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Heat affected zone (HAZ) microfissuring was investigated in electron beam welds of two Allvac 718 PLUS (718 Plus) alloys with different boron and phosphorus contents. Two preweld solution heat treatments were used and microfissuring susceptibility was evaluated by measuring the HAZ crack lengths in both alloys after bead on plate welding. The segregation behaviour of boron and phosphorus was studied before and after welding by secondary ion mass spectrometry (SIMS). Hot ductility behaviour of the alloys in the standard solution preweld heat treatment was also examined. The results of crack measurements, segregation studies and hot ductility tests correlated well with each other. It was observed that besides constitutional liquation of Nb rich MC type carbides, segregation of B and P largely influenced the microfissuring susceptibility of the alloy. Heat affected zone microfissuring increased with an increase in BzP concentrations and with an increase in the preweld solution heat treatment temperature. Segregation behaviour of B and P and its contribution to microfissuring in Allvac 718 Plus alloy were discussed.

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Weldability of two versions of newly developed 718 Plus alloy, HC49 and HC20 was studied and compared with that of Inconel 718. 5.0" x 0.5" x 0.5" thick plates were heat treated at 950º and 1050ºC, water-quenched then EB welded. Microstructures of the base metal and welds were examined by optical and analytical electron microscopy. Intergranular microcracks were observed in HAZ in all the three alloys, and their susceptibility to cracking was established by measuring total and maximum crack lengths. It was observed that HC49, with higher B and P, suffered most cracking and Inconel 718 the least. HAZ cracking was observed to be associated with resolidified liquated phase suggesting it to be liquation cracking. The EDS analysis suggested that the grain boundary liquation was due to the constitutional liquation of NbC and due to the GB segregation of B during the pre-weld heat treatment. The increased cracking in 718 Plus as compared to Inconel 718 was attributed to the softer matrix of Inconel 718, which could withstand the welding stresses better. More severe cracking in higher B version of 718 Plus than in the lower B 718 Plus could be due to the increased segregation of B on its grain boundaries.

A microstructural analysis has been carried out on the commercially solution treated and aged (STA) Incoloy 909 superalloy to evaluate the phases present using optical, SEM, TEM and differential scanning calorimetry. Comparison is made where possible with non-standard STA analyses reported in the literature. In the STA condition, the main phase present besides the γ matrix was a deliberately produced Laves phase to control grain growth. Increasing the solution temperature above that recommended for the alloy resulted in eventual solution of the Laves phase and concomitant grain growth. In the STA condition, in addition to the Laves phase, a copious precipitation of a γ′ phase (Ni3TiNb) was observed. In addition, a limited observation was made of the early stages of the transition of some of the γ′ phase to the ϵ phase for the over aged condition. There was no ϵ phase observed in the STA condition. A final observation was the presence of ‘black grain’ previously reported in the literature, in both the ST and STA conditions in the authors’ research. The present view is that the phenomenon is an etching artefact and not a genuine microstructural feature.