Stress relaxation of nickel-based superalloy helical springs at high temperatures

The creep resistance of materials in spring applications is generally acknowledged to be well below that observed in other applications. Helical springs formed from three candidate nickel-based superalloys, Nimonic 90, René 41 and Haynes 282, have been tested under compression in order to gain some insight into this phenomenon. Stress relaxation tests conducted at 600–700 °C found that, under constant displacement, the degradation of the spring force is one to three orders of magnitude faster than would be predicted from creep data from extruded samples under equivalent tensile loading. An analytical model for torsional creep in helical springs is derived from a modified version of the Dyson creep model. The effects of various microstructural features on the deformation rate are considered. Effects such as the coarsening of the precipitate-strengthening gamma-prime phase, tertiary creep due to dislocation multiplication, damage evolution and hardening due to transfer of the stress to the particles from the matrix are concluded to make negligible contributions. It is predicted that the poor performance of the springs is due to the very high population of geometrically necessary dislocations that result from the bending and twisting of the wire into a helical coil. It is expected that these dislocations are resistant to conventional heat treatments, resulting in a persistent residual stress field and a large number of dislocations to facilitate the creep process. In some cases, the stress relaxation is found to be so fast that the precipitate hardening of the alloy is too slow to prevent significant initial degradation of the spring

Effects of Long-Term High Temperature Exposure on the Microstructure of Haynes Alloy 230

Haynes Alloy 230 was specifically designed to have excellent long-term thermal stability and resistance to the precipitation of damaging phases. This paper describes in detail studies on the effects of long-term high temperature exposure on the hardness, microstructural changes and tensile properties of thermally exposed samples of Haynes Alloy 230. The samples from the 2mm thick sheet material have been investigated using X-Ray diffraction and advanced electron microscopy techniques (FEGSEM, TEM etc.). The evolution of the precipitating phases was monitored across a wide range of temperatures (from 500°C to 1170°C) and durations (from 24 hours up to 30000 hours) and several key phases have been identified. In addition to the primary W-rich carbide and the precipitation of Cr-rich M23C6, a new brittle phase/carbide was observed within the microstructure at the highest exposure temperatures (above 930°C). The microstructurally based model assists in the assessment of in-service operating temperatures as a means of evaluating the remaining operational life of the components

Microstructural analysis of creep exposed IN617 alloy

Nickel base alloys such as IN617 are one of the preferred choices for steam turbine components used by fossil fuelled power generation plants. IN617 is a solid-solution-strengthened nickel-based superalloy containing ~23% Cr, 12% Co, and 9% Mo with low content of precipitation-strengthening elements Al, Ti and Nb. In the ‘as-received’ (solution-annealed condition), the microstructure consists of primary carbides (M23C6) and occasional TiN particles dispersed in a single-phase austenitic matrix. Owing to high temperature exposure and the creep deformation processes that occur in-service, evolution of the microstructure occurs. This results in secondary precipitation and precipitate coarsening, both on grain boundaries and intragranularly in areas of high dislocation density. The influence of creep deformation on the solution-treated IN617 alloy at an operating condition of 650˚C/574 Hrs, with emphasis on the morphology and distribution of carbide/nitride precipitation is discussed. The applied stress was at an intermediate level

Microstructural Evolution in High Temperature Creep and Thermally Aged HA230

Haynes Alloy 230 is a sheet material used for combustor components in a number of small industrial gas turbines manufactured by Siemens. During normal operating service the material is subjected to high temperatures and cyclic mechanical and thermal stresses, which can lead to degradation of the microstructure and mechanical properties of the alloy, and hence limit component design life. As a result of this a long-term programme has been initiated to investigate the effects of thermal and creep exposure on the microstructure of this material using advanced FEGSEM and analytical TEM techniques with the objectives of: - determining the effects of turbine operating factors on the microstructural evolution of the alloy during service exposure; - identification of alloy phases which could potentially act as indicators of the average exposure temperatures experienced for specific service periods; - development of a microstructurally based model to enable the assessment of in-service operating temperatures as an aid to evaluation of the remnant life of HA230 combustor components. Originally this alloy 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 the case of thermal exposure, there is growing evidence from the studies conducted to date that in addition to M6C and intergranular precipitation of M23C6 resulting from thermal exposure, other types of phases may also precipitate in the alloy due to time dependent plastic deformation during long-term creep and/or thermo-mechanical fatigue exposure leading to reductions in both ductility and high temperature strength. This paper describes initial studies on the effects of long-term high temperature exposure on hardness and microstructural changes of creep rupture tested and thermally exposed samples of HA230 being carried out as part of the current COST 538 technology programme

High Temperature Microstructural Degradation of Haynes Alloy 230

Haynes Alloy 230TM 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 M6C and the precipitation of M23C6 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

Microstructural evolution in creep exposed IN617

Inconel alloys are currently being investigated for high temperature applications such as HP and IP valve chest and rotor forgings in advanced steam power plant operating at temperatures of 700°C and above. One of the preferred alloys for these components is IN617. This is a solid solution strengthened austenitic Ni-based alloy containing ~23% Cr, 12% Co, and 9% Mo with small additions of Ti and Al which can contribute some additional precipitation strengthening. In the solution treated condition, the microstructure consists of equiaxed austenite containing M23C6 at the grain boundaries and occasional TiN particles within the matrix. Owing to high temperature exposure and the creep deformation processes that occur in-service, evolution of the microstructure occurs in the form of precipitation, precipitate coarsening and recovery effects. This paper discusses microstructural evolution occurring in this alloy in samples that have been exposed to temperatures up to 700°C and for durations up to 45,000 hours using advanced FEGSEM, TEM and XRD techniques

TEM studies of microstructural evolution in creep exposed E911

Transmission electron microscopy has been used to investigate precipitate evolution in E911 steel samples creep tested to a range of temperatures (600-650°C) for durations of up to 75,000 hours. E911 is a 9%Cr 1% MoNbVNW creep resistant ferritic/martensitic steel that is used for boiler applications in power generation plant. The initial microstructure consists of tempered martensite containing M23C6 precipitates at the prior austenite and martensite grain boundaries together with fine M2X and MX precipitates in the matrix. A small amount of primary MX is also observed within the matrix. After prolonged exposure at high temperature and stresses, coarsening of the original M2X and M23C6 was found to occur together with the precipitation of Laves phase and Z-phase. The paper discusses the evolution of the microstructure and relates this to the hardness and strength changes observed owing to creep testing of the alloy