Journal of Mechanical Science and Technology 26 (7) (2012) 2015~2018

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-012-0505-5

Evaluation of welding characteristics for manual overlay and

laser cladding materials in gas turbine blades† Hyung-Ick Kim1, Hong-Sun Park2, Jae-Mean Koo3,*, Chang-Sung Seok3, Sung-Ho Yang4 and

Moon-Young Kim4

1Center for Composite Materials, University of Delaware, USA 2KEPCO E&C, Korea

3School of Mechanical Engineering, Sungkyunkwan University, Korea 4Gas Turbine Technology Service Center, Korea Plant Service & Engineering, Korea

(Manuscript Received February 22, 2012; Revised March 16, 2012; Accepted April 10, 2012)

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Abstract A Ni-based super-alloy is widely used in manufacturing the first stage blade of high power land-based gas turbines as these first stage

blades operate under high temperature and high pressure. The blade of a gas turbine must withstand the most severe combination of tem-perature, stress, and environment. After continued operation, the blade may be damaged by the turbine operation mode. To recover its initial mechanical properties, the blade of the Ni-based super-alloy undergoes a replacement repair process. Typical repair processes include blending, welding, re-machining and precision grinding. In this paper, the effects of manual overlay and laser cladding were in-vestigated as part of the welding characteristic evaluation. Results are compared with those for post-heat treatment known as hot isostatic processing (HIP).

Keywords: Gas turbine blades; Ni-based super-alloy; Post-heat treatment; Welding; Manual overlay; Laser cladding ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

As the demand for electric power steadily increases, the ca-pacity of power generation equipment must increase as well. To enhance gas turbine efficiency, the inflow temperature of the first-stage blade of the turbine system is raised to more than 1300°C. When the gas turbine blade is used at high tem-perature for a long time, its base material can become dam-aged. To mitigate this damage, regenerative maintenance is performed after the number of equivalent operating hours (E.O.H. - approximately 24000 hours) recommended by the manufacturer so that the part may be restored for reuse [1, 2].

After the first-cycle operation, nondestructive testing and a durability diagnosis of the material quality are performed to determine whether the gas turbine blade should be reused or scrapped. In the primary restoration process, which may in-clude cleaning, stripping, heat treatment, defect removal, non-destructive testing, welding processing, restoration heat treat-ment, and coating, the tip of the damaged blade is removed and replaced by overlaying after the first-cycle operation.

This paper describes the mechanical properties of a manual

overlay method and a laser cladding method.

2. Chemical compositions of GTD 111DS and Rene 80

materials

The materials used were GTD 111DS and Rene 80, a Ni-based super-alloy, the nominal composition of which is given in Table 1. GTD 111DS and Rene 80 are different chemical compositions of Mo, C and Ta. However, the primary chemi-cal components, Ni, Cr, Co, Ti and Al, are not different. GTD-111 super-alloy, in fact, is a modification of Rene 80 and has a multi-phase microstructure consisting of fcc γ matrix, bimodal γ’ precipitates (primary and secondary), γ-γ’ eutectic, carbides

*Corresponding author. Tel.: +82 31 299 4759, Fax.: +82 31 290 5849 E-mail address: kjm9000@chol.com

† This paper was presented at the ICMR2011, Busan, Korea, November 2011. Recommended by Guest Editor Dong-Ho Bae

© KSME & Springer 2012

Table 1. Nominal chemical compositions of GTD111 DS and Rene 80 materials (wt. %) [5].

Ni Cr Co Mo Ti Al C

GTD111 DS Bal. 13.6 9.14 1.6 4.9 2.97 0.09

Rene 80 Bal. 13.8 9.30 3.9 5.0 3.00 0.16

Ta W Cb Zr B Hf V

GTD111 DS 2.87 3.44 < 0.01 - 0.010 - -

Rene 80 - 4.00 - - - - -

2016 H.-I. Kim et al. / Journal of Mechanical Science and Technology 26 (7) (2012) 2015~2018

and a small amount of deleterious phases such σ, δ and η [3, 4].

Fig. 1 shows the effect of Al and Ti content on the post-weld heat treatment cracking tendency in Ni-based super-alloys. As can be seen, the γ’-strengthened Ni-based super-alloys with high Al and Ti content are particularly difficult to weld because of their high susceptibility to cracking.5 Such alloys tend to harden rather rapidly with age and their ductility is low. Al and Ti levels of GTD 111DS and Rene 80 are about 6%. Therefore, such welding conditions are difficult to satisfy and tend to undergo post-weld heat treatment cracking. Weld-ing conditions in the present work were chosen based on those from the preceding work. They were adjusted with each trial to minimize or eliminate weld defects and cracks.

3. Specimens of manual overlay and the laser clad-

ding

The blade (E.O.H.: 24,512 hr) platform made with GTD 111DS was used as the substrate for manual overlay welding. To remove the residual stress of the base material, pre-heat treatment was applied before welding. Then, using a welding rod made from the same material as the base material, the specimen was manufactured by GTAW (gas tungsten arc welding) in an inert (argon) atmosphere maintaining a tem-perature exceeding 1000°C.

The CNC laser cladding system (HP-115CL) from Huffman Corporation was used for laser cladding. The experiment was completed by varying power and powder flow rate, which are the most important variables in laser cladding, as the main variables. The cladding part was analyzed for microstructures to check for existing cracks to ensure that the cladding speci-men was manufactured without any cracks.

The post-processing conditions for laser cladding and man-ual welding included HIP (hot isostatic processing), which is effective in removing porosities and cracks; solution heat treatment which restores the mechanical characteristics of a precipitation strengthened Ni-based super-alloy; and aging heat treatment [7, 8]. The two post-welding processing condi-

tions were without HIP and with HIP at 1204°C. The pressure with HIP was 103 MPa for 4hr, which is the same as in the condition for manual overlay welding. The condition for solu-tion heat treatment was 1204°C (2 hr) and 1121°C (2 hr), while the condition for aging heat treatment was 843°C (4 hr).

With manual overlay welding, half of the specimen was the substrate in the lengthwise direction, while the other half was the part to be welded. Meanwhile, with Rene 80 powder clad-ding, the tensile specimen was extracted from the cladding build-up of the block in order to make the overall specimen a fully clad part, consisting only of Rene 80 powder without any substrate. The tensile specimen was processed to have a paral-lel diameter of 4mm and a parallel length of more than 16mm. After the specimen was processed, materials appeared normal and identical to the naked eye.

4. Result of the tensile test and related considerations

The tensile test was completed at a high temperature (760°C) using the Instron electro-hydraulic UTM with a ca-pacity of five tons. As a result of the tensile testing described in Fig. 2 and Table 2, the yield strength and tensile strength of ‘Rene80 non-HIP,’ the specimen with laser cladding, were found to be lower than those of ‘GTD111 non-HIP,’ the man-ual overlay welding specimen without HIP. Meanwhile, the

Fig. 1. Effect of aluminum and titanium contents on post-weld heat treatment cracking (modified) [6].

Fig. 2. Engineering stress-engineering strain curve and fracture appear-ance at 760°C for Rene 80 powder cladding specimen, Non-HIP and HIP.

H.-I. Kim et al. / Journal of Mechanical Science and Technology 26 (7) (2012) 2015~2018 2017

yield strength and tensile strength of ‘GTD111 HIP,’ the man-ual overlay welding specimen with HIP, and ‘Rene80 HIP-a,’ the first specimen with laser cladding, were almost the same. However, the tensile strength of ‘Rene80 HIP-b’ was lower than the specimens above.

By applying HIP, the tensile strength increased about 4-5% for both the manual overlay welding specimen using the GTD 111 welding rod and the laser cladding specimen using Rene 80 powder. Fig. 3 shows a comparison of the fracture surfaces between the Rene 80 powder cladding specimens with or without HIP. Here, fractures are made along specific surfaces with dimples showing the characteristic ductile trans-granular fraction. While it was difficult to clearly point out the peculi-arities or differences between the two conditions (with or without HIP), many voids with large diameters were found in the fractures of the specimen that did not receive HIP.

This agrees with the result of the tensile test at room tem-

perature described in Fig. 4. When voids are made during cladding, they are greatly reduced during HIP. Unlike the test at 760°C, however, no fracturing trend existed along specific surfaces or dimples found in the tensile test at room tempera-ture.

Meanwhile, MC carbides were not found in either tensile test at high temperature or room temperature.

Fig. 5 shows the cavity (L) and hollow powder (R) micro-structure after Rene 80 power cladding. The cavity is a result of melting bubbles caused by the rapid solidification. If melt-ing bubbles are made to limit supply more than powder, melt-ing bubbles are left in the solidification state of the melting. After the cladding, microstructure was observed in many cavi-ties. The size of the cavity is 0.200~11.9 μm. Hollow powder occurs during the manufacturing process. This is a nearly round shape, unlike the cavity caused by melting bubbles, and size is 8.60~60.5 μm.

Fig. 6 shows the cavity (L) and hollow powder (R) micro-structure after solution heat treatment and aging heat treatment. Despite solution heat treatment and aging heat treatment, a defect of the cavity and hollow powder exists.

Fig. 7 shows a defect after solution heat treatment, aging heat treatment and HIP. The cladding of a defect has consid-erably disappeared. The cavity, relatively small size, by melt-ing bubbles is not observed. Only the large size of the defect by hollow powder was found. Cavities, caused by the melting

Table 2. The tensile test results at 760°C for GTD111 DS manual overlay welding specimen and Rene 80 powder cladding.

No. Yield strength(MPa)

Tensile strength (MPa)

Fracture location

GTD111 non-HIP-a 872 1,026 Welding

GTD111 non-HIP-b 936 1,092 Welding

Average 904.0 1,059.0 -

GTD111 HIP-a 870 1,098 Welding

Average 870.0 1,098.0 -

No. Yield strength(MPa)

Tensile strength (MPa)

Fracture location

Rene80 non-HIP-a 840 1,007 Welding

Rene80 non-HIP-b 845 1,052 Welding

Average 904.00 1,059.00 -

Rene80 HIP-a 871 1,104 Welding

Rene80 HIP-b 867 1,057 Welding

Average 435.00 549.00 -

Fig. 3. Fracture surface at 760℃ for Rene 80 powder cladding specimen.

Fig. 4. Fracture surface at room temperature for Rene 80 powder clad-ding specimen.

Fig. 5. Cavity and hollow powder microstructure of Rene 80 cladding part (as-cladding).

2018 H.-I. Kim et al. / Journal of Mechanical Science and Technology 26 (7) (2012) 2015~2018

bubbles during laser cladding and the hollow powder, remain as deficiency inside the cladding to decrease mechanical strength even after applying post-heat treatment. Although HIP can ef-fectively remove most of the cavities, it did not have much ef-fect on removing cavities larger than a certain size.

5. Conclusion

After manufacturing a manual overlay welding specimen with a GTD 111 welding rod commonly used during the re-pair of industrial gas turbine blades, and a laser cladding specimen with the recently developed Rene 80 powder, both specimens were tensile tested.

When specimens did not receive HIP, the mechanical strength of the laser cladding specimen using Rene 80 powder was lower than that of the manual overlay welding specimen using a GTD 111 welding rod. After HIP was applied, how-ever, the mechanical strength of both specimens became simi-lar. The application of HIP increased the tensile strength about 4-5%, suggesting that voids made during cladding were greatly reduced during HIP. Although HIP can effectively

remove most of the cavities, it did not have much effect on removing cavities larger than a certain size. Future work will include quantitative studies on HIP and heat-processing after welding.

Acknowledgments

This work was supported by the Korea Research Founda-tion Grant funded by the Korean Government [KRF-2008-357-D00002], the National Research Foundation of Korea (NRF) (No. 2011-0020024) and the BK21 funded by the MKE (Ministry of Knowledge Economy).

References

[1] H. I. Kim, Y. Huh, H. S. Park, C. S. Seok and M. Y. Kim, Int. J. Mod. Phys., B20 (25-27) (2006) 4135.

[2] H. I. Kim, H. S. Park, J. M. Koo, S. H. Yang, M. Y. Kim and C. S. Seok, Key Eng. Mater., 353 (2007) 519.

[3] S. A. Sajjadi and S. Nategh, Mater. Sci. Eng. A-Struct. Ma-ter. Prop. Microstruct. Process., 307 (2001) 158.

[4] G. Frederick, Laser welding repair of service exposed IN 738 and GTD 111 buckets (TR 1004345, EPRI, 2003).

[5] R. Viswanatan, Gas turbine blade superalloy material prop-erty handbook (TR 1004652, EPRI, 2001).

[6] T. J. Kelly, Weldability of materials, ASM International, Materials Park (1990) 151.

[7] L. C. Lim, J. Z. Yi, N. Liu and Q. Ma, Mater. Sci. Technol., 18 (2002) 413.

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Chang-Sung Seok received his B.S., M.S., and Ph.D in Mechanical Engineer-ing from Sungkyunkwan University, Korea in 1981, 1983 and 1990, respec-tively. Prof. Seok is currently a Professor of the School of Mechanical Engineer-ing of Sungkyunkwan University, Korea. His research fields are the integrity

evaluation and durability design of machine structures, devel-opment of component parts and analysis of stress and strength.

Fig. 6. Cavity and hollow powder microstructure of Rene 80 claddingpart (after solution and aging heat treatment).

Fig. 7. Cavity and hollow powder microstructure of Rene 80 cladding part (after solution and aging heat treatment and HIP).