Gamma Prime Solutioning Tests of Alloy 247 Specimens

Abstract

Abstract Strictly speaking, this contribution is not about a case study in failure analysis. However, the investigation described herein was inspired by a failure case. A heavyduty gas turbine engine used in a power and desalination plant in the Middle East experienced creep damage and cracking in some of its row 4 turbine blades after only some 15,000 operating hours. Microstructural alterations detrimental to the creep strength of the alloy were determined to be the metallurgical root cause of the failure. It was believed that said microstructural alterations were brought about by unusual transient operating conditions in engine service. Heat treatment tests were ordered to verify or disprove this hypothesis. It was established that the peculiar gamma prime morphology found in the failed blades can be produced by very high solutioning temperatures and subsequent rapid cooling. Such conditions in engine service are conceivable if sufficient unburnt fuel enters the turbine and ignites downstream of the combustion chamber (high temperature), and if water used in fuel line purging is injected into the turbine section immediately thereafter (rapid cooling). Solutioning above 1240 °C, followed by a water quench, produced said mono-modal fine γ' morphology.