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The room/high temperature mechanical property of the Inconel 617B joined by multi-layer narrow gap tungsten arc welding technology (NG-TIG) is investigated, and thermal simulation is utilized to reveal the behavior of M 23 C 6 phase in Inconel 617B superalloy during welding. Eutectic microstructures and micro fissures resulting from constitutional liquation due to thermal exposure in welding occur in the heat affected zone (HAZ), which leads to slight softening in the HAZ after welding. Room/high temperature tensile tests show that little adverse effects on the mechanical strength of joint has been caused by the welding process. It is shown by the thermal simulation that constitutional liquation of this micro-scale M 23 C 6 carbides would occur when the peak temperature (T p ) reached 1300 ℃. The behavior of the micro-sized M 23 C 6 in multi-layer welding cycle could be described as: M 23 C 6  + (γ-Ni) → liquid → M 23 C 6  + γ-Ni → M 23 C 6  + γ-Ni + secondary M 23 C 6 . [ABSTRACT FROM AUTHOR]

• Local deformation mode determines the fatigue crack initiation mechanism. • Pile-up of dislocations on HAGBs leads to intergranular cracking. • Dislocation bands inside grains contribute to intragranular cracking. In this study, fatigue crack initiation behaviors around defects induced by welding thermal cycle in IN617B were investigated by in-situ fatigue tests combined with DIC (Digital Image Correlation) and EBSD (Electron Back Scattering Diffraction) characterization. Two mechanisms of fatigue crack initiation around defects were clarified and the distance of crack initiation position from defects was proposed to determine which mechanism was dominant. Intergranular fatigue crack initiation caused by pile-up of dislocations was dominant in the far-field region from defects, while intragranular fatigue crack initiation caused by cracking at the interface between dislocation bands and matrix was dominant in the near-field region from defects. Welding defects altered the fatigue crack initiation mechanism by changing the deformation mode of the near-field region, thereby accelerating the fatigue failure process of IN617B alloy. [ABSTRACT FROM AUTHOR]

The microstructure and impact toughness of weld metals (WMs) and heat-affected zones (HAZs) of a low-alloy CrMoV steel gas turbine rotor which had served for 14 years were investigated. The ex-service joints in the turbine part (serving at 500–540 °C) and the compressor part (serving below 300 °C) of the rotor were selected for comparative research. The microstructure of the WMs and HAZs between the turbine part and the compressor part was similar, indicating that there was no significant deterioration in microstructure of the turbine part during service. However, compared with the compressor part WM, the impact energy of the turbine part WM decreased significantly, and FATT50 increased greatly. The degraded toughness of turbine part WM was related to more serious intergranular cracking caused by higher segregation level of phosphorus (P) at prior austenite grain (PAG) boundaries. Welding and post-weld heat treatment led to obvious segregation of P at PAG boundaries in WMs, and the segregation of P in turbine part WM was further aggravated during serving at 500–540 °C. Additionally, the inhomogeneous microstructure of the WMs also aggravated the segregation of P. The toughness of the HAZs in both turbine part and compressor part was high, which was because of fine grains. Furthermore, due to there being more grain boundaries and low P content, the segregation of P in HAZs was slight and its adverse effect on the toughness could be ignored. [ABSTRACT FROM AUTHOR]

The mechanism for HAZ (heat-affected zone) liquation of alloy 617B during gas tungsten arc welding (GTAW) was investigated. Welding thermal simulation work was conducted to investigate the effects of thermal parameters (peak temperature, holding time, and thermal cycle numbers) on M23C6 carbides' evolutionary behavior in nickel-based alloy 617B. OM (optical microscopy), SEM (scanning electron microscopy), TEM (transmission electron microscopy), and SIMS (secondary ion mass spectrometry) were employed to characterize HAZ carbides. It was found that the constitutional liquation of M23C6 carbides is responsible for HAZ liquation in alloy 617B. Rapid heating meant that solute atoms released from partially dissolved M23C6 carbides did not have enough time to sufficiently diffuse into the matrix, resulting in eutectic reaction M23C6 + γ → liquid in the temperature range from 1250 °C to 1300 °C. In the following cooling process, the liquid phase transformed into γ and M23C6 (or M6C) carbides simultaneously, creating a eutectic microstructure. Subsequent thermal cycles with peak temperature 1100 °C and proper holding time brought about a tempering effect to precipitate fine M23C6 carbides from the Cr supersaturated zone around the eutectic microstructure. Boron was found to be enriched in carbides and was expected to promote HAZ liquation by two mechanisms. No cracking caused by HAZ liquation has been found, indicating that GTAW is a suitable method for joining alloy 617B. [ABSTRACT FROM AUTHOR]

This research focused on the impact of temperature and driving force ΔK on the fatigue crack growth (FCG) mechanism of C-276 Ni-based weld metal in a 9% Ni steel joint. On one hand, the FCG resistance increased due to higher yield strength at 77 K than at room temperature (RT), and the decrease of FCG rate can be well explained by Weertman's model. On the other hand, critical ΔK values (37.5 MPa m1/2 at 77 K and 30.2 MPa m1/2 at RT) existed in the process of FCG, accompanied by the transition of FCG mechanism at both 77 K and RT. In detail, the fatigue crack mainly propagated in a quasi-cleavage mode at a lower ΔK level at 77 K, in a single-slip mode at a higher ΔK level at 77 K or a lower ΔK level at RT, and in a multi-slip mode at a higher ΔK level at RT. Furthermore, the dislocation-precipitate interaction was enhanced and even provided favourable locations for FCG at a higher ΔK level at RT, leading crack to propagate via linking after the formation of micro-cracks and micro-voids near precipitates. [ABSTRACT FROM AUTHOR]