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Proceedings of the

1st International Conference on Engineering Materials and Metallurgical Engineering

22- 24 December, 2016 Bangladesh Council of Scientific and Industrial Research (BCSIR)

Dhaka, Bangladesh

METALLOGRAPHIC CHARACTERIZATION BETWEEN GAS TUNGSTEN ARC WELDED AND FLUX CORED ARC WELDED HIGH-MN BASED TWIP STEELS

Maksudur Rahmana,*, Yeong-Do Park b

a,bDepartment of Advanced Materials Engineering, Dong-Eui University, Busan, South Korea.

Abstract Austenitic high-Mn based twinning-induced plasticity (TWIP) steels having their excellent tensile strength-ductility property combination provide great potential in applications for structural components. In certain cases, these steels might also replace austenitic Cr-Ni stainless steels due to lower price and higher welding speed. Hence a significant increase in the research activity on the weldability of TWIP steels has been proposed in the past decades. In this study, continuous butt welding of Gas Tungsten Arc [1st and 2nd pass] and Flux Cored Arc [2nd pass] welding of pipe elbows were examined. The weldments were characterized by metallography and microhardness tests. Tensile specimens, cut perpendicular to the welding line, were also tested and then subjected to fractographic examinations. Fusion zones, containing dendritic microstructure had lower hardness than HAZ and base metal. Internal cracks, present in the Flux Cored Arc welded samples, were responsible for lowering the tensile properties. Keywords: Butt Welding, Tensile fracture, SEM-EDS, Microstructure, Grain size, Solidification.

1. INTRODUCTION

Due to superior strength, ductility, and impact resistivity; high-Mnaustenitic steels are considered as an excellent candidate for variouscrash-worthy and low temperature application. Recently high-Mnaustenitic steels are getting more attention for low temperatureapplication such as transportation and storage system of liquefied natural gas (LNG) [1].Specifically, the austenite stabilizing capacities of Mn and C are beingexploited to create lower cost alternatives to other cryogenic materials(9Ni steel, Invar, 304 SS, etc.) which are commonly used during construction of LNG tanks [2]. Mn could be used inhigh concentration (between 20~28 wt%) as an alloying addition to createan austenitic steel with mechanical properties similar to traditional LNG tank materials. In response to achieve the desirable properties of LNG,high Mn Steels are eventually arc welded.Welding and post-weld inspections are essential parts of the constructionprocess for LNG tanks [3]. Weldability of high-Mn steels needs to beconsidered during their development for LNG applications. This wouldinclude both the weldability of high-Mn base materials as well asdeveloping compatible and weldable filler materials. One immediateconcern that arises when considering the weldability of austenitic steelsis the potential for solidification cracking during fabrication [4]. A detail investigation on metallographic characterization between gas tungsten arc welded (GTAW) and flux cored arc welded (FCAW) high-Mn based TWIP steelshas been done.

2. MATERIALS AND METHODS

3.0 mm thickness of high-Mn austenitic TWIP steels was used for thisstudy. The chemical compositions of used steel are 0.38% C, 23.31% Mn,0.39% Si, 0.01%Ni, 3.38%Cr and other minor alloying elements.Base metal micrograph is shown in the Fig. 1.

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FIG. 1: MICROSTRUCTURE OF THE BASE METAL (AS RECEIVED).

Specimens were cleaned properly with ethanol to remove dirt and oilbefore experiment. Welding was carried out in a FCAW welding machinefrom Huosung power and industrial system performance group and in a Fronious GTAW welding machine. Groove joint was done by manually to make the elbow for theapplication of practical uses. Schematic diagram of the elbow is shown in Fig. 2.

FIG. 2: SCHEMATIC DIAGRAM OF ELBOW

Sample 1 (GTAW+GTAW): Root pass was done by Gas Tungsten Arc Welding (GTAW) and fillpass was done by the GTAW welding process. Sample 2 (GTAW+FCAW): Root pass was done by Gas Tungsten Arc Welding (GTAW) and fillpass was done by the FCAW welding process. 1.5 mm diameter welding wire for GTAW contains 26% Mn, 0.2% C &0.2% Ni. 1.2 mm diameter welding wire for FCAW contains 18.2 %Mn, 0.4% C &8.0% Ni. Welding Parameters are summarized in the following Table 1.

Table 1: Welding Parameters

Process Name of layers Welding Current (A)

Welding Voltage (V)

Heat Input (kJ/m) Shielding gas

GTAW (a) Root Layer 77.5 11.0 548 Argon

GTAW (b) Filler Pass 77.0 11.5 708 Argon

FCAW (c) Filler Pass 155.0 23.0 436 CO2

The microstructure was revealed using both Nital (4% HNO3; 96%ethanol), 20s; and by successive color etching of Nital (3%), 30s; HCl(10%), 5s; solution of Na2S2O5 (10g) indistilled water (100mL), 10s. EPMA was done by an Electron Probe Micro Analyzer (CAMECA, Model: SX100, 2003). Element Mapping conditions are- HV=20kV, Current=200 nA, Beam size=0.1 µm, Focal Length=165.6 µm, Step size=0.6 µm, and Peak Time=10s. Tensile Tests were carried out for the welded metal. The specimen standard size was followed according ASTME E-8M for welded metal and fractography analysis of tensile fractured samples were done by SEM.

3. RESULTS & DISCUSSIONS

3.1. MACRO ANALYSIS

Fig.3 shows the bead appearance and Fig. 4 shows cross section of the groove joint of GTAW+GTAW & GTAW+FCAW samples. Bead width is little higher at GTAW+GTAW welding process due to high heat input.FCAW creates lots of spattering due to unstable arc and GTAW has no spattering due to stable arc. Full penetration did to occur at fill pass for GTAW+FCAW.

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(a) GTAW+GTAW (b) GTAW+FCAW

FIG.3:BEAD APPEARANCE OF WELDED SAMPLES

(a) GTAW+GTAW (b) GTAW+FCAW

FIG. 4:CROSS SECTIONS OF WELDED SAMPLES THROUGH WELD ZONE

3.2. MICROSTRUCTURAL ANALYSIS GTAW and FCAW welding processes change the microstructure of variouswelding zones, including the HAZ, partially melted zone (PMZ), and fusion zone (FZ).The temperature in the FZ is above the liquidus temperature, whichresults in the formation of molten metal during welding [5].Equiaxed dendritic solidification mode is observed in GTAW and columnar dendritic solidificationmode is observed in FCAW welded high-Mn steels (shows in Fig.5).

(a) GTAW+GTAW

(b) GTAW+FCAW

FIG.5:MICROSTRUCTURE FROM PMZ TO FZ (FILL PASS) CONTAINING AUSTENITIC DENDRITES. The evolution of microstructures in the Fusion Zone (FZ) during weld solidification is dependent on various parameters including crystal growth rate (R), liquid temperature gradient (G), undercooling, and alloy constitution [6]. These parameters dictate the non-equilibriumsolidification conditions that cause solid-liquid interface instabilities as crystallization occurs.Weld solidification starts with an initial crystal nucleation and growth.Crystal nucleation in the FZ of weldments is a highly heterogeneousprocess and is commonly described as being epitaxial in nature [7].Solidification growth morphologies can generally be described as beingplanar, cellular, or dendritic [6].In FCAW, L → L + γ → γ reaction occurs during cooling to roomtemperature in the

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microstructure reactions. Metallographic examinationof the FZ revealed one columnar dendritic type of solidification withdirectional solidification toward the center of the FZ.Grain growth occurs at the root pass fusion zone which is also knownas re-heated FZ (shows in Fig.6).

(a) GTAW+GTAW

(b) GTAW+FCAW

FIG.6: MICROSTRUCTURE OF THE RE-HEATED FUSION ZONE (ROOT PASS)

Grain size distribution is not uniform through the total HAZ area. From top to bottom the grain size is gradually increases and from FZ to BM the grain size of HAZ gradually decreases. Because root pass (GTAW) is very slow welding process and hence HAZ near root pass fusion zone (re-heated FZ) gets more heat than the top HAZ near fill pass (FCAW). The difference between the top HAZ and the bottom HAZ length is quite significant for GTAW+FCAW. Fig. 7 shows the grain size variations for GTAW+GTAW and GTAW+FCAW.

FIG.7:GRAIN SIZE VARIATIONS AT THE TOP AND THE BOTTOM HAZ AREA.

Difference between top and bottom HAZ length is more prominent at GTAW+FCAW due to variations of heat input during welding. These grain size variations are responsible for hardness variations at HAZ areas. Hardness profiles are shown in Fig. 8.Smaller gran size gives higher hardness and larger grain size give lower hardness.

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FIG.8:HARDNESS PROFILE AT THE TOP AND THE BOTTOM HAZ AREA

3.3. EPMA ANALYSIS

Mode of solidification, partitioning coefficient, and segregation ratio areresponsible for alloying element segregation. Among the types ofsolidification modes in austenitic stainless steel weld metals, primaryaustenitic solidification (A type) has the strongest micro segregationeffects [8-11]. In the experiment high-Mn steel containing 23.31%Mn isused, and therefore a strong variation of Mn at the weld zone isexpected. EPMA test revealed that extensive segregation of C and Mn in the fusion zone (shows in Fig. 9).

FIG.9:EPMA-ELEMENT MAPPING SHOWING MN & C SEGREGATION.

The affinity of C toward Mn is known tobe stronger than that of Fe [12].The equilibrium segregation coefficients (κ) of Mn and C areapproximately 0.75 and 0.28, respectively [8]. Thus, C and Mn show segregation behavior according to the condition of κ < 1; that is,segregation occurs along the boundaries between cells or dendrite arms,and the region in which segregation occurs is known as interdendriticregion [12].

3.4. TENSILE PROPERTIES & FRACTOGRAPHY ANALYSIS Tensile tests were done after grinding the weld zone of the samples. Table 2 shows the tensile properties of GTAW+GTAW & GTAW+FCAW samples. GTAW+GTAW samples show better tensile properties than GTAW+FCAW. In all cases, tensile fracture occurred at the fusion zone. Hence the variation of tensile properties is due to the fusion zone characteristics, which would be explained through fractography analysis.

Table 2: Tensile Properties

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Material condition As received Basemetal GTAW+GTAW GTAW+FCAW Fracturelocation - Fusion zone Fusion zone

Tensilestrength (MPa) 900 780 710 Elongation (%) 60 28 10

Fig. 10 shows the cross section of the tensile fracture zone. Grain elongation occurs towards the tensile stress at GTAW+GTAW sample. But no such elongation was visible at GTAW+FCAW sample.

FIG. 10:CROSS SECTION OF TENSILE FRACTURE AT WELD ZONE.

Fig. 11 shows the tensile fracture surface of the samples. Some defects (micro cracks) are found at the fracture surface of GTAW+FCAW.

FIG. 11:TENSILE FRACTURE SURFACE SHOWING SOME MICRO CRACKS.

4. CONCLUSIONS 1. GTAW+GTAW sample has higher heat input than the GTAW+FCAW due to slower welding speed. 2. Spattering occurs during the FCAW welding. 3. Non uniform HAZ for GTAW+FCAW welding process. 4. Full penetration occurs at the 2nd pass of GTAW+GTAW. 5. Mn and C segregation at the FZ of the weldments. 6. Tensile properties are better at GTAW+GTAW than GTAW+FCAW. 7. Microcracks and defects are more in case of GTAW+FCAW than GTAW+GTAW.

5. REFERENCES [1] Choi, J.K., et al. High Manganese Austenitic Steel for CryogenicApplications. inTewnty-second

International Offshore and PolarEngineering Conference. 2012. Rhodes, Greece. [2] API, 620: Design and Construction of Large, Welded, Low-PressureStorage Tanks, in Appendix Q: Low-

pressure Storage Tanks forLiquefied Hydrocarbon Gases. 2008. [3] Kruzic, R., LNG storage tanks: advancements in weld inspections.New inspection method improves jobsite

examinations andverification of welds: LNG and GTL Developments. Hydrocarbonprocessing 2004. 83(7): p. 53-55.

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[4] Brooks, J.A. and A.W. Thompson, Microstructural Development andSolidification CrackingSusceptibility of Austenitic Stainless-SteelWelds. International Materials Reviews 1991. 36(1): p. 16-44.

[5] K. A. Lyttle and W. F. G. Stapon, “Select the best shielding gasblend for the application”, Welding Journal, 69, pp. 21–27,November 1990.

[6] David, S. and J. Vitek, Correlation between solidification parameters and weld microstructures. International Materials Reviews 1989.34(1): p. 213-245.

[7] Savage, W. and A. Aronson, Preferred orientation in the weldfusion zone. WELD J, 1966. 45(2): p. 85. [8] V. P. Kujanpaeae, and S. A. David, Laser application conference,pp. 63-70(1986). [9] Y. Arata, F. Matsuda, and S. Katayama, T. JWRI., 5, pp.35-51(1976). [10] N. Suutala, T. Takalo, and T. Moisio, Metall. Mater. Trans. A., 11,pp. 717-725(1980). [11] L. Mujica, S. Weber, G. Hunold, and W. Theisen, Steel. Res. Int.82, pp. 26-31(2011). [12] S. Kou, Hoboken, New Jersey, J. Wiley & Sons, Inc, (2003).