The continued drive for increased efficiency, performance and reduced costs for industrial gas turbine engines demands extended use of high strength-high temperature capability materials, such as nickel based superalloys. To satisfy the requirements of the component design and manufacturing engineers these materials must be capable of being welded in a satisfactory manner. The present paper describes the characteristic defects found as a result of welding the more difficult, highly alloyed materials and reviews a number of welding processes used in the manufacture and repair of nickel alloy components. These include gas tungsten arc (GTA) and electron beam (EB) welding, laser powder deposition and friction welding. Many of the more dilute nickel based alloys are readily weldable using conventional GTA processes, however, high strength, precipitation hardened materials are prone to heat affected zone and strain age cracking defect formation. A number of factors are found to affect the propensity for defects: composition (aluminium and titanium content), grain size, pre and post-weld heat treatment, as well as the welding process itself (control of heat input and traverse speed). Process parameter identification is still largely empirical and a fuller understanding of the joining processes is dependent upon the development and application of more sophisticated numerical modelling techniques.

No abstract

No abstract

Haynes Alloy 230 TM is a solid solution strengthened nickelbased material used for combustion components in industrial gas turbines. In this application a primary limit to the component design life is the response of the material to high temperature exposure with and without mechanical loading. Over long periods of operation in service, this leads to degradation of the microstructure of the alloy and consequently to its mechanical properties. A detailed understanding of the processes associated with in-service the microstructural degradation of the alloy and its effects on the mechanical properties of the material is therefore of great importance in the drive for increased component life and reliable extended plant operation.In order to investigate the effects of thermal and long-term creep exposure on the degradation behaviour of Haynes Alloy 230 sheet material during service, detailed studies of the microstructural changes taking place in this material have been made using advanced analytical FEGSEM, EDX and XRD techniques. The objective of the programme is to quantify the microstructural changes and phase precipitation reactions occurring as a result of service exposure, based upon the use of laboratory controlled thermally aged and creep tested samples.Haynes Alloy 230 was specifically designed to have excellent long-term thermal stability and resistance to the precipitation of damaging phases. However, whilst this appears to be true for unstressed thermal exposure, there is growing evidence from the studies to date that, in addition to the primary M 6 C and the precipitation of M 23 C 6 resulting from thermal exposure, other phases can precipitate in the alloy, under the influence of timedependent plastic deformation during long-term creep exposure, which can lead to reductions in both ductility and high temperature strength. This paper describes some detailed studies on the effects of long-term high temperature exposure on the hardness and microstructural changes in creep rupture tested and thermally exposed samples of Haynes Alloy 230.

IN738LC is a polycrystalline superalloy which is still widely used for gas turbine blading in many industrial applications. It has been the centre of many research programmes during the last four decades in areas such as alloy design, processing, various degradation regimes and even after heat treatment during repair. The subject of this study has been the microstructural evolution during multiple reheat treatments combined with long term aging. Material has been exposed at two different aging temperatures (875 and 950uC) for a period of 4000 h. This has been followed by a reheat treatment and then further aging, up to a total of 16 000 h. The microstructural evolution of the alloy, c9 coarsening and carbide transformations, have been investigated in detail after the heat treatments and further aging periods. The results highlighted that although the c9 structure has largely been recovered by the heat treatments, that of the carbides did not follow a similar trend. This partial recovery of the alloy microstructure and its potential impact on the long term integrity of gas turbine blading which has been heat treated periodically will be discussed in this paper.