1,721,048 research outputs found
Contact and fretting wear properties of superalloy Rene® 77
No full textThe nickel-based superalloy Rene® 77 is widespread used in turbine blades. The reason is the excellent mechanical properties and resistance to the oxidation at high temperature. In turbine blades, a further resistance to the fretting stresses is needed. This research is focused on the fretting wear characterization of Rene® 77. A point contact test rig was used to study fretting wear and contact parameters at room temperature and 600 C. Results at room temperature shows a non-linear increasing in wear volume at higher number of cycles or higher dissipated energy. At high temperature oxidation prevails at low energy and while wear prevails at high energy
Reciprocating Wear Damage in Steam Turbine Grid Valves: Influence of Coatings and Thermochemical Surface Treatments
No full textFretting fatigue analysis of additively manufactured blade root made of intermetallic Ti-48Al-2Cr-2Nb alloy at high temperature
No full textSlots in the disk of aircraft turbines restrain the centrifugal load of blades. Contact surfaces between the blade root and the disk slot undergo high contact pressure and relative displacement that is the typical condition in which fretting occurs. The load level ranges from zero to the maximum during take-off. This cycle is repeated for each mission. In this paper, a fretting fatigue analysis of additively manufactured blades is presented. Blades are made of an intermetallic alloy γTiAl. Fretting fatigue experiments were performed at a frequency of 0.5 Hz and at a temperature of 640 °C to match the operating condition of real blades. The minimum load was fixed at 0.5 KN and three maximum loads were applied, namely 16, 18 and 20 kN. Both an analytical and a two-dimensional finite element model were used to evaluate the state of stress at the contact interfaces. The results of the analytical model showed good agreement with the numerical model. Experiments showed that cracks nucleate where the analytical model predicts the maximum contact pressure and the numerical model predicts the maximum equivalent stress. A parametric analysis performed with the analytical model indicates that there exists an optimum geometry to minimize the contact pressure. Tests showed that the component life changed dramatically with the maximum load variation. Optical topography and scanning electron microscopy (SEM) analysis reveals information about the damage mechanism
Reciprocating wear damage of grooved non-grooved flat surfaces for grid valves of steam turbines
No full textGrid valves are used in extraction type steam turbines for the steam extraction in different sections of the turbine before the last stage. This valve is based on the relative rotation of two holed disks by means of an actuator, steam is extracted when the holes of these disks are fully or partially overlapped. When the valve is closed the holes of rotating disk are overlapped to a grooved flat surface of the statoric disk. If these grooved parts are too worn the valve does not rightly close steam extraction. This study investigates the wear damage mechanism of the grooved parts of the statoric disk.
Research was based on experiments where grooved and non-grooved flat surfaces were kept in contact and worn by relative reciprocating motion. Test rig was characterized by free approaching of contact surfaces and self-positioning of specimens. Specimens may be disassembled, in order to make the surface topography measurements by means of optical instrument based on focus variation to evaluate the wear volume, and subsequently reassembled to continue the wear process. Friction force and relative displacement was continuously measured during the processes in order to display hysteresis loops and consequently evaluate the friction coefficient. Relative displacement was measured in the vicinity of contact surfaces directly on the specimen in order to avoid the inclusion of test rig component deformations in relative displacement measurements.
The choice of wear process parameters was based on operating conditions of grid valves, three temperatures, two normal loads, one sliding amplitude (500 micrometers), two frequencies (5, 50 Hz) and two sliding distances (300, 500 m). This set of process parameters were used to study the wear and the friction of five different materials based on different thermochemical treatments (nitriding and carbonitriding) and coatings (ENP, CrC, and two Stellites) of the steel 39NiCrMo3
Parametric numerical analysis of induction hardening problems
No full textThe prediction of residual stresses in induction hardened steel parts is practically
important since they affect component life and performance. The computation of residual
stresses involves the integration of a set of strongly coupled partial non linear differential
equations and a relevant number of input data. The object of this work is to perform a
parameter sensitivity analysis, with respect to residual stress computation. The results
show that the dilatometric data, as well as the spray cooling coefficient and the yielding
stresses of martensitic and ferrite-pearlite phase are critical input parameters in induction hardening processes. Transformation plasticity also plays a significant role, though, more systematic experimental study is necessary in order to quantitatively assess its actual role in surface induction hardened parts
Fretting wear damage mechanism of CoMoCrSi coatings
No full textSuperalloy coatings of the CoMoCrSi family (e.g. Tribaloy® T800) are applied to mitigate wear effects at high temperature. These coatings are extensively used on the contact surfaces of the shroud of turbine blades. If severe wear occurs on these contact surfaces the blade interlocking decreases, reducing the stiffness of the assembly, altering its dynamic behaviour, and increasing the risk of fatigue failure. Fretting is the expected damage mechanics on these mating surfaces. The study presented in this paper investigates the fretting damage mechanism of interfaces coated with CoMoCrSi alloys. The experimental plan includes fifteen combinations of the test parameters: two contact geometries, three deposition processes, four temperatures, three normal loads and three strokes. Wear at different number of cycles was also explored. Moreover, two types of contact geometries were investigated, namely point contact (sphere-on-flat) and flat-on-flat. The friction coefficient was computed using the hysteresis loops measured during the fretting tests. The topography of the contact surfaces was measured at different fretting wear cycles to estimate the volume loss. Wear grooves were observed by scanning electron microscopy. Results of point contact experiments at room temperature exhibited a steady friction coefficient independent of the normal load. Wear volumes showed a sharp increasing in wear rate at high dissipated energy while the trend was linear at lower dissipated energy. Oxidation was found more dependent on substrate than on temperature, stroke and wear cycles. Wear volumes and wear rate on flat-on-flat specimens coated with welded T800 were higher at 400 C than at room temperature and at high temperature (800 C). At room temperature, wear volumes of welded T800 applied by single layer were much higher than in dual layer. At room temperature and at low dissipated energy the wear rate of the point contact geometry was lower than flat-on-flat. At high energy, the wear rate of point contact tends to the flat-on-flat wear rate
A numerical method to solve the normal and tangential contact problem of elastic bodies
No full textRecently, interest has been growing among the engineering community to develop predictive models for the effect of joints on the tribology of jointed structures. The ability to predict contact forces and force–displacement relations of joints is key in enabling simulations to predict forced response and wear of jointed structures.
Only for a limited number of contact geometries has a solution in closed-form been found, and it is available in the literature. The finite element method has been used to a great extent to solve problems of elastic bodies in contact, but the iterative solution of large models is very demanding. This work deals with the development of a numerical procedure that utilizes the stiffness matrices of the bodies in contact modeled with the finite element method. The matrices are reduced with a lossless static reduction, and their small dimensions make the iterative solution of the contact problem very fast. Results are compared with contact models found in the literature and the sample results agree well with corresponding analytical solutions
- …
