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Evolution of the microstructure and mechanical properties of Sanicro 25 austenitic stainless steel after long-term ageing

Sroka M., Zieliński A., Golański G., Pawlyta M., Purzyńska H., Novy F.

Archives of Civil and Mechanical Engineering

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2023

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Vol. 23, no. 3

art. no. e149, 2023

EN

A newly developed heat-resistant austenitic steel, Sanicro 25 is currently considered the leading candidate material for an advanced ultra-supercritical installation. The test material was subjected to long-term ageing (up to 30,000 h) at 700 and 750 °C, after which investigations into the microstructure, identification of precipitates, and testing of mechanical properties were conducted. Sanicro 25 had an austenitic microstructure with annealed twins and numerous large primary NbX and Z-phase precipitates in the as-received condition. It was found that the long-term ageing of the steel resulted in numerous precipitation processes. For example, M23C6 carbides, Laves, σ and G phases occurred at the grain boundaries. However, Z-phase precipitates, ε_Cu particles, and Laves phase were observed inside the grains. At the same time, compound complexes of precipitates based on the primary Z-phase precipitates were revealed in the microstructure. The ageing process increased the particle size of M23C6 carbides and the σ phase. After longer ageing times, a precipitate-free zone (PFZ) near the grain boundaries was observed. The precipitation processes initially lead to an increase in the strength properties of the steel. However, after 5000 h, an over-ageing effect was observed at 750 °C, which was not observed at 700 °C.

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Microstructure and mechanical properties of the Sanicro 25 steel after ageing

Merda A., Sroka M., Klimaszewska K., Golański G.

Journal of Achievements in Materials and Manufacturing Engineering

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2018

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Vol. 91, nr 1

5--11

EN

Purpose: The purpose of the research was to determine and analyse the changes in the microstructure and mechanical properties of the Sanicro 25 steel in the as-received condition and after ageing at 600, 650 and 700°C for up to 10,000 hours. Design/methodology/approach: The scope of the investigations included: microstructural investigation – SEM microscopy, analysis of precipitation performed using TEM microscopy, investigation of mechanical properties, Vickers hardness measurement. Findings: In the as-received condition, the Sanicro 25 steel was characterised by austenitic microstructure with annealing twins and numerous primary precipitates. The analysis of Sanicro 25 steel microstructure after ageing at 600 and 700°C for up to 10,000 hours revealed significant changes in the microstructure consisting mainly in a tendency to create unfavourable morphology of secondary precipitates – M23C6 carbides that form continuous carbide systems along the grain boundaries. The observations have shown that during long-term ageing the secondary carbides were also precipitated inside the grains and at the interface of three grain boundaries – σ phase. Research limitations/implications: The analysis of the microstructure of the examined steel using SEM and TEM was performed to determine the influence of ageing on the processes of changes in the precipitate morphology. Practical implications: The results obtained based on the performed research constitute a building block for the degradation characteristics of the microstructure and mechanical properties of the 23/25-type austenitic steels. Originality/value: The results of the investigation and analysis of the metallographic and mechanical properties of the Sanicro25 austenitic steel in as-received condition and after ageing are presented.

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Evaluation of Creep Strength of Heterogeneous Welded Joint in HR6W Alloy and Sanicro 25 Steel

Zieliński A., Dobrzański L., Purzyńska H., Sikora R., Dziuba-Kałuża M., Kania Z.

Archives of Metallurgy and Materials

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2017

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Vol. 62, iss. 4

2057--2064

EN

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 long-term 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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Właściwości mechaniczne złączy spawanych ze stali Sanicro 25 oraz stopu HR6W

Jamrozik P., Sozańska M., Pasternak J.

Przegląd Spawalnictwa

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2013

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R. 85, nr 10

39--45

PL

Materiały wykorzystywane w energetyce muszą spełniać określone wymagania, do których należy zaliczyć przede wszystkim wysoką wytrzymałość na pełzanie i zmęczenie, żaroodporność w środowisku spalin i odporność na utlenianie w parze wodnej, odpowiednią ciągliwość oraz stabilność mikrostruktury podczas długotrwałej eksploatacji. W przemyśle energetycznym, w zależności od przeznaczenia danego elementu, stosuje się różne gatunki stali i stopów. Przedmiotem badań była ocena właściwości mechanicznych jednorodnych i niejednorodnych złączy spawanych ze stali Sanicro 25 oraz stopu HR6W przeznaczonych do zastosowania na wężownice przegrzewaczy pary w nowoczesnych blokach energetycznych. Przeprowadzono statyczną próbę rozciągania w temperaturze pokojowej oraz podwyższonej (700ºC i 750ºC) i próby udarnościowe w temperaturze pokojowej. Badano wytrzymałość na pełzanie złączy spawanych w podwyższonej temperaturze (700ºC) oraz rozkład twardości po długotrwałym wyżarzaniu w temperaturze 700ºC.

EN

The materials used in the power industry must meet certain requirements, which include primarily high resistance to creep and fatigue, heat resistance in an exhaust gas and oxidation resistance in water vapour, adequate ductility and microstructural stability during long-term use. In the energy industry, depending on the purpose of the element are various types of steels and alloys. Subject of research was to evaluate the mechanical properties of homogeneous and heterogeneous welded joints in Sanicro 25 steel and HR6W alloy designed for use on steam superheater coils in modern its energy units. Static tensile test was carried out at room and elevated temperatures (700ºC and 750ºC) and the impact tests at room temperature. Investigated creep strength of welded joints at elevated temperatures (700°C) and hardness distribution after prolonged an annealing at 700°C.