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ICM11

Application of small punch creep test for Inconel 617 alloy weldment

B. J. Kima,b, Y. B. Sima, J. H. Leea, M. K. Kima and B. S. Lima*

aMaterial Strength & Computational Bioengineering Lab, School of Mechanical Engineering, Sungkyunkwan University

300, Cheoncheon-dong, Jangan-gu, Suwon, gyeonggi-do 440-746, Korea bSchool of Mechanical Engineering, Dongyang Mirae University, 62-160, Kochuck-dong, Guro-gu, Seoul city, 152-714, Korea

Abstract

In this work, SP creep tests were applied to the Inconel 617 alloy and its weldment, which is one of the candidate materials for future advanced fossil power plant. To find the creep characteristics of the weakest local part in Inconel 617 alloy weldment, test specimens of thin squares (10 10 0.5mm) were machined at three different locations: weld metal, base metal, and HAZ near base metal. The test results of different microstructures were compared to one another. To investigate the effect of microstructure on the creep behavior of Inconel 617 alloy weldment, the rupture parts were also observed by a scanning electron microscope.

© 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11 Keywords: Small punch creep ; Cross weld ; Weldment ; Fusionline ; Heat affected zone.

1. Introduction

The small punch(SP) creep test is a highly useful method for creep life assessment of service components such as steam pipe and turbine rotor operating at high temperatures because it requires only miniature sized test specimens which can be acquired from the actually operating components in nondestructive manner[1-8]. The SP creep tester is equipped by a ceramic (Si3N4) ball of 2.4 mm in diameter to apply a concentrated load to the specimen. Because of its small ball size, it has been successfully employed to evaluate the local creep characteristics of weldment of various microstructures[9-12]. In this work, the SP creep test was applied to the Inconel 617 alloy and its weldment. Inconel 617 (UNS N06617: Ni-22Cr-12Co-9Mo) is primarily a solid solution nickel-base superalloy, which is one of the promising candidate materials for an advanced ultra super critical(usc) fossil-fuel power plant with superior engineering properties[13-16]. Because of its excellent properties of elevated temperature strength and creep resistance, this alloy has been already widely used in fossil fuel and nuclear power plants, aerospace, and chemical plants. Based on a literature survey, we found that no previous work has been reported on the SP creep test results for Inconel 617 alloy and its weldment. Therefore, the main purpose of this research was set to evaluate the local

* Corresponding author. Tel.: +82-31-290-7471; fax: +82-31-290-7934. E-mail address: bslim@skku.edu

1877-7058 © 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11doi:10.1016/j.proeng.2011.04.425

Procedia Engineering 10 (2011) 2579–2584

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creep behavior of various microstructures such as, HAZ(heat affected zone), weld metal, and base metal of Inconel 617 alloy weldment using the SP creep tests. The test results were evaluated and compared to one another. Also, to investigate the effect of microstructure on the creep behavior of Inconel 617 alloy weldment, its microstructure and fracture surface were observed by a scanning electron microscope (SEM).

Nomenclature

minimum displacement rate

A material constant

P applied load

n load exponent

2. Experimental Procedure

2.1. Materials

Table 1 shows the chemical composition of Inconel 617 alloy, which is base metal of this study, and of HAYNES and Thyssen 617(filler metal)[17]. Welding procedure was performed in seven passes by TIG(Tungsten Inert Gas arc Welding).

Table 1. The chemical composition of Inconel 617 alloy(base metal) and Tissen 617(filler metal), wt(%).

2.2. Creep test and specimen preparation

To examine the local creep characteristics by SP creep tests, a small weld block was etched to determine the microstructure and machining position of SP specimens. Thin square shape (10 10 0.5mm) specimens were machined at three different locations from the block, i.e., weld metal, base metal, and HAZ(cross-weld specimen) as shown in Fig. 1. In case of HAZ specimen, it was machined to locate the fusion line at the center of specimen. The specimens were grounded and polished following the standard metallographic sample preparation method. The final polishing process was carried out using 0.1 μm Al2O3 powder to avoid the possible stress concentration by machining scratch. The thickness was controlled to within ±10 μm with a digital micrometer. Fig. 2 shows the SP creep jig used in this work. It consists of puncher, ball, upper die, and lower die. The specimen is first placed on the lower die. The upper die is then assembled on the lower die. This assembly is finally clamped by four clamping screws. Constant creep loads ranging from 392N to 589N were applied to SP creep specimen by a ceramic (Si3N4) ball with diameter of 2.4mm through the puncher. The displacement of the specimen was measured by a linear variable displacement transducer (LVDT) to the accuracy of 10–3 mm. All the tests were performed at 700 under argon gas condition to prevent oxidation during tests. The temperature of the SP-creep tester furnace was controlled to an accuracy of 1 .

Material Ni Cr Co Mo Al C Fe Mg Si S Ti Co B Mn

Alloy 617 44.5 20.0

~24.0

10.0

~15.0

8.0

~10.0

0.8

~1.5

0.05

~0.15

3.0 1.0 1.0 0.015 0.6 0.5 0.006

Tissen617 Bal 21.5 11.0 9.0 1.0 0.05 1.0 0.1 0.5 0.1

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Fig. 1. Inconel 617 alloy weldment block and SP creep specimens. Fig. 2. Schematic illustration of SP-creep test apparatus

2.3. Microstructure and Fracture

To examine the effect of microstructure on the local creep behavior, the different microstructures of 617 alloy weldment were observed by SEM. After SP creep tests, to investigate the fracture mode of local microstructural region in weldment, those three specimens of different microstructures were investigated using an SEM at the creep fracture surface.

3. Result and Discussion

3.1. Microstructure characteristics

Typical microstructures of different specimens are presented in Fig. 3, which shows (a) base metal, (b, e) weld metal, and (c, d, f) HAZ. The microstructures of base and weld metal were dissimilar at the boundary of fusion line. The interfacial regions of the Thyssen 617 filler metal and Inconel 617 base metal are shown in Fig. 3(d). However, no considerable unmixed zones can be observed. It can be attributed to the similarity of 617 filler and base metal from the viewpoint of the melting temperature and chemical composition. As shown in Fig. 3, the microstructure of (a) base metal shows an austenitic(γ) matrix with an average grain size of 50μm and some inter- and intra-granular precipitates while that of weld metal shows completely austenitic with a dendrite structure and contains precipitates dispersed in the matrix. The weld metal is mostly composed of fine equiaxed dendrites as shown in Fig. 3 (b, e). The 617 filler material is reported to have better ultimate tensile strength and total elongation values compared with the other fillers[17]. This can be related to the fine dendrite microstructure of filler metal. Therefore, this fine equiaxed dendrite microstructure is thought to contribute to the creep resistance and lead to different creep fracture behavior from the base metal. Generally, this fine microstructure has a lower segregation ratio of Mo which reduces the solidification cracking during the welding process[18]. Fig. 3(d) shows the higher magnification of HAZ region. In the interfacial HAZ region, both coarse and mixed grains of base and filler metal were observed. This microstructural transition region can be resulted in creep strength mismatch. As shown in Fig. 3(g, h), the crack and microvoids were also observed. This crack formation is believed to be caused by solidification cracking[18]. These defects will have an effect on the scatter of creep data. Also, as shown in Fig. 3(f), the migrated grain boundaries(MGB) and the ductility dip cracking(DDC) was observed at the HAZ region adjacent to the weld metal and it can be resulted from the formation of precipitates along the solidification grain boundaries (SGB). Generally, it is well known that the DDC in nickel-based alloys typically occurs on the migrated grain boundary (MGB) in regions reheated by a multipass welding and is unavoidable in welding operations[19]. Therefore, it is important to control the MGB, which is the primary factor in reducing cracking susceptibility. According to report, it is postulated that precipitate on MGB plays an important role as a pinning effect, leading to increased resistance of MGB evolution during reheating by multipass operation[20]. These MGB and DDC are believed to have a detrimental effect the detrimental effect on the creep rupture strength and life of HAZ.

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Fig. 3. SEM microstructure of 617 alloy weldment; (a)base metal; (b, e)weld metal; (c, d, f)HAZ; ( g, h) solidification cracking in base metal

3.2. Local creep behavior of 617alloy weldment

Fig. 4 shows typical displacement-rupture time curve obtained from SP creep tests for base metal, weld metal and HAZ in various load conditions. As shown in Fig. 4, the SP creep curves were similar to the unaxial creep curves. Fig. 5 shows the comparison results of creep curves for base metal, weld metal and HAZ. The longest creep rupture life was found in weld metal for the same load conditions, whereas HAZ revealed the shortest rupture life and the greatest slope of displacement. As mentioned above, the fine equiaxed dendrite microstructure of weld metal is closely related with the creep strength and rupture life. The rupture time of HAZ specimens were lower than those of the base and weld metal. This behavior becomes more significant when decreasing test loads. It indicates that the creep strength of HAZ was lower than those of the base and weld metal. There must have been some contribution from the surrounding materials to the creep deformation of HAZ which has the width of 0.2mm. This effect was considered because the crack initiation and propagation caused by the 2.4mm ball indenter were all observed to take place inside of the HAZ region. According to a report, a tri-axial stress state is found to exist in HAZ which lowers creep strength [9]. Generally, the creep deformation of HAZ is mechanically constrained by weld metal and base metal of relatively higher creep strength and it generates a tri-axial stress condition in HAZ. In prior research, we demonstrated by a series of interrupted creep rupture tests that the necking of creep rupture occurred in Type IV region where a relatively large number of creep cavities were found [5]. Creep voids were frequently observed at the interface of large M23C6 carbides at prior austenite grain boundaries. Fig. 5 shows the relationship between minimum displacement rate and applied load of each microstructure of 617 alloy weldment obtained from SP creep tests. In the SP creep tests, the relationship between displacement rate, , and load, P , can be established by the following equation,

nAP , (1)

where A is a constant and n is load exponent. The load exponents of the weld metal and base metal were 4.6 and 6.19, respectively, while that of HAZ was

10.37. The displacement rates of base and weld metals showed a very similar tendency. The weld metal specimen shows the lowest minimum displacement rate, whereas HAZ has the highest minimum displacement rate. This difference becomes larger at higher load conditions. It implies that the HAZ with inhomogenious microstructures is

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highly sensitive to the load conditions compared to base and weld metals having relatively homogenious microstructures. This result is strongly related with fracture behaviors during the SP creep test.

Fig. 4. SP creep curves of (a) base metal; (b) weld metal; (c) HAZ (cross weld) at 700 .

Fig. 5. Comparison results of SP minimum displacement rates of (a) base metal; (b) weld metal; (c) HAZ (cross weld) at 700 .

3.3. Fracture behavior

Fig. 6 shows the bottom fracture surface of each of the SP creep rupture specimens of the weldment under the load of 392N. For the base metal, the creep brittle intergranular fracture mode was observed and several circumferential brittle branch cracks were formed as shown in Fig. 6 (a). The intergranular creep cavities or ductile dimples were not observed in the creep ruptured part. But, for the weld metal, the fracture mode almost revealed ductile fracture as shown in Fig. 6 (b). From the close-up view of Fig. 6 (b), red square area, it was observed that the fracture occurred at MGB and SGB. Fig. 6 (c) shows the fracture surface of HAZ specimen. A mixed fracture of base and weld metals was observed at the interfacial region which is the locally weakest part of weldment, HAZ.

Fig. 6. Bottom fracture surface of SP creep rupture specimens (a) base metal; (b) weld metal; (c) HAZ at 392N (T= 700 )

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4. Conclusion

In this work, the local creep characteristics of various microstructures in Inconel 617 alloy weldment were investigated by using an SP creep tester at 700 and the following conclusions were obtained. Interfacial HAZ region showed the greatest slope of displacement rate and had the shortest rupture life, implying it is the weakest part of the whole weldment. From the results of metallurgical and fracture mode observation, the microstructural transition region was found to cause creep strength mismatch leading to the early failure of this region. But in the base and weld metal specimens, although the fracture mode is brittle, it showed longer creep lives. Finally, the SP creep tests for Inconel 617 alloy weldment showed that it is experimentally possible to derive the creep properties for different microstructures of weldment with the good validity.

Acknowledgements

This work was supported by the Power Generation & Electricity Delivery of the Korea Insitute of Energy Technology Evaluation and Planning(KETEP) grant funded by the Korea government Ministry of Knowledge Economy. (No. R-2007-1-001-01.)

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