Scholarly article on topic 'Evaluation of the Effects of Thermal Aging of Austenitic Stainless Steel Welds Using Small Punch Test'

Evaluation of the Effects of Thermal Aging of Austenitic Stainless Steel Welds Using Small Punch Test Academic research paper on "Materials engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Procedia Engineering
OECD Field of science
Keywords
{"Austenitic stainless steel welds" / "Small punch test" / "Thermal aging" / "Fracture toughness" / "Tensile test"}

Abstract of research paper on Materials engineering, author of scientific article — S.H. Hong, M.-G. Seo, C.H. Jang, K.-S. Lee

Abstract Austenitic stainless steels and associated welds have been widely used for the pressurizer surgeline of the pressurized water reactors (PWRs). Though the amount of the ferrites in the welds is generally low, the relatively high operating temperature of surgeline may cause the degradation of fracture resistance due to thermal aging during the long-term operation. In this study, the mechanical property changes caused by the thermal aging were evaluated for the ER316L and ER347 welds as well as CF8M. The materials were thermally aged at 400 ̊C for 5,000h and the properties were measured at room temperature and 320 ̊C. The fracture resistance was measured using 1/2T-CT specimen by normalization method. In addition, small punch (SP) test was performed at both temperatures and the results were used to estimate the tensile property and fracture resistance of the stainless steel welds and CF8M. Based on the results, correlations were developed to estimate the fracture resistance using the load-displacement curve of SP tests. Also, the fracture surfaces of CT and SP test specimens were compared and discussed in view of the effect of thermal aging on microstructure.

Academic research paper on topic "Evaluation of the Effects of Thermal Aging of Austenitic Stainless Steel Welds Using Small Punch Test"

CrossMark

Available online at www.sciencedirect.com

ScienceDirect

Procedía Engineering 130(2015) 1010-1018

Procedía Engineering

www.elsevier.com/locate/procedia

14th International Conference on Pressure Vessel Technology

Evaluation of the Effects ofThermal Aging of Austenitic Stainless Steel Welds Using Small Punch Test

S.H. Honga, M.-G. Seoa, C.H. Janga<*, K.-S. Leeb

aDepartment of nuclear and quantum engineering, Korea Advanced Institute of Science and Technology,291 Daehak-ro, Yuseong-gu,

Daejeon, 34141, Rep. of Korea bCentral Research Institute, Korea Hydro and Nuclear Power Co., Ltd., 1312 Gil, 70 Yuseongdae-ro, Yuseong-gu,

Daejeon, 34101, Rep. of Korea

Abstract

Austenitic stainless steels and associated welds have been widely used for the pressurizer surgeline of the pressurized water reactors (PWRs). Though the amount of the ferrites in the welds is generally low, the relatively high operating temperature of surgeline may cause the degradation of fracture resistance due to thermal aging during the long-term operation. In this study, the mechanical property changes caused by the thermal aging were evaluated for the ER316L and ER347 welds as well as CF8M. The materials were thermally aged at 400 °C for 5,000 h and the properties were measured at room temperature and 320 °C. The fracture resistance was measured using 1/2T-CT specimen by normalization method. In addition, small punch (SP) test was performed at both temperatures and the results were used to estimate the tensile property and fracture resistance of the stainless steel welds and CF8M. Based on the results, correlations were developed to estimate the fracture resistance using the load-displacement curve of SP tests. Also, the fracture surfaces of CT and SP test specimens were compared and discussed in view of the effect of thermal aging on microstructure.

©2015 The Authors.PublishedbyElsevierLtd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ICPVT-14

Keywords: Austenitic stainless steel welds; Small punch test; Thermal aging; Fracture toughness; Tensile test

* Corresponding author. Tel.: +82-42-350-3824; fax: +82-42-350-3810. E-mail address: chjang@kaist.ac.kr

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ICPVT-14

doi: 10.1016/j.proeng.2015.12.253

1. Introduction

Austenitic stainless steel welds are widely used to join the pressurizer surgeline pipes of the pressurized water reactors. Such welds typically contain certain amounts of ferrite to prevent hot cracking as well as to achieve higher strength [1]. In addition to welds, cast austenitic stainless steels (CASSs) have been used in primary pressure boundary components of nuclear power plant (NPP) utilizing their high strength and good corrosion resistance. However, because of the existence of ferrites, austenitic stainless steel welds and CASS are susceptible to thermal aging embrittlement during long-term exposure to service temperature of NPP [2-7]. Therefore, the degradation of mechanical properties such as tensile and fracture toughness tests caused by the thermal aging should be properly evaluated.

For the conventional test methods for tensile and J-R properties, relatively large size specimens are used. However, sometimes the volume of welds is not enough for the standard test specimen. Meanwhile, small punch (SP) test has advantages of using small size samples [8]. But as SP test is an indirect test method, the proper correlation between SP tests results and standard tests should be established for practical application. Therefore, in this study, the changes in mechanical property caused by thermal aging were evaluated for the stainless steel welds and CASSs by using tensile, J-R, and SP test. The measured property degradation was compared with the values estimated from the existing models. Also, correlations were developed to estimate the tensile properties and fracture toughness from the load-displacement curve of SP tests.

2. Materials and experiment

2.1. Materials

In this study, austenitic stainless steel (SS) welds made of ER316L and ER347 weld wires were used. The filler metals were selected to produce the finished welds with ferrite contents of approximately 10 vol. %. For comparison purpose, CF8M with 34 vol. % ferrite was also included in the study. The chemical compositions and ferrite contents of welds and CF8M blocks are listed in Table 1. Ferrite contents of the welds were calculated using the Schaeffler diagram. The welding blocks of ER316L and ER347 were fabricated by using manual gas-tungsten arc welding (GTAW) with build-up welding process on the type 316LSS plate.

Table 1. Chemical compositions and ferrite contents of the test materials.

wt.% Fe Cr Ni Mo C Si Mn Nb Ferrite contents (vol. %)

ER316L Bal. 18.4 11.0 2.56 .008 .4 1.74 - 11

ER347 Bal. 19.0 9.04 .17 .045 .38 1.53 .69 10

CF8M Bal. 20.5 8.19 2.63 .05 .66 .86 - 34

2.2. Experiment

The test materials were thermally aged at 400 0C for 5000 h in air environment. The aging temperature (400 0C) represents accelerating condition compared to operating condition (343 0C) of pressurizer surgeline in PWR. To evaluate the mechanical properties of SS welds and CF8M, tensile, fracture toughness, and SP tests were conducted at room temperature and 320 0C. Tensile test was performed using round bar type specimens with a strain rate of 5 x 10"4 /s following the procedures of ASTM E8/E8M-13a [9]. The J-R curve is measured using 1/2T-CT specimen by the normalization method following the procedures of ASTM E1820-13 [10]. The machined notch of the CT specimens was aligned with the direction of welding (L-S orientation). The ratio of initial crack length to specimen width (a/W) was approximately 0.57. Fracture toughness test was conducted at a cross-head speed of 1 mm/min.

The schematic design of SP test is shown in Fig. 1. The SP specimen is a square type with 10 mm each side and 0.5 mm of thickness. SP test specimens were mechanically polished with up to 600 SiC paper before the tests. The SP test was carried out at a cross-head speed of 0.12 mm/min. An alumina ball of 1.2 mm in radius with hardness above HRC 65 is used. In order to prevent oxidation of the specimens, SP tests were performed in a vacuum

condition (10~3 torr) at room temperature and 320 °C. Also, the micro structure change after the thermal aging was observed using both scanning electron microscope (SEM) and transmission electron microscope (TEM).

Fig. 1. The schematics ofthe small punch testjig.

3. Results and discussion

3.1. Effect of thermal aging on mechanical properties

„ 600 (0

t C CD

■t 300 to

Test condition: room temperature, air

I I uri-aged

1 aged at 400 "C for 5,000 h

ER316L

1Û0Û

-C 700

Tri boo

I- 200

Test condition: room temperature, air

I_lun-aged

m aged at 400 "C for 5,000 h

ER316L

— 50

LU 20 10 0

Test condition: room temperature, air

• ASME Sec. II - Part. C requirement

I lun-aged

aged at 400 °C for 5,000 h

ERJ16L

- ASME Sec II - Part. C requirement

Fig. 2. Tensile test results at room temperature for un-aged and aged specimen.

Fig. 3. Tensile test results at 320 °C for un-aged and aged specimen.

The results of tensile tests at room temperature and 320 0C for the un-aged and 5000 h aged specimens are shown in Figs. 2 and 3. The tensile strength and elongation of the un-aged weld materials are within the ASME specification (red line in Fig. 2). As shown in the figures, after 5000 h aging at 400 0C, changes in tensile properties

are observed for all test materials. In case of the ER316 and ER347 welds, tensile strength increased up to 96 MPa while elongation decreased by 11 %. For CF8M, the change was much bigger such that tensile strength increased up to 129 MPa and elongation decreased to 23 %. The larger reduction of ductility of CF8M could be related to the higher ferrite content (34 vol. %) compared to weld materials (10-11 vol. %). Similar mechanical property change was also observed at 320 °C. Such changes in mechanical properties are caused by the hardening of ferrites during aging treatment by spinodal decomposition and other secondary phase precipitation [2,4,5].

Figure 4 and Figure 5 show the J-R test results for the un-aged and aged specimens respectively. For ER316L and CF8M, a significant decrease in fracture resistance was observed after aging compared to the un-aged condition. For ER316L welds, room temperature Jic decreases from 453 kJ/m2 to 253 kJ/m2 or by 43% after aging. For CF8M, the decrease in room temperature Jic was even greater that it dropped from 1105 kJ/m2 to 144 kJ/m2, or by 87 % after aging. Similar behaviour was also observed at 320 °C. Meanwhile, the decrease in fracture resistance was less significant for ER347 welds after aging at 400 °C for 5000 h both at room temperature and 320 °C. It is interesting that the ferrite content and tensile property change are similar for ER316L and ER347 welds, but the fracture resistance change is somewhat different between the two SS welds. The observed change of the mechanical properties will be discussed in view ofthe effect ofthermal aging on microstructure in section 3.2.

Fig. 4. J-R test results at room temperature for un-aged and aged specimen.

Fig. 5. J-R test results at 320 °C for un-aged and aged specimen.

3.2. Effect of thermal aging on microstructure

Figure 6 shows the microstructure of the un-aged and aged CF8M. As shown in Fig. 6(a), the microstructure of the un-aged CF8M is typically composed of delta ferrite and austenite phases while precipitates such as carbides are not observed. On the other hand, after the thermal aging at 400 0C for 5000 h, a mottled contrast in nano-scale was observed in TEM bright field image as shown in Fig. 6(b), which is a typical indication ofthe presence of Cr-rich region and Fe-rich region formed by the spinodal decomposition [2, 11-13]. Formation ofthe Cr-rich a' phase in

ferrite provides the strengthening mechanisms that increase strain hardening and local tensile stress [14]. Previously, it was reported that M23C6 type carbides were observed at austenite/ferrite inter-phase boundaries after thermal aging at 343 °C for 20000 h [15]. However, in this study, such carbides were not observed.

Ferrite

211- 200

B=Z=[011]

Austenite

Austenite

220200 020'

B=Z=[001]

(a) TEM micrograph ofthe un-aged CF8M

(b) TEM micrograph ofthe aged CF8M at 400 °C for 5000 h

Fig. 6. TEM images of (a) the un-aged and (b) aged CF8M.

3.3. Small punch test and development of correlations

The maximum load values of SP test are used to develop the correlation with tensile strength. Fig. 7 shows the relationship between tensile strength and the maximum load measured in the small punch test. Previously, Mao et al. have derived an empirical linear relationship between SP load and tensile properties [16]. Currently, tentative correlation is proposed for stainless steel welds and CF8M as following equation (1).

oUTS = 2.15 x Pm

where, outs and Pmax are tensile strength of standard tensile tests and maximum load of SP tests respectively. Fig. 7 shows maximum SP load and tensile strength based on the above linear relationship. Wang et al. reported that J-integral could be expressed as following equation (2) [17].

integral

2rcrt 2 (n +1)'

where, SPE (small punch energy) is area under the load-deflection curve, r and t represent radius of small punch ball and specimen thickness, respectively. At the same time, oys and n are the yield strength and hardening exponent from tensile tests. £ is equivalent strain, expressed as a function of angles (a, P) [18]:

_ , I 1 + cos a , ,

£ = In I - I + In

1 + cosß

2 + 2 cos a (1 + cosß )2

where, a and P are angle at the boundary and angle made by the surface normal [18].As the angles are not easily measurable parameters, previous studies related angles (a, P) to deflection as (4):

8 = rsin2pln[ tan{-Pl2\ + r(l - cosp) tan(a / 2)

Meanwhile, angles a and P are related by the following the equation (5) [18]:

sin a = —sin P b

In this study, radius of ball is 1.2 mm, while angle a is set to values ranging from 0 to 900. The value for b, defined as the effective specimen radius was 1.95 mm. As shown in Fig. 8, fitting the values of 5 and equivalent strain by a polynomial expression, a relationship between 5 and equivalent strain is obtained as follows.

в = 0.0965S2 + 0.14548 Then, the Jmtegrai is expressed as: SPE

integral

2nrt 2(n +1)

(0.0965Ô2 + 0.14545)"

900 800 700 600 500 400 300 200 100

Test condition: room temperature, 320 °C

J Â U.T.S = 2.15 * P +228 max

■ ■B

open simbol: un-aged

I • Дд close simbol: aged at 400 °C for 5,000 h

/в □ ER316L at room temperature

□ ER316L at 320 °C

О ER347 at room temperature

О ER347 at 320 °C

A CF8M at room temperature

A CF8M at 320 °C

100 200 300 400 500 600 700 800 900

P [kgf]

may L о J

Fig. 7. Correlation between maximum load (Pmax) in SP tests and tensile strength (outs.)-

This process to calculate Jic is termed modified energy method (MEM), proposed by Yang et al [19]. Fig. 9 shows the relation between Jic obtained from standard test and that from MEM. A linear relationship could be fitted as the following equation (8).

J = 2 014* J -137 5

J1C (s tan dard) 1C (MEM)

However, it should be noted that there is considerable scatter in the data. Therefore, it is necessary that additional tests and analysis should be conducted to develop a more reliable correlation.

Fig. 8. Relationship between 8 and equivalent strain.

Fig. 9. Correlation between Jic estimated from modified energy method and Jic measured using CT specimen.

4. Conclusions

Stainless steel welds of ER316L and ER347 as well as CASS (CF8M) were thermally aged at 400 0C for up to 5000 h and the mechanical property changes were measured. In case of ER316L and ER347 welds, the increase in tensile strength and reduction in elongation was moderate both at room temperature and 320 0C. Meanwhile, for CF8M, the changes in tensile property were somewhat greater. The larger reduction of ductility for CF8M could be related to the higher ferrite content (34 vol. %) compared to that of SS welds (10-11 vol. %). Consequently, degradation of fracture toughness was greatest for CF8M, while less significant for stainless steel welds, which could be explained by the large difference in ferrite content. The degradation of mechanical properties is thought to be attributed to the effect of the spinodal decomposition as observed in the microstructure analysis. Meanwhile, the change in fracture resistance was minimal after aging for ER347 weld compared to ER316L weld despite the similar change in tensile property.

Small punch test was employed to develop a correlation with standard mechanical tests such as uniaxial tensile test and J-R test. A linear relationship was observed when maximum SP load was compared to tensile strength. Fracture toughness value was estimated using SP results by modified energy method. Though a linear relationship is also observed between the estimated Jic values and those measured from CT specimens, the scatter was rather large. Further test and analysis are in progress to develop a more reliable correlation for fracture toughness.

Acknowledgements

This study was mainly supported by the Korea Hydro and Nuclear Power Co., Ltd. as the Proactive Material Aging Management Project. Financial support for two of the authors is provided by the BK-Plus Program of the MSIP of the Republic ofKorea.

References

[1] O. K. Chopra, and A. Sather, Initial assessment of the mechanisms and significance of low-temperature embrittlement of cast stainless steels in LWR systems. NUREG/CR-5385, Argon National Laboratory, (1990).

[2] H. M. Chung, and T. R. Leax, Embrittlement of laboratory and reactor aged CF3, CF8, and CF8M duplex stainless steels. Materials Science and Technology, 6 (1990) 249-262.

[3] F. H. Huang, Fracture toughness of aged stainless steel primary piping welds evaluated by multiple and single-specimen methods, Journal ofPressure Vessel Technology, 115 (1993) 201-206.

[4] J. S. Cheon, and I. S. Kim, Evaluation ofthermal aging embrittlement in CF8 duplex stainless steel by small punch test. Journal ofNuclear Materials, 278 (2000) 96-103.

[5] D. J. Alexander, K. B. Alexander, M. K. Miller, R. K. Nanstad, Y. A. Davidov, The effect of aging at 343 oC in the microstructure and mechanical properties oftype 308 stainless steel weldments, NUREG/CR-6628, (2000).

[6] C. Jang, H. Jang, S. Hong, and J. G. Lee, Evaluation of the recovery of thermal aging embrittlement of CF8M cast stainless steels after reversionheattreatments, JournalofPressure Vessels andPiping, 131 (2015) 67-74.

[7] D. J. Gavenda, W. F. Michaud, T. M. Galvin, W. F. Burke, O. K. Chopra, Effects of thermal aging on fracture toughness and charpy-impact strength of stainless steel pipe welds, NUREG/CR-6428, (1996)

[8] X. Mao, Small punch test to predict ductile fracture toughness JIC and brittle fracture toughness KIC, Scripta Metallurgica et Materialia, 25 (1991)2481-2485.

[9] ASTM-E8/E8M-13a, Standard test methods for tension testing ofmetallic materials, ASTM International, (2013)

[10] ASTM-E1820-13, Standard test method for measurement offracture toughness, ASTM International, (2013)

[11] M. K. Miller, J. Bentley, APFIM and AEM investigation of CF8 and CF8M primary coolant pipe steels, Materials Science and Technology, 6 (1990) 285-292.

[12] H. Abe, Y. Watanabe, Low-temperature aging characteristics oftype 316L stainless steel welds: dependence on solidification mode, Metallurgic and Materials Transactions A, 39 (2008) 1392-1398.

[13] H. Jang, S. Hong, C. Jang. J. G. Lee, The effects fo reversion heat treatment onthe recovery of thermal aging embrittlement ofCF8M cast stainless steels, Materials and Design, 56 (2014) 517-521

[14] O. K. Chopra, Estimation offracture toughness of cast stainless steels during thermal aging in LWR systems. NUREG/CR-4513, Argon National Laboratory, (1994).

[15] D. J. Alexander, K. B. Alexander, M. K. Miller, R. K. Nanstad, The effect of aging at 343 oC on the microstructure and mechanical properties oftype 308 stainless steel weldments, NUREG/CR-6628, Oak Ridge National Laboratory, (2000).

[16] X. Mao, H. Takahashi, Development of a future-miniaturized specimen of 3 mm diameter for TEM disk small punch tests, Journal of Nuclear Materials, 150 (1987) 42-52.

[17] Z-X. Wang, H-J Shi, J. Lu, P. Shi, X-F Ma, Small punch testing for assessing the fracture properties ofthe reactor vessel steel with different thicknesses, Nuclear Engineering and Design, 238 (2008) 3186-3193.

[18] J. Chakrabarty, A theoty of strtch forming over hemispherical punch heads, international Journal of Mechanical Sciences, 12 (1970) 315325.

[19] Z. Yang, Z-W. Wang, Relationship between strain and central deflection in small punch creep specimens, International Journal of Pressure Vessels and Piping, 80 (2003), p.397-404.