Creep-resisting austenitic steels constitute a group of construction materials which can work in the conditions of creep for the temperature range from 550 to 700°C. The service of austenitic steels leads to the progressive degradation of their microstructure, which results in the changes of functional properties. The main mechanisms of degradation of the austenitic steel microstructure include the processes of matrix softening, the processes of precipitation and matrix depletion of the interstitial and substitution elements. Precipitation processes in austenitic steels are a very important indicator, which allows the advancement of microstructure degradation processes in these steels to be determined. Hence, the knowledge of the impact of individual secondary phases on the microstructure and properties of austenitic steels plays a very important role in diagnosing the components and equipment of the power boiler system and makes it possible to forecast the time of safe operation of systems made from these steels. Based on own studies and data from literature, this paper will present the characteristics of secondary phase precipitates occurring in creep-resistant austenitic steels during their operation at an elevated/high temperature. The effect of secondary precipitates on mechanical properties of these steels will be discussed too.

This paper presents selected material characteristics of Super 304H austenitic steel used for elements of boilers with super- and ultra-critical steam parameters. In particular, abridged and long-term creep tests with and without elongation measurement during testing and investigations of microstructural changes due to long-term impact of temperature and stress were carried out. The practical aspect of the use of creep test and structure investigation results in forecasting the durability of materials operating under creep conditions was presented. The characteristics of steels with regard to creep tests and structure investigations developed in this paper are used for estimation of changes in functional properties of the material of elements operating under creep conditions.

This article presents the results of investigations on HR6W alloy and Sanicro 25 steel and the dissimilar welded joint made of them. The characteristic images of microstructure of the investigated materials in the as-received condition and following the creep test, observed with a scanning electron microscope (SEM), are shown. The X-ray analysis of phase composition of the existing precipitates was carried out. The method for evaluation of creep strength based on abridged creep tests carried out at a temperature higher than the design one is presented. The obtained results do not deviate from the values of creep strength determined in longterm creep tests. The maximum difference in creep strength of the investigated materials is ±20%, which is in compliance with the acceptable scatter band. The methodology presented can be used for verification of creep strength (life time) of the material of finished components to be operated under creep conditions.

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This paper presents the characteristics of the performance of P91 (X10CrMoVNb9-1), P92 (X10CrWMoVNb9-2) and VM12 (X12CrCoWVNb12-2-2) steels used for condition assessment of the pressure components of boilers with supercritical steam parameters. Studies on the mechanical properties, microstructure tests using scanning and transmission electron microscopy, and X-ray analysis of the phase composition of precipitates were performed for selected steels in the as-received condition and after long-term annealing. These steel characteristics are used for the evaluation of the microstructural changes and mechanical properties of the material of components after long-term service. The result of this study is the database of material characteristics representing the mechanical properties related to the microstructure analysis and it can be used for diagnosis of the components of pressure parts of power boilers.Keywords: steel P91, P92, VM12, microstructure, mechanical properties, long-term annealing inspections, and possible repairs are carried out during the boiler's lifetime to ensure safe and failure-free operation of power units. In safety assessment of the pressure equipment in which new creep-resisting steels were used, it is essential that a number of tests should be carried out to allow the evaluation of their condition and suitability for further operation beyond the design work time under further operation conditions [7,8,[12][13][14][15][16][17][18][19].In cooperation with other research centres (Silesian University of Technology, Czestochowa University of Technology, AGH University of Science and Technology in Cracow) as well as the boiler factories Rafako S.A. and SEFKAO S.A. -the leading manufacturers of boilers, the Institute for Ferrous Metallurgy performs the works on the advance and verification testing of new-generation steels for the power industry [2,7,8,19,21,23].This study presents the results of investigations on the mechanical properties in the as-received condition as well as the effect of long-term impact of elevated temperature and stress on the change in properties of 9-12%Cr steel. Test sampleThe test sample was P91 (X10CrMoVNb9-1), P92 (X10Cr-WMoVNb9-2) and VM12 (X12CrCoWVNb12-2-2) steels in the as-received condition after heat treatment and after long-term exposure at elevated temperature. The chemical composition of the examined steels is summarised in Table 1.

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This paper presents the results of material tests of repair welded joints carried out for samples of materials taken from high pressure steam pipelines after over 200,000 operating hours in creep conditions, made of the 13HMF steel (14MoV6-3). The aim of the study was to assess the mechanical properties and changes in the structure of repair welded joints (new material + material after use), including the original material, heat affected zone and the weld, occurring as a result of the welding process and heat treatment after welding.

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