摘要:

AbstractThis paper presents the fatigue crack prop agati on results obtained at 600°C from CT specimens of the nickel-based superalloy IN718 used in turbine discs of aeroengines. Firstly, the influence of the stress ra tio on the fatigue crack growth rate was assessed for a baseline trapezoid al load waveform with a frequency of 0.25 Hz and a Is duration in each step. These results were then compared for thr ee crack growth laws. The results of a prelim inary study of dwell time at maximum load are also presented. The da/dN versus∆K results were obtained for dwell times of 2 and 5 min. A fatigue crack propagation analysis was carried out and the fatigue crack growth dat a were compared against the results obtained with the baseline trapezoidal load waveform. Scanning electron microscopy observations of the fatigue fracture surfaces are also presented.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11752529

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe creep properties of simulated heat-affected zone, weld metal and cross-weld specimens of tCr0.5Mo and cross-weld specimens of 2.25Cr tMo have been studied with reference to the effect of postweld heat treatment (PWHT). A reduction in the creep rupture strength of up to 20% has been observed after PWHT. Contrary to what has been previously proposed, PWHT was in the present investigation found to have only a marginal influence on the creep ductility. The creep rate can increase by up to a factor of 2-4 after postweld heat treatment, probably because of-carbide coarsening during the heat treatment. Preheating at 200°C did not affect the creep properties of 2.25Cr lMo. Overheating, on the other hand, at 650°C for 1000h reduced the rupture time of tCr0.5Mo by almost an order of magnitude. The tensile strength and yield strength of tCrO.5Mo were significantly reduced after postweld heat treatment. Reductions of up to 100-200MPa were observed both at room temperature and at 550°C in some cases.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11752530

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractUniaxial creep tests at 80 MPa with different temperatures and at 600°C with different loads were carried out for a 12%CrMoV martensitic steel. The creep strain behaviour was analysed using an ana lytical expression where. the creep strain rate is proportional to the exponential of the creep strain. The creep curves, rupture times and strains could be well reproduced. The observed and predicted rupture times satisfied the Monkman-Grant relationship.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11752531

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractTh e effect of silicon on the microstructure and solidification segregation of directionally solidified IN738 nickel-based superalloy was studied. Directional solidification at various solidification rates and partial directional solidification plus rapid quenching were applied. Metallographican alysis and an electron microp robe were mainly used to observe and measure the micro structure and elemental segregation of the alloy, respectively. It was found that silicon affected the morphology of the liquid-solid interface of the alloy during solidification and gave the alloy a tendency to form well-developed dendrites. The addition of siliconen larged the solid us-liquidus temperature interval. and the solidification rate also greatly influenced the interval. The interval increased with increasing solidification rate. Silicon promoted the precipitation of theγ/γ' eutectic, and also affected its precipit ation temperature. Silicon segregated mainly in interd endritic regions, and promoted the segregation of other elements. All of the effects of silicon on the alloy related to the solidification rate.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11752532

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractA study of the behaviour of Ag-Cu-Ti braze in contact with a ceramic - its wetta bility, evolution and reactivity-by different means of investigation gives some insight into the mechanisms involved in reactive brazing. Joints between siliconnitride and a structural steel were made by brazing under various conditions of temperature, hold time and a tmosphere. The room temperature mechanical strength measured in a shear test may be correlated to some microstructural and physicochemical features. The heterogeneous microstructure observed for almost all Ag-Cu-Ti brazes is explained by immiscibility in the liquid state with an equilibrium established between Ag-Cu-rich and Cu-Ti-rich liquids. It is shown that high temperatures (>910°C) ensure a more efficient reaction of the titanium-rich phases and so a good wetting of the siliconnitride. Consequently, the shear strengths of bonds between siliconnit ride and steel achieved by using a sufficiently high brazing temperature are optimal (123-158 MPa).

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11752533

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractTh e interdiffusion between the Pt-modified aluminide coating RT 22 and the single-crystal supera lloys CMSX-6 and SRR 99 has been investigated at 1000 and 1200°C up to 1600 h of exposure time in air and vacuum. Both coating/substrate systems possess remarkable stability at 1000°C, in air as well as in vacuum. It appears that the plate-like precipitates of intermetallics in the interd iffusion zone between coating and substrate act as a barrier layer for interdiffusion. Some inward diffusion of Pt and Al and outward diffusion of Ti occur at 1000°C. Rapid degradation of the coating takes place at 1200°C mainly by interdiffusion. The AI di ffusion into the substrates leads to conversion ofβ-NiAI intoγ'-Ni3AI and finally intoγ-phase. Tirapidly diffuses outwards and becomes incorporated into the scale rich in AI. Thus, the Ti content of the substrate affects the scaling behaviour of the coa ting. The intermetallic precipitates are dissolved and, therefore, so is the interdiffusion barrier. The somewhat better behaviour of the RT 22/ SRR 99 system is probably caused by the greater stability of the W-rich intermetallics in this system.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11752534

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe present paper attempts to assess the many aspects which are involved in the old, interesting and somewhat perplexing grain boundary segregation phenomenon commonly known as reverse temper embritlement (RTE). The actual mechanisms involving grain boundary failure, the mechanics of impurity segregation at grain boundaries and the speciality of grain boundaries are discussed at length . The thermodynamics of impurity segregation, which is treated as aneq uilibrium-type process, together with the segregation kinetics, which are required for an estimation of the extent of grain boundary impurity segregation in terms of time and temperature, are also considered. The various methods of portraying the effects of reverse temper embrittlement in terms of bulk chemistry, grain boundary chemistry, therm al history (tempera ture, time and impurity segregation characteristics) and material properties (micro structure, hardness and strength) are critically assessed and compared. Interms of chemistry it is shown that the extent of grain boundary segregation, viz. phosphorus monolayers, exhibits very con sistent and small datascatter trends. Consideration is given to other aspects such as thermal history, together with impurity diffusion and material properties such as hardness, tensile strength and grain size, and it is established that RT E effects can be adequately expressed in terms of (I) grain sizebulk phosphorus trends and/or (2) a grain boundary phosphorus factor.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11752535

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractSuperalloy 718 in the solution-treated condition was welded autogenously by electron beam welding and gas tungsten are welding processes. The weldrnents after suitable heat treatment were subjected. to creep/stress rupture testing at 650°C and 690 MPa. The results showed that the Laves phase resulting in the weld metalis deterimental to the creep rupture life of weld metals when present With the continuous morphology found in gas tungsten are welds. The lower amounts and discontinuous morphology of the Laves phase in electron beam weldments in combination With favourable gramonentauon resulted in relatively better rupture properties for these weldments.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11752536

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor