Tomáš Vlasák

Creep behaviour of steel P23 weldments

Low-alloy creep resistant steel P23 (2.25%Cr-1.6%W-0.25%V) has been developed for pipelines in power engineering. Creep resistance of this steel was already evaluated. Continuation of this work is the investigation of possibilities of weldment production. This paper reports some of this possibilities and mentioned further are the creep strength and metallographic evaluation of the optimally prepared weldments P23 – P23.

High-temperature creep properties of Fe-Al alloys modified by Zr

Abstract Two Fe – Al alloys with Zr additions were prepared by vacuum induction melting and rolling at 1200 °C: a disordered (ferrite – A2) and a D03-ordered one. The data are compared to a similar Fe3Al-based alloy with lower Zr content investigated earlier. Tensile and creep tests were performed at 600, 700, and 800 °C, where the disordered alloy maintains its A2 structure, and D03-ordered alloys are transformed to B2-ordered structure. Transmission electron microscopy of the alloys crept at 600 °C showed precipitation of fine particles on dislocations, subgrain boundaries and also in the matrix. Coarsening of these particles during creep at 800 °C takes place. The particles in Fe3Al-based alloys are identified mostly as zirconium carbides, whereas (Fe, Al)2Zr precipitates were observed in the ferritic alloy. The increase of Zr content from 0.25 to 0.6 at.% does not have a significant effect on the high temperature (HT) mechanical properties of the Fe3Al-type aluminide, because both alloys contain the same amount of carbon. In consequence no additional volume fraction of ZrC particles could be formed. The creep resistance of the ferritic alloy is considerably lower, but this material can easily be rolled to prepare sheets for HT corrosion protection.

Creep of Cast Steel P91 with Weld Joint

High-temperature martensitic steel P91, internationally marked GX12CrMoVNbN91, is the material used in the energy industry. Creep and high-temperature corrosion resistances are important properties that affect the application of this material at higher temperatures. Weldment reduces creep properties. This work deals with the quantification of this decrease in the case of material P91. The main focus is except the evaluation of creep test results given to the mathematical description of the weld creep strength reduction. Further metallographic analyses of weld joint after creep exposures were performed.

The Microstructure Changes in IN713LC during the Creep Exposure

Nickel-based creep resisting alloys (strengthened by γ´) are the basic materials for high-temperature constructional parts in aircraft engines and energy units. These parts are exposed to combined effects of mechanical stresses, high temperature and dioxide-corrosion conditions. The microstructure changes of cast polycrystalline Ni-based superalloy IN713LC after creep exposure were studied. Three specimens with three different diameters were used for creep tests. The degradation stage (damage parameter π) was determined for all parts of specimens. Individual parts of specimens were metallographic observed and analyzed by image analysis after rupture. The results were compared with model of stress distribution in the specimen with potential damage in the centre of the specimen.

Influence of Al-Si Layer on Structure and Properties of Cast Ni-Based Superalloys

High-temperature constructional parts of aircraft engines and energy units are exposed to high dynamic stress (fatigue processes and creep) and various temperatures in dioxide-corrosion condition (hot corrosion, oxidation and erosion). The improvement of aero-engine and turbine efficiency is possible through the increase of temperature in front of turbine. This requires the use of heat-resistant and creep-resistant materials, especially nickel-base superalloys which resist mentioned effects for a limited period of time. A deposition of protective layers should improve hot corrosion resistance. This paper is focused on microstructure of protective layers created by codeposition of Al and Si on nickel-base superalloys INCO 713 LC and INCO 738 LC after thermal and thermal-stress exposition and on microstructure of basic materials (substrates). The contribution also shows creep tests results for both superalloys with and without a protective layer.