No abstract

The fact that Gamma titanium aluminides (-TiAl) offer strong potential for replacing conventional titanium and nickel-base alloys in future gas turbine engine designs has resulted in the requirement to accurately describe the creep behaviour of these complex alloys. Consequently, the system Ti-45Al-2Mn-2Nb has been extensively studied under high temperature creep conditions where creep ductilities are shown to far exceed the relatively low tensile ductilities characteristic of the alloy at room temperature. Modern lifing approaches have been applied to the alloy, helping to develop understanding of the deformation behaviour, and are shown to accurately describe the stress-rupture and minimum creep rate behaviour of the alloy, promoting confidence for the derivation of computational models which are used to describe the behaviour of the alloy for in service conditions. When compared to an existing, conventional, high temperature titanium alloy, Timetal 834, significant improvements in creep life were demonstrated for equivalent stress conditions with a comparable level of creep ductility between the two alloys. In addition, the gamma titanium aluminide showed a superior resistance to surface oxidation and associated cracking with notable  case formation in Timetal 834 leading to premature failure.

Cast 25Cr-35Ni alloys are extensively being used in the petrochemical and petroleum refining industries for high-temperature applications. A typical application of such alloys in the industry is in the manufacture of cast catalyst reformer tubes for the production of hydrogen. The cast 25Cr-35Ni catalyst reformer tubes possess the required mechanical properties, creep resistance, oxidation resistance, and high-temperature stability. Though reformer tubes are designed to give a service life of over 100,000 hours at temperatures beyond 900°C, there are incidents of failure due to creep damage, which is the predominant failure mechanism in reformer tubes. The paper discusses an investigation conducted on the premature failure of a 25Cr-35Ni reformer tube. The investigation involved microstructural assessments and the evaluation of mechanical properties. The microstructure and mechanical properties of the service-exposed reformer tube were also compared with a new tube. The investigation revealed that the failure of the tube was due to creep embrittlement. The creep embrittlement was due to the microstructural degradations that occurred as a result of overheating. Adherence to the design and operational parameters is critical in mitigating creep embrittlement failures.