Improvement of Microstructure and Mechanical Behavior of Gas Tungsten Arc Weldments of Alloy C-276 by Current Pulsing

doi:10.1007/s40195-014-0186-4

Abstract

Abstract:

Superalloy C-276 is known to be prone to hot cracking during fusion welding by Gas Tungsten Arc method. Microsegregation occurring during cooling of fusion zone with consequent appearance of topologically close-packed phases P and µ has been held responsible for the observed hot cracking. The present work investigated the possibility of suppressing the microsegregation in weldments by resorting to current pulse. Weldments were made by continuous current gas tungsten arc welding and pulsed current gas tungsten arc welding using ERNiCrMo-4 filler wire. The weld joints were studied with respect to microstructure, microsegregation, and mechanical properties. Optical microscopy and scanning electron microscopy were employed to study the microstructure. Energy-Dispersive X-ray Spectroscopy was carried out to evaluate the extent of microsegregation. Tensile testing was carried out to determine the strength and ductility. The results show that the joints fabricated with pulsed current gave rise to narrower welds with practically no heat affected zone, a refined microstructure in the fusion zone, reduced microsegregation, and superior combination of mechanical properties.

Key words: Alloy C-276, Pulsed current gas tungsten arc welding, Microsegregation

M. Manikandan, N. Arivazhagan, M. Nageswara Rao, G. Madhusudhan Reddy. Improvement of Microstructure and Mechanical Behavior of Gas Tungsten Arc Weldments of Alloy C-276 by Current Pulsing[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(2): 208-215.

Figures/Tables 12

Table 1 Chemical composition of base metal and filer wire (wt%)

Material	Mo	Cr	W	Co	Mn	Fe	Si	V	Cu	P	S	C	Ni
Hastelloy C-276	16.36	15.83	3.45	0.05	0.41	6.06	0.02	0.17	-	0.005	0.002	0.005	Bal
ERNiCrMo-4	17.00	16.5	4.5	2.50	1.0	7.0	0.08	-	0.5	0.04	0.03	0.02	Bal

Table 2 Process parameters for welding of alloy C-276 using GTA and PCGTA methods, employing ERNiCrMo-4 filler

Welding method	Current (A)	Voltage (V)	Peak current (A)	Background current (A)	Pulse frequency (Hz)	Pulse on time	Welding speed (mm/s)	Heat input (J/mm)
GTA	150	13.2					1.54	900
PCGTA			200	100	6	50%	1.54	798

Fig. 1 Photographs of weld joints: a ERNiCrMo-4 GTAW; b ERNiCrMo-4 PCGTAW

Fig. 2 Macrostructure of weld joints: a ERNiCrMo-4 GTA weld; b ERNiCrMo-4 PCGTA weld

Fig. 3 Microstructure of alloy C-276 in the as-received condition

Fig. 4 Micrographs of weld joints produced by ERNiCrMo-4 GTAW: a Fusion Zone; b Weld Interface; c Weld Pass Interface

Fig. 5 Micrographs of weld joints produced by ERNiCrMo-4 PCGTAW: a Fusion Zone; b Weld Interface; c Weld Pass Interface

Fig. 6 SEM/EDAX analysis of GTA welded alloy C-276 with ERNiCrMo-4 filler wire for different regions of the weldments: a SEM image of weld zone; b EDAX result of weld subgrain boundary (Site 1); c EDAX result of weld subgrain body (Site 2); d SEM image of HAZ and Interface; e EDAX result of weld interface-subgrain boundary (Site 3); f EDAX result of weld interface-subgrain body (Site 4)

Fig. 7 SEM/EDAX analysis of PCGTA welded C-276 alloy with ERNiCrMo-4 filler wire for different regions of the weldment: a SEM image of weld zone; b EDAX result of weld subgrain boundary (Site 1); c EDAX result of weld subgrain body (Site 2); d SEM image of HAZ and Interface; e EDAX result of weld interface-subgrain boundary (Site 3); f EDAX result of weld interface-subgrain body (Site 4)

Table 3 Distribution coefficient (k) of different alloying elements in GTA and PCGTA welded alloy C-276 using ErNiCrMo-4 filler

Welding method	Ni	Cr	Mo	W	Fe
GTA	1.01	1.01	0.92	1.22	0.91
PCGTA	1.02	1.00	0.94	1.19	0.94
Arc welding^(Dupont)	1.08	0.95	0.82	1.01	1.01

Fig. 8 Photographs of broken tensile test specimens: a GTAW- ERNiCrMo-4; b PCGTA ERNiCrMo-4

Table 4 Tensile properties of base metal and ERNiCrMo-4 weldments of alloy C-276 using GTAW and PCGTAW

Material	UTS (MPa)	Elongation (%)
Base metal	750	75
GTA weldment	766	34
PCGTAW weldment	815	41

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