Microstructural modification in full penetration and partial penetration electron beam welds in INCONEL-718 (IN-718) and its effect on fatigue crack initiation

Detailed microstructural analysis, as well as fatigue crack initiation evaluation, was carried out for electron beam (EB) welded IN-718. Fatigue test specimens were EB welded (full penetration) along their length, and a second weld pass, incorporating a slope-out from full to zero penetration along the gage length, was also applied. The specimens were fatigue tested at 523 degrees C and maximum stress (R = 0) in the range 579 to 820 MPa. Early fatigue failure (<100,000 cycles at 0.25 Hz) was directly associated with the initiation at solidification porosity formed during "spiking" in the partial penetration weld metal at the start of the slope-out. The base metal, full penetration weld metal, and slope-out region were characterized using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), which indicated that the microstructures of the base metal and full penetration weld metal should give good fatigue resistance. The rapid solidification of the full penetration weld metal gave an interdendritic terminal solidification product consisting of gamma + NbC + Laves phase instead of the usually reported eutectic gamma + Laves phase. Microstructural and chemical heterogeneities in the full penetration weld metal, combined with the sharp perturbations in penetration and solidification conditions (spiking) in the partial penetration weld metal, resulted in locally embrittled regions and interdendritic regions containing large numbers of fine pores as well as a higher volume fraction of mixed, hard interdendritic phases. These features would be consistent with a lower resistance to fatigue crack propagation in the partial penetration weld metal.