The Effect of welding method and heat treatment on creep resistance of Inconel 718 sheet welds
Abstract— The paper presents results of a study on the effect of welding technique and heat treatment on structure and creep resistance of welded joints made on thin (0.5–1.2 mm) Inconel 718 sheet with the use of GTAW and laser method without filler metal. Technological high-temperature creep tests consisted in measuring the time to rupture for sheet metal without and with welded joints under constant stress σ = 150 MPa and at constant temperature tc = (860 ± 2)°C.
Sheet metal joints made with the use of GTAW method and laser-welded are characterized with a three-zone macrostructure, including the weld with size of dendritic grains depending on welding method, the heat-affected zone (HAZ), and the base material (BM). For GTAW method, with increasing sheet metal thickness, the observed values of parameter λ2s (distance between axes of second-order branches of dendritic grains) decrease from about 12 µm to 8 µm, and similarly, in case of laser welding, λ2s values decrease from about 9.6 µm to 5 µm with increasing sheet metal thickness.
Thin sheet Inconel 718 in as-delivered condition high-temperature creep-tested have the time to rupture on the level of 12 h at elongation of about 48%. Similar time-to-rupture values characterize the joints made by laser welding. GTAW joints are characterized by about 30% shorter time to rupture and elongation less by about 45%.
The microstructure change induced by heat treatment (solution treatment and two-stage ageing) resulted in significant increase of creep resistance properties of both base metal and laser-welded joints. The time to rupture of heat-treated specimens of thin Inconel 718 sheet and laser-welded joints creep-tested at 860°C ± 2°C under stress of 150 MPa was about 19.5 h with elongation ranging from 23% to 33%.
Keywords— Inconel 718, GTAW, laser welding, heat treatment, microstructure, high-temperature creep.
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