Microstructural characterization of INCOLOY 903 weldments

2007

The laser welding of a directionally solidified INCONEL 738 superalloy (DS IN738) was investigated. Microstructural analysis of the fusion zone (FZ) and the heat-affected zone (HAZ) revealed that cracking occurred mainly in HAZ with only some cracks being extended into the FZ. However, the frequently observed centerline cracking in FZ of DS and single crystal (SX) alloys did not occur, which was attributed to the presence of negligible volume fraction of c-c¢ eutectic in FZ. Constitutional liquation of secondary solidification constituents (MC carbides, M 3 B 2 borides, M 2 SC sulphocarbides, and c-c¢ eutectic) and c¢ precipitate particles was found to be the major cause of grain boundary liquation and the resultant intergranular microfissuring in the HAZ. The extent of HAZ microfissuring was, however, observed to be smaller in samples welded along the transverse direction (perpendicular to solidification direction) than when welding was done along the longitudinal direction (solidification direction). Nonetheless, more severe HAZ cracking occurred in samples of similarly welded conventionally cast (CC) alloy, and the present results indicate that the severity of HAZ liquation cracking in IN738 superalloy can be reduced by using a DS version of the alloy.

Section: Haz Microstructurementioning

confidence: 84%

Microstructural Analysis of Laser-Beam-Welded Directionally Solidified INCONEL 738

Sidhu

2007

“…[17,18] The reverse segregation behavior of Al in the fusion zone as compared to its negative segregation during casting of IN 738, as reported by Taha and Kruz, [19] could be due to the back-diffusion effect. It is known that during dendritic solidification, the extent of segregation can be significantly reduced by solidstate diffusion, [20] which would be more pronounced with slowly cooled casting compared to the rapidly solidified weld metal.…”

Section: Elemental Partitioning In the Fusion Zonementioning

confidence: 85%

Study of the fusion zone and heat-affected zone microstructures in tungsten inert gas-welded INCONEL 738LC superalloy

Richards

2006

Self Cite

127

The fusion zone and heat-affected zone (HAZ) microstructures obtained during tungsten inert gas (TIG) welding of a commercial superalloy IN 738LC were examined. The microsegregation observed during solidification in the fusion zone indicated that while Co, Cr, and W segregated to the ␥ dendrites, Nb, Ti, Ta, Mo, Al, and Zr were rejected into the interdendritic liquid. Electron diffraction and energydispersive X-ray microanalyses using a transmission electron microscope (TEM) of secondary phases, extracted from the fusion zone on carbon replicas, and of those in thin foils prepared from the fusion zone showed that the major secondary solidification constituents, formed from the interdendritic liquid, were cubic MC-type carbides and ␥-␥Ј eutectic. The terminal solidification reaction product was found to consist of M 3 B 2 and Ni 7 Zr 2 formed in front of the interdendritic ␥-␥Ј eutectic. Based on the knowledge of the Ni-Ti-C ternary system, a pseudoternary solidification diagram was adapted for IN 738 superalloy, which adequately explained the as-solidified microstructure. The HAZ microfissuring was observed in regions surrounding the fusion zone. Closer and careful microstructural examination by analytical scanning electron microscopy revealed formation of re-solidified constituents along the microfissured HAZ grain boundaries, which suggest that HAZ cracking in this alloy involves liquation cracking. Liquation of various phases present in preweld alloy as well as characteristics of the intergranular liquid film contributing to the alloy's low resistance to HAZ cracking were identified and are discussed.

“…Constitutional liquation of second-phase particles during rapid heating of twophase alloys was first proposed by Pepe and Savage [29] and has been subsequently observed by other investigators in various alloy systems involving carbide, boride, and sulfide particles. [29][30][31][32] Constitutional liquation of the main strengthening phase of precipitation-hardened nickel-base superalloys, c¢ precipitates, however, was only reported recently for the first time by the present authors. [33] The liquation phenomenon is known to occur generally by a eutectic-type liquation reaction between a second-phase particle and the matrix producing a nonequilibrium solute-rich film at such a particle/ matrix interface.…”

Section: Liquation Of Microconstituent Containing M 23 X 6 Particlesmentioning

confidence: 93%

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

2007

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.