The effect of filler metal on microstructure and mechanical performance of GTAW inconel 625 at room and elevated temperature

Abstract

The aim of this study was evaluating the influence of ER321 and ERNiCr-3 filler metals on the mechanical performance of the Inconel 625 gas tungsten arc welded joints at room and elevated temperature and correlating with the microstructure. Optical and scanning electron microscopy was conducted to characterize microstructure features at the fusion and the interfaces. Microhardness measurements, tensile test, Charpy impact test, and creep rupture test were applied to the weldments to investigate the effect of filler metal on mechanical properties of the joints. Creep rupture test was performed at 600 °C under load stresses of 150, 200 and 300 MPa. At high stress levels (300 MPa), nucleation and coalescence of voids accelerates and a ductile fracture with dimple morphology for both filler metals was seen. At lower stress level of 150 MPa, the creep deformation becomes dominant for both weldments and the voids which are mainly micro-creep voids get smaller in size and are located near the precipitates. A 100% increase in stress level (from 150 MPa to 300 MPa), reduces the creep rupture life of the ERNiCr-3 and ER321 welds by about 11 and 30 times, respectively. The results show that stringers of δ-Ferrite in inter-dendritic region of austenite appear in ER321 weldments along with some precipitates which are highly enriched in Ti and C. The formation of the unmixed zone with a different concentration gradient of the elements is observed at the Inconel 625/ER321 stainless steel interface. XRD pattern reveals the formation of (Fe2Mo) Laves phase in ER321 weld metal where the amount of Fe in the weld increases and Mo solubility decreases. ERNiCr-3 weldments show fully austenitic structure and some NbC precipitates. EDS analysis of ERNiCr-3 weld metal confirms high amount of Mo and Nb at the inter-dendritic region, whereas the dendritic core is depleted from Mo and Nb. By applying different filler metals, different fracture morphology was revealed under different loads. A non-uniform hardness profile and the minimum microhardness value of 120 HV were seen in the wide unmixed zone of ER321 weld metal due to its heterogeneous structure and also due to lack of weld metal dilution. It was shown that ER321filler metal does not give an appreciable tensile and impact properties comparing to ERNiCr-3 weldments. The tensile elongation was greater for ERNiCr-3 weldments (13.1%) than ER321 joints (10.2%) andthe average impact toughness of ERNiCr-3 weldments was about two times higher than ER321.