1. A procedure was developed for repair welding cracked steam turbine shrouds made of 410 martensitic stainless steel using matching ER 410 filler wire. This included optimizing post-weld heat treatment at 735°C for 1 hour, qualification welding, and mock-ups. 2. The procedure was applied to repair a cracked shroud on a nuclear power plant turbine. Non-destructive testing after localized post-weld heat treatment showed no discontinuities, and microhardness and microstructure were similar to qualified welds. 3. In-situ metallography of the repair weld confirmed adequate tempering of the heat-affected zone, matching the microstructure of procedure

1. Proceedings of International Symposium on Materials Ageing and Life
Management, October 3-6,2000, Kalpakkam, India
Eds: Baldev Raj, K. Bhanu Sankara Rao, T. Jayakumar and R.K. Dayal
Allied Publishers Limited, Chennai (2000)
REPAIR WELDING OF CRACKED TURBINE SHROUDS USING MATCHING
COMPOSITION CONSUMABLE
T.P.S. Gill, A. K. Bhaduri, S.K. Albert, V. Ramasubbu, G. Srinivasan,
K. Shanmugam and K. Balachander
Indira Gandhi Centre for Atomic Research, Kalpakkam – 603 102, India
Abstract
The procedure for repair welding of cracked steam turbines shrouds has been developed using the GTAW
process and ER 410 matching composition filler wires. The procedure development included optimisation
of post-weld heat treatment parameters, microstructural examinations, welding procedure qualification
and mock-ups. This procedure has been recently applied for repair welding of a cracked shroud in the HP-
III stage of a steam turbine in an Indian nuclear power plant. The details of the procedure development
and the experience of in-plant repair welding are presented.
1. Introduction
In steam turbines the shrouds, which are used for packeting the blades, are subjected to very high
centrifugal and bending forces during operation. In turbine stages prone to resonance, shrouds have
to bear additional high stresses due to blade resonance. For these stages, any relaxation in the
stringent specifications during fabrication, assembly and quality assurance may lead to shroud
failure. A large number of shroud failures have been reported in literature [1–3]. The conventional
remedial approaches, requiring either total replacement of blades or reduction in the height of
blades and re-shrouding, considerably increase the duration of turbine outage, and hence reduce the
performance and availability factor of the turbine. Weld repair of the shroud, on the other hand, can
save considerable down-time required for shroud repair using the conventional approaches, and
hence would be highly economical for the comparative power utility industry.
The material of the shroud is AISI 410 martensitic stainless steel (410 SS). In view of the low
operating temperature of the PHWR steam turbines and the non-availability of matching ER 410
filler wire in Indian market, repair welding of the shroud cracks have been successfully carried out
using ER 316L austenitic stainless steel filler wires for eight shroud cracks in four steam turbines
of Indian nuclear power stations [4-7].
A partial-width shroud crack was recently detected on the HP-III stage of a steam turbine. As it is
now possible to procure ER 410 filler wire, the procedure for weld repair of this cracked shroud
using matching composition ER 410 filler wire was developed [8]. The development of repair
welding strategy involved optimisation of post-weld heat treatment (PWHT) parameters,
microstructural examinations, welding procedure qualification and mock-ups.
2. Optimising PWHT parameters
For optimising PWHT parameters for welding with matching composition filler wire, an
autogenous GTA weld was made on an actual turbine shroud material made of AISI type 410
martensitic SS. The autogenous weldment was subjected to 1 h PWHTs at four different
temperatures, viz. 700, 725, 750 or 775 ºC. The microhardness profiles across the weld interface
and the average hardness of the different weldment regions for this autogenous weldment before
PWHT (as-welded) and after the different PWHTs are given in Fig. 1 and Table 1, respectively.

2. Table 1: Average hardness of autogenous weldment of turbine shroud material
Weldment
Region
Average Hardness, HV (load: 200 g)
As-welded
After 1 hr. PWHT at
700 ºC 725 ºC 750 ºC 775 ºC
Weld metal 444 253 253 243 236
HAZ 430 249 247 234 229
Base metal 251 245 242 227 221
Fig. 1: Microhardness profiles across autogenous shroud weldments
In the as-welded autogenous weldment, the average hardness of the weld metal (HV 444) and HAZ
(HV 430) is greatly different from that of the base metal (HV 251). However, on PWHT, the
average hardness reduces drastically in the weld metal (by HV 190–210) and HAZ (by HV 180–
200), but only marginally in the base metal (by HV 5–20). This results in an even hardness
distribution across all autogenous weldments subjected to PWHT, and clearly brings out the
necessity of carrying out a PWHT. Further, there is only a marginal reduction in the average
hardness across the different weldment region with increase in PWHT temperature from 700 °C to
775 ºC. This indicates that selecting a PWHT temperature in the mid-range between 700 °C and
775 °C would give optimum benefits. Hence, the optimum PWHT for 410 SS welded with
matching composition filler wire was chosen as 735 ± 10 °C for 1 h.
The photomicrograph across the weld interface of the autogenous weldment in the as-welded
condition (Fig. 2a) shows untempered martensite in the weld and HAZ with a well-defined fusion
boundary. After the 750 °C/1 h PWHT of this autogenous weldment only tempered martensitic
microstructure is observed with no well-defined fusion boundary (Fig. 2b). Further, it is observed
from Fig. 2(b) that the HAZ has been effectively tempered by the PWHT, as is indicated by the
presence of carbides at grain boundaries.

3. Fig. 2: Weld interface microstructure of autogenous shroud weldment: (a) as-welded; and
(b) after 750 °C/1 h PWHT
3. WPQ and Mock-up
The welding procedure qualification (WPQ) and mock-up for repair welding of 410 SS were
carried out on pipes of 88.9 mm diameter and 3.2 mm thickness using the GTAW process with
matching composition ER 410 filler wire of 1.6 mm diameter. A single-V groove joint geometry
with a 70° groove-angle was employed. The shielding gas used had a purity of at least 99.99% to
minimise the probability of hydrogen-induced cold cracking in the weldment. A preheat
temperature of 250 °C and PWHT at 735 ± 10 °C for 1 h was used. An as-welded mock-up
weldment (i.e., without PWHT) was also prepared and examined for purpose of comparison. The
chemical composition of the shroud material, the 410 SS pipe used, and the ER 410 weld metal are
given in Table 2. It is observed from Table 2 that the shroud material is similar to 410 SS. The
welding parameters employed for the WPQ and mock-up weldments are given in Table 3.
Table 2: Chemical composition (wt.-%)
Element Shroud material 410 SS pipe 410 weld metal
C 0.126 0.127 0.091
Cr 11.5 12.8 12.3
Mn 0.55 0.34 0.55
Si 0.26 0.32 0.55
Mo 0.37 0.12 <0.20
Ni 0.45 0.20 0.28
P 0.024 0.024 0.026
S 0.007 0.006 0.017
Co 0.02 0.02 0.027
Cu – – 0.14
Nb < 0.015 < 0.015 < 0.07
V 0.020 0.024 0.016
Ti < 0.004 < 0.004 < 0.004
Fe Balance Balance Balance

4. Table 3: Welding parameters used
Welding parameters WPQ Mock-up
Pass Root Final Root Final
Welding process GTAW GTAW
Filler wire ER 410 ER 410
Filler wire diameter (mm) 1.6 1.6
Current (amp.) 77.5 72.5 – –
Voltage (volts) 9 10 – –
Welding speed (mm.min-1
) 63 56 – –
Mode of preheating Flame Electrical resistance
Preheat temperature (°C) 250 250
Interpass temperature (°C) 250 250
Polarity DCSP DCSP
Purity of Argon gas (%) 99.99 99.99
Argon gas flow rate (l.min-1
) 7 – 9 7 – 8 – –
Welding position 1G 1G
PWHT 735 ± 10 °C for 1 hr. 735 ± 10 °C for 1 hr.
Non-destructive examination, by dye penetrant testing (DPT), fluorescent magnetic particle testing
(FMPT) and/or X-radiography, of the WPQ and mock-up weldment were carried out only after
PWHT, as it was imperative that PWHT is carried out immediately after completion of welding to
obviate the risk of hydrogen-assisted cracking in the martensitic SS weldment. NDT of WPQ and
mock-up weldments did not reveal any discontinuities (Table 4). Figure 3 and Table 4 show that
the average hardness across the various weldment regions of the WPQ and mock-up weldments,
after optimised PWHT at 735 ± 10 °C for 1 h, is fairly even varying between HV 198–221 in base
metal, HV 217–256 in HAZ, and HV 259–272 in weld metal. The microhardness profiles across the
WPQ and mock-up weldment after PWHT are compared with that for the as-welded mock-up
weldment in Fig. 4. In addition, Table 4 shows that the tensile properties and face/root bend tests
results of the WPQ and mock-up weldments are satisfactory.
Fig. 3: Average hardness of weldments

5. Table 4: Test results for WPQ, mock-up and as -welded weldments
TESTS WPQ Mock-up As-welded
735 ± 10 °C for 1 hr. PWHT Yes Yes Yes
Non-destructive
Tests
DPT Passed Passed –
X-radiography Passed – –
FMPT – Passed –
Hardness
(average HV,
200 g load)
Weld metal 272 259 425
HAZ 256 217 377
Base metal 221 198 200
Tensile
Properties
UTS (N.mm2
) 774 579 689
Elongation (%) 13.7 11.0 9.6
Fracture location Base metal Base metal Base metal
Bend Test
Results
Face Bend Passed Passed Passed
Root Bend Passed Passed Passed
Fig. 4: Microhardness profiles across weld interface in WPQ/mock-up weldments
4. Procedure for Localised Preheating and PWHT
After a number of trials, with alternative procedures on mock-up assemblies simulating actual
shroud and blade geometries, the procedure of local PWHT using electrical resistance heating on
one surface of the weldment and monitoring the temperature by placing a thermocouple on the
other surface of the weldment was found to give the most satisfactory result [5]. The same set-up
was adopted for preheating as well. A schematic diagram of the localised preheating and PWHT
set-up is shown in Fig. 5. Figure 6 shows the time–temperature record during actual localised
preheating and PWHT, following the above procedure, for the mock-up weldment.

6. Fig. 5: Schematic of set-up for preheating / PWHT by electrical resistance heating
Fig. 6: Preheating (250 ± 10 °C) and PWHT (735 ± 10 °C for1 h) cycle used for the mock-up
weldment
5. Repair Welding of Cracked Shroud
5.1 Removal of crack and groove preparation
The crack was completely removed by grinding with special tools and edge preparation was carried
out. The groove angle was kept as close to 70° as possible. As the crack initiated under a tenon
crown, a part of the crown was ground off to expose the crack. Thereafter, the crack underneath the
tenon crown was also removed and a groove prepared. DPT and FMPT were carried out to ensure
that the shroud crack had been completely removed.

7. 5.2 Repair Welding
Localised preheating of the shroud, in the repair-welded groove area, to 250 ± 10 °C by electrical
resistance heating was first carried out using the set-up shown schematically in Fig. 5. Repair
welding using ER 410 filler wire by the GTAW process was then carried out on the shroud. The
welding parameters were maintained as per the qualified procedure. After deposition of the root
pass, the repair weldment was allowed to cool to 150 °C to ensure completion of the martensitic
transformation in the 410 SS weld metal. The fill pass was then deposited. As a part of the tenon
crown had been ground off, the weld groove in that region was first filled up with the filler metal.
Subsequently, a separately piece of 410 SS (of the shape and size of the removed part of the tenon
crown) was welded to the remaining part of the old tenon crown taking extreme care to ensure that
there was no joining between the shroud and the tenon crown. The repair weldment was finally
cooled to 150 °C before starting the PWHT.
5.3 Localised PWHT
The repair weldment was subjected to localised PWHT at 735 ± 10 °C for 1 h by electrical
resistance heating in accordance with the procedure developed during the mock-up. The PWHT set-
up schematically shown in Fig. 5 was used. Figure 7 shows the time–temperature record during the
actual localised PWHT of the repair-welded shroud.
Fig. 7: Preheating (250 ± 10 °C) and PWHT (735 ± 10°C for 1 h) cycle actually used for the repair
welded shroud crack
6. Non-destructive Examination
After PWHT, DPT and FMPT were performed, and the repair weldment cleared these tests.
Besides DPT and FMPT, in-situ metallography was also carried out to confirm adequate tempering
of the HAZ during PWHT of the repair-welded shroud. In addition, the micrograph obtained by in-
situ metallography was compared with the micrographs of the WPQ, mock-up and as-welded
weldments obtained by traditional metallography.
6.1 In-situ metallography
In-situ metallography of the repair-welded region after PWHT was carried out using portable
polishing unit, etching unit and microscope. The surface of the weldment was polished up to a

8. diamond finish and etched in Vilella’s reagent. The in-situ microscopic observations at site
indicated proper tempering of the weldments. Subsequently, replicas of the weldments were
transferred on to plastic films, which were then gold-coated to improve reflectivity for further
microscopic observations in the laboratory. The photomicrograph of the weld interface region in
the repair-welded shroud, after PWHT, is shown in Fig. 8(a). Figures 8(b) and 8(c) show the typical
microstructural features in the weld interface region in WPQ and mock-up weldments after PWHT;
Fig. 8(d) shows the microstructure of the same region in as-welded condition for comparison.
Fig. 8: Optical micrographs of the weld interface region in weldments of: (a) repair-welded shroud
after PWHT (in-situ); (b) WPQ after PWHT; (c) mock-up after PWHT; and (d) as-welded.
Figures 8(a-c) show that the typical microstructural features after 735 ± 10 °C/1 h PWHT observed
in the repair weld (Fig. 8a) are similar to those observed in WPQ and mock-up weldments (Figs. 8b
and 8c, respectively). All these micrographs show the typical tempered martensitic microstructure
in the weld metal and HAZ. Due to the near matching of composition of the 410 SS shroud (base
metal) and ER 410 weld metal, no well-defined fusion boundary is observed. It is also observed
that the HAZ has been effectively tempered by the PWHT given. Hence, the quality of repair-
welded shroud is considered satisfactory for intended service.
7. Comparison of Repair Procedures using ER 316L and ER 410 Filler wires
As pointed out earlier, repair welding of the earlier eight shroud cracks in different steam turbines,
employed a procedure using ER 316L austenitic SS filler wire. However, the present shroud crack
in the HP-III stage of a steam turbine was repair-welded employing the newly developed procedure
using matching composition ER 410 filler wire. Therefore, a comparative evaluation of both the
repair procedures was also carried out. The microhardness profiles across the mock-up weldments
obtained by the two procedures (Fig. 9) clearly shows that the weldment made using ER 410 filler
wire has an appreciably more uniform hardness gradient across the weld fusion line. More
significantly, the weldment made using ER 316L filler wire shows that adjacent to the fusion line a

9. hard zone is formed in the weld metal and a soft zone is formed in the HAZ. In contrast, the
weldment made using ER 410 filler shows a smooth transition in hardness across the fusion line.
Fig. 9: Microhardness profiles across weld interface of mock-up weldments after PWHT obtained
by procedures using ER 316L and ER 410 filler wires.
The microstructural reason for this microhardness profile observation is clearly brought out in the
optical photomicrographs obtained by in-situ metallography for the repair-welded shroud
weldments made by the two procedures (Fig. 10). The micrograph for the repair weldment made
with ER 410 filler wire, taken after the 735 ± 10 °C/1 h PWHT (Fig. 10b), shows a typical
tempered martensitic microstructure in both the weld metal and HAZ, with no well-defined fusion
boundary due to the near matching of composition of the 410 SS shroud material and the ER 410
weld metal. In contrast, the micrograph for the weldment made with ER 316L filler wire, taken
after the 600 °C/1 h PWHT (Fig. 10a), shows a dark etching region along the fusion line and ferrite
grains growing into the HAZ. The formation of a soft and a hard zone at the fusion line of such
410 SS/316L SS dissimilar weldments is a well-known phenomenon and occurs when carbon
diffuses down the carbon activity gradient. In this case, the carbon migrated from the martensitic
SS to the austenitic SS during the PWHT. The ferrite grains, which are essentially, free from any
precipitates form on the martensitic SS side of the fusion line and a carbide-rich band forms on the
austenitic SS side of the fusion line. The formation of soft and hard regions in a dissimilar weld
does not impair the room temperature tensile properties of the joint. The microstructure on the
316L weld-metal side shows that most of the delta-ferrite transformed to M23C6 carbides, secondary
austenite and isolated particles of sigma-phase during the 600 °C/1 h PWHT. The formation of a
continuous sigma-phase network is considered deleterious to the mechanical properties of the
welds but sigma-phase when present as isolated particles, as in this case, does not result in any
significant loss in properties.

10. Fig. 10: In-situ metallography microstructures of weld interface region in repair-welded shrouds
welded with: (a) ER 316L after 600 °C/1 h PWHT; and (b) ER 410 after 735 °C/1 h
PWHT
Based on the above observations, it was decided to use the procedure using matching composition
ER 410 filler wire for repair welding the present and all future steam turbine shroud cracks.
8. Conclusions
(1) Matching composition ER 410 filler wire has now been employed for repair welding of a
cracked shroud in the HP-III stage of steam turbine. This repair procedure is superior to that
using an ER 316L austenitic SS filler wire.
(2) The repair welding procedure using matching composition ER 410 filler wire involves
preheating at 250 ± 10 °C and PWHT at 735 ± 10 °C for 1 h.
(3) The local preheating and PWHT procedure employing electrical resistance heating (carried out
immediately after completion of the repair welding operation) gives excellent control of
temperature at site.
(4) DPT and FMPT are to be carried out only after completion of PWHT.
(5) After PWHT in-situ metallography should be performed to ascertain the adequacy of the heat
treatment.
Acknowledgements
The authors gratefully thank Mr. Ch. Surendar, Mr. K. Hariharan, Mr. S. Krishnamoorthy and their
colleagues of Nuclear Power Corporation of India Ltd., and Dr. Baldev Raj, Dr. S.L. Mannan and
Dr. S.K. Ray of Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam for their whole-
hearted support and encouragement in completing the work in time. This technical support given by
Mr. A. Joseph, Mr. N. Dhakshanamoorthy and Mr. R. Gananasekaran of IGCAR, Kalpakkam
during the execution of the repair work is also gratefully acknowledged. Last but not the least, the
authors would like to place on record their indebtedness to Dr. Placid Rodriguez, Director, IGCAR
whose training in their formative years has stood them in good stead now.
References
1. Dewey R P, and Rieger N P, Steam turbines blade reliability, EPRI, Boston, USA (1982).
2. Dewey R P, and McCloskey T H, Analysis of steam turbine blade failures in the utility
industry, ASME, Paper 83-JPGC-Pwr-20 (1983).
3. Dewey R P and Rieger N F, Report RP 1856-1, EPRI, Boston, USA (1983).

11. Fig. 2: Weld interface microstructure of autogenous shroud weldment: (a) as-welded; and
(b) after 750 °C/1 h PWHT
3. WPQ and Mock-up
The welding procedure qualification (WPQ) and mock-up for repair welding of 410 SS were
carried out on pipes of 88.9 mm diameter and 3.2 mm thickness using the GTAW process with
matching composition ER 410 filler wire of 1.6 mm diameter. A single-V groove joint geometry
with a 70° groove-angle was employed. The shielding gas used had a purity of at least 99.99% to
minimise the probability of hydrogen-induced cold cracking in the weldment. A preheat
temperature of 250 °C and PWHT at 735 ± 10 °C for 1 h was used. An as-welded mock-up
weldment (i.e., without PWHT) was also prepared and examined for purpose of comparison. The
chemical composition of the shroud material, the 410 SS pipe used, and the ER 410 weld metal are
given in Table 2. It is observed from Table 2 that the shroud material is similar to 410 SS. The
welding parameters employed for the WPQ and mock-up weldments are given in Table 3.
Table 2: Chemical composition (wt.-%)
Element Shroud material 410 SS pipe 410 weld metal
C 0.126 0.127 0.091
Cr 11.5 12.8 12.3
Mn 0.55 0.34 0.55
Si 0.26 0.32 0.55
Mo 0.37 0.12 <0.20
Ni 0.45 0.20 0.28
P 0.024 0.024 0.026
S 0.007 0.006 0.017
Co 0.02 0.02 0.027
Cu – – 0.14
Nb < 0.015 < 0.015 < 0.07
V 0.020 0.024 0.016
Ti < 0.004 < 0.004 < 0.004
Fe Balance Balance Balance