The Mechanism for HAZ Liquation of Nickel-Based Alloy 617B During Gas Tungsten Arc Welding

Abstract

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.

Evolution of microstructure in nickel-based C-HRA-2 alloy during welding thermal simulation

“…Constitutional liquation occurs during welding or other high-temperature processes, where the melting of components at the grain boundaries of the alloy leads to the formation of a eutectic mixture. Li et al [18,19] suggested that nickel-based alloy 617B will have eutectic microstructure due to constitutional liquation at T p above 1300 °C, basically consistent with their conclusions. Table 3 lists the chemical compositions of points A and B obtained by EDS.…”

“…The simulated HAZ with T p of 1450 °C showed contiguous microcracks distributed along the grain boundaries, indicating that the matrix was melted to a large extent. The evolutionary behavior of micron-scale M 23 C 6 carbides during the welding process can explain the liquefaction phenomenon [17][18][19]. The melted structure and micron-scale M 23 C 6 carbide were rich in Ni, Cr, Mo, and Co.…”

“…Furthermore, they found that the HAZ appears as a laminar eutectic structure composed of M 23 C 6 carbides and austenite matrix. Li et al [18,19] revealed the evolutionary behavior of M 23 C 6 carbides in Inconel 617B alloy through thermal simulation and investigated the effect of T p , holding time and the number of thermal cycles on the evolutionary behavior of M 23 C 6 carbides. It was indicated that constitutional liquation of some M 23 C 6 carbides would occur when T p reached 1300 °C, which is the reason for the liquefaction of the HAZ.…”

Evaluation of Liquation Cracking Behavior and Susceptibility in Heat-Affected Zone of CM247LC Superalloy Welds for Turbine Blade Application

“…Schematic description of liquation cracking at γ/MC: (a) pseudo-binary diagram of γ-MC and (b) schematic description of constitutional liquation at γ/MC[16,17,30].…”