Low cycle fatigue behaviour of nickel base superalloy IN 740H at 760 °C: Influence of fireside corrosion atmosphere

Interaction of fireside corrosion atmosphere and fatigue damage in A-USC boiler alloy IN 740H was studied by conducting fatigue experiments on bare and salt coated (58%Na2SO4 + 39%K2S2O7 + 2%NaCl + 0.5%CaSO4) specimens at 760 °C followed by extensive electron microscopy. The salt mixture fused and released SO3 gas during fatigue tests. Fatigue life was significantly reduced due to fireside corrosion atmosphere. Fluxing of complex oxide layer and extensive surface cracking is ascertained to be the reason of premature fatigue failure at lower Δεt/2 (e.g. 0.3%), however, at higher Δεt/2 e.g. 0.7%, breaking of oxide scales and surface pitting are the causes of early fatigue failure.