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%.
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Introduction
Inconel 718, a nickel-based alloy, belongs to a large group of heat-resisting and creep-resisting metallic materials used to manufacture a variety of welded structures, especially in aircraft industry where they are used for exhaust components in PW400 and CF6 jet engines [1–4]. The alloy shows good weldability and can be heat-treated by precipitate strengthening [5– 9]. Components made of thin Inconel 718 sheet can work at high temperatures which in case of combustion chambers can reach 1300°C [2, 8].
In operating conditions, welded structures with components made of the alloy are subject to both high temperatures and large stresses. In view of the above, creep-resisting properties of welded joints in such structures are of great importance.
The sources of concentrated heat streams used to weld Inconel 718 sheet include: electric arc in GTAW welding method [10– 12] and a beam of monochromatic light in case of laser welding.
The paper concerns the effect of the two welding methods used for welding thin Inconel 718 sheet [13, 14] and heat treatment on structure and the time to rupture of obtained joints measured in high-temperature creep test.
Conclusion
The initial material selected for the study were three grades of thin (0.5-mm, 0.9-mm, and 1.2-mm thick) sheet metal in asdelivered condition, rolled out of nickel-based Inconel 718 alloy. The material contained about 17% Cr, 16% Fe, 5.0% Nb, 2.5% Mo, 1.0% Ti, 0.60% Al, and 0.05% C. Total content of other elements such as Si, Mn, B, and N did not exceed 0.25%.
The microstructure of sheet metal reveals equiaxial grains of phase γ with dimensions of about (100 ± 15) μm with a small quantity of irregular phases σ and spherical carbides of MC-type sparsely distributed in the matrix. Intermetallic (TPC-type) phases σ have complex and diversified chemical composition (Fig. 13). These phases contain Nb and can be therefore described as Ni(Cr, Fe, Nb, Ti). There are also phases without Nb, Ni(Cr, Fe, Ti), as well as those without Ti, Ni(Cr, Fe, Nb). Carbides of MC type sparsely distributed in structure of the sheet metal contain mainly Nb and Ti and small quantities of nickel. They can be described as (Nb, Ti)C.
Thin Inconel 718 sheet in as-delivered condition with such microstructure shows good tensile strength on the level of about (850 ± 30) MPa at high plasticity, and elongation on the level of (30 ± 3)% [15]. The creep resistance of sheet metal in asdelivered condition determined by means of the time to rupture measured in high-temperature creep tests at temperature 860°C and under stress 150 MPa was 12 h at elongation of 48%.
Structure change induced by thermal treatment of Inconel 718 sheet consisting in solution treatment starting from temperature 1010°C/8 h/H2O and two-stage ageing at temperatures 720°C/8 h and 620°C/8 h resulted in significant improvement of their strength properties. Tensile strength on the level of 1240 MPa was obtained with elongation similar to this measured for sheet metal in as-delivered condition [15].
Heat treatment (solution treatment and ageing) of Inconel 718 alloy sheet resulted in changes of morphological features characterizing its microstructure.
Mixtures σ-phase with diversified chemical composition irregularly distributed in the alloy’s microstructure (Fig. 13) were almost entirely dissolved in solid solution of phase γ. Unsolved phase σ remaining in microstructure of Inconel 718 occurs on the form of fine rounded particles (Fig. 14). Similarly, MC-type carbides rich in niobium and titanium remaining in the alloy’s microstructure have spherical shapes and are present in the form of fine particles on boundaries of γ phase grains.
It has been found that the current intensity I on the level of 80 A and the welding rate Vs = 200 mm/min were GTAW parameters most favorable from the point of view of thermal efficiency ηc and HAZ size. Such parameters of Inconel 718 sheet welding allowed to obtain good welded joints (Figs. 4–7) with minimum HAZ widths ranging from about 50 µm to 100 µm (Figs. 10–12).
Limited heat efficiency of laser light beam means that higher power levels must be used when welding thin Inconel 718 alloy sheet. Good joints on 0.7–0.9 mm thick sheet were obtained for laser with power of 400 W and welding rate of the order of 25 mm/s. For such laser welding parameters, the welded joints had virtually no HAZ (Figs. 4B, 5B, and 6B) or widths of their HAZs did not exceed 40 µm (Figs. 9 and 11).
The seams in the joints welded both with electric arc and laser reveal dendritic structure. Value of the structural parameter λ2s characterizing dendritic grains of the joints depends on the welding method and decreases with increasing sheet metal thickness. In case of GTAW, increase of sheet metal thickness from 0.5 mm through 0.9 mm to 1.2 mm is accompanied by reduction of λ2s parameter value from 12 µm through 10 µm to 8 µm. Similarly, in case of laser-welded joints, with sheet metal thickness increasing from 0.5 mm to 1.2 mm, λ2s values decrease from 9.6 µm to 5µm.
Boundaries of second-order branches of dendritic grains observed in welded joints on sheet metal in as-delivered condition made with the use of both GTAW and laser method are the locations where irregular eutectic (Fig. 13A) composed of σ phase and MC-type carbides can be found. Heat treatment (solution treatment and ageing) of welded sheet metal joints resulted in almost complete dissolution of σ phases and disappearance of irregular eutectic. On boundaries of joint grains’ second-order branches there are fine rounded MC-type carbides surrounded with residues of σ phase (Fig. 14C).
A measure of creep-resisting properties of thin sheet Inconel 718 in as-delivered condition, heat-treated and welded with the use of GTAW and laser method is the time to rupture at adopted high-temperature creep conditions (Table 2, Fig. 17). The obtained results in combination with conclusions drawn from of microstructure examination of welded joints before and after heat treatment show that the use of laser welding allows to increase the time to rupture of heat-treated joints by nearly 62% in comparison with BM in as-delivered condition and by about 80% with respect to the time to rupture observed in GTAW joints made on sheet in as-delivered condition.
Specimens of sheet metal in as-delivered condition welded with the use of laser method and heat treated get ruptured outside the welded joint areas (Fig. 15B and 16B). It can be therefore concluded that laser-welded joints demonstrate higher creep resistance than the base material.