Materials Letters 270 (2020) 127680

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Materials Letters
journal homepage: www.elsevier.com/locate/mlblue

Insight into the microstructural features and corrosion properties of wire

arc additive manufactured super duplex stainless steel (ER2594)
A. Rajesh Kannan a, N. Siva Shanmugam a,⇑, V. Rajkumar b, M. Vishnukumar c
a
Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli, Tamil Nadu 620015, India
b
Department of Mechanical Engineering, Coimbatore Institute of Engineering and Technology, Coimbatore, Tamil Nadu 641109, India
c
Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli, Tamil Nadu 620015, India

a r t i c l e i n f o a b s t r a c t

Article history: This work investigates the understanding between microstructure and corrosion behaviour of SDSS 2594
Received 17 February 2020 wall component fabricated using gas metal arc welding (GMAW) based wire arc additive manufacturing
Received in revised form 16 March 2020 (WAAM) process, with the help of optical micrographs and electrochemical corrosion tests.
Accepted 18 March 2020
Potentiodynamic polarization tests and Electrochemical impedance spectroscopy (EIS) analysis in 3.5%
Available online 19 March 2020
NaCl solution revealed excellent pitting resistance while the corrosion rate ranged between 1.51 and
1.61 mils per year (mpy) with pitting resistance equivalent number (PREN) > 40. The intergranular cor-
Keywords:
rosion (IGC) test results confirmed the absence of sensitization and highlights that the formation of pas-
Super duplex stainless steel
WAAM
sive film is compact for corrosive environments.
Microstructure Ó 2020 Elsevier B.V. All rights reserved.
Corrosion
Welding

1. Introduction process. Jia et al [6] fabricated Inconel 718 via ultra-high frequency
pulsed arc WAAM process and observed reduction in Nb segrega-
Super duplex stainless steels (SDSS) are mainly designed to tion with refined grain matrix. Chakkravarthy et al [7] deposited
handle extreme corrosive environments such as the oil and gas, SS316L using CMT process and reported that h1 1 1i was the pre-
petrochemical, pressure vessels and seawater equipment indus- ferred texture orientation for better corrosion resistance. SDSS
tries due to their excellent properties in terms of corrosion behav- 2594 fabricated via TOP-TIG-WAAM revealed the higher wt% of
ior and strength [1–2]. The higher wt% of chromium, nitrogen and austenite with low ferrite content, while the presence of detrimen-
molybdenum offer exceptional resistance to IGC, crevice corrosion tal intermetallics like r and k did not affect the mechanical prop-
and pitting [3]. The fabrication of SDSS wrought alloys is costly erties [1,8]. Nevertheless, only few pieces of established literature
compared to austenitic stainless steels, and thus, it is required to are available on the fabrication and characterization of SDSS using
explore other techniques to fabricate structural components of WAAM. Hence this investigation focused on some fundamental
SDSS with better mechanical and corrosion properties. WAAM is concerns of microstructure and corrosion behavior of as-built SDSS
a fast manufacturing technology based on the deposition of metals specimens through WAAM process.
in a layer-by-layer technique with enhanced material utilization
[4]. In this sense, WAAM process is preferred for fabricating
medium-large size parts of complex geometry, because it offers 2. Experimental
higher deposition rate and is cost-competitive compared to pow-
der based additive manufacturing systems. The welding processes The GMAW based WAAM system consists of OTC-Daihen six-
such as GMAW, Gas Tungsten Arc welding (GTAW), Laser Arc axis robot with DM350 MIG DC power source. Shielding gas of
Welding (LAW) and Cold Metal Transfer (CMT) process have been 99.9% pure argon with a constant flow rate of 20 l/min was used
predominantly used to deposit and construct the feed material in to deposit feeding material, ER2594 (/1.2 mm) and its nominal
the WAAM process [5]. Industrial materials such as stainless steels, composition (wt%) is C-0.02, Mn-0.8, W-0.01, Si-0.3, Cr-24.6,
titanium and nickel alloys are deposited widely using WAAM Ni-8.6, Mo-3.8, N-0.25, Cu-0.01 and Fe-balance. Two multi-
layered thin walls of SDSS were fabricated over SDSS substrate.
The deposition parameters are: welding current = 140 Amps,
⇑ Corresponding author. voltage = 16.2 V and welding speed = 300 mm/min. A dwell time
E-mail address: nsiva@nitt.edu (N. Siva Shanmugam). of 60 s was considered for the partial solidification and to reduce

https://doi.org/10.1016/j.matlet.2020.127680
0167-577X/Ó 2020 Elsevier B.V. All rights reserved.
2 A. Rajesh Kannan et al. / Materials Letters 270 (2020) 127680

the delineations. The wall was fabricated by depositing 62 layers microstructural characteristics of the wall component at bottom
and mean height is 2.09 mm for each layer. The wall dimensions (SDSS-B), middle (SDSS-M) and top (SDSS-T) regions. The
were 130  120  7 mm before machining and 120  114  4.2 microstructure revealed the existence of intragranular austenite
mm after machining, respectively. Microstructure at various (IGA), grain-boundary austenite (GBA), and Widmanstätten
regions of the WAAM processed SDSS wall were captured using austenite (WA). The previous investigations revealed that austenite
metallurgical microscope and Scanning electron microscope content in the DSS is influenced by cooling rate [8]. Hence more wt
(SEM). SEM attached with energy dispersive spectrometry (EDS) % of Ni and N is added to filler wire for better corrosion properties.
was employed to analyze the elemental distribution. Tafel scan The microstructure varied across the built-up direction and is
and EIS measurements were performed in 3.5% NaCl solution using attributed to the complex cyclic thermal history (CCTA) during
classical three electrode system with a scan rate of 1.33 mv/s. As the deposition of adjacent layers. The CCTA influenced the propa-
1% of NaCl is not enough to examine the pitting resistance of SDSS, gation of secondary austenite (c2) to IGA and the peak tempera-
3.5% NaCl is used [9]. Double-loop electrochemical potentiody- ture was sufficient to form intragranular c2 and intergranular c2
namic reactivation (DLEPR) test was carried out to examine the (Fig. 1k) [1]. Further, the coarsening of WA, GBA and IGA controls
IGC susceptibility. The standard solution of 2 mol/L H2SO4 + 0.01- the growth of excessive austenite [11]. The overlapping melt pools
mol/L KSCN + 0.5 mol/L NaCl was modified and a more aggressive revealed the growth of randomly oriented columnar and equiaxed
solution of 2 mol/L H2SO4 + 1.5 mol/L HCl with a scan rate of grains (Fig. 1(c)) cutting along the deposition direction [12]. The
0.67 mV/s is used [10]. as-deposited DSS via selective laser melting revealed fully ferritic
microstructure and annealing was carried out to improve the
3. Results and discussion austenite-ferrite ratio [13]. The microstructure was predominantly
austenitic with less ferritic phases and traces of detrimental inter-
The as-built wall component was devoid of any shrinkage or metallics (r, k, v, Cr2N etc.) were not observed in the present
solidification defects (Fig. 1a). Fig. 1(a)–(j) illustrates the work. The austenite propagated primarily along grain boundaries

Fig. 1. Microstructural features of the as-built wall component (a) Macrostructure (b–c, e–f and h–i) microstructure, (d, g and j) corresponding SEM-EDS elemental maps of
Fe, Cr, Ni and Mo at top, middle and bottom regions, (k) SEM micrograph and (l) EDS line scan of the intergranular c2 precipitation region.
 A. Rajesh Kannan et al. / Materials Letters 270 (2020) 127680 3

and secondarily as IGA dispersions and WA plates. The GBA is Fig. 2(a-b) illustrates the Tafel curves and Nyquist plots of the
mainly columnar with orientation to the build direction [14]. WAAM processed SDSS samples at various locations. There were
SEM elemental maps of Fe, Cr, Ni and Mo from the SDSS-T, SDSS- no noticeable variation in the corrosion rate (mpy) for all the sam-
M and SDSS-B reveals the absence of elemental partitioning ples. The corrosion results are tabulated and presented as an inset
(Fig. 1 (d), (g) and (j)). The wt% of Mo and Cr were comparable with in the Fig. 2a. The corrosion current density (Icorr) for SDSS 2507
AISI 2507, whereas Fe wt% increased with deposition height [8]. base metal (BM) was 3.6 ± 0.2 mA/cm2 at room temperature
SEM photomicrographs confirmed the presence of intergranular [10]. The Icorr values of WAAM processed SDSS samples (Fig. 2(a))
c2 (Fig. 1(k)) while intragranular c2 was observed as clusters in are less and consistent with the SDSS BM results (Icorr < 10 mA/
the ferrite grain interiors. As pointed by Ref. [1], higher Cr wt% is cm2) [9–10]. This is attributed to the higher wt% of Mo and Cr in
observed at temperature>1100 °C due to the c2 formation and the as-built wall compared to BM. According to Fontana [15], cor-
the EDS line scan results confirms the Cr wt% (the green line in rosion resistance is excellent (1–5 mpy). These results confirm that
Fig. 1(l)). the WAAM process can effectively retain the corrosion resistance

Fig. 2. Electrochemical test results: (a) Tafel and (b) Nyquist plots of various samples in as-built condition.

Fig. 3. Optical micrographs showing pits obtained from samples subjected to Tafel scans: (a) SDSS-B, (b) SDSS-M and (c) SDSS-T.
4 A. Rajesh Kannan et al. / Materials Letters 270 (2020) 127680

to the absence of detrimental phases and comparable microstruc-

ture [9].

4. Conclusions

The following inferences concerning the microstructural and

corrosion properties of WAAM processed SDSS 2594 have been
drawn:

 WAAM processed SDSS wall exhibited higher austenite propor-

tion than ferrite.
 The microstructure revealed the existence of IGA, GBA, and WA
with c2.
 The corrosion rate ranged between 1.54 and 1.61 mpy and pit-
ting resistance was excellent compared to wrought alloy with
stable micro-level pits.
 The PREN was > 40 and this is attributed to the increased wt% of
Cr and Mo in the wall component.
 DLEPR test revealed no sensitization, which is due to the
absence of detrimental phases and comparable microstructure.
Fig. 4. DLEPR polarization curves for WAAM processed SDSS samples.

of SDSS. Fig. 2(b) shows the corresponding Nyquist plots of the Declaration of Competing Interest
WAAM processed SDSS samples. The radius of the capacitive loop
of the SDSS samples are almost similar and this highlights the pit- The authors declare that they have no known competing finan-
ting resistance. This trend is identical to the results attained in cial interests or personal relationships that could have appeared
Tafel plots. The PREN is calculated with the wt% of important ele- to influence the work reported in this paper.
ments (Mo, Cr and N). The PREN calculated from SEM-EDS analysis
for SDSS-T, SDSS-M and SDSS-B were 41.38, 41.29 and 41.90, References
respectively. This confirms that the SDSS wall meets the require-
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Fig. 3 demonstrate the pit morphologies in the different loca- John Wiley & Sons, USA, 2014.
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the lack of a reactivation peak in the reverse scan, which is due