The effect of post-weld heat treatment temperature on the microstructure of Inconel 625 deposited metal
Introduction
Inconel 625 is a solid-solution-strengthened nickel-base superalloy that is extensively used in applications that require a combination of moderate strength and excellent corrosion resistance at temperatures below 800 °C and, in some cases, at temperatures below 1200 °C [1], [2], [3]. Inconel 625 is primarily strengthened by the addition of substitutional alloying elements, such as Cr and Mo, which provide solid-solution strengthening of the austenite microstructure. Because of the existence of precipitate strengthened elements, such as Nb, Al and Ti, it can also be strengthened by the dispersed γ′ phase [Ni3(Al, Ti, Nb)], the metastable phase (Ni3Nb) and blocky MC (M denotes Nb, Ti), M6C (M denotes Si, Ni, Cr), and M23C6 (M denotes Cr) carbides [4], [5], [6], [7], [8].
Previous studies [9], [10], [11] have shown that the use of nickel-base superalloy Inconel 625 wire to weld high yield strength steels or stainless steels, which are commonly employed in modern industry, can significantly improve the high-temperature mechanical properties and corrosion resistance of weld structures. However, the toughness, fatigue strength and creep rupture strength of the weld may obviously decline due to the precipitation of the intermetallic Laves phase (A2B type phase: A denotes Ni, Cr, Fe; B denotes Nb, Mo, Ti) and the δ phase (Ni3Nb) when a weld structure is employed under high temperatures for extensive periods [12], [13], [14], [15]. Therefore, many studies have been performed to control the formation of the Laves phase and the δ phase in weld joints. The results revealed that the morphology, distribution and content of the Laves phase is highly dependent on the segregation of high concentration refractory elements, such as Nb and Cr, and the Nb segregation and Laves formation are difficult to control [12], [16]. Sundararaman et al. [17] observed the δ phase in Inconel 625 heat-treated at 750 °C for 100 h but did not observe the δ phase after a similar heat treatment at 700 °C. A large number of δ phases were observed when alloy 625 was heat-treated at 800 °C, which was the maximum temperature at which the phase was observed. Although the δ phase is considered to be a harmful phase, the δ phase was recently reported [18], [19] to block grain boundary sliding and to control the grain size. However, the formation process of the δ phase was not mentioned. Mechanical properties such as the yield strength and fatigue strength of the heat-affected zone of the weld structure significantly degrade due to reheating when nickel-base superalloy Inconel 625 wire is used to weld high yield strength steels or stainless steels [20], [21], [22]. In these cases, the toughness, fatigue strength and creep rupture strength of the weld, as well as the mechanical properties of the heat affected zone, should be preserved through post-weld heat treatment (PWHT). Although numerous studies have focused on the microstructure and properties of nickel-base superalloys, few studies on the PWHT for deposited metal (DM) of nickel-base superalloy Inconel 625 wire are available.
This study investigated the effect of PWHT temperature on the microstructure of nickel-base superalloy Inconel 625 DM, which was fabricated by gas tungsten arc (GTA) overlay welding, using OM, SEM and TEM. The transformation mechanism of the → δ phase and the growth kinetics law of the γ′ phase during PWHT were revealed.
Section snippets
Material and methods
A DM sample was GTA overlay welded with Inconel 625 (ERNiCrMo-3) filler wire on a low-alloy steel plate using a current of 110 A DC and a voltage of 13 V. The interpass temperature was controlled at 100 °C or less, and a welding speed of 25 mm/min was maintained. The chemical composition of the superalloy DM in this study consisted of (wt.%) 22.66 Cr, 8.71 Mo, 3.53 Nb, 0.01 C, 0.09 Mn, 0.32 Fe, 0.08 Si, 0.01 Cu, 0.14 Al, 0.21 Ti, and margin Ni. The as-welded DM sample was evenly divided into 5
Microstructure characterization of as-welded DM
Fig. 2 shows OM images of the as-welded DM, which was etched with mixed acid (hydrochloric acid:nitric acid:acetic acid = 1:1:1) or aqua regia. The microstructure of as-welded DM is composed of columnar grains, as shown in Fig. 2. The columnar grain sizes in this study ranged between 100 μm and 200 μm, with an average value of 160 μm. Two different types of cellular-dendritic microstructures were observed in the OM image of the as-welded DM: the first microstructure is continuous cellular–dendritic,
Formation process of secondary phases in the microstructure of DM
Based on the experimental results, secondary phases, such as the dispersed γ′ phase, blocky MC-type carbide and irregular shape Laves phase, precipitate in the microstructure of the as-welded DM, in addition to the γ-matrix. According to the literature [2], [25], the equilibrium solidification process of as-deposited Inconel 625 superalloy is as follows:
In the GTA overlay welding process, the solidification in DM of Inconel 625 wire begins with the reaction which
Conclusions
The microstructure of Inconel 625 DM, which is generated by a PWHT of 4 h at 650 °C, 750 °C, 850 °C and 950 °C, has been investigated. The following conclusions were attained through this study:
- 1.
The microstructure of as-welded DM is composed of columnar grains with different sizes. The average grain size is approximately 160 μm. There are some dispersed γ′ phase, blocky MC type carbide and irregular shape Laves phase precipitating in the microstructure of the as-welded DM.
- 2.
No significant change in
Acknowledgements
This research is financially supported by Tianjin Natural Science Foundation, No. 11JCYBJC06000, and Key Project of Tianjin Municipal Science and Technology Support Program, No. 11ZCGYSF00100.
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