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Creep behaviour of 316L stainless steel prepared by 3D printing
Dymáček, Petr ; Kloc, Luboš ; Gabriel, Dušan ; Masák, Jan ; Pagáč, M. ; Halama, R.
The 3D printed 316L stainless steel produced by selective laser melting (SLM) was subjected to short term creep testing at 700 °C. Two directions of printing: i) horizontal and ii) vertical, were selected to test the creep performance of the steel. Comparison with the open literature data shows very good short-term creep properties of 3D printed steel that are superior to conventional steel. Both studied directions of printing show similar results so the steel can be considered from creep point of view as isotropic. The solution annealing prior to the creep testing slightly, but not substantially, lowers the creep performance of the steel.
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Low Strain Creep of ex-service 0.5CrMoV Steel
Kloc, Luboš
Helicoid spring creep testing technique with high strain sensitivity was applied on specimens of low-alloy steel after 197 thousands hours in service. The creep testing conditions were selected to be close to that of the service, that is 560 °C and 35MPa. The ex-service specimens creep much faster than the unused one and an anisotropy in creep properties was also observed. The method is capable to provide some additional information for the residual creep life assessment, but interpretation of the results is quite difficult. The large volume of the material needed for testing is also considerable disadvantage.
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Reliability Problems in Methods of Residual Creep Life Assessment
Kloc, Luboš ; Sklenička, Václav
The service life of components designed to operate for long periods under loading at elevated temperatures is limited by the creep deformation and fracture phenomena. Possibility of a reliable prediction of the residual creep life is of major practical and economical relevance. A number of approaches has been used for the estimation of the creep damage and residual life; these approaches could be divided into two principal branches. The microscopic methods are based on the microstructural observations of accumulated damage in the samples of material after service, while macroscopic meth- ods use mechanical testing of the samples under some reference conditions. However, both approaches still do not provide enough reliable results for wide practical use, be- cause of result scatter and missing generally applicable models for the creep damage evolution. The most important factors which make the results unreliable are analyzed in the present paper. These are mainly: i) production scatter in the material's properties, ii) inhomogeneity of the material, iii) sampling problem in microscopic methods, iv) extrapolations involved in macroscopic methods. Some suggestions to overcome these problems are made and the need for a new approach and new experimental techniques is presented.
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Problems in Theories of Low Stress Creep Mechanisms
Kloc, Luboš ; Sklenička, Václav ; Fiala, J.
Viscous creep behaviour of many materials including creep resistant steels was observed at temperatures close to one half of absolute melting point and at very low stresses. The experimental results were interpreted as Coble diffusional creep and/or Harper-Dorn dislocation creep. Some of the results are in a very good agreement with the Coble theory of diffusional creep and support an idea that the diffusional creep is an important deformation mechanism under certain loading conditions. Nevertheless, another results are not fully compatible with the theoretical predictions. Some authors dispute the role or even the very existence of the diffusional creep and offer the Harper-Dorn creep mechanisms instead. In fact, there are many theories trying to deescribe Harper-Dorn creep mechanism, but none of them is capable to explain all observed properties. Transition (or primary) stage observed at low stress creep regime exhibits also some properties which contradict all existing theories. The observed effects which cannot be explained by the current theories are: i) a large scatter of creep rates observed for coarse grain materials, ii) creep rates much higher than those predicted by the diffusional creep theory while the creep rate dependence on grain size is maintained, iii) transition stage duration is independent of stress, iv) transition strain is dependent on temperature. Despite the problems in theoretical description, the experiments show that the viscous creep regime in structural materials must be considered as an important behaviour of the structural materials.
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