Journal of Minerals & Materials Characterization & Engineering, Vol. 7, No.2, pp 115-126, 2007 Printed in the USA. All rights reserved
Characterization of the Weld Regions within Duplex Stainless Steels
using Magnetic Force Microscopy
B. Gideon1, L. Ward2 and K. Short3
1ARV Offshore, Bangkok, Thailand
2School of Civil, Environmental and Chemical Engineering, RMIT University, GPO
Box 2476V, Melbourne, Vic. 3001, Australia
3Australian Nuclear Science and Technology Organization, Lucas Heights, NSW, 2234,
Standard metallography and optical microscopy are well established techniques for
the characterization of duplex stainless steels (DSS), which consist of approximately
50% ferrite and 50% austenite. Recently, the use of atomic and magnetic force
microscopies (AFM and MFM respectively) have been employed to differentiate
between magnetic and non magnetic phases in materials. Such techniques would be
valuable to identify different phases in duplex stainless steels, particularly the weld
regions, and would thus compliment standard metallographic and optical microscopy
techniques. In particular, AFM and MFM would be particularly valuable for
identification of phases within the different weld regions (root, fill and cap).
In the present study, Gas Tungsten Arc Welded (GTAW) DSS samples, as a function of
heat input and weld configuration, were subject to standard metallographic practices
(ferrite content determination, Vickers hardness measurements, Charpy impact studies
and transverse tensile testing) in addition to MFM analysis. The metallographic tests
revealed that the weld properties were acceptable in accordance with current industrial
standards. The MFM results of the weld metal shows the formation of both a finer
and coarse structure within the weld metal, which is dependent on the level of
Key Words: GTAW Welding, Duplex Stainless Steels, Mechanical Properties,
Magnetic Force Microscopy, Characterization
116 B. Gideon, L. Ward and K. Short Vol.7, No.2
Traditionally, the common methods for studying the microstructure of duplex stainless
steels (DSS), in particular the weld regions, have been quantitative metallography and
microhardness techniques. However, a technique has been developed to compliment
these conventional mehods, utilizing a scanning probe microscope in magnetic imaging
mode, known as magnetic force microscopy (MFM), which enables the ferrite regions
to be distinguished from the austenite regions, using their magnetic characteristics. The
ferrite regions are ferromagnetic, in contrast to the austenite regions, which are
paramagnetic [1]. The spatial variation of the magnetic force interaction between these
regions can be studied using MFM and is now recognized as a powerful tool for the
characterization of Duplex Stainless Steels (DSS) [2, 3].
MFM imaging mode is based on non-contact Atomic Force Microscopy (AFM), with
the tip modulated at or near its resonant frequency by means of a piezoelectric element
and the cantilever coated with a magnetic material. When resonated over the sample
surface, the tip–sample interaction includes both surface and magnetic forces. A
limitation of this technique is the ability to accurately align the information obtained
on surface topography characteristics using the scanning probe microscopy in atomic
force microscopy (AFM) mode, with information obtained on the magnetic contrast of
differently magnetized domains using the scanning probe microscope in MFM mode.
However, these problems have been overcome by adopting a two-pass procedure,
whereby a second signal is measured in addition to AFM surface topography. For MFM
measurements, this is possible by using a CoCr-coated tip. This set-up allows for a very
easy combination of the two techniques by simply changing some software parameters.
Consequently, this specific procedure has been adopted in the current investigation.
Previous studies by Takaya et al [4] on the application of MFM for studying Cr
depleted regions of 304 stainless steel showed a strong correlation between the
depleted regions and the degree of sensitization to stress corrosion cracking. AFM and
MFM studies by Dias and Andrade [5] showed that the clarity of magnetic patterns was
strongly dependant on the type of magnetic tip employed and the tip – surface
separation distance.
In the present investigation, MFM studies were carried out on the weld and parent metal
regions of four duplex stainless steel weld samples, in order to compliment information
provided by conventional metallography and microscopy techniques, for structural and
Vol.7, No.2 Characterization of the Weld Regions within Duplex Stainless Steels 117
morphological characterization of the DSS welds.
In MFM, the magnetic fields adjacent to a sample are detected with sub-micron
resolution, by scanning a magnetic probe over the surface and recording the changes in
its phase or resonant frequency [6]. Once set in place in the instrument, the tip is
oscillated at its resonant frequency by a piezoelectric element, and scanned over the
sample surface. The topography of the sample surface is obtained in the first pass by
lightly tapping the surface with the tip. In the second pass, the tip is lifted off the
surface by a predetermined distance (in this study, between 50 and 100 nm) so that only
the magnetic forces affect the tip, thus avoiding interference from the surface
topography [6, 7]. The tip is then scanned along the same line following the
topographic surface contour recorded during the first pass, so that the tip-sample
distance, and hence the resolution, are maintained constant. In this way, the phase shift
induced by the magnetic force gradient between the tip and the sample can be recorded,
yielding an image of the magnetic patterns over the surface, which in the case of DSS
can be associated to the microstructure of the sample (Fig. 1).
Fig. 1 Schematic diagram showing the principle of MFM imaging for DSS Welds
Phase shifts (Δθ) between oscillations of the cantilever and the piezoelectric
actuators measured by equation 1 for small amplitudes cantilever as follows:
----------------------------------------------------------------------Equation 1
Q is the free oscillation quality factor (165
10 in air), k is the spring constant
(3 N/m) and F’ is the vertical gradient of the magnetic force on the tip of the
Therefore, F’ can be calculated as shown in equation 2:
sampletip dVrrHrM
rF ')'()'()(' 2
---------------------------Equation 2
Mtip(r’) is the magnetization of the volume element in the tip and Hsample(r+r’) is the
stray field from the sample [8]. The liftoff between the tip and the sample surface is
about 100 nm. Specimens were polished mechanically before the MFM observation.
3.1 Welding of Duplex Stainless Steels
118 B. Gideon, L. Ward and K. Short Vol.7, No.2
The parent material chosen for the investigation was a 10mm wall thickness, 250mm
diameter DSS linepipe corresponding to UNS 31803 specifications. The filler material
used was the conventional ER2209 AWS A5.9-93 classification. Full details of the
chemical composition of both the parent material and filler material are listed in Table
1, confirming that the primary solidification mode was ferrite.
Table 1. Chemical composition of DSS pipe and filler material.
Pipe C Mn P S Si Ni Cr Mo N Cu Pren Creq Nieq
Min - - - - - 5.00 21.50 3.00 0.15 - 35 - -
Max 0.030 2.0 0.025 0.015 1.0 6.50 23.00 5.50 0.20 0.16 - 32.04 10.78
Max 0.016 1.69 - - 0.42 8.60 23.07 3.20 0.160 0.16 35 28.09 11.90
Note; Creq = Cr+1.37Mo+1.5Si+2Nb+3Ti and Nieg = Ni+22C+0.31Mn+14.2N+Cu
Welding was performed using the manual Gas Tungsten Arc Welding (GTAW)
technique. Two different join configurations were adopted, namely double bevel single
V bevel and double bevel single U joint configuration. Full details of the weld
parameters, to include number of passes, arc travel speed and heat input, are given in
Table 2. For the V bevel configuration, welding was performed at low (1515.38 J/min
average) and high (2000.97 J/min average) heat input powers. For the U groove
configuration, two welding operations were performed at similar heat input powers
(1429.35 J/min). These will be referred to as weld conditions 1 to 4 respectively.
Upon completion of welding, all test conditions were visually inspected for surface
defects both internally and externally. Liquid dye penetrant tests were performed 4
hours after completion of welding, and radiography (X-ray) was performed 24 hours
later to determine the integrity of the girth welds.
Table 2. Weld conditions for 4 DSS samples used in this study.
Weld Condition Weld
Pass Travel Speed Heat Input
Vol.7, No.2 Characterization of the Weld Regions within Duplex Stainless Steels 119
mm/min J/min
Condition 1
V groove
1 (weld root) 51.00 1474.71
2 (weld fill) 123.00 883.12
3 (weld fill) 66.00 1745.45
4 (weld fill) 64.00 1788.00
5 (weld cap) 64.00 1685.63
Average 1515.38
Condition 2
V groove
1 (weld root) 45.00 1591.20
2 (weld fill) 105.00 1440.46
3 (weld fill) 79.00 2756.05
4 (weld cap) 94.00 2216.17
Average 2000.97
Condition 3
U groove
1 (weld root) 110.00 419.78
2 (weld fill) 62.00 757.55
3 (weld fill) 38.00 1733.05
4 (weld fill) 40.00 2194.80
5 (weld cap) 43.00 2041.67
Average 1429.37
Condition 4
U groove
1 (weld root) 115.00 420.31
2 (weld fill) 66.00 942.55
3 (weld fill) 73.00 2219.18
4 (weld cap) 57.00 2135.37
Average 1429.35
3.2 Mechanical Testing of Welded Duplex Stainless Steels
Ferrite contents of the four weld conditions were measured using both the Magna-
Gauge and Fischer ferrite-scope methods and, as a comparison, determined
metallographically by the point count method [9].
Vickers hardness measurements were made with a 10 kg load in the parent material,
heat affected zone (HAZ), weld cap, weld fill and weld root regions. Transverse tensile
test specimens were used to determine the tensile values whilst the more restrictive
120 B. Gideon, L. Ward and K. Short Vol.7, No.2
transverse side bend specimens were used in lieu of the root and face bend tests,
commonly adopted in accordance with weld procedure qualifications. Charpy impact
tests were performed to assess the notch toughness of samples extracted from the
weldments, in accordance with ASTM A 370 standards [10].
3.3 Magnetic Force Microscopy Studies of Welded Duplex Stainless Steels
MFM studies were conducted on metallographically prepared cross-sections of the
welds, after grinding and polishing using SiC abrasive papers and diamond paste. The
scanning probe microscopy (Digital Instruments), operating in tapping and lift modes
was employed to study the topographic and magnetic features of the DSS samples.
Topographic and magnetic force data were taken in the same scan. In order to produce
reliable images, repeated scans in different directions were done to ensure
reproducibility of the features. Various scan sizes and speeds were tested to enhance
height and magnetic induced signals, thus minimizing tip hysteresis and the delay
between line scans.
A cantilever equipped with a special coated tip was scanned over the surface of the DSS
sample, using tapping mode in order to obtain surface topography profiles. To obtain
magnetic images, lift mode was adopted, whereby the tip was then raised just above the
sample surface. The surface topography was then scanned while being simultaneously
monitored for the influence of magnetic forces. These influences were measured using
the principle of force gradient detection [8, 11, 12]. In the absence of magnetic forces,
the cantilever has a resonant frequency that is shifted by an amount proportional to
vertical gradients in the magnetic forces on the tip. In the present work, the frequency
modulation detection method was used to measure the resonance frequency shift. The
cantilever is maintained by a feedback loop using the signal from the deflection sensor;
thus, changes in the oscillation frequency caused by variations of the force gradient of
the tip–sample interaction are directly measured [13].
4. Results and Discussion
4.1 Mechanical Properties of Welded Duplex Stainless Steels
Vickers hardness testing revealed that for all the four fusion zones, hardness values in
the range of 235-285 HV10 were exhibited. A summary of the results of the mechanical
properties of the welded duplex stainless steels is given in Table 3. It can be seen that
Vol.7, No.2 Characterization of the Weld Regions within Duplex Stainless Steels 121
the notch toughness values obtained by Charpy impact testing at -43 °C show that,
although slight differences exist in the ferrite content among the four test conditions,
condition 1 and 2 had impact values of 116 joules while condition 3 and 4 have impact
energy values of 97 joules and 100 joules, indicating a reduction of 16.4% and 13.8%
respectively. This suggests that the differences in the toughness cannot be explained
simply in terms of the changes in ferrite content. The fact that all four weld conditions
retain much of their toughness even at -43 °C may not be due to a reduction in ferrite–
austenite ratio alone. Tensile strength values for all 4 conditions were within the range
of 774 N/mm2 to 794 N/mm2. In summary, the metallographic tests revealed that the
weld properties were acceptable in accordance with current industrial standards.
Table 3 Summary of the results of the mechanical properties of the welded duplex
stainless steels
Weld Condition Weld
Imp act Te st
Magna-Gauge Fisher Ferrite-
Point Count
N/mm2 J % % %
Condition 1
V groove
1 (weld root) 779.59 116 49.50 35.00 41.02
2 (weld fill) - - -
3 (weld fill) - 36.00 38.67
4 (weld fill) - - -
5 (weld cap) 48.25 37.00 43.67
48.88 36.00 41.12
Condition 2
V groove
1 (weld root) 794.53 116 39.75 35.00 35.31
2 (weld fill) - 35.00 36.48
3 (weld fill) - - -
4 (weld cap) 48.00 45.00 46.33
43.88 38.33 39.37
Condition 3
U groove
1 (weld root) 774.03 97 43.00 40.00 42.58
2 (weld fill) - - -
3 (weld fill) - 35.00 36.95
4 (weld fill) - - -
5 (weld cap) 43.75 36.00 40.03
43.38 37.00 39.85
Condition 4
U groove
1 (weld root) 783.75
100 54.50 42.00 45.47
2 (weld fill) - 35.00 33.75
3 (weld fill) - - -
4 (weld cap) 43.75 36.00 37.19
122 B. Gideon, L. Ward and K. Short Vol.7, No.2
49.13 37.67 38.80
4.2 Magnetic Force Microscopy Studies of Welded Duplex Stainless Steels
Topographic images and corresponding magnetic force images for all weld conditions
(conditions 1 to 4) are shown in Figs. 2 – 5 respectively. Each figure consists of MFM
images taken from three regions of the weld, namely the root, fill and cap, in addition
to the AFM image. In general, the topographic images, although showing the surface
topography of the metallographically prepared weld, reveal no information about the
phases present. However, two distinct regions are easily identified in the MFM
images. The lighter regions represent the paramagnetic austenite regions, while the
striated, darker regions represent the ferromagnetic ferrite regions. Variations in the
relative distributions of the austenite and ferrite regions can be observed not only in the
different weld areas for any given configuration, but also between specific weld
regions over all configurations examined.
For weld condition 1 (V configuration, low heat input), the root and fill regions of the
weld consist mainly of austenite in the forms of large elongated platelets and small fine,
eqiaxed dendritic regions (Figs. 2 (a) and 2 (b)), surrounded by fine discontinuous
regions of ferrite. In contrast the cap region, as shown in Fig. 2 (c), show larger
continuous regions of ferrite, in addition to a long continuous band of austenite forming
at the grain boundary. For the same configuration with higher heat input (Fig. 3),
similar observations were evident in all three regions, compared with the lower heat
For weld conditions 3 and 4 (Figs. 4 and 5 respectively), using the U configuration at
low heat input, similar structures were observed compared with conditions 1 and 2.
However, large regions of ferrite were observed in the fill regions of the weld for
condition 4 (Fig. 5 (b)). Further, the root region for condition 3, as shown in Fig. 4 (c),
consisted of elongated bands of austenite surrounded by ferrite. Such a structure is
typical of the parent metal structure, which would have elongated austenite grains
embedded in a ferrite matrix.
The results of the MFM imaging in general, suggest that the austenite regions observed
Vol.7, No.2 Characterization of the Weld Regions within Duplex Stainless Steels 123
in the DSS weld metal regions are formed from ferrite in three modes, viz., as
allotriomorphs at the prior-ferrite grain boundaries, as Widmanstätten side-plates
growing into the grains from these allotriomorphs and as intragranular precipitates. In
the micrographs, the grain boundary allotriomorphs and Widmanstätten austenite are
clearly seen. However, the austenite seen within the grain could be either intragranular
precipitates or Widmanstätten austenite intercepted transverse to the long axis. The
findings from the MFM work conducted in this study here confirm the findings from a
previous study [14] which looked at the structure of these duplex stainless steels using
conventional microscopy.
The different austenite / ferrite microstructures observed in the weld regions, such as
the presence of discontinuous grain boundary austenite layers (Figs. 2(a) and 2 (b)),
Widmanstätten austenite side-plates, austenite intragranular precipitates and
intragranular acicular ferrite are thought to be associated with, and hence explained in
terms of variations in transformation rates and the degree of undercooling [15].
The formation of grain boundary and side-plate fractions require a relatively smaller
driving force [16] and therefore can occur at higher temperatures with little
undercooling. The formation of intragranular acicular ferrite, on the other hand,
requires a greater degree of undercooling and therefore occurs at lower transformation
temperatures. It is likely that a similar transformation sequence is adopted during the
microstructural evolution of the regions associated with the DSS weld. Thus the grain
boundary austenite and Widmanstätten side-plates form early at higher temperatures,
while the intragranular austenite particles require a greater driving force and precipitate
later at a lower temperature. This can also be seen in the MFM images, where the
intragranular precipitates are seen to form in the regions partitioned by the
Widmanstätten plates. When cooling occurs rapidly in the cap region of the welds, it is
expected that the transformation product requiring a higher degree of undercooling is
formed, and hence the greater volume of ferrite observed in the cap region compared
with the root and fill regions.
The MFM technique was capable of clearly imaging the magnetic domain structure of
the ferrite phase that surrounds the “islands” of austenite, which appear flat and uniform
due to their paramagnetic properties. Clear bands of ferrite could be easily
distinguished, but a closer examination of the images revealed other regions, considered
to be ferrite, that did not exhibit the more typical striped magnetic domain configuration
124 B. Gideon, L. Ward and K. Short Vol.7, No.2
associated with ferrite. The different appearances of the magnetic domains in the
ferrite phase can be explained in terms of the orientation of the magnetic domains.
One of the most important factors in MFM imaging is the actual orientation of the
magnetic domains of the sample, which, in turn, depends on the crystallographic
orientation of the ferrite. Therefore, it should be expected that ferrite grains with
different crystallographic orientations yield different magnetic patterns in the MFM
images. Other factors that can affect the contrast in the MFM images are the geometry
of the magnetic domains, and the fact that even domains that are underneath the surface
(i.e. non superficial) can be detected, which may render different contrast than the
superficial ones. A further drawback from the use of this technique is the ability to
resolve features associated with the magnetic domains and the possible inclusion of fine
precipitates such as secondary austenite, χ and σ phases.
1. The metallagraphic tests carried out on the different weld regions, revealed that the
weld properties were acceptable in accordance with current industrial standards
2. Magnetic force microscopy was successfully conducted on the various regions of the
duplex stainless steel welds using the scanning probe microscope in both lift and
tapping mode.
3. The magnetic force microscopy images revealed (i) the formation of both fine
and coarse structure within the weld metal and (ii) clearly defined austenite and
ferrite regions within the different weld passes (root, fill, cap) for the different
weld configurations adopted in the study here.
4. The different austenite / ferrite microstructures observed in the weld regions, such as
the presence of discontinuous grain boundary austenite layers, Widmanstätten austenite
side-plates, austenite intragranular precipitates and intragranular acicular ferrite are
thought to be associated with, and hence explained in terms of variations in
transformation rates and the degree of undercooling
5. MFM is a powerful tool to use for differentiating the austenite and ferrite phase
in duplex stainless steels.
Vol.7, No.2 Characterization of the Weld Regions within Duplex Stainless Steels 125
The authors would like to thank Dr. Lim Chiang Liang of Metacos for preparing the
duplex stainless steel samples for metallographic studies conducted. The authors would
like to acknowledge support provided by the Australian Institute of Nuclear Science
and Engineering (AINSE - grant no. AINGRA06184P), to allow the magnetic force
microscopy studies to be conducted at the Australian Nuclear Science and Technology
Organization (ANSTO). Acknowledgement goes to ARV Offshore for their continuing
support towards this research program.
[1] D. Peckner, I.M. Bernstein, in: H.B. Crawford, B. Gatewood (Eds. ), Handbook of
Stainless Steels, McGraw-Hill, Caledonia, 1977.
[2] M.S. Andrade, J.M.R Vilela, A.C.C. Reis, J.A. Sluss, V.T.L. Buono, Acta Microsc.
5B (1996 ) 266.
[3] B.R.A. Neves, M.S. Andrade, Appl. Phys. Lett. 74 (1999 ) 2090.
[4] S. Takaya, T. Suzuki, Y. Matsumoto, K. Demachi and M. Uesaka, J. Nuclear
Materials 327 (2004) 19-26
[5] A. Dias and M.S. Andrade, Appplied Surface Science 161 (2000) 109-114
[6] J. Wittborn, Nanoscale Studies of Functional Materials using Scanning Probe
Microscopy, Doctoral Thesis. Royal Institute of Technology, Stockholm, Sweden, p. 10
[7] B. R. A. Neves and M. S. Andrade, Identification of two Patterns in Magnetic Force
Microscopy of Shape Memory Alloys, Appl. Phys. Lett., 74, 2090 (1999).
[8] R. Wiesendanger, Scanning Probe Microscopy and Spectroscopy — Methods and
Applications, Cambridge Univ. Press, Cambridge, 1996.
[9] ASTM E562, Standard Test Method for Determining Volume Fraction by
Systematic Manual Point Count
[10] ASTM A370, Standard Test Methods and Definitions for Mechanical Testing of
Steel Products
[11] J.L. Hunter, J. Bechhoefer, J. Vac. Sci. Technol., B 12 (1994 ) 2251.
[12] D. Sarid, in: M. Lapp, H. Stark (Eds. ), Scanning Force Microscopy, Oxford Univ.
Press, New York, 1991.
[13] C. Schonenberger, S.F. Alvarado, S.E. Lambert, I.L. Sanders, J. Appl. Phys. 67
(1990) 7278.
126 B. Gideon, L. Ward and K. Short Vol.7, No.2
[14] Gideon B, Ward L P and Biddle G, Metallurgical Characterisation of Duplex
Stainless Steel and their Susceptibility to Intergranular Corrosion, Proc. European
Corrosion Federation (EUROCORR 2006) Conference Maastricht, Netherlands, 24 –
28 Sep (2006) Session K, pp. 190-191
[15] L. Karlsson: 'Intermetallic Phase Precipitation in Duplex Stainless Steels and
Weld Metals: Metallurgy, Influence on Properties and Testing Aspects', Welding in the
World, vol. 43, no. 5, 1999.
[16] B.J. Ginn and T.G. Gooch: `Effect of Intermetallic Content on Pitting Resistance
of Ferritic-Austenitic Stainless Steels', proc. conf. Stainless Steels'91 Science and
market, Chia Laguna Sardinia, Italy, 1999, vol. 3, p. 81-89