Abstract
The microstructural features that influenced the room and elevated temperature fatigue-crack growth behavior of as-welded, conventional heat-treated, and modified heat-treated Alloy 718 GTA weldments were studied. Electron fractographic examination of fatigue fracture surfaces revealed that operative fatigue mechanisms were dependent on microstructure, temperature and stress intensity factor. All specimens exhibited three basic fracture surface appearances at temperatures up to 838{sup 0}C: crystallographic faceting at low stress intensity range ({Delta}K) levels, striation formation at intermediate values, and dimples coupled with striations in the highest {Delta}K regime. At 649{sup 0}C, the heat-treated welds exhibited extensive intergranular cracking. Laves and {delta}particles in the conventional heat-treated material nucleated microvoids ahead of the advancing crack front and caused an overall acceleration in crack growth rates at intermediate and high {Delta}K levels. The modified heat treatment removed many of these particles from the weld zone, thereby improving its fatigue resistance. The dramatically improved fatigue properties exhibited by the as-welded material was attributed to compressive residual stresses introduced by the welding process. 16 figures.
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