Creep resistant 9% Cr steels are frequently used in civil engineering projects of supercritical coal‐fired power plants operating at temperatures of superheated steam around 610 °C. This work focus on analysis of microstructure degradation and degradation of mechanical properties of orbital narrow‐gap welding of P91 and P92 steels. Nowadays welding technology of orbital narrow‐gap using tungsten inert gas is not limited to welding of high‐pressure steam pipelines. This method is highly productive and efficient and leads to an increase in productivity, quality, and repeatability of welding process. It replaces the original method of manual welding with coated electrode into a standard bevel, which is a time‐consuming process that costs a high amount of weld metal. Welds are degraded at 650 °C/30 000 h without applied stress and the effect of degradation is evaluated across the weld based on the observed changes in microstructure and hardness. Attention is focused mainly on precipitation and coarsening of Laves phase in the main areas of heat affected zone (HAZ): coarse grain HAZ, fine grain HAZ, and intercritical HAZ. The results show that frequency and rate of Laves phase coarsening in the fine grain and coarse grain HAZ are higher than in the base material.

The paper presents the results of the analysis of secondary phases formed during thermal exposure in creep resistant austenitic steels of SUPER 304H and HR3C types. These steels were worldwide used for construction of the superheaters (the heat exchangers) of supercritical and ultra-supercritical (USC) coal-fired power plants. In order to accelerate precipitation processes, the steels were isothermally aged at 675 °C for 20 000 h. The investigations of the precipitates were primarily focused on the occurrence of brittle phases. Changes in mechanical properties caused by occurrence of secondary phases were documented by tensile testing and measurement of the impact strength. For comparison reasons, the impact tests of the initial state of the steels were also performed. The results showed that long-term ageing had led to the significant decrease of the impact strength. Correlation between precipitation of secondary phase and measured values of mechanical properties is shown. The influence of brittle phases on long-term durability of the degraded steel regarding its insufficient impact strength has been discussed.

The aim of this paper is to design a method for assessing the life of steam pipes for Czech power plants. The most widely-used material in Czech power plants is steel 15 128. Our findings may also be applied for international equivalents of this steel. The paper shows the classification of cavitation damage and microstructure classification status, based on the German VGB Act, with references to EPRI law in the USA. Calculations of remaining life on the basis of Russian experience are also shown. The possibility of applying this method to increase the operating parameters for power plants is discussed.

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This article deals with the results of mechanical testing and structural analysis of sections of narrow gap orbital welded P91 steel on tube OD 355.6 x 40 mm. The evaluation of mechanical properties was based on tensile test at room temperature on mini-tensile specimens and on measurement of modulus of elasticity. Weld was cut longitudinally into 9 narrow slices by using waterjet. From these slices 108 flat mini-tensile specimens (dimensions of gauge is 2 x 2 mm) were prepared. In experimental part microstructure evaluation and documentation of fracture surface of each mini-tensile specimen were carried out. The aim of these experiments was to assess the mechanical properties of the individual sections of the weld (base metal, heat affected zone and weld metal). These data can be used for new approaches of FEM modelling of welds considering heat affected zone like a combination of different materials with different mechanical properties, which connect the thermally unaffected base metal and weld metal.