PSI - Issue 41

Paolo Ferro et al. / Procedia Structural Integrity 41 (2022) 430–438

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Paolo Ferro et al. / Structural Integrity Procedia 00 (2022) 000–000

1. Introduction Inconel 625 (IN625), UNS N06625, is a nickel base superalloy developed in the 1960s with the purpose of creating a material that could be used for steam-line piping. Some modifications were then made to its original composition that have enabled it to be even more creep-resistant and weldable. Today, it is used in marine applications, nuclear technology, industrial processing, and aerospace equipment. Its chemical composition is optimized to resist against challenging conditions. In short, nickel and chromium provide resistance to oxidizing environments while nickel and molybdenum assure resistance to non-oxidizing atmospheres. Molybdenum can also prevent pitting and crevice corrosion [Li et al., 2015; Rajani, et al., 2013; Yin et al., 2009; Xu et al., 2013]. In a critical raw materials perspective, its criticality index defined by Ferro and Bonollo (2019), is only 0.32, which indicates that IN625 is poorly affected by supply risk, despite its high economic importance for the European Community (EC). As it is expected for all the alloys with complex chemical compositions [Ferro et al., 2005], IN625 is very sensitive to industrial processes and in particular to welding operations that can compromise its properties because of sensitization phenomenon (secondary phases precipitation). Moreover, like welding, additive manufacturing also suffers almost the same problems. A rapid solidification induces the precipitation of Laves phases that negatively affect the mechanical and chemical properties of the additively manufactured components. For this reason, different corrective post-heat treatments were investigated in literature. Hu et al. (2018) studied the effect of solution heat treatment on the microstructure and mechanical properties of Inconel 625 superalloy fabricated by laser solid forming. They confirmed that the recrystallization phenomenon occurs at temperatures above 1200 °C with an increased dissolution of Laves phases as the temperature increases. Guo et al. (2016) investigated the effect of heat treatment temperatures (ranging from 650 to 950 °C) on microstructure and corrosion properties of Inconel 625 weld overlay deposited by pulsed tungsten inert gas welding. The weld overlay, in rough welding condition, showed the presence, in inter-dendritic spaces, of Leave and MC phases that are rich in corrosion resistant elements, Nb and Mo. This was attributed to alloy elements segregation during solidification. The depletion in inter-dendritic spaces of useful elements due to secondary phases precipitation, reduces the corrosion resistance of the alloy. PWHTs, with temperatures ranging from 650 to 850 (holding time, 2h), dissolved the Leave phases with the consequent precipitation of  phase, enriched in Nb, Mo, Cr and Ni. On the other hand, PWHT at 950 °C, reduced the formation of  phase. With reference to corrosion mechanism, the galvanic coupling was identified. During heat treatments, Cr, Mo and Nb diffuse from dendrite cores to inter-dendritic spaces where they precipitate as secondary phases. Such precipitation depletes the precipitate boundaries of alloys elements making them more vulnerable to corrosion initiation. Authors concluded that heat treatments at high temperature boost the precipitation of secondary phases, and spoiling corrosion resistance. It is worth mentioning that, even if no corrosion test were carried out, Arjun et al. (2017) found that generation of carbides (MC, M 23 C 6 and M 6 C) and precipitates (Ni 2 Cr, Mo),  , etc., took take place at 625-925 °C, while these particles dissolution occurred at 1025 °C. Cortial et al. (1994) found that 8h PWHT in the range between 750 and 950 °C, has detrimental effects on mechanical and chemical performances of IN625 because of intense precipitation, in the inter-dendritic spaces, of the stable orthorhombic intermetallic  Ni 3 (Nb, Mo, Cr, Fe, Ti) phase. Temperatures beyond 1000 °C are able to restore the corrosion resistance at the expense, however, of a decrease in mechanical properties as the temperature increases. Induction PWHT is certainly preferred to furnace heat treatment because of its rapid heating and perhaps because it could be the only possibility when long welded pipes need to be treated [Ferro et al., 2008]. Compared to previous works, the influence of short induction PWHTs, 20 min holding time and temperatures above 950 °C, on the corrosion resistance of IN625, was investigated. Both the heat treatments showed an improvement on corrosion resistance compared to the as-welded sample, being the highest temperature (for kinetics and thermodynamic reasons), 1050°C, the best one in restoring the chemical properties of the alloy. This effect was attribute to both possible detrimental nanometric intermetallic phases (i.e.,  phase) dissolution (thermodynamics) and chemical homogenization (kinetics). . 2. Material and methods The alloy nominal composition is summarized in Table 1. The analyzed welds were taken from pipes obtained by a