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Technical Evaluation of Ultrasound Phased Array Inspection in Welded
Joints of AISI 304L Austenitic Stainless Steel
Ramon Ferreira Ferreira¹, Maurício Saldanha Motta², Lincoln Silva Gomes³, Maurício Ogawa4, André
Rocha Pimenta5
1, 3, 4- SENAI Welding Technology Centre. São Francisco Xavier Street, 601, Maracanã, Rio de
Janeiro,
Brazil. Email: [email protected], [email protected], [email protected], phone number:
+55 21 7120-0734 / +55 21 3978-8701
2- Co-Author. Federal Centre of Technology Education Celso Suckow da Fonseca. Maracanã Avenue,
229, Maracanã, Rio de Janeiro, Brazil. [email protected], phone number: +55 21 8778-5939 / +55
21 2566-3195
5 – Federal Institute of Rio de Janeiro. Sebastião Lacerda Street, Paracambi, Rio de Janeiro, Brazil.
Email: [email protected], phone number: +55 21 3693-2378
Abstract
Stainless steels are materials with increasing use in industries, because they have good corrosion
resistance and mechanical properties. However, these materials require rigorous welding and
inspection procedures.
Conventional ultrasound techniques are rarely used to evaluate the austenitic stainless steel
welds, because it has high attenuation of ultrasonic waves. As a result, the detection of defects is
hampered by weak signal to noise ratio, causing problems to the inspection.
A study was developed to evaluate the phased array ultrasound technique effects austenitic welds,
identifying a correlation between microstructure with ultrasonic properties, the ultrasonic beam
behavior and a better characterization of welding defects in this inspection.
During the research, samples with 16 inches of diameter were welded and were analyzed with
micrographic, austenitic and ferritic structure phase’s percentage, hardness measurement, attenuation
and ultrasonic speed measurement, all to identify the influence in the detection.
Keywords: Phased Array, Ultrasound, microstructure phase’s, Austenitic Stainless Steel, Welding.
1. Introduction
The issue of transport in pipelines for oil and its derivatives dates back to the 19th century,
however it only started effectively use in the last century. This fact occurs principally because of
three important points in this sector: sources of production, refineries and consumer centers, all of
them need to be connected.
The mesh of pipelines in Brazil corresponds to 26 thousand miles, data from September 2011, this
makes the country the number 16 in the world rankings, which is still little if it is taken into
account the massive production in the oil and mining sectors, such considerable leading.
The occurrences of accidents with this transport system have shown the enormous complexity and
the extension of the damage done not only to companies, but mainly with the ecosystems and the
communities around these events.
Stainless steels are materials with increasing use in industries, because of their specific features,
notably those related to the corrosion resistance and mechanical properties. However, these
materials require special attention to work with them, under the risk of compromising their
specific properties. Among these risks, we highlight the welding of austenitic stainless steels,
because admittedly this is one of the operations that may lead to a compromise of these materials.
The use of improper welding procedures can permanently affect some characteristics of the
material,
resulting in significant changes in base metal, mainly in the mechanical behavior, causing the
welding defects, and the corrosion resistance (SENAI, 1998).
Through the choice of tests and selection of the best inspection parameters it is possible to inspect
welded joints of complex materials, such as the joints made of austenitic stainless steels,
evaluating possible defects from the welding process during the construction or operation of the
equipment.
The ultrasound inspection technique, for example, has been routinely used in the industry for
about 30 years and this technique has always been known for presenting difficulties in inspection
of components manufactured from austenitic stainless steel castings or welded (LOBERTO,
2007). Approximately 10 years ago the research found much information about the ultrasonic
inspection in this material. The weld region microstructure generates attenuation in the ultrasound
velocity (PIRES, 2009).
The advanced ultrasound, uses phased array linear transducers, this technique improves the
sonic pressure and it allows better control of the ultrasonic beam during the inspection.
2. Bibliography review
2.1 Principles of ultrasound Phased Array
The phased array Ultrasound started the creation of a patent in 1959 by Tom Brown, where he
developed the annular transducer focused dynamically. From 1960 this device was restricted to
the use of laboratories, where in the year 1968, Jan C. Somer published the first medical work on
the electronic scanning ultrasound diagnosis (Figure 2.1 a). In the 1970’s, physicists encouraged
the development of new research in the development of body image in the medical field
(GINZEL, 2003).
In ultrasound Phased Array to pulsar individual elements or groups of elements with different
delays creates a lot of waves with points of origin that combine in a single wave front travelling at
an angle selected. This electronic effect is similar to mechanical delay generated by a
conventional wedge, but can be more controlled, by changing the pattern of delays (OLYMPUS,
2010).
In addition to change the direction of the primary wave front, this combination of individual
components allows beam focusing of the beam (Figure 2.1 b) anywhere in the near field
(OLYMPUS, 2010).
(a) (b)
Figure 2.1 – (a) Ultrasound image of the fetus by multi-element transducer. (Source: OLYMPUS,
2010). (b) Graphic example ultrasonic pulsed beam shape for several individual elements or
groups, provided by the selection of propagation angles. (Source: Olympus, 2010).
The returns of the echoes are received by the various elements or groups of elements and then are
summed. Unlike a conventional single element transducer, a multi-element transducer selects
different wave front and spatially return, according to the time of arrival and amplitude of each
element (OLYMPUS, 2010).
Phased array ultrasound machines offer various forms of visualization of the results obtained
during the scan. Among them stands out the types: A-scan, B-scan, C-scan and S-scan (Figure
2.2).
Figure 2.2 - Representation of the types of views on the phased array. (a) C-scan; (b) S-scan; (c)
B-scan; (d) A-scan.
2.2 Austenitic Stainless Steels
The austenitic stainless steels have a predominantly austenitic microstructure, not being hardened
by heat treatment. Contains about 6 and 26% nickel, 16 and 30% chromium and less than 0.30%
carbon, with a total content of alloying elements of at least 26%. (MODENESI, 2001).
The microstructure of this is, in general, all for austenite. While the fused zone can retain varying
amounts of delta ferrite at room temperature. The fused zone can be analyzed with the aid of the
Fe-Cr-Ni system to 70% iron.
The final microstructure of cast zone of an austenitic stainless steel will depend on the form of
steel solidification and subsequent transformations in solid form. This microstructure can be
classified according to the morphology of the ferrite. The main microstructures found (Figure
2.3), to increasing values of the ratio chrome/Nickel are: austenite; austeníta + eutectic ferrite;
austenite + ferrite in the spine; austenite + ferrite laminate; Widmanstatten austenite + ferrite
(MODENESI, 2001).
Figure 2.3 - examples of the δ ferrite morphologies in austenitic stainless steels cast. Source:
MODENESI, 2001.
The increase in the percentage of ferrite in the microstructure of molten zone results in increase of
the limit of resistance due to the creation of winding contours, making the spread of cracks, where
(A) (B)
(C) (D)
Austenite Ferrite
Relation with Cr/Ni
the increase in hardness of the material, causing a loss of ductility, thus making the fragile
material (RUIZ, 2009).
The increase of the limit of resistance of stainless steel by the larger presence of ferrite is detected
by ultrasound technique, where the ultrasonic velocity and attenuation increases, when the
percentage of ferrite increases. (T.R.G. Kutty, 1987), (p. Li, 1992), (GOMES, 2007).
3. Methodology
3.1 Materials and methods
The material used for the manufacture of specimens, in this study, has been austenitic stainless
steel AISI 304 l. Evidence soldiers were made from a stainless steel tube AISI 304/304 l with 16
outer diameter 30 "Schedule (406.4 mm/9.53 mm) with V-Groove, bevel angle of 35°, height of 1
mm nose and opening of 4 mm bevel used in pipelines for compressed natural gas, as shown in
Figure 3.1.
In the research, 4 specimens were prepared with the GTAW welding process, two samples were
welded with AWS 5.9-ER308L (diameter of 2,4 mm) and the other two samples with AWS 5.9-
ER316L (diameter of 2,4 mm), Sandvik consumables.
The GTAW welding machine used was FRONIUS TRANSTIG model brand 3000, for manual
welding. The values of qualified welding procedures used were two current values, 90 and 120
amps, reverse polarity (CC) and the voltage between the 12.0 10.5 Volts. The nozzle used was a
number 7, using a flow rate of 7 liters per minute for the shielding gas and 12 liters per minute to
the purge gas. The tungsten electrode used was the electrode with 2% Thorium and 2.4 mm
diameter (figures 3.1 and 3.1 (b)).
(a) (b)
Figure 3.1 – (a) Step of preparation of specimens. (b) Four specimens prepared for analysis.
3.2 Micrographic analysis, Percentage of measurement and analysis by EDS
After weld specimens were sampled for the execution of the micrograph, a sample of each welded
joint was removed, where they were cut by a cutter brand, Panambra model metallographic
Pancut 100, went through a sequence of files with different sample sizes: 220, 320, 400, 600, 800,
1000, 1200. After this sanding step, samples were polished with 1 μm alumina. The samples were
attacked chemically with aqua regia (HCl + HNO3 + H2O), where the dissolution used was
composed of 10 ml of nitric acid, chloric acid 20 ml, 30 ml of distilled water.
After processing, the samples were analyzed for texture and the resulting microstructures stages
of balance through a reversed lens optical microscope Olympus GX51 model brand. In each
sample were observed root regions, filling and finishing of welded region (Figure 3.2). To the
percentage analysis of phases (Figure 3.3) was used the same lens inverted microscope Olympus
GX51 model, brand, and also an image Analyzer, analySIS software. This step aims to correlate
the percentage of phases in the area with the ultrasound parameters (speed and Sonic attenuation).
Figure 3.2 - Micrographs of proof in 1 (a); (b) filling; (c) finishing; (d) base metal. Aqua regia
Attack. 100 x increase.
Figure 3.3 - Percentage analysis of phases.
In order to ensure the presence of ferrite and their format in the microstructure of base metal and
welded, the samples were subjected to analysis by scanning electron microscopy (SEM) and
analysis by EDS (Energy Dispersive x-ray Detector) (Figure 3.4). For this test we used a scanning
electron microscope of Hitachi brand, model TM3000, and also the Olympus software Stream
Motion v. 1.6, Olympus brand, for analysis of the images.
Figure 3.4 - EDS analysis in the samples for the confirmation of the presence of ferrite in the
microstructure of welded region.
3.3 Vickers hardness testing
The four samples were submitted to the Vickers hardness testing to correlate these values with the
ultrasound parameters (speed and ultrasonic attenuation). For this test we used a Vickers hardness
tester Wolpert brand. In the essay was used a load of 5 kgf (HV5 .0) and a Vickers Indenter
(diamond pyramid of 136). The hardness was taken at the root, filling and finishing. In this
analysis was prepared 18 measurements per sample, within an average of 3 measures in each
region of the samples (Figure 3.5).
Figure 3.5 - Vickers hardness testing on the samples.
3.4 Ultrasonic Inspection
This step aimed to determine the speed and Sonic attenuation in each sample prepared in the two
techniques, conventional ultrasound and phased array (Figure 3.6). Getting these two parameters
will allow the establishment of a correlation with the tests with the microstructure. The correlation
will allow demonstrating that from the knowledge of the resulting microstructure, i.e. after the
development of a welding procedure, inspection parameters can be adjusted. In this step we used
ultrasound equipment and the heads are described in table 3.1.
Table 3.1 - Equipment used in the inspection.
(a) (b)
Figure 3.6 - ultrasound Equipment (a) and (b) phased array.
In this study a software was used to simulate the behavior of the parameters in the inspection. The
ESBEAM tools was used for conventional (figure 3.7a) and Phased Array inspection (figure
3.7b).
(a)
(b) Figure 3.7 – (a) US configuration. (b) US phased array configuration.
4. Results and discussions
Through studies it was possible to perform a correlation with all results found. The welding
parameters chosen directly affect the result of the weld microstructure, which in turn affect in
propagating Sonic during an inspection. Having the knowledge of micro-structural results (Figure
4.1, 4.3) you can perform inspection parameters settings to achieve better discoverability and
dimensioning of defects, improving the efficiency of the technique.
4.1 Results for SEM and EDS analysis The results of SEM and EDS analysis showed the presence of ferritic phase in the microstructure
of the weld. The values of the percentage of mass of the Chrome and Nickel.
(a) (b) Figure 4.1 – Results of SEM and EDS analysis. EDS measurement in the (a) ferritic phase, (b) austenitic matrix.
4.2 Results of the Percentage Phase Analysis
The diagram of Schaefler was used to quantify the percentage of ferrite in the weld samples
(figure 4.2). This diagram was used to make a comparison with the results of the percentage of
ferrite obtained with micrograph and image analysis. The results of the percentage of the
microstructure analysis showed a high percentage in samples CP1 and CP2 welded with ER308L
consumable, described in table 2 (figure 4.2).
(a) (b) Figure 4.2 – Results from the Schaefler’s diagram for AISI 304L welds. (a) Consumable
ER308L, (b) consumable ER316L.
Figure 4.3 - Percentage Analysis of phases on the bodies, in the regions of finishing, filler and
root.
The values of percentage of ferrite and austenite analyzed with the image software, are described
in the table 2. The four welded samples were analyzed in the three regions: top, middle and
bottom.
Table 2 - Percentage of ferrite and austenite in the welded samples in different areas.
4.3 Results of the Correlation of all Analysis
In the research developed by BABIDI in 2003, presented the effect of microstructural variation on
speeds and Sonic attenuation. Samples with varying structures of ferrite-Pearlite to martensite, to
steel with 0.4% carbon, were obtained by the method of Jominy. In this study it was observed that
the speed and attenuation values are directly proportional to the hardness of the material.
The same correlation between hardness and speed was observed in the study of (SHIGEYUKI,
2000) in samples of aged duplex stainless steels to 475º C, but the ultrasonic attenuation values
were not sensitive to structural changes.
The study of (PALANICHAMY, 1995) showed the application of sonic speed for determining the
grain size of austenitic stainless steels. It was observed that the speed of sound is affected by grain
size, i.e. an increase of grain size will mean that the ultrasonic waves will lead a longer path to
cover the material densities, thus reducing of the speed of sound.
In the study of (GARCIA, 2007) with 1045 steel noted that the variation of sonic speed is
inversely proportional to the variation of hardness of the material studied. It was also verified that
the sonic speed is dependent on the elastic modes, ranging from those with the microstructure, it
follows that the sonic speed is a promising parameter for the identification of heat treatment of
steel.
The increase in the percentage of ferrite in the microstructure of molten zone results in increase of
the limit of resistance, decreasing the grain size in weld of austenitic stainless steel, due to the
meandering contours creation, hindering the spread of cracks.
According to the study (FONSECA, 2011) the limit of resistance of a steel depends on its elastic
constant that can relate the sonic speed with the limit of resistance of this material. Ultrasonic
attenuation is caused by the presence of defects and reflectors as outlines of material grain and
can, therefore be related to the limit of resistance of the material as it suffers the effect of grain
size and the presence of defects.
In the study of (T.R.G. Kutty, 1987) and (p. Li, 1992) showed the application of ultrasound in
measuring the percentage of ferrite stainless steel super duplex. In this work, the increase in the
limit of resistance of stainless steel is generated by the larger presence of ferrite, which is detected
by ultrasound technique, where the ultrasonic speed increases, when the percentage of ferrite
increases.
The fact of the duplex and super duplex steels have an array and austenite phase ferritic structure,
allows a better spread of ultrasonic beam, allowing the sonic speed increase, due to formation of
delta ferrite in the shape of the spine. On the microstructure of austenitic stainless steels after weld
presents the delta ferrite on the spine, which has winding contours, which prevent the spread of
the beam, producing the effect of scattering and sonic speed reduction. The results are correlated
in table 4.1.
Table 4.1 - Correlation of results obtained in the tests.
Through the results, correlations were found which allowed to demonstrate that the increase in the
percentage of ferrite in the microstructure, resulted in the decrease of the sonic speed, and also, in
Sonic attenuation increase. We also observed that the samples with higher percentages of ferrite,
had the highest average hardness values, but with few variations. The study showed agreement
with the statement of the study (PALANICHAMY, 1995) on the reduction of the speed as a
function of grain size be higher in specimens with larger ferrite levels, causing the sound to go a
long way. The study showed agreement with the statement of (GARCIA, 2007) regarding the
reduction of speed, the greatest value of hardness.
5. Conclusions
The study made it possible to evaluate the technique of ultrasound phased array inspection in
welded joints of austenitic stainless steel AISI 304L, about the effects of microstructure on sizing
of discontinuities and thickness.
Based on the results, it was possible to observe that the specimens with the highest value of
current soldiers have thinned out more ferrite on its microstructure, rendering the texture more
refined, and, in addition, the filler metal ER308L formed more ferrite in the microstructure
comparing with ER316L.
The result of the scanning electron microscopy allowed observing and ratifying the presence of
ferrite in steels by EDS analysis. The confirmation came from the chrome (alpha’s phase
percentage) and nickel (gamma’s phase percentage), where it was observed that, in the suggestive
microstructures of ferrite, the percentage of chromium increased and percentage nickel decreased.
The opposite occurred in the matrix region, the percentage of chromium fell and the percentage of
nickel increased.
In ultrasonic attenuation, measurement showed that the values detected by conventional
ultrasound techniques and phased array were higher for samples with higher percentage of ferrite.
In the study it was possible to demonstrate that the sonic speed is directly proportional to the
sizing of discontinuities and thickness measurement. The sonic speed adjustment to a welded
austenitic stainless steel region affect the sizing of discontinuities and depth of defect.
Correlations have been found through the results, it was concluded that the percentage of ferrite
microstructure increases in this austenitic stainless steel, and then, results in a reducing of the
ultrasonic speed and increases the ultrasonic attenuation.
Phased array ultrasound technique has greater Sonic pressure than the conventional technique,
allowing greater reliability in the inspections. In addition, it allows configuration of ultrasonic
beam groups with different ultrasonic speeds, improving detection and sizing of discontinuities in
a welded joint.
6. Acknowledgements
The Advisor, Mauricio S Motta, teacher from CEFET-RJ, by friendship, encouragement and
guidance to the implementation of this work.
The Coordenação de Aperfeiçoamento de Pessoal de Nível Superior-CAPES, by scholarship
granted during the masters.
The SENAI Technology Center, for the encouragement and support in the use of equipment,
materials and software, for the carrying out of the trials and tests for this dissertation.
The Instituto Federal do Rio de Janeiro – Campus Paracambi, by Prof. André Rocha pepper, in the
execution of the scanning electron microscopy analysis.
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