Effect of Heat-Treatment Upon the Fatigue-Crack Growth Behavior of Alloy 718 Weldments—Part II: Microscopic Behavior

The microstructural features that influenced the fatigue-crack growth behavior of as-welded, conventional heat-treated, and modified heat-treated Alloy 718 GTA weldments were studied. Electron fractographic examination revealed that operative fatigue mechanisms were dependent on microstructure, temperature and stress intensity factor. All specimens exhibited three basic fracture surface morphologies at temperatures up to 538°C: crystallographic faceting at low stress intensity range (ΔK) levels, striation formation at intermediate values, and dimples coupled with striations in the highest ΔK regime. At 649°C, extensive amounts of intergranular cracking were observed. Laves and δ 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 ΔK 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 were attributed to compressive residual stresses introduced by the welding process.