A study of HAZ microfissuring in a newly developed Allvac® 718 PLUS™ superalloy

Allvac (R) 718 PLUS (TM) (718 Plus) is a newly developed gamma' strengthened Ni-base superalloy with superior high temperature properties compared to its baseline superalloy, Inconel 718, but only limited information about its weldability is available. Electron beam welding response of this new superalloy was studied in two different pre-weld heat treated conditions. Microstructural characterization, prior to and after bead-on-plate electron beam welding, was done by optical microscopy, scanning electron microscopy and transmission electron microscopy. Two versions of 718 Plus with different concentrations of boron and phosphorus were investigated to study the effect of minor alloying elements on weldability. Grain boundary segregation of these minor alloying elements was studied using Secondary Ion Mass Spectrometer (SIMS). HAZ cracking due to liquation at the grain boundaries was observed in the alloys. The welding response of 718 Plus alloys was compared with that of Inconel 718 using two weldability evaluation techniques viz. - weld crack measurements in actual welds and hot-ductility measurements using Gleeble thermo-mechanical simulator. The results obtained by both the techniques were in agreement with each other and indicated that weldability of Inconel 718 was slightly better than that of 718 Plus alloy with normal boron and phosphorus concentration, although weldability of the higher boron and phosphorus version of 718 Plus alloy deteriorated significantly. The causes of cracking in 718 Plus alloys and the parameters affecting it are discussed and the welding response of the newly developed superalloy is assessed.