Failure Investigation on Failed Blades Used in a Locomotive Turbocharger

A failure investigation has been conducted on the turboblades used in a locomotive turbocharger. These blades are made from as-cast K418 nickel-base (Ni-base) superalloy. Fracture occurred at the mid-airfoil and tip locations. In the mid-airfoil regions, a high density of intergranular cracks and dent marks are present. The microstructure at the mid-airfoil is typical of overheat damage, such as melting of the boundary or interdendritic region and degraded interdendritic carbides and remelting of eutectic phases (γ + γ′). The fracture surfaces of the failed blades have a dendritic appearance without evidence of fatigue features. It is suggested that these fractures were formed instantaneously. Overheat damage made the strength in interdendritic region decrease so that microcracking occurred preferentially in the interdendritic region. Under the action of bending stress and the spin centrifugal tensile stress, the microcracks propagated within the interdendritic region and converged so that the transverse fracture occurred at the mid-airfoil location where the maximum stress acted. The fragments from previously fractured blades hit neighboring blades, ultimately causing damage to all the turboblades.