness increases<br />
appreciably from ~290 VHN at the interface to<br />
~410 VHN in the deposit across a distance of<br />
0.2 mm. The short distance over which the hardness<br />
increases across the interface is indicative of the<br />
narrow dilution zone in this PTAW deposit.<br />
4. CONCLUSIONS<br />
Fig. 6 : Micrograph across WT- 50/316 SS interface in asdeposited<br />
condition<br />
(1) Thermal cycling reduces the peak value and<br />
gradient in compressive residual stress across<br />
the hardface deposit in both the as-deposited<br />
and stress-relieved conditions.<br />
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TRANS. INDIAN INST. MET., VOL. 57, NO. 3, JUNE 2004<br />
(2) Stress relieving heat treatment has a marginal<br />
effect on the residual stress distribution across<br />
the deposit subjected to thermal cycling.<br />
(3) The integrity of the hardface deposit is not<br />
affected on thermal cycling.<br />
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