Studies on Corrosion Behaviour of IN718 alloy Gas Tungsten Arc Welds

Studies on Corrosion Behaviour of IN718 alloy Gas Tungsten Arc Welds

1* K. Prakash

Department of Metallurgical Engineering Andhra University College of Engineering (A) Visakhapatnam-AP, India

1Dilkush

Department of Metallurgical & Materials Engineering Rajiv Gandhi University of Knowledge Technologies (AP-IIIT), Nuzvid-AP, India

2K. Srinivasa Rao

l Department of Metallurgical Engineering Andhra University College of Engineering (A) Visakhapatnam-AP, India

3G. Madhusudhan Reddy

Defence Metallurgical Research Laboratories, Hyderabad-AP,

India

AbstractInconel 718 alloy is a material that has been extensively used in gas turbine engine components in marine- based industrial applications because of its superior mechanical and corrosion resistant properties. When it comes to welding of IN718 alloy, corrosion resistance gets affected due to the segregation of niobium and formation of intermetallic laves phase in the fusion zone of the welds. The present work is aimed to evaluate the corrosion behaviour of IN718 alloy welded with Gas tungsten arc welding (GTAW) process. In order to improve the corrosion resistance of welds, a post weld heat treatment (PWHT) was employed, i.e., solutionizing treatment at 10800C followed by direct aging condition (1080STA). In the present work, the corrosion resistance of IN718 alloy GTA welds was studied by employing Cyclic Potentiodynamic Polarization test on the fusion zones of as-welded and PWHT conditions in a salt environment of 3.5Wt% NaCl solution. The established cyclic potentiodynamic polarization curves in as-welded and 1080STA conditions shown better corrosion resistance for the 1080STA condition.

KeywordsInconel 718 alloy, Gas tungsten arc welding (GTAW), Solutionizing and Post weld heat treatments (PWHT), Cyclic Potentiodynamic Polarization and 3.5wt % NaCl solution.

1. INTRODUCTION

   Inconel 718 is a nickel-based superalloy extensively used in many industrial applications, significantly in gas turbine engine blades in marine applications, space vehicles, reciprocating engine components and many more [1,2]. On account of the slow precipitation kinetics of principal strengthening precipitates (Ni3Nb) and (Ni3Al) in annealed IN718 alloy, it renders high strength, good mechanical characteristics, corrosion resistance and resistance to strain age cracking [3]. Nevertheless, IN718 alloy also confronts weldability problems such as microfissuing and solidification cracking in the heat affected zone and formation of Nb rich intermetallic Laves phase at interdentritic regions of weld during solidification of weld metal [4]. In order to overcome the above weldability problems, several researchers worked on welding parameters and post weld heat treatments (PWHT), on mechanical and microstructural behaviour [5-10].

   Very few studies were reported on the corrosion resistance behaviour of IN718 alloy welds [11].

   In present work, IN718 alloy plates of 3mm were welded using Gas Tungsten Arc Welding process with similar IN718 alloy plates. Welding is followed by a post weld heat treatment at 10800C chosen as a modified route to overcome the welding associated problems. The primary objective of the work is to study the effect of PWHT (1080STA) on corrosion behaviour of the GTA weldment and to compare with that of the as-welded condition.

2. METHODOLOGY

   1. Material and Fabrication

      IN718 plates of 3mm thickness were welded by gas tungsten arc welding process, using a filler wire of same alloy (TABLE I). TABLE II contains the list of welding parameters that were used and the gas tungsten arc welds of IN718 alloy is shown in Fig.1. Specimens were cut from welded joints using a wire electric discharge machine (Wire EDM). These specimens were subjected to post weld heat treatment, i.e., solutionizing treatment at 10800C/20min/FC followed by direct aging (7200C/8h/FC followed by 6200C/8h/AC). Both the PWHTed specimen and the as-welded specimen were prepared for further characterizations.

      Fig. 1. Images GTAW IN718 alloy weldments

      TABLE I. WEIGHT% COMPOSITION OF BASE METAL IN718 AND WIRE FILLER

      Material	Composition
      Filler (IN718)	Ni	Cr	C	B	S	P
      	50	17	0.05	0.005	0.015	0.015
      	Si	Ti	Al	Mo	Nb	Fe
      	0.35	0.65	0.45	2.80	4.75	Balance
      IN718 alloy	Ni	Cr	C	B	S	P
      	53	18.2	0.02	0.003	0.002	0.0005
      	Si	Ti	Al	Mo	Nb	Fe
      	0.12	0.97	0.51	3.13	5.08	Balance

      TABLE II. WELDING PARAMETERS USED DURING GTA WELDING PROCESS

      Welding parameter	Selection
      Current	110A
      Voltage	18V
      Speed	6mm/s
      Heat Input	330J/mm

   2. Microstructural characteristics

      The specimens of as-welded and PWHT conditions are flattened by grinding. For metallography, specimens are polished using emery papers of grade 1/0, 2/0, 3/0 and 4/0 followed by disc polish with a diamond paste of 0.5 and 1 micron until the mirror finish is obtained. The mirror finished metallographic specimens were etched using a mixed acid solution, i.e. prepared using 76mL H2O + 20mL HNO3+ 100mL HCl+ 2mL HF. Microstructural studies were carried by Optical microscopy for both as welded and post weld heat treated specimens after the corrosion test.

   3. Corrosion Studies

   GillAC 1130 equipment was used to study the cycle polarization studies in a salt environment that contain 3.5% NaCl solution with a sweep rate of 0.1667mV/sec. An area of 0.1257cm2 is exposed to the test environment. Based on the loop area, repassivation (Erp) potential, and the hysteresis obtained, the corrosion resistance of specimens was evaluated.

3. RESULTS AND DISCUSSIONS

   1. Microstructure Studies

      Optical microscopy of fusion zone in both as welded and 1080STA conditioned IN718 alloy welds are shown in Figs. 2. (a)&(b). It clearly depicts, fusion zone of as-welded IN718 alloy comprise of coarse laves and delta phases. Whereas 1080STA resulted in less Nb rich intermetallic Laves phase and complete dissolution of the delta phase resulting in better corrosion resistance of PWHTed specimen.

      Fig. 2. (a) Microstructure of as-welded specimen

      Fig. 2. (b) Microstructure of 1080STA specimen

   2. Corrosion Studies

      The cyclic potentiodynamic polarisation curve of as welded and 1080STA conditioned IN718 alloy welds are shown in Fig.3.2 from the curves, it is clear that corrosion potential (Ecorr) of PWHTed specimen is more positive when compared to the as-welded specimen indicating improved pitting corrosion resistance. The difference between pitting otential (Epit) and protection potential (Erp) is known as the relative corrosion resistance E [12-15]. Its magnitude indicates the pitting resistance, i.e., higher the E magnitude lower is the pitting resistance. Also, the area under the hysteresis loop is an indicator of corrosion resistance, smaller the area higher the corrosion resistance vice versa.

      Obtained cyclic potentiodynamic polarisation curves Fig.3 depict that both the conditions have lesser current density during reverse scan than forward scan that indicates both conditions are showing native hysteresis. But E of 1080STA specimens is less compared to the as-welded specimen. TABLE.III describes the magnitudes of Ecorr, Epit, Erp, E. Hysteresis loop area of the 1080STA specimen was smaller than that of the as-welded specimen, indicating that 1080STA specimen is having higher corrosion resistance compared to as-welded specimen. This can be attributed to the relatively less amount of Nb segregation.

      Fig. 3. Cyclic Potentiodynamic Polarization Curves of As-Welded (Red) and

      1080STA (Blue) IN718 alloy welds.

      TABLE III. RESULTS FROM CYCLIC POLARISATION TEST

4. CONCLUSIONS

• From the obtained results, it is clear that solutionized and aged at 10800C specimen of IN718 GTAW alloy weldment has shown better corrosion resistance than as- welded IN718 GTAW welds.

• Improved corrosion resistance is attributed to the less segregation of Nb rich intermetallic laves phases in the fusion zone of the post weld heat treated specimen.

ACKNOWLEDGMENT

The author thanks the Department of Metallurgical Engineering, Andhra University, and Dept. of Metallurgical & Materials Engineering, RGUKT, Nuzvidu for their support.

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   Specimen	Ecorr	Epit	Erp	E
   As-Welded	171mV	988mV	473mV	515mV
   1080STA	281mV	970mV	498mV	472mV

   Specimen	Ecorr	Epit	Erp	E
   As-Welded	171mV	988mV	473mV	515mV
   1080STA	281mV	970mV	498mV	472mV

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