Failure analysis of nickel-based superalloy tube – thermal fatigue crack
The austenitic structure of the material contains a transgranular crack (the crack propagates across the grains, cutting the crystallographic planes) with the presence of strong oxidization along the edges. The crack formed as a result of frequent thermal shocks and contemporaneous mechanical vibrations.
Failure analysis of cupronickel tubing – denickelization
Stagnation phenomena in marine waters, aided by localized overheating due to failed heat transfer, can induce dealloying, the selective removal of one element from an alloy, in Cu Ni alloy. In this case a loss of nickel occurs, with consequent formation of a copper “sponge”.
The phenomenon occurs in the weld seam zone. The heating of the material during the weld and the subsequent rapid cooling causes the precipitation of fragile structures at the grain border (martensite). This causes a decrease in mechanical resistance and corrosion of the weld seam as the edges of the ferritic grains are less resistant, thereby inducing premature failure phenomena.