Scholarly article on topic 'Acoustic emission characteristics of stress corrosion cracks in a type 304 stainless steel tube'

Acoustic emission characteristics of stress corrosion cracks in a type 304 stainless steel tube Academic research paper on "Materials engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Nuclear Engineering and Technology
OECD Field of science
Keywords
{"Acoustic emission" / Signal / "Stainless steel" / "Stress corrosion crack" / Waveform}

Abstract of research paper on Materials engineering, author of scientific article — Woonggi Hwang, Seunggi Bae, Jaeseong Kim, Sungsik Kang, Nogwon Kwag, et al.

Abstract Acoustic emission (AE) is one of the promising methods for detecting the formation of stress corrosion cracks (SCCs) in laboratory tests. This method has the advantage of online inspection. Some studies have been conducted to investigate the characteristics of AE parameters during SCC propagation. However, it is difficult to classify the distinct features of SCC behavior. Because the previous studies were performed on slow strain rate test or compact tension specimens, it is difficult to make certain correlations between AE signals and actual SCC behavior in real tube-type specimens. In this study, the specimen was a AISI 304 stainless steel tube widely applied in the nuclear industry, and an accelerated test was conducted at high temperature and pressure with a corrosive environmental condition. The study result indicated that intense AE signals were mainly detected in the elastic deformation region, and a good correlation was observed between AE activity and crack growth. By contrast, the behavior of accumulated counts was divided into four regions. According to the waveform analysis, a specific waveform pattern was observed during SCC development. It is suggested that AE can be used to detect and monitor SCC initiation and propagation in actual tubes.

Academic research paper on topic "Acoustic emission characteristics of stress corrosion cracks in a type 304 stainless steel tube"

Available online at www.sciencedirect.com

ScienceDirect

journal homepage: http://www.journals.elsevier.com/nuclear-engineering-and-technology/

Original Article

ACOUSTIC EMISSION CHARACTERISTICS OF STRESS CORROSION CRACKS IN A TYPE 304 STAINLESS STEEL TUBE

WOONGGI HWANG a, SEUNGGI BAE a, JAESEONG KIM b, SUNGSIK KANG c, NOGWON KWAG d, and BOYOUNG LEE a'*

a School of Aerospace and Mechanical Engineering, Korea Aerospace University, 76, Hanggongdaehang-ro, Daegyang-gu, Goyang-si, Gyeonggi-do 412-791, Republic of Korea

b Center for Robot Technology and Manufacturing, Institute for Advanced Engineering, Goan-ro 51beon-gil, Baegam-myeon, Cheoin-gu, Yongin-si, Gyeonggi-do 449-863, Republic of Korea

c Department of Nuclear Safety Research, 62 Gwahak-ro, Yuseong-gu, Daejeon 305-338, Republic of Korea d Ultrasonic Division, RM910, Byucksan Digital Valley II, 481-10, Gasan-Dong, Geumchun-gu, Seoul 153-803, Republic of Korea

ARTICLE INFO

ABSTRACT

Article history: Received 4 July 2014 Received in revised form 28 December 2014 Accepted 6 February 2015 Available online 4 April 2015

Keywords:

Acoustic emission Signal

Stainless steel Stress corrosion crack Waveform

Acoustic emission (AE) is one of the promising methods for detecting the formation of stress corrosion cracks (SCCs) in laboratory tests. This method has the advantage of online inspection. Some studies have been conducted to investigate the characteristics of AE parameters during SCC propagation. However, it is difficult to classify the distinct features of SCC behavior. Because the previous studies were performed on slow strain rate test or compact tension specimens, it is difficult to make certain correlations between AE signals and actual SCC behavior in real tube-type specimens. In this study, the specimen was a AISI 304 stainless steel tube widely applied in the nuclear industry, and an accelerated test was conducted at high temperature and pressure with a corrosive environmental condition. The study result indicated that intense AE signals were mainly detected in the elastic deformation region, and a good correlation was observed between AE activity and crack growth. By contrast, the behavior of accumulated counts was divided into four regions. According to the waveform analysis, a specific waveform pattern was observed during SCC development. It is suggested that AE can be used to detect and monitor SCC initiation and propagation in actual tubes.

Copyright © 2015, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society.

1. Introduction

Because mechanical structures are getting bigger and more efficient, the component materials demand high strength and

toughness as well as increased corrosion and thermal resistance. Furthermore, the working environment for mechanical structures is getting severe. Therefore, to ensure the integrity of structural materials, regular inspection of these materials is

* Corresponding author. E-mail address: bylee@kau.ac.kr (B. Lee).

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http:// creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. http://dx.doi.org/10.1016/j.net.2015.04.001

1738-5733/Copyright © 2015, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society.

increasingly emphasized. The state of structural components is monitored through a periodic inspection using nondestructive tests such as radiographic test, ultrasonic test, etc. However, these tests have certain limitations, and therefore, during an in-service inspection not all existing defects can be identified. Because cracks can only be detected after they have grown large enough, preventive detection is difficult. These disadvantages may be reduced by remote and continuous monitoring. Acoustic emission (AE) is one of those solutions. AE is a nondestructive technique based on the rapid release of energy within a material generating transient elastic wave propagation. It can detect very tiny defects in the structural material and has a relatively low test cost.

Many authors have reported the application of AE for evaluating stress corrosion cracks (SCCs) and pitting corrosion using slow strain rate test (SSRT) or compact tension (CT) specimens [1—4]. They found that SCC occurred in specific combinations of three essential conditions, namely, tensile stress or strain of a sufficient level, an aggressive electrolyte, and a susceptible material. The synergetic combination of mechanical and electrochemical processes could lead to two different modes of crack propagation, namely, (1) intergranular SCC in which cracks advance along crystal grain boundaries and (2) transgranular SCC in which cracks advance through crystal grains [5]. Whatever the SCC mechanism is, the AE signals show similar parameters and amplitude distribution [6]. Many studies have reported on the characteristics of AE signals generated by SCCs and pitting corrosion [1,3—12]. Parameters of AE signals from SCCs were studied by Leinonen et al [3], Alvarez et al [6], Sunget al [7], Shaikh et al [11], and Perrin et al [12]. Fast Fourier transform analysis of AE waveforms in SCCs was studied by Chang et al [4] and Kovac et al [5]. Mazille et al [1], Fregonese et al [8], and Xu et al [9] studied AE signals generated in pitting corrosion. Although many studies were carried out on corrosion and cracking, their exact characteristics are not revealed clearly. Moreover, because the previous studies were performed on SSRT or CT specimens, rather than on real tubes used in nuclear power plants, it is difficult to make certain correlations between AE signals and behaviors of SCC in a real tube. Therefore, it is necessary to conduct the test with a tube in similar SCC environments and analyze the AE signal behavior, which transients during the SCC process.

In this study, we made SCCs in a real 304 austenitic stainless steel tube using our own designed equipment system, and a specific AE signal pattern for the SCC process was observed and analyzed.

2. Methods

2.1. Materials and equipment

The dimensions of the AISI 304 austenitic stainless steel tube specimen (from POSCO, Pohang Steel Corporation, Korea) is as follows: diameter, 89 mm; thickness, 7.6 mm; and length, 150 mm. The composition and mechanical properties of the specimen are presented in Table 1. The inner surface of the tube was welded by gas tungsten arc welding to give the specimen residual stress (Fig. 1). According to previous studies

Table 1 - Chemical composition (wt%) and mechanical properties of the AISI 304 stainless steel.

Alloy element C Si Mn P S Cr Ni Composition 0.005 0.12 1.65 0.029 0.008 18.23 8.16 Yield strength (MPa) Tensile strength (MPa) Elongation (%) 410 669 66.5

[6,13], the maximum residual stress is formed at the heat-affected zone. In general, the welding zone is the most sensitive region for SCC [14], because 5-ferrite is transformed in the fusion line and Cr23C6 is partially distributed in the grain boundaries [15].

To make an SCC, the AISI 304 tube was installed in our own designed equipment system, and was filled with a mixture of 1M Na2S and 4M NaOH aqueous solution to simulate a corrosive environment. The amount of corrosive aqueous solution added was 50% of the volume of the tube specimen. Fig. 2 shows the test equipment layout and a cross section of the tube specimen.

To reproduce environmental conditions similar to that of a nuclear power plant, the tube was heated on the outer surface by a heating coil. The maximum temperature and pressure were measured as 383°C and 73 bar (Fig. 3).

2.2. AE testing setup

The system for recording and analyzing the AE signals included a four-channel data-acquisition instrument, storage media, sensor, and amplifier. The data-acquisition instrument was designed and manufactured by Physical Acoustics Corporation (PAC). The sensor was also manufactured by PAC. The sensor has a 400-kHz resonance frequency for the high-temperature purpose, with a maximum operation temperature of 500°C. The sensor output was amplified by a gain of 40 dB using a 1,222-charge preamplifier. A threshold level of 40-45 dB was set as a float type that can control the sensitivity of detection by keeping the voltage threshold of detection above the average background noise to minimize noise.

One high-temperature sensor was installed on each side of the flange. Once the signal had been detected by the sensors, the data were sent to the acquisition instrument and then stored immediately in the specific storage. The AE data from the specimen were filtered from electrical and environmental noise based on parameter characteristics, which have lower amplitude values than environmental noise. To distinguish the AE signals from the mechanical noise, the noise signal was

Fig. 1 - Equipment for gas tungsten arc welding and the welding bead (inner surface of tube).

Fig. 2 - Diagram of the stress corrosion crack-forming equipment and cross section of the tube specimen. AC, alternating current; AD, analog-to-digital convertor; D/A, digital-to-analog convertor; I/O, input/output.

characterized by recording the signals generated during the heating test without inserting solutions in the tube.

Results and discussion

3.1. Formation of SCC

temperature condition, it contributed a suitable condition to induce the SCC easily for the cylindrical specimen (i.e., tube specimen). In a closed tube, the pressure force affecting the cylindrical tube wall would induce hoop stress. It is estimated that the SCC was accelerated by hoop stress as well as by susceptible material, corrosive environment, and residual stress.

An SCC was developed using the custom-made manufacturing system. Fig. 3 shows the temperature and pressure variation during the test. The maximum vapor temperature recorded was 383° C and the maximum vapor pressure was 73 bar, which was measured by the pressure sensor. As shown in Fig. 3, the vapor pressure had a sudden drop at approximately 53 minutes (3,200 seconds) after the test began. It could be explained that the tube specimen was fractured at about 3,200 seconds by an SCC due to the accelerated corrosive condition. In other words, the material had already experienced SCC, which had propagated to the tube wall before the signs of leakage were evident. Because the vapor pressure induced circumferential stress, which is called "hoop stress," in the tube wall at the high-

3.2. MetaUographic observation

Signs of leaking were observed on the outer surface of the tube when the test was stopped (Fig. 4). To detect cracks, a fluorescent penetrant test (Fig. 5) was conducted on the outer and inner surfaces. Longitudinal cracks were confirmed on the surface in contact with the aqueous solution (90°). It is estimated that the cracks were accelerated by additional hoop stress as well as by susceptible material, corrosive environment, and residual stress by welding. Fig. 6 shows the metallographic images obtained with an optical microscope. A deep crack was observed with many small branches. The cracks propagated through grain boundaries.

Fig. 3 - Temperature and vapor pressure variation in the specimen.

Fig. 4 - Signs of leaking on the outer surface of the tube. (A) Top view (0°) and (B) side view (90°).

Fig. 5 - Fluorescent penetrant testing result. (A) outer surface and (B) inner surface of longitudinal cracks (90°).

Fig. 6 - Metallographic images obtained with an optical microscope. (A-E) Magnification, 200 x.

Fig. 7 - Variation of cumulative acoustic emission counts with time during stress corrosion crack of AISI 304 stainless steel in 1M Na2S and 4M NaOH aqueous solution.

3.3. AE signal

Fig. 7 shows the AE counts variation with time. The cumulative counts increased smoothly until about 2,000 seconds (region A), and the rate of increase of cumulative counts was gradually reduced (region B). After about 2,800 seconds, the cumulative counts increased rapidly until the test was over (regions C and D). Fig. 8 shows that the AE signals (rise time, amplitude, and energy) were intensively generated from region C, and the value of energy and amplitude was higher in region C than in other regions. As shown in Fig. 3, the vapor pressure indicates a sudden drop at about 53 minutes (3,200

seconds) after the test began. After region C, the energy and amplitude decreased except for the rise time in region D, which corresponds to the tube rupture.

The variation of AE counts can be divided into four stages. The first stage is a start of the test, during which homogeni-zation and chemical stabilization occur in the corrosive solution. The second stage is SCC initiation and propagation. During the third stage, a number of processes occurred such as the rupture of the passive film, dissolution of exposed bare material, repassivation of the bare metal subsequent to its dissolution, plastic deformation. Finally, pure crack propagation affects the AE signal generation [9,10,16-18].

Fig. 9 shows four specific waveforms detected during the SCC process. Waveform (A), detected at heating time, looks like continuous emission. When temperature and pressure were almost saturated, waveform (B) was generated. Waveform (B) shows that the peak amplitude takes precedence and the value of amplitude falls off. Near the maximum temperature and pressure, waveform (C) was sprinkled with the former AE signal-generating pattern. The characteristic feature of waveform (C) is the time delay, which exists until the peak amplitude is detected. Waveform (D), with an irregular waveform pattern, occurred when leakage occurred.

3.4. Concluding remarks

Based on the study results, the following conclusions have been made. (1) SCC could be artificially induced on the AISI 304 austenitic stainless steel tube in our own designed system. (2)

»-WOO-

y.fSOOO-

„ moo-

K-moo-

a-»»-

7030000-

A B ci D ■

—-(-

» ■

• • •

• •

■ ■ • • " B ■

* * ■ • • ■ •y.

1 . ■» . V

" • * (■ , • 1 ■ hi' I • • fi-C

.. '..... . ■ 'AV'.U .. "J"

r i;t >iti ! ¡.ji .■!•;(I !i • : • : • - —r *-i ' ,—'—- • 1 • , • ," • r

Rise Time

Amplitude

Energy

Region Energy Amplitude Risetime

A, B Low v 1

C High High High

D Low Low High

Fig. 8 - Energy, amplitude, and rise time distribution with time during stress corrosion crack of AISI 304 stainless steel in 1M Na2S and 4M NaOH aqueous solution.

Fig. 9 - Specific waveforms during stress corrosion crack development process. (A) Heating section. (B) Saturated section. (C) Maximum temperature and pressure section. (D) Leaking section.

The events of the AE signals increase under the pressurized condition with high temperature. The increasing tendency of AE counts is similar to previous studies using flat bar or CT specimens. The number of AE counts rapidly increased until the pressure dropped. The AE signals can be divided into four stages. (3) During the SCC development process, specific AE waveforms were generated, which can distinguish the SCC stages by heating, saturation, maximum temperature and pressure, and leakage. This could be used to distinguish SCC damage signals from normal signals and help the supervisor to repair the damage before a deep crack forms.

Conflicts of interest

There were no conflicts.

REFERENCES

[1] H. Mazille, R. Rothea, C. Tronel, An acoustic emission technique for monitoring pitting corrosion of austenitic stainless steels, Corros. Sci 37 (1995) 1365-1375.

[2] J.K. Lim, D.S. Chung, S.H. Chung, Evaluation of SCC susceptibility of weld HAZ in structural steel (I)—material properties and strain rate, J. Korean Weld. Soc 11 (1993) 48-60.

[3] H. Leinonen, T. Schildt, H. Hanninen, Stress corrosion cracking—crevice interaction in austenitic stainless steels characterized by acoustic emission, Metall. Mater. Trans. A 42 (2011) 424-433.

[4] H. Chang, E. Han, J.Q.. Wang, W. Ke, Acoustic emission study of corrosion fatigue crack propagation mechanism for LY12CZ and 7075-T6 aluminum alloys, J. Mater. Sci 40 (2005) 5669-5674.

[5] J. Kovac, C. Alaux, T. James Marrow, E. Govekar, A. Legat, Correlations of electrochemical noise, acoustic emission and complementary monitoring techniques during intergranular stress-corrosion cracking of austenitic stainless steel, Corros. Sci 52 (2010) 2015-2025.

[6] M.G. Alvarez, P. Lapitz, J. Ruzzante, AE response of type 304 stainless steel during stress corrosion crack propagation, Corros. Sci 50 (2008) 3382-3388.

[7] K.Y. Sung, I.S. Kim, Y.K. Yoon, Characteristics of acoustic emission during stress corrosion cracking of Inconel 600 alloy, Scr. Mater 37 (1997) 1255-1262.

[8] M. Fregonese, H. Idrissi, H. Mazille, L. Renaud, Y. Cetre, Monitoring pitting corrosion of AISI 316L austenitic stainless steel by acoustic emission technique: choice of representative acoustic parameters, J. Mater. Sci 36 (2001) 557-563.

[9] J. Xu, X. Wu, E.-H. Han, Acoustic emission during pitting corrosion of 304 stainless steel, Corros. Sci 53 (2011) 1537-1546.

[10] G. Du, J. Li, W.K. Wang, C. Jiang, S.Z. Song, Detection and characterization of stress-corrosion cracking on 304 stainless steel by electrochemical noise and acoustic emission techniques, Corros. Sci 53 (2011) 2918-2926.

[11] H. Shaikh, R. Amirthalingam, T. Anita, N. Sivaibharasi,

T. Jaykumar, P. Manohar, H.S. Khatak, Evaluation of stress corrosion cracking phenomenon in an AISI type 316LN stainless steel using acoustic emission technique, Corros. Sci 49 (2007) 740-765.

[12] M. Perrin, L. Gaillet, C. Tessier, H. Idrissi, Assessment of stress corrosion cracking in prestressing strands using AE technique, J. Acoust. Emiss 26 (2008) 32-39.

[13] Y.K. Woo, Study on the Prediction of the Stress Corrosion Crack Occurrence in the Tube and Control the Crack Depth Using Acoustic Emission Method (Master's thesis),

Korea Aerospace University, Goyang-si, Gyeonggi-do, South Korea, 2013.

[14] H.S. You, J.K. Lim, S.H. Chung, A study on the stress corrosion cracking evaluation for weld joint of steel by using miniaturized small specimen, J. Korean Weld. Soc 12 (1994) 411-423.

[15] K.I. Kim, I.C. Kang, A study on stress corrosion cracking of weld zone in 304-stainless steel, J. Korean Weld. Soc 5 (1987) 35-43.

[16] H.S. Yu, E.G. Na, S.H. Chung, A study on the stress corrosion cracking behaviors for weld joint of steel with various pH values in synthetic sea water, J. Korean Weld. Soc 13 (1995) 510-520.

[17] A.J. Russell, D. Tromans, A fracture mechanics study of stress corrosion cracking of type-316 austenitic steel, Metall. Trans. A 10 (1979) 1229-1238.

[18] J.H. Kim, J.S. Kim, J. Lee, N.G. Kwag, B.Y. Lee, The basic study on the method of acoustic emission signal processing for the failure detection in the NPP structures, J. Korean Soc. Nondestruct. Test 29 (2009) 485-492.