Liquation Microfissuring in the Weld Heat-Affected Zone of an Overaged Precipitation-Hardened Nickel-Base Superalloy

Abstract

The effect of preweld overaging heat treatment on the microstructural response in the heataffected zone (HAZ) of a precipitation-hardened nickel-base superalloy INCONEL 738LC subjected to the welding thermal cycle (i.e., rapid) was investigated. The overaging heat treatment resulted in the formation of an interfacial microconstituent containing M 23 X 6 particles and coarsening of primary and secondary c¢ precipitates. The HAZ microstructures around welds in the overaged alloy were simulated using the Gleeble thermomechanical simulation system. Microstructural examination of simulated HAZs and those present in tungsten inert gas (TIG) welded specimens showed the occurrence of extensive grain boundary liquation involving liquation reaction of the interfacial microconstituents containing M 23 X 6 particles and MC-type carbides. In addition, the coarsened c¢ precipitate particles present in the overaged alloy persisted well above their solvus temperature to temperatures where they constitutionally liquated and contributed to considerable liquation of grain boundaries, during continuous rapid heating. Intergranular HAZ microfissuring, with resolidified product formed mostly on one side of the microfissures, was observed in welded specimens. This suggested that the HAZ microfissuring generally occurred by decohesion across one of the solid-liquid interfaces during the grain boundary liquation stage of the weld thermal cycle. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that the temperature at which the alloy exhibited zero ductility during heating was within the temperature range at which grain boundary liquation was observed. The on-cooling ductility of the alloy was significantly damaged by the on-heating liquation reaction, as reflected by the considerably low ductility recovery temperature (DRT). Important characteristics of the intergranular liquid that could influence HAZ microfissuring of the alloy in overaged condition are also discussed.

Improvement in Laser Weldability of INCONEL 738 Superalloy through Microstructural Modification

“…This may occur at temperatures below that at which constitutional liquation of c¢ could occur. Ojo et al [18] reported by Gleeble thermal simulation that significant liquation occurs at temperatures below c¢ liquation temperature in the UMT condition. This may be related to liquation reaction of the M 5 B 3 particles that form along grain boundaries during the UMT preweld heat treatment.…”

“…Liquation of c¢ is caused by incomplete dissolution of c¢ particles before the c-c¢ eutectic reaction temperature is reached in the HAZ during the heating cycle of welding. [18] A deviation from the equilibrium precipitate dissolution mechanism occurs during rapid heating such as that experienced during welding. Soucail et al [19] have shown that the solvus temperature of c¢ particles may markedly depart from its equilibrium value during rapid heating, causing these precipitates to survive to significantly higher temperatures than those predicted by the equilibrium phase diagram.…”

“…[23] The Cr-rich particles that form on grain boundaries during aging heat treatments of IN 738, such as the UMT, have been generally reported to be M 23 C 6 particles. However, Ojo et al [18] suggested that these particles may not be exclusively M 23 C 6 particles. Hence, a careful microstructural study of the intergranular particles in the UMT condition was performed to determine the presence of any other second-phase particles besides the M 23 C 6 carbides.…”

On the Mechanism of Serrated Grain Boundary Formation in Ni-based Superalloys with Low γ' Volume Fraction

“…The test is based on the premise that the deformation behavior of a material, as evaluated by its hot ductility, reflects its capability to accommodate tensile stresses and resist cracking during the welding thermal cycle [21]. The test results indicate that the serrated sample shows a lower susceptibility to liquation cracking than the unserrated one: the GB serration leads to about 15°C decrease in the brittle temperature range (BTR) where HAZ liquation cracking is most likely to occur.…”

Fatigue Life of the Repair TIG Welded Hastelloy X Superalloy

“…The inferiority of WZ of some superalloy materials are originated from brittleclose-packed (TCP) phases as investigated by some researches. 2,3,[7][8][9][10] Fabrication and repair of shroud are performed using conventional Tungsten inert gas (TIG) welding. Nevertheless, the high heat input generates a huge effect on the HAZ resulting in the grain growth and melting of eutectic constituents.…”