Influence of Deleterious Phases in Super-Duplex Steel SAF 2507 in the

Ultrasonic Lamb Waves Propagation

Cláudia Teresa T. FARIAS1, Ygor Tadeu B. SANTOS

1, Diego Henrique S. ZANINI

2,

Rodrigo E. COELHO3, Eduardo F. SIMAS FILHO

4, Ivan C. SILVA

1

1 Ultrasound Laboratory, Federal Institute of Bahia; Salvador, Brazil

2Mechatronic Engineering, Salvador University; Salvador, Brazil

3Materials Characterization Laboratory, Federal Institute of Bahia; Salvador, Brazil

4Electrical Engineering Program, Federal University of Bahia; Salvador, Brazil

(GPEND) Nondestructive Testing Research Group - Phone: +55 71 21029423; e-mail: gpend@ifba.edu.br

Abstract This work aims to detect possible microstructural alterations in the SAF 2507 steel using the non-destructive

ultrasound technique of Lamb waves, allied to a proper digital signal processing chain. Super-duplex stainless

steel was developed, for the offshore work. In service, generally, this materials are submitted to high

temperatures where, under specific conditions, can occur microstructural transformations, such as, different

deleterious phases precipitation, which promotes reduction of the tenacity and the corrosion resistance.

Inspections were conducted in shaped plate super duplex steel samples, isothermally treated at different

temperatures, in order to induce microstructural changes. Ultrasonic testing was performed by immersion in the

pitch-catch configuration and the collected signals were pre-processed through the discrete Fourier transform.

The results showed the efficiency of the method, and the importance of the ultrasonic Lamb waves in the

characterization of thermally treatment exposed materials. Keywords: Super-duplex steel, nondestructive testing, ultrasonic Lamb waves, materials characterization.

1. Introduction

The super-duplex stainless steels are iron, chrome, nickel, and molybdenum alloys, presenting

a matrix with approximately equal proportions of ferrite and austenite, in order to ally good

mechanical properties and corrosion resistance, which are higher than the duplex steel’s.

Despite that, these materials might have their integrity compromised when exposed to high

temperatures (thermal treatment, welding, hot forming, etc.), as changes occur in their

microstructure in these conditions, mainly because of the sigma phase precipitation, which is

one of the phases that promote the greater embrittlement effect, because it’s chrome-rich and

it depletes this element matrix [1].

Proper evaluations procedures are required in order to ensure that the changes occurred in the

material's matrix do not compromise its operation. Among non-destructive techniques,

ultrasonic evaluation is widely used because of its versatility. The propagation of ultrasonic

waves in polycrystalline materials subjects to interactions with the structural components of

the material. The super-duplex steel has a ferritic matrix with precipitations of the austenite

phase with round and elongated shape. These features may come to cause variation in the

ultrasound propagation velocity in the material [2].

Ultrasonic Lamb waves are guided elastic waves that propagate in the plane of a plate and,

like bulk elastic waves, their interaction with discontinuities and defects means they can be

used for inspection purposes. The main advantage of ultrasonic Lamb waves over bulk waves

is that these guided waves can propagate through a much farther distance, and thus they

enable long range inspection. The characteristics of guided waves, namely dispersive

propagation and attenuation, are directly related to the properties of the system in which they

11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic

are propagating, so the measurement of these wave characteristics can be used for material

characterization and condition monitoring [3, 4].

The objective of this work was to analyze the influence of deleterious phases in super duplex

steel SAF 2507 by analyzing the frequency spectrum of the Lamb wave symmetric S0 mode.

Ultrasonic tests were performed in the immersion pith-catch configuration in specimens

isothermally treated in order to promote the subsequent precipitation of deleterious phases.

The microstructures were determined through metallographic analysis and optical

microscopy. To characterize the changes in the material micro-structure from exposure to

different isothermal treatments the group and phase velocities were determinate for the

selected Lamb wave mode propagation.

2. Theoretical basis

2.1 Super duplex stainless steels

Super-duplex steels are ternary system Fe-Cr-Ni alloys, characterized by a two-phase

structure with fractions approximately equal of ferrite and austenite, and may present

variation in the order proportions of 30% to 70% of ferrite. It has high mechanical fatigue-

resistance, good tenacity, and resistance to under voltage corrosion and pitting, in several

environments.

The Pitting Resistance Equivalent (PRE) number on super-duplex stainless steels is higher

than 40, which is a higher number than those of the 300 family steels (austenitic steels). For

that reason, the super-duplex steel can be widely applied on chemical and petrochemical

industries, especially in offshore environments, in centrifugal pumps, flow control valves, and

seamless tubes under concentration up to 120.000 ppm of chloride [5]. The increase in

chrome, molybdenum, and nitrogen contents promotes the achievement of these properties

combination: high resistance to corrosion, and in the nitrogen’s case, it has a very favorable

effect on the mechanical resistance [6]. Mostly, they are produced with equal portions of

ferrite and austenite in their structure, and to that end, great control in cooling speed and when

adding the alloy's elements during their production is necessary. A great variety of phases can

be formed on super-duplex stainless steels in the 400 – 1000°C temperature interval, during

the isothermal aging (processing or use) or welding [7].

In Figure 1, a diagram of phases transformations (TTT) for the steel SAF 2507 can be seen,

showing the different deleterious phases found in different thermal exposure times. The

micro-structural transformations that occur on the duplex family steels are due to the high

amount of alloy's elements, and thus constitute thermodynamically metastable systems in

solubilized state at room temperature, since any heat adsorption that they may receive

promotes a more stable energetic condition, which is achieved through precipitation of

carbides and intermetallic phases [8, 9].

The inter-metallic phases precipitation is usually associated to undesirable consequences such

as, impoverishment of the matrix in alloy's elements like chrome, molybdenum, and niobium,

and with that, the loss of ductility, tenacity, and resistance to corrosion, specially pitting

corrosion [10].

Figure 1. TTT Diagram showing the formation of intermetallic phases on steel SAF 2507 [7].

2.2 Ultrasonic Lamb Waves Propagation

In Lamb waves, a large number of particle vibration modes are possible with specific energy

quantities (specific modes), which depends substantially of some factors, such as: the pulse

system, the ultrasonic beam incidence angle, the transducer central frequency, the bandwidth

frequency and others parameters described in [11, 12]. The complex movement of the

particle is similar to the elliptical orbit to the surface. The most common propagation modes

are the symmetric and the asymmetric modes. The velocity of guided Lamb waves is not only

dependent on the material (like longitudinal, shear and surface waves) but it is also affected

by the thickness of the material and the signal frequency.

Dispersion curves are used to describe and predict the relationship between frequency, phase

velocity and group velocity, incidence angle, mode and thickness [13]. These curves are

originated from solutions that satisfy boundary conditions of the wave equation for a

determined system and are described, as seen before, in terms of Lamé Constants. Figure 2

shows dispersion curves to symmetric (S0) and asymmetric (A0) modes for steel plate

immersed in water. Through the analysis of the Lamb waves propagation modes using

dispersion curves in terms of frequency-thickness, it is possible to determine the frequency

range of interest and the incidence angle to be applied on the practical experiments to ensure

the less dispersive guided wave propagation mode only.

Figure 2. Dispersion curves for stainless steel: (a) Phase velocity; (b) Group velocity; (c) Incidence angle (d)

Attenuation [13].

3. Methodology

3.1. Specimens

In this work were used 5 specimens of super-duplex steel SAF 2507 (composition: Si -

0.497%, Cl – 0.262%, V- 0.117%, Cr - 24.8%, Mn - 0.934%, Fe – 62%, Co – 0.332%, Ni –

7.11%, Cu – 0.179%, Mo- 3.59%). All of the specimens were confectioned with the

dimensions 205 x 85 x 2.7 mm. The specimen NT did not receive heat treatment. The

specimen SS was solubilized at 1120 ºC for 30 min. Other specimens: ST-30, ST-60 and ST-

120 were subjected to isothermal treatment at 900 ° C for different times (30, 60 and 120

minutes, respectively). In order to evaluate the influence of the precipitates in the intermetallic

phases, ultrasonic spectral analysis was performed. The study of sigma phase is very

important because its presence causes a decrease in the mechanical properties of super duplex

steels. Temperatures were selected based on the diagram (TTT), Figure 1. After isothermal

treatment all specimens were cooled in water.

3.2 Metallographic Testing

The super-duplex steel specimens used in the metallographic testing were embedded in

Bakelite, grinded by abrasive paper (220, 320, 400, 600 and 1200 mesh), polished with

diamond paste (3µm and 1µm) and alumina (0.3µm). The specimens were etched by the

solution of Behara containing: 0.6 g of potassium metabisulfite, 20 mL of HCl, 100 mL of

water. It was used to delineate particles of second phase [14]. After this stage, an optical

microscope UNIOMET model Union 9117 and a photographic camera Nikon D50

allow the capture and storage of images related to the microstructure revealed by the testing.

3.3 Ultrasonic Inspection by Immersion in Pitch-Catch Configuration

The specimens inspection was performed with the transmitter and receiver transducers placed

in line. The experimental set-up was a pulse generator Olympus®, model 5077PR, two

transducers diameter of 25 mm, central frequency of 0.5 MHz. The signals were collected

using a digital oscilloscope Tektronix©, model TDS2024B, with sample frequency of 250

MHz and an interface to a microcomputer to store the signal. Signals were acquired with a

distance of 200 mm between transducers. Figure 3 shows the positions of the equipment,

sensors and the experimental scheme of inspection by immersion in the configuration of

pitch-catch.

Figure 3. Experimental setup for Lamb Wave generation using pitch-catch configuration by immersion [15].

3.4 Dispersion Curves

The dispersion curves were simulated using Disperse® software [13]. By the analysis of the

simulated dispersion curves of the group and phase velocities against frequency-thickness the

frequency and the incidence of angle beam to be used in the experimental work was

determined in order to generate only the fundamental Lamb waves modes.

3.5 Spectral Analysis of Lamb Ultrasonic Signals

In order to evaluate of the influence of microstructure and grain size after the precipitations,

the Discret Fourier Transform (DFT) was applied to the measured Lamb wave signals in

order to determine the frequency spectrum for each specimen (MATLAB® software was used

in this work). The used features for spectral analysis were the bandwidth and the summation

of the product of frequency by the transform coefficient value, that is equivalent to integration

[16], eq.(1).

��� = ��(�)(�) (1)

Where f(i) and A(i) are the frequency and value of the ith DFT coefficient, respectively. For

the summation calculus were used the normalized values of DFT.

4. Results and Discussions

4.1 Simulation Disperse Curves

In Figure 2 are shown the simulated dispersive curves related to immersed testing for steel

specimens by Disperse Software®. S0 mode was selected because it is less attenuating than A0

mode in a frequency-thickness band less dispersive for practical experiments. As shown in

Figure 2, the incident angle more adequate for the ultrasonic sensors was 16° for all

specimens.

4.2 Microstructural Analyses

The microstructures of the specimens as received (NT) and solubilized for 30 minutes (SS)

are shown in Figures 4(a) and 4(b), respectively. It is observed in both Figures that the grain

boundaries are well defined, ferrite (δ) in blue and austenite (γ) in yellow, without the

presence of deleterious phases, which proves the condition of the specimen as to its initial

state .

Figura 4 . Optical microscopy of specimens in steel SAF 2507 etched with Behara reagent, 200X magnification:

(a) as received (NT); (b) solubilized (SS).

Micrographs of test samples subjected to isothermal aging 900 ° C for 30 minutes and 60

minutes are displayed in Figures 5 (a) and 5 (b), respectively. It is observed that the samples

displayed no precipitates of phases in both Figures. The yellow phase composed of austenite

the others by ferrite and precipitates. The sigma phase (white regions) is precipitated in the

ferrite / austenite interfaces. It was observed that with increasing treatment time increased the

amount of precipitate in the test samples. Figure 5(c) shows the micrograph of TS-120 sample

subjected to isothermal aging 900°C for 120 minutes. Comparing with Figures 5(a) and 5(b)

can be seen the increase in sigma phase in the matrix.

Figure 5. Optical microscopy of specimens in steel SAF 2507 etched with Behara reagent, 200X magnification:

(a) ST-30; (b) ST-60 (c) ST-120.

4.3 X-Ray Diffraction

The evolution of the presence of deleterious phases in the matrix of super-duplex steel during

thermal treatment can be observed in the diffractograms, Figure 6.

(a) (b) (c) (d) Figure 6. Diffractograms of Super-duplex specimens: (a) solubilized; (b) 900ºC / 30 min; (c) 900ºC / 60 min; (d)

900ºC / 120 min .

Figure 6(a) shows the specimen diffractogram for the solubilized condition. Peaks of ferrite

(δ) and austenite (γ) demonstrate that solubilization was perfect, which is consistent with No

Heat Treatment condition (NT). For 30 minutes of heat treatment, Figure 6(b), is observed

two small peak from sigma phase (σ) according with optical microscopy, Figure 5(a). The

amount of precipitated sigma phase increase with heat treatment time, Figures 6(c) and 6(d),

in agreement with microscopy results.

4.4 Spectral Analysis of Lamb Ultrasonic Signals

In Figure 7 are viewed the frequency spectrum for each thermal condition. The analysis

shows that the frequency bandwidth and the product sum of DFT coefficient increases with

heat treatment time, see Table 1, because the precipitation generates components of high

frequency due ultrasonic wave dispersion across your path in material.

Figure 7. Frequency spectrum of ultrasonic Lamb waves in specimens with different thermal treatment times.

Table 1. Features extracted from frequency spectrums.

Features\Condition As received Solubilized 30min 60min 120min

Bandwidth (kHz) 170 170 200 210 225

Summation (x106) 3.53 3.90 4.08 4.25 4.71

5. Conclusions

There was a good correlation between phase precipitation due heat treatment and the spectral

analysis of the Lamb wave signals. The precipitation increased the frequency bandwidth and

the summation of the signals. The results were corroborated by optical microscopy and X-rays

diffraction.

Acknowledgements

The authors thank to Federal Institute of Bahia and FAPESB for funding this study. The

authors wish to thank Sandvik for the supply of specimen materials

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