in situ repair welding of steam turbine shroud

7/27/2019 In Situ Repair Welding of Steam Turbine Shroud

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IN-SITU REPAIR WELDING OF STEAM TURBINE SHROUD

FOR REPLACING A CRACKED BLADES.K.Albert, C.R.Das, V.Ramasubbu, A.K.Bhaduri, S.K.Ray and Baldev Raj

Metallurgy and Materials Group,Indira Gandhi Centre for Atomic Research, Kalpakkam - 603 102, India

Abstract

A root-cracked blade in a high-pressure steam turbine of a nuclear power plant had to be

replaced with a new blade by removing the cracked blade by cutting the shroud. Thisnecessitated in-situ welding of a new shroud piece with the existing shroud after the

blade replacement. The in-situ welding of the shroud, a 12%Cr martensitic stainlesssteel with tempered martensite microstructure, was carried out using the gas-tungsten

arc welding and ER316L austenitic stainless steel filler metal followed by localized post-weld heat treatment at 873K for 1 h using a specially designed electrical resistanceheating furnace. Mockup trials were carried out to ensure that sound welds could be

made under the constraints present during the actual in-situ repair welding operation.In-situ metallography of the repair-weld after post weld heat treatment confirmedadequate tempering of the martensitic structure in the heat-affected zone. The

successful completion of this repair welding resulted in enormous saving both in terms ofreducing the down-time of the plant and cost of the repair. The turbine has since been

put back in to service and has been operating satisfactorily for the last 14 months.

Introduction

Steam turbine blades are critical components in power plants, which convert the linearmotion of high temperature and high-pressure steam flowing down a pressure gradientinto a rotary motion of the turbine shaft and these blades are subjected to very highcentrifugal and bending forces during operation. In a steam turbine, the root of the

blades are locked to the rotor while their free-ends are held together by a shroud orlacing wire or lacing rod, depending on the size of the blade and their position in the

rotor. This reduces the stresses due to vibration of the blade during rotation of theturbine at about 3000 rpm. As the steam enters turbine from the boiler, it passes

through different stages such as high-pressure (HP) to low-pressure (LP) zones. Reportsavailable in literature [1-3] show that the blades of the LP turbine blades are generallymore susceptible to failure that those of the HP turbine.

Routine inspection of the turbines during an annual maintenance shut down of a nuclearpower plant, revealed a crack in the root region of one of the closing blades in the III-

stage of the HP turbine. It was decided to remove the cracked blade by cutting a portionof the shroud just above it, and replace it with a new blade. The steps involved in thisprocess include inserting the tenons of the new blade into the tenon holes in the new

shroud piece of size identical to the one that had been removed by cutting, repairwelding of the new shroud piece to the rest of the existing shroud and finally riveting the

tenons of the new blade to the welded shroud piece.

This decision was taken in view of the successful performance of shrouds whose cracks

were repair welded two years earlier during the previous shut down of this plant [4-5].


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Further, this repair procedure is less expensive and time consuming than other optionsavailable and could be completed without extending the duration of the shutdown.

Choice of Welding ProcedureThe chemical composition of the new shroud piece, a martensitic stainless steel, is givenin Table 1. A welding procedure using gas tungsten arc welding (GTAW) process andaustenitic stainless steel consumable ER316L had already been developed for repair

welding of cracks in the shrouds and blades of steam turbines [5,6]. The crackedshrouds repair-welded using this procedure has performed successfully in service for

almost three years. In situ metallography of the repair welds after two years ofsuccessful service (Fig. 1) confirmed the good health of the repair welds. Using austenitic

stainless steel consumable and GTAW process, preheating, which is normally required forwelding of martensitic stainless steel to prevent hydrogen assisted cracking, can be

avoided provided post weld heat treatment (PWHT) is carried out soon after welding.

Table 1: Chemical composition (wt.-%) of the shroud for the III-Stage of the HP turbine

C Cr Ni Mo Mn Si P S Co Cu V Sn

0.15 11.50 0.40 0.60 0.60 0.30 0.025 0.006 0.025 0.06 0.020 < 0.004

Fig. 1:In-situ metallography microstructures of repair weldments executed in 1998:(a) fusion zone in a IV-stage LP-turbine blade; and(b) HAZ near fusion line in a III-stage HP-turbine shroud.

Mock-up Trials prior to Repair

As the in-situ welding of a new shroud piece to the existing shroud was being attemptedfor the first time (earlier only cracks were repair welded), a mock-up trial welding wascarried out simulating constraints that would present during the actual in-situ repair

welding operation. From Fig. 2, which shows the portion of III-stage of the HP turbineafter removal of the cracked blade, it is evident that the blades on either side of the new

replaced blade would limit access to the underside of the weld joint. Hence, a mock-upassembly was fabricated using 1 mm thick austenitic stainless steel sheets that had the

contours of inner and outer surfaces of the closing blades of III-stage of the HP turbine.These austenitic stainless steel sheets were then welded to two separate 3 mm thickaustenitic stainless steel plates having the same width as that of the actual shroud. Thisassembly was so prepared that it closely simulated the actual arrangement of the III-

stage HP turbine blades. Figure 3 shows a simple schematic representation of the mock-up assembly.


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Fig. 2: The III-stage HP turbine shroud after cutting and removal of the cracked blade.

Fig 3: Schematic representation of the mock-up assembly of the III-stage HP turbine blades andshroud piece.

After suitable edge preparation, the mock-up welding of the 3 mm plates was carried out

in the vertical down position. Next the mock-up assembly was rotated by 90 (as this

was the maximum rotation of the turbine possible during actual in-situ repair), the

underside of the weld fused, and final grind operation of the root region executed.Inspection by liquid penetrant testing (LPT) of the mock-up welding assembly confirmed

the successful welding by the welding technique adopted.

In-Situ Repair Welding of the Turbine Shroud:

1. Edge Preparation of the Old and New Shroud Pieces and Fit-up

The in-situ repair welding of the III-stage HP turbine shroud was taken up afterthe successful demonstration of the welding procedure on the mock-up assembly.While cutting the shroud piece above the root-cracked closing blade, it was

ensured that the cutting operation is carried out at an angle of about 45 to the


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plane of the shroud so that the adjacent blades are not damaged during cuttingand subsequent repair welding operations. Both the cut edges of the original

shroud were then edge prepared for welding by grinding them at an angle ofabout 35. After edge preparation, LPT was carried out on the edges and nodefects were observed.

As the dimensions of the new shroud piece were different from that of the original

shroud in the turbine, some machining was required to be carried out to matchtheir profiles precisely. The new shroud piece was then edge prepared by grinding

them at an angle of about 35 and maintain a root gap of about 2 mm betweenthe old and the new shroud pieces. After the new blade was inserted into its slot in

the turbine rotor, the positions and dimensions of the two tenon holes on the newshroud piece were precisely marked. The tenon holes on this new shroud piece

was then made by drilling and machining, and inserted on to the tenons of thenew blade to complete the fit-up for repair welding.

2. Repair welding of the Turbine Shroud

During the welding operation, the new blade and shroud piece were held inposition but without riveting the tenons to the shroud piece. The welding wascarried out in the vertical down position by the GTAW process, using ultra-high

purity argon as shielding gas and ER 316L filler wires, adopting the proceduredeveloped and approved earlier [4] and the technique finalized using the mock-up

assembly. As the new shroud piece had to be welded on either side to the existingshroud and these two weld joint locations were very close to each other, the

welding was carried out successively on both the locations to minimize distortionduring welding. After depositing one layer of weld metal on both the joints, themisalignment between the exiting shroud and the new shroud piece, due to minordimensional differences between them, were machined. Subsequently three more

passes/layers of weld metal were deposited in each weld joint. Welding voltageand current employed were 10-12V and 80-85A respectively. Welding speed

varied between 1-2 mm/s

After completion of welding from the topside, the turbine was rotated by 90 to

access the underside of the joints in the horizontal position for grinding the weldroot and a final dressing pass was made at the weld root. Subsequently, the root

of the weld was made flush with the shroud by grinding. Finally, ridges were builtup on the new shroud piece by weld deposition, which was then made flush to

with the exiting shroud by grinding. After completion of the welding, the jointswere subjected to inspection by LPT, and were found to be free of defects. Figure

4 shows the shroud after the repair welding operation.


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Fig. 4: The III-stage HP turbine shroud after in-situ repair welding.

3. Localised Post-Weld Heat Treatment of the Repair-Welded Turbine Shroud

The repair-welded joints of the turbine shroud were subjected to localized PWHTat 873 K for 1 h soon after completion welding using the procedure earlier adopted

for the repair-welded shrouds and blades [5,6]. The localized PWHT was carriedout using a specially designed electrical resistance-heating furnace by which both

the weldments could be heat treated simultaneously. This furnace was providedwith an on-off controller and a solid-state relay capable of controlling thetemperature with an accuracy of 2 K, while the power supply was provided from

an auto-transformer for controlling the heating and cooling rates. The heat from

the furnace was transferred to the weldments on the shroud through a copper

block, which has a flat top-surface (on which heater is placed) and a curvedbottom-surface with the dimensions and shape matching the shape and curvatureof the shroud piece (on which it was seated during heat treatment). The use of the

copper block ensured good thermal contact with the shroud for efficient heattransfer during heat treatment. A K-type thermocouple was welded to the

underside of the shroud and its output fed to the controller for both monitoringand controlling the PWHT temperature. A schematic diagram of the localized PWHT

set-up is given in Fig. 5.

During the PWHT, the weldments were slowly heated to 873 K over a span of five

hours, and after holding at this temperature for one hour, the furnace was

switched off and the weldments allowed to slowly cool down to 573 K.Subsequently, the furnace was removed and the joint was allowed to cool in air.


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Fig. 5: Schematic diagram showing set-up used for localized PWHT by electricalresistance heating.

4. Inspection of the Repair-Welded Joints

After successful PWHT, liquid penetrant testing (LPT) and wet florescent magnetic

particle (WFMPT) technique was performed on the weldments, and no defect was found.

WFMPT was also carried out after subsequent riveting of the tenons of the new blade tothe new shroud piece (now welded to the original shroud). Finally, in-situ metallographywas carried out after PWHT, using portable polishing unit, etching unit and microscope,

for which the martensitic stainless steel shroud material was etched with Vilellas reagentand the 316L austenitic stainless steel weld metal was electrolytically etched with 10%

ammonium per sulphate solution. The in-situ metallography microstructure of the HAZ,shown in Fig. 6, indicated successful and adequate tempering of martensite structure of

HAZ during PWHT.

Fig.6:In-situ metallography microstructure of the HAZ in the repair-welded shroud after localized PWHT at 873 K for 1 h

Conclusions

1. In-situ repair welding is a technically and economically viable solution forreplacement of cracked steam turbine blades, and significantly reduces the

down-time of turbine accompanied by enormous savings as it avoidsreplacements of entire set blades attached to a shroud segment. The repair-

welded turbine has been operating satisfactorily for the last 14 months.2. Inspection by LPT and WFMPT after PWHT confirmed the soundness of the

repair-welded shroud weldments.


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3. In-situ metallography on the repair-welded shroud showed that the procedurefor localised PWHT at 873 K for 1 hour that was adopted has been successful, as

the microstructures of the HAZ after PWHT confirmed adequate tempering of themartensitic HAZ.

Acknowledgements

The authors would like to thank Mr. G. Nageswara Rao and Mr. M.P. Hansora , KakraparAtomic Power Station for their whole-hearted co-operation during the execution of thiswork at Kakrapar. Thanks are also due to Mr. P. Sukumar and Mr. N. Dhakshinamoorthy

of Indira Gandhi Centre for Atomic Research, Kalpakkam for meticulously carrying outthe in-situ metallography of the cracks and repair welds. The authors also acknowledge

the co-operation of Mr. V.P. Sampath and his colleagues from M/sTEAM, Chennai, Indiafor carrying out the welding operation.

References

1. Atrens, A., Meyer, H., Faber, G. and Schneider, K., in Corrosion in Power

Generating Equipment, ed. M.O. Speidel and A. Atrens. Plenum Press, 1983, p.299.

2. Modern Power Station Practice, 3rd ed., British Electicity International, PergamonPress, 1992, p. 73.

3. Viswanathan, R., Damage Mechanisms and Life Assessment of High TemperatureComponents, ASM International, Metals Park, OH, USA, 1989, p. 313.

4. Albert, S.K., Gill, T.P.S., Iyer, D.R., Shanmugham, K. and Sampath, V.P., Repairwelding of under straps, shrouds and blades of KAPS-1 and 2 stream turbines,

Report no. IGC/MJS/MTD/1/99, Indira Gandhi Centre for Atomic Research,Kalpakkam, 1999.

5. Albert, S.K., Gill, T.P.S., Bhaduri, A.K. and Iyer, D.R., Repair Welding Of Cracked

Shroud In Steam Turbine. Proceeding of International Symposium on CaseHistories on Intregrity and Failures in Industry, Milan, Italy, 28 September 1October 1999, pp. 863-872.

6. Bhaduri., A.K., Gill., T.P.S., Albert., S.K., Shanmugam, K. and Iyer., D.R., RepairWelding of Cracked Steam Turbine Blades using Austenitic and Martensitic

Stainless-Steel Consumables, Nuclear Engineering and Design, 206, 2001, pp.249-259

http://www.igcar.ernet.in/igc2004/cg/mcrg-research/Art-03/e-comm3.HTM

in situ repair welding of steam turbine shroud

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