Application of ultrasonic methods for early detection of thermal damage in 2205 duplex stainless steel

NDT&E International 54 (2013) 19–26

Contents lists available at SciVerse ScienceDirect

NDT&E International journal homepage: www.elsevier.com/locate/ndteint

Application of ultrasonic methods for early detection of thermal damage in 2205 duplex stainless steel A. Ruiz a,n, N. Ortiz a, A. Medina a, J.-Y. Kim b, L.J. Jacobs b,c a

´ rgicas, UMSNH, Morelia, Me´xico Instituto de Investigaciones Metalu School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0355, USA c G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0355, USA b

a r t i c l e i n f o

abstract

Article history: Received 28 May 2012 Received in revised form 19 November 2012 Accepted 26 November 2012 Available online 4 December 2012

Early thermal damage in 2205 duplex stainless steel which is caused by the precipitation of second phases during a short term exposure to high temperature (700 1C) is investigated. Nonlinear ultrasonic measurements are performed and their results are compared with those from ultrasonic velocity and attenuation measurements. Experimental results show that the measured acoustic nonlinearity parameter is more sensitive than the ultrasonic longitudinal velocity and attenuation to the precipitation of chi and sigma phases early in the aging treatments. The results from the nonlinear ultrasonic measurements are also supported with those from the scanning electron microscopy (SEM), Rockwell C hardness and Charpy impact test. Especially notable is the close correlation between the hardness and the nonlinearity parameter. This research therefore proposes the nonlinear ultrasonic method as a nondestructive assessment means for early detection of thermal degradation of mechanical properties in 2205 duplex stainless steel. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Nonlinear Rayleigh waves Second order harmonic Longitudinal velocity Attenuation coefficient Thermal damage

1. Introduction Nonlinear ultrasonic techniques have been used to characterize inherent material nonlinearity and material damage such as plasticity. When a finite amplitude sinusoidal ultrasonic wave of frequency o is introduced in a nonlinear elastic material, a second harmonic of frequency 2o is generated as a result of its interaction with the material microstructure; this is called second harmonic generation. Measurements of the amplitude ratio of the fundamental and second harmonic waves have been performed by several authors for the purpose of understanding the relationship between the material nonlinearity and the material microstructural state [1–4]. The effect of second phase precipitates on the nonlinear parameter has also been investigated and the nonlinear parameter in some cases linearly increases as the volume fraction of second phase increases [5,6]. Recently, a reliable experimental technique for nonlinear ultrasonic measurements using Rayleigh surface waves has been developed and applied to characterize fatigue damage and residual stresses in various metals including a nickel base superalloy and an aluminum alloy [7,8]. Some advantages of using nonlinear Rayleigh surface waves as a NDE technique are: Rayleigh waves do not require access to both sides of a component as opposed to most

n

Corresponding author. Tel.: þ52 443 316 8355; fax: þ 52 443 322 3500x4010. E-mail address: [email protected] (A. Ruiz).

0963-8695/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ndteint.2012.11.009

techniques based on bulk waves; the energy of Rayleigh waves is concentrated near the stress-free surface which can lead to a stronger nonlinear effect as compared to bulk waves. In addition, Rayleigh waves propagate far distances, making them an ideal tool to interrogate large areas. Duplex stainless steels have a wide range of applications in different industrial fields such as shipbuilding, petrochemical, and chemical industries. The main applications of these steels are related to their high strength and excellent toughness properties as well as their high corrosion resistance [9]. The excellent mechanical properties exhibited by these steels are due to a duplex microstructure that consists of approximately the same amounts of austenite (g) and ferrite (d). To some degree austenite ensures the toughness of the material while ferrite is responsible for the strength [10]. However, during some manufacturing processes such as welding, these steels are inevitably subjected to elevated temperatures raising the susceptibility of these steels to the precipitation of second phases. Of main concern is the precipitation of s-phase during the exposure to elevated temperatures in the range from 650 1C to 900 1C [11]. The sigma phase is a hard and brittle intermetallic phase and studies on the microstructural evolution in 2205 duplex stainless steel aged at high temperatures show that s-phase precipitates at the g/d or d/d grain boundaries in short aging times throughout a heterogeneous nucleation process [12–16]. It has been observed that the fast formation kinetics of s-phase dramatically decreases impact properties and corrosion resistance [17,18]. Several researchers

20

A. Ruiz et al. / NDT&E International 54 (2013) 19–26

have investigated the phenomena of ferrite decomposition and sigma phase precipitation [19–23]. The mechanism of ferrite transformation is relatively well understood. Few investigations have been conducted to nondestructively characterize these thermal effects on duplex stainless steels. Albuquerque et al. [24] studied the thermal effect at 475 1C on a duplex stainless steel (UNS S31803) using ultrasonic wave velocity measurements and found that the longitudinal velocity is sensitive and could detect phase transformations by the spinodal decomposition. Ruiz et al. [25] investigated the thermal effect on duplex stainless steel using ultrasonic attenuation and shear wave velocity measurements. Their results indicate that the measured high frequency attenuation can predict thermal degradation after 30 min holding time at 700 1C. Elmer et al. [26] studied the microstructural transformation of ferrite, austenite, and sigma phases of 2205 duplex stainless steel using synchrotron radiation. Based on the amounts of sigma, ferrite, and austenite phases measured at 850 1C, they concluded that the dissolution temperature is underestimated by the thermodynamic calculations. The main concern is that regardless of the temperature in the 650 1C–900 1C range, s-phase begins to precipitate at relatively low aging times and according to the reported literature, most of the damage to the impact capabilities occurs at these low aging times, e.g. less than 30 min at 700 1C [25]. No study has shown a sufficient sensitivity of linear ultrasonic methods to detect changes in this early stage of thermal degradation. In an effort to find a reliable NDE technique capable of detecting the precipitation of sigma phase at early stages, this investigation undertakes an interdisciplinary research in which the linear and nonlinear ultrasonic measurements as well as destructive test and scanning electron microscopy are jointly performed for the purpose of assessing thermal damage produced at low aging times in 2205 duplex stainless steel specimens.

2. Experimental procedure 2.1. Material preparation, thermal aging, and mechanical test Type 2205 duplex stainless steel was used in this study, the material was received in a plate shape (thickness¼12.7 mm) and with the following chemical composition in wt%: 0.05C, 22.5Cr, 5.8Ni, 3.1Mo, 66.01Fe, 0.159V, 0.02W, 0.055Co, and 1.5Mn. Seven specimens were obtained from the plate with each specimen measuring 300  50  12.7 mm3 and the specimen’s longitudinal direction coincident with the rolling direction. Aging treatments were performed at 700 1C for different holding times ranging from 10 min to 24 h; these treatments were interrupted and the specimen was water-quenched. The ultrasonic measurements (linear ultrasonic velocity and attenuation, and nonlinear acoustic parameter) were performed as described below. Charpy impact test and Rockwell C hardness test were selected to measure the performance of the 2205 duplex stainless steel during the aging treatment. Charpy impact test is commonly used to determine the brittleness of a material. This test detects small changes in the impact property of the material that has been affected by different fabrication processes and generally these changes are not detected by the quasi-static tension test. The test is conducted on a specimen with a square cross section (10  10 mm) and contains a 2 mm deep notch with a 0.25 mm root radius and a 451 angular aperture. The specimen is supported horizontally as a beam. An impact pendulum is used to strike behind the notch forcing the specimen to bend and fracture at high strain rates. The energy absorbed during the fracture is usually expressed in joules. Charpy impact test was performed at the room temperature. Hardness measurements were made by using a Rockwell C hardness tester with a load of 150 kgf.

In addition, for the microstructural analysis and sigma phase content measurements, specimens were prepared by conventional metallographic polishing and etching in a KOH electrolyte (50 g KOH, 100 ml water) with a voltage of 3 V for approximately 10 s. Microstructural examination of specimens was done using a JEOL JSM-5910LV scanning electron microscope (SEM). Finally, sigma phase content was measured using commercial software and compared to an existing sigma phase predicting model.

2.2. Ultrasonic measurement 2.2.1. Nonlinear Rayleigh surface wave measurements Consider a Rayleigh wave propagating near the free surface of a half-space. It has been shown [7] that the acoustic nonlinearity parameter is related with the surface normal displacements in the following equation: qffiffiffiffiffiffiffiffiffiffiffiffiffi cl c2l c2R 8u2o ð1Þ b¼ 2 o X 1 u2o 2cs =cR 2 1 where X1 is the distance of Rayleigh wave propagation, uo and u2o are the out-of-plane displacement amplitudes of the fundamental and second-order harmonics, cl, cs and cR are the longitudinal, shear and Rayleigh wave speeds in the solid, respectively, and o is the angular frequency. The nonlinearity parameter measured from nonlinear surface waves are related to the microstructural changes occurring at different holding times in the 2205 duplex specimens during thermal aging. For measuring the nonlinearity parameters an ultrasonic measurement setup shown in Fig. 1 is used. A high power gated amplifier RAM-5000 Mark IV (RITEC Inc, Warwick, RI) is used for the generation of nonlinear Rayleigh surface waves. As shown in [7,8] the input voltage level needs to be sufficiently high in order to make a reliable nonlinear ultrasonic measurement; this study uses a constant input voltage at 90% of the maximum output level of the amplifier (  734 Vpp with the transducer loading). A 34 cycle tone burst signal is used to obtain a sufficient length of steady-state portion in the measured time domain signal. Commercial narrow-band piezoelectric transducers having center frequencies of 2.25 and 5 MHz are used as the transmitter and the receiver, respectively. Wedges are specially designed and manufactured from a Plexiglas block to generate and detect Rayleigh waves in the steel specimens. The transducers are coupled to the wedges with light lubrication oil and these transducer/wedge assemblies are coupled to the sample using the same couplant. Measurements are performed at multiple propagation

PC trigger low voltaje high voltaje

GPIB surface wave Ch1 Ch2 Ch3 Ch4

High Power Amplifier RAM-5000 50 Ω terminator emitter surface wave

receiver

specimen Fig. 1. Experimental setup for nonlinear surface wave measurements.

A. Ruiz et al. / NDT&E International 54 (2013) 19–26

distances that vary from 30 to 200 mm from the edge of the transmitting wedge. This experimental setup provides a detection system that measures a voltage signal proportional to the out-of-plane (normal) surface displacement. The received signals are low-pass filtered with a cut-off frequency of 20.4 MHz and then sent to an oscilloscope (Tektronix TDS 420) that records these signals with a sampling frequency of 250 MHz and 15,000 sample points. 512 signals are averaged to increase the signal-to-noise ratio (SNR). The digitized time-domain signal is then transferred to a PC through GPIB for postsignal processing. The internal trigger signal from the RAM-5000

0.8

transient portion

steady state portion ringing effect

Amplitude [V]

0.4

0

-0.4

-0.8 65

70

75

80

85

90

Time [s]

14

1

Fundamental Amplitude

0.8 10

0.7 0.6

8

0.5 6

0.4 0.3

4

0.2 2

2nd harmonic amplitude

0.9

12

0.1

0

0 0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Frequency MHz Fig. 2. (a) Time-domain signals, and (b) Fourier spectrum (FFT that shows the amplitudes of the fundamental and second harmonics).

21

amplifier is used as a reference trigger. Fig. 2(a) shows a typical Rayleigh wave signal, generated and detected with the wedge transducers at a propagation distance of 130 mm. The entire length of this time-domain signal consists of a transient part (2 cycles) at the beginning, a steady-state portion (34 cycles), and a few cycles of transient ringing at the end. A Hann window is imposed on the steady-state part of the signal, and a fast Fourier transform (FFT) is performed on the windowed signal. Fig. 2 also shows the amplitudes of the fundamental (A1) and second harmonic (A2) signal in the frequency domain. Note that (A1) and (A2) are uncalibrated quantities and the nonlinearity parameter is determined from the slope of the relation A2/(A1)2 versus the propagation distance. The objective of this research is to track the relative change of nonlinearity during the aging process and thus the plot of the acoustic nonlinearity parameter normalized by its initial value for the intact base material is shown. Since the method relies on the use of fluid coupled contact transducers, a large variation of the fundamental amplitude is unavoidable. So, if the nonlinearity is determined from the relationship A2 versus A21, the effect of the fundamental amplitude will be considerable. The present method instead takes a different approach, i.e., the nonlinearity is determined from the relationship between the normalized second harmonic amplitude (A2/A21) versus the propagation distance with the transmitter at a fixed location. In this approach, the nonlinearity is the slope of this relationship, the variation and other nonlinearities from the transmitter appears as the offset of (A2/A21), and the variation from the receiver appears as an error bar. While not completely free from the effects of other nonlinearities and the varying fundamental, this approach has shown to provide consistent results even for varying fundamental amplitudes (see for example, Walker et al. [27]). In other words, the slopes of the linear fits – the nonlinearity – with different offsets are quite close for a given material state. 2.2.2. Linear ultrasonic measurements The longitudinal wave velocity and attenuation were also measured on the aged 2205 duplex stainless steel specimens. Fig. 3(a) shows a schematic diagram of the experimental setup used in the ultrasonic velocity and attenuation measurements. For longitudinal velocity measurements, a single transducer of 5 MHz nominal central frequency was used in a pulse-echo immersion configuration. The transducer is excited by a Panametrics 5073PR broadband pulser/receiver. The ultrasonic signal is digitized and averaged by a LeCroy Wavejet 332 oscilloscope, and the time of flight between two consecutive back wall echoes is measured at five different positions on each samples. For accurate time measurement,

BW1

BW2

Amplitude [a.u.]

transducer

P1

P2

steel specimen Time [a.u.] Fig. 3. (a) Experimental setup for attenuation and longitudinal wave velocity measurements, and (b) explanation of positive zero crossings used for time measurements.

22

A. Ruiz et al. / NDT&E International 54 (2013) 19–26

we pick the corresponding positive zero crossing points in the first (P1) and second (P2) back wall echo signals as shown in Fig. 3(b). In the attenuation measurement, two transducers of 15 and 20 MHz nominal central frequency were used in pulse-echo immersion configuration. The transducer is excited by a Panametrics 5073PR broadband pulser/receiver. The ultrasonic signal is digitized and averaged by a LeCroy Wavejet 332 oscilloscope and then sent to the computer for further processing. The attenuation as a function of frequency is calculated from the difference in the measured spectra of two broadband signals. In our case, the specimen’s first (BW1) and second (BW2) back wall echoes are used. Spectrum analysis is performed to calculate the attenuation of the longitudinal wave for this; the two echo signals are FastFourier-Transformed to calculate their frequency spectra. The specimens were carefully polished with 2000-grade sandpaper to minimize surface roughness induced attenuation. The attenuation coefficient is calculated after accounting for all experimental factors affecting measurements, i.e., impedance mismatch and diffraction losses. The impedance mismatch loss was calculated using Eq. (2) from [25]. The diffraction losses associated with the beam spreading are calculated using the magnitude of the Lommel diffraction correction formula from Rogers and Buren [28].

3. Results and discussion 3.1. Effect of aging treatment on microstructural evolution In the temperature range between 650 1C and 900 1C and depending on the holding time, duplex stainless steel experiences

a phase transformation through the eutectoid reaction d-g2 þ s where secondary austenite (g2) forms and sigma phase (s) nucleates. These microstructural changes reduce the ferrite content and significantly affects the impact properties of the 2205 duplex stainless steel. It has been reported [22] that in the same temperature range another intermetallic compound called (Mo-rich) chi (w) phase precipitates in the d/d interface but it has a smaller volume fraction and different structure, composition and morphology from those of the s phase. To show the effects of temperature and holding time on the ferrite transformation, the SEM micrograph in Fig. 4(a) shows the microstructure of the duplex stainless steel after 30 min aging time at a temperature of 650 1C where no appreciable changes in the content and morphology of g and d phases, i.e., the duplex phase characteristic of this steel is maintained with large and elongated island-like austenitic grains (bright) in a more or less continuous matrix of ferrite (dark). The sequence of SEM micrographs 4(b)–(d) shows the microstructural changes that take place in the 2205 duplex stainless during this isothermal aging at 700 1C for different times. Fig. 4(b) shows slight changes in the microstructure after 10 min of exposure but the amount of g and d remains similar to those of Fig. 4(a). However there is a gradual formation of new grain boundaries that break down the initially elongated ferrite matrix to form small ferrite grains in cellular shape. Also, the g/d interface is more clearly defined showing bright areas that indicate the beginning of the precipitation of second phases. This condition leads to a loss of chromium in the neighboring ferrite region, and the migration of the initial g/d grain boundary into the ferrite grain [21–23]. Also, it has been reported that at 700 1C, chi phase was observed to appear earlier

γ δ

γ δ

γ

γ σ δ Fig. 4. Microstructure of the 2205 duplex stainless steel (a) aged for 30 min at 650 1C, (b) aged for 10 min at 700 1C, (c) aged for 30 min at 700 1C and (d) aged for 2880 min at 700 1C.

A. Ruiz et al. / NDT&E International 54 (2013) 19–26

23

than sigma phase [29]. Elmer et al. [30] continuously monitored the transformation of sigma phase using synchrotron X-Ray diffraction and found that sigma phase is detectable at approximately 30 min. In Fig. 4(c) it can be seen that at 30 min holding time the initially elongated ferrite grains break down to form ferrite grains with cell like shape, while austenite remains with out change in its shape. Other than that, no significant difference is observed between 10 min and 30 min samples. Finally, Fig. 4(d) shows the effect of longer aging times where relatively big particles of sigma phase are present. From the microstructural analysis in Fig. 4, it is clear that at low aging times the changes in the morphology of d and g phases are very subtle and difficult to distinguish. On the other hand, as it will be discussed, the effect of these slight changes in the mechanical properties is significant and therefore it is of great importance to qualitatively detect early degradation of the microstructure of 2205 duplex stainless steel. 3.2. Nonlinear and linear ultrasonic measurements

Fig. 6. Normalized slopes versus the aging time at 700 1C.

Normalized second harmonic amplitude A2/(A1)2

The measured relative acoustic nonlinearity parameters, A2/(A1)2 (or normalized second harmonic amplitude) versus the propagation distance for these samples are shown in Fig. 5. From Eq. (1) it is inferred that the slope of A2/(A1)2 versus the propagation distance is proportional to b. It is observed that these normalized second harmonic amplitudes get saturated with the increase of the propagation distance due to the beam spreading effect. Therefore, the slope is determined from the data in a distance before this saturation occurs such as between 20 and 60 mm taking four significant digits in MATLAB fitting tool. Fig. 6 shows the calculated slope as function of aging time, the curve of the normalized slope show three distinctive stages of change. Specifically, it clearly shows that initially the slope increases abruptly as aging times increases (stage I): i.e., the slope of the sample aged for 10 min is larger than the untreated base material (BM) sample. It has been discussed that the precipitations of chi and sigma phases begin on grain boundaries at short aging treatments [23,24]. Consequently the initial large change in the measured acoustic nonlinearity parameter may be related to the precipitation of these second phases and as it has been reported, sigma phase plays the more important role in the degradation of the mechanical properties of the duplex stainless steel due to the fact that the amount of sigma phase increases constantly with aging time until it reaches a maximum volume fraction. On the contrary, chi phase fraction remains almost constant after it had

3

×10-4 24 hr 10 min. 1 hr 30 min 2 hr BM

2.5 2 1.5

1 0.5

precipitated in the initial stage of aging treatment [29]. Stage II is characterized by small changes of the slope since after 30 min the slope begins to decrease rapidly showing a minimum at 2 h and finally on stage III, the slope increases again at 24 h. Similar up-and-down trend of the nonlinear parameter as a function of aging time is found in a long term thermal degradation in ferritic Cr–Ni alloy steel plates using nonlinear Lamb waves [4], aged M250 maraging steel [31] and thermally embrittled Inconel 718 [32]. The higher sensitivity of the nonlinear measurements to the thermal embrittlement is demonstrated when Fig. 6 is compared against the results obtained from the linear longitudinal wave velocity and attenuation measurements. In Fig. 7(a) the longitudinal velocity remains almost unchanged from the untreated condition to almost 30 min showing a small drop of approximately 0.37% in contrast to the 60% change in the nonlinear parameter. Similarly, Fig. 7(b) shows the attenuation coefficient obtained at five different frequencies as function of holding time. As in the velocity measurements the attenuation shows no appreciable changes between 10 MHz and 15 MHz; it is important to mention that typical NDE evaluations are performed in the 5 MHz–10 MHz range. At higher frequencies, the attenuation coefficient starts to significantly decrease after 30 min holding time. The fact that longitudinal velocity and attenuation measurements remain almost unchanged for the time up to 30 min indicates that they are not sensitive enough to the subtle microstructural changes that take place at these short times. This could be due to the fact that the initial phase content and initial elongated shape of the duplex microstructure remains similar for these times as it is seen in Fig. 4(a) through (c). The attenuation coefficient and longitudinal velocity begin to change only when the microstructure undergoes a dramatic change in the initial shape and the volume fraction of ferrite continuously decreases to the point where the main products of transformation are austenite and small particles of sigma phase shown in Fig. 4(d). 3.3. Hardness measurements

0 0

20

40

60

80

100

120

140

Propagation distance [mm] Fig. 5. Variation in the normalized second harmonic amplitude as a function of propagation distance for the reference sample MB, aged samples 10 and 30 min, 1, 2 and 24 h. The frequency of excitation is 2.25 MHz.

Fig. 8 shows the effect of holding time on hardness. It can be observed that hardness exhibits a trend very similar to the nonlinearity, i.e., hardness linearly increases for holding times up to 30 min (stage I), begins to decrease after this time to reach a minimum value at 2 h (stage II), and then increases again to a maximum value at 24 h (stage III). To understand the changes in

24

A. Ruiz et al. / NDT&E International 54 (2013) 19–26

stages they could act as counter acting mechanisms that operate simultaneously. At initial stages of aging it seems that the mechanism is attributed to the precipitation of small second phase particles that could act in a similar manner to classical agehardened materials. The reduction in hardness could be related to the transformation of ferrite and the increase of hardness to the precipitation and coarsening of sigma phase.

Longitudinal velocity [mm/μs]

6

5.9

5.8

3.4. Impact energy determination 5.7

5.6

5.5 0.001

0.01

0.1

1

10

100

Aging time [h]

1.6 30 MHz 25 MHz 20 MHz 15 MHz 10 MHz

Attenuation coefficient [dB/mm]

1.4 1.2 1 0.8 0.6

3.5. Sigma phase content measurements 0.4 0.2 0 0.001

0.01

0.1

1

10

100

Aging time [h] Fig. 7. (a) Longitudinal velocity measurements and (b) attenuation coefficient as function of aging time for 2205 duplex stainless steel at 700 1C.

35

30 Rockwell C hardness

The importance of the early detection of sigma phase precipitation is corroborated by the results of the absorbed energy capabilities of the 2205 duplex stainless steel. Fig. 9 shows the impact characteristics of the 2205 duplex stainless steel as a function of aging time. The excellent impact resistance of this steel is deteriorated very fast depending on the temperature of the aging treatment [25]. It is important to mention that the base metal impact sample did not break during the impact tests therefore the result for the base material is not shown in Fig. 9. This corroborates the excellent impact characteristics of this steel. The slight changes in the microstructure for the sample aged for 10 min caused an important effect in the impact energy since the sample breaks at approximately 350 J. Even though the microstructure of the 10 min and 30 min are very similar, the impact properties are decreased by as much as about 50% at 30 min. After 30 min, impact energy decrease rapidly for the remaining holding times. These results corroborate the need of an early detection of thermal damage.

25

From the literature it is not possible to establish a universal master model that can predict the kinetics of ferrite decomposition since numerous physical and metallurgical parameters could affect the transformation kinetic and the modes of ferrite transformation. Among the parameters are: variations in the chemical composition, ferrite grain size and its volume fraction as well as solution treatment and conformation treatment, and mainly the effect of aging time and holding temperature. Among different models to predict the fraction of sigma phase, one of the most used is the Johnson–Mehl–Avrami (JMA) model. At early stages, sigma phase is difficult to detect with conventional methods such as metallographic analysis. In order to estimate the fraction of sigma phase at low holding times we have established the fraction of sigma phase from the SEM micrographs in which the sigma phase is easily measured using a commercial software and the results were compared to the JMA theory that relates the transformed fraction of the initial phase to the transformation

20 400 350

10 0.001

0.01

0.1

1

10

100

Aging time [h] Fig. 8. Rockwell C hardness as function of aging time.

hardness throughout the aging treatment it is important to mention that the microstructural changes of this steel are complex processes that include: precipitation of second phases, transformation of ferrite and coarsening of the precipitated second phases. Therefore, the changes in hardness are attributed to the combined effect of these phenomena and at certain aging

Absorbed energy [J]

15

300 250 200 150 100 50 0 0.01

0.1

1

10

100

Aging time [h] Fig. 9. Changes in absorbed energy in Charpy-impact test of the aged 2205 duplex stainless steel.

A. Ruiz et al. / NDT&E International 54 (2013) 19–26

hardness measurements show a trend very similar to the measured nonlinearity, which indicates that the changes in nonlinearity are caused, at least in part, by the precipitation of sigma phase. The nonlinear parameter can be used as an NDE tool since quantitative assessment of early transformation of ferrite phase in duplex stainless steel is of primary importance from the point of thermo-mechanical stability of the beneficial effects of the duplex phase.

1 0.9

Sigma phase fraction,

0.8

25

Measured sigma phase at 700° C JMA Model

0.7 0.6 0.5 0.4 0.3 0.2

Acknowledgments

0.1 0 0.001

0.01

0.1

1 Aging time [h]

10

100

1000

Fig. 10. Theoretical and experimental determination of sigma phase fraction.

time by [21,23].    1 ¼ nlnðt Þ þ lnðbÞ ln ln 1X

This work was performed in a joint effort between Georgia Institute of Technology and the Universidad Michoacana de San Nicolas de Hidalgo with funding from CONACYT Me´xico under project: CB-2010-01-152406. The authors also wish to thank CONACYT-Me´xico for its support of the Doctoral student Noemı´ Ortiz Lara during her visit to Georgia Institute of Technology.

References ð2Þ

where X is the fraction of the initial phase transformed at time t, n is the Avrami exponent that varies from 0.2 to 0.8, the constant b  can be expressed by the Arrhenius equation:b ¼ b0 exp Q =RT that depends on the temperature T and activation energy Q for transformation, and R is the universal gas constant. In order to apply this model to predict the phase transformation kinetics the following assumptions are made: The transformation occurs in isothermal conditions, the nucleation frequency is either constant, or maximum at the beginning of transformation and decreases towards the end of transformation, and the nucleation is random. Under certain conditions, the phase transformation kinetic obeys the classical JMA model, i.e., when the evolution of ln[ln(1/(1 X)] as a function of ln(t) gives a straight line. The kinetic parameters n and b are related to the transformation mechanism and transformation rate, respectively, and they can be found using simple linear regression of the experimental data. From Ruiz et al. [25] we use the values for n700 1C, b700 1C along with Eq. (2) to compare it to measured experimental values of sigma phase precipitation and the experimental and predicted values are in good agreement as it is shown in Fig. 10. From the model predictions it is clear that at times before 30 min the fraction of sigma phase is so small that it will be difficult to detect even by destructive methods. Current nondestructive methods show sensitivity at aging times where the precipitation of sigma phase has caused a significant damage to the impact properties of the 2205 duplex stainless steel and therefore these results show the importance of detecting the precipitation of sigma phase at low aging times.

4. Conclusions The capability of nonlinear measurements as an alternative NDE means for early detection of sigma phase precipitation in 2205 duplex stainless steel was investigated and the results were compared to those from the conventional ultrasonic measurements. The observations performed lead to the following conclusions: the nonlinear parameter is more sensitive to early thermal damage in aged 2205 duplex stainless steel than in ultrasonic velocity and attenuation. The changes in the nonlinear parameter presented at low aging times can be related to the precipitation of sigma phase during the isothermal aging treatment. The results of

[1] Cantrell JH, Yost WT. Nonlinear ultrasonic characterization of fatigue microstructures. Int J Fatigue 2001;23(Suppl. 1):487–90. [2] Kim J-Y, Jacobs LJ, Qu J, Littles JW. Experimental characterization of fatigue damage in a nickel-base superalloy using nonlinear ultrasonic waves. J Acoust Soc Am 2006;120:1266–73. [3] Cantrell JH. Substructural organization, dislocation plasticity and harmonic generation in cyclically stressed wavy slip metals. Proc R Soc Lond Ser A: Math, Phys Eng Sci 2004;460:757–80. [4] Xiang Y, Deng M, Xuan F-Z, Liu C-J. Experimental study of thermal degradation in ferritic Cr–Ni alloy steel plates using nonlinear Lamb waves. NDT & E Int 2011;44:768–74. [5] Cantrell JH, Yost WT. Effect of precipitate coherency strains on acoustic harmonic generation. J Appl Phys 1997;81:2957–62. [6] Hurley DC, Balzar D, Purtscher PT. Nonlinear ultrasonic assessment of precipitation hardening in ASTM A710 steel. J Mater Res 2000;15:2036–42. [7] Herrmann J, Kim J-Y, Jacobs LJ, Qu J, Littles JW, Savage MF. Assessment of material damage in a nickel-base superalloy using nonlinear Rayleigh surface waves. J Appl Phys 2006;99:124913. [8] Liu M, Kim J-Y, Jacobs L, Qu J. Experimental study of nonlinear Rayleigh wave propagation in shot-peened aluminum plates—Feasibility of measuring residual stress. NDT & E Int 2011;44:67–74. [9] Gunn RN. Duplex stainless steels: microstructure, properties and applications. Abington, Cambrige: Abington Publishing; 1997. [10] Solomon HD, Devine TM. Duplex stainless steels. Metals Park. OH: American Society for Metals; 1983. [11] Chen TH, Weng KL, Yang JR. The effect of high-temperature exposure on the microstructural stability and toughness property in a 2205 duplex stainless steel. Mater Sci Eng A 2002;338:259–70. [12] Weng KL, Chen HR, Yang JR. The low-temperature aging embrittlement in a 2205 duplex stainless steel. Mater Sci Eng A 2004;379:119–32. ¨ R. Sigma phase precipitation in duplex stainless steel [13] Sieurin H, Sandstrom 2205. Mater Sci Eng A 2007;444:271–6. [14] Sathirachinda N, Pettersson R, Wessman S, Pan J. Study of nobility of chromium nitrides in isothermally aged duplex stainless steels by using SKPFM and SEM/EDS. Corros Sci 2010;52:179–86. [15] Kordatos JD, Fourlaris G, Papadimitriou G. The effect of cooling rate on the mechanical and corrosion properties of SAF 2205 (UNS 31803) duplex stainless steel welds. Scripta Mater 2001;44:401–8. [16] Kim SB, Paik KW, Kim YG. Effect of Mo substitution by W on high temperature embrittlement characteristics in duplex stainless steels. Mater Sci Eng A 1998;247:67–74. [17] Calliari I, Zanesco M, Ramous E. Influence of isothermal aging on secondary phases precipitation and toughness of a duplex stainless steel SAF 2205. J Mater Sci 2006;41:7643–9. [18] Deng B, Jiang YM, Gao J, Li J. Effect of annealing treatment on microstructure evolution and the associated corrosion behavior of a super-duplex stainless steel. J Alloy Compd 2010;493:461–4. [19] Chen TH, Yang JR. Effects of solution treatment and continuous cooling on s-phase precipitation in a 2205 duplex stainless steel. Mater Sci Eng A 2001;311:28–41. [20] Lai JKL, Wong KW, Li DJ. Effect of solution treatment on the transformation behaviour of cold-rolled duplex stainless steels. Mater Sci Eng A 1995;203:356–64. [21] Badji R, Bouabdallah M, Bacroix B, Kahloun C, Bettahar K, Kherrouba N. Effect of solution treatment temperature on the precipitation kinetic of s-phase in 2205 duplex stainless steel welds. Mater Sci Eng A 2008;496:447–54.

26

A. Ruiz et al. / NDT&E International 54 (2013) 19–26

[22] Jackson EMLEM Visser PEd, Cornish LA. Distinguishing between Chi and Sigma phases in duplex stainless steels using potentiostatic etching. Mater Charact 1993;31:185–90. [23] Sasikala G, Ray SK, Mannan SL. Kinetics of transformation of delta ferrite during creep in a type 316(N) stainless steel weld metal. Mater Sci Eng A 2003;359:86–90. [24] de Albuquerque VHC, de Macedo Silva E, Pereira Leite J, de Moura EP, de Arau´jo Freitas VL, Tavares JMRS. Spinodal decomposition mechanism study on the duplex stainless steel UNS S31803 using ultrasonic speed measurements. Mater Des 2010;31:2147–50. [25] Ruiz A, Ortiz N, Carreo´n H, Rubio C. Utilization of ultrasonic measurements for determining the variations in microstructure of thermally degraded 2205 duplex stainless steel. J Nondestruct Eval 2009;28:131–9. [26] Elmer JW, Palmer TA, Specht ED. In situ observations of sigma phase dissolution in 2205 duplex stainless steel using synchrotron X-ray diffraction. Mater Sci Eng A 2007;459:151–5.

[27] Walker SV, Kim J-Y, Qu J, Jacobs LJ. Fatigue damage evaluation in A36 steel using nonlinear Rayleigh surface waves. NDT & E Int 2012;48:10–5. [28] Rogers PH, Buren ALV. An exact expression for the Lommel-diffraction correction integral. J Acoust Soc Am 1974;55:724–8. [29] He Y-L, Zhu N-Q, Lu X-G, Li L. Experimental and computational study on microstructural evolution in 2205 duplex stainless steel during high temperature aging. Mater Sci Eng A 2010;528:721–9. [30] Elmer J, Palmer T, Specht E. Direct observations of sigma phase formation in duplex stainless steels using in situ synchrotron X-ray diffraction. Metall Mater Trans A 2007;38:464–75. [31] Viswanath A, Rao BPC, Mahadevan S, Parameswaran P, Jayakumar T, Raj B. Nondestructive assessment of tensile properties of cold worked AISI type 304 stainless steel using nonlinear ultrasonic technique. J Mater Process Technol 2011;211:538–44. [32] Barnard D, Dace G, Rehbein D, Buck O. Acoustic harmonic generation at diffusion bonds. J Nondestruct Eval 1997;16:77–89.