Failure analysis of a gas turbine blade made of Inconel 738LC alloy

• Published in: Engineering failure analysis 2005-06, Vol.12 (3), p.474-486
• Main Authors: Mazur, Z., Luna-Ramírez, A., Juárez-Islas, J.A., Campos-Amezcua, A.
• Format: Article
• Language: English
• Subjects: Creep–fatigue interaction; Failure analysis; Gas turbine failures; Metallurgical examination; Turbine blade failure
• ISSN: 1350-6307; 1873-1961
• DOI: 10.1016/j.engfailanal.2004.10.002

The failure analysis of the 70 MW gas turbine first stage blade made of nickel-base alloy Inconel 738LC is presented. The blades experience internal cooling hole cracks in different airfoil sections assisted by a coating and base alloy degradation due to operation at high temperature. A detailed analysis of all elements which had an influence on the failure initiation was carried out, namely: loss of aluminium from coating due to oxidation and coating phases changing; decreasing of alloy ductility and toughness due to carbides precipitation in grain boundaries; degradation of the alloy gamma prime (γ′) phase (aging and coarsening); blade airfoil stress level; evidence of intergranular creep crack propagation. It was found that the coating/substrate crack initiation and propagation was driven by a mixed fatigue/creep mechanism. The coating degradation facilitates the crack initiation due to thermal fatigue. The substrate intergranular crack initiation and propagation were due to a creep mechanism which was facilitated by grain boundary brittleness caused by formation of a continuous film of carbides on grain boundaries, the degradation of γ′ due to elongation (rafting) and coalescence, and high thermomechanical stress level.