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1 Zeszyty Naukowe. Mechanika / Politechnika Opolska

1 2010

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Microstructural analysis of P23/P91 dissimilar welds after creep exposure at 500 - 600 C

Střílková L., Vodárek V.

Zeszyty Naukowe. Mechanika / Politechnika Opolska

2010

z. 97

27-28

The development in power plant technology towards larger units and higher efficiencies depends on the availability of advanced creep resistant steels 1 . The attention has to be paid to weld joints which usually represent the weakest point in any power equipment working in the creep regime. This paper is focused on microstructural evolution in two designs of heterogeneous P23/P91 welds during long-term creep exposure in the temperature range from 500 to 600oC. The attention has been paid to microstructural changes in the vicinity of the fusion boundary between a low alloy steel and a high alloy steel. Results of microstructural investigations were compared to the results of thermodynamic and kinetic modeling, which had been carried out using the ThermoCALC and DICTRA software packages. Welds A and B were manufactured using E CrMo91 B and Thyssen Cr2WV filler materials, respectively. Cross-weld samples for creep rupture tests involved base materials, heat affected zones and weld metal. Creep tests were carried out at 500, 550 and 600°C in the stress range between 55 and 200MPa. Specimens for metallographical investigations were prepared in the longitudinal direction. Studies of minor phases were performed on the Weld A specimens using SEM and TEM techniques. Results of creep rupture tests on the P23/P91 dissimilar welds are close to, or below, the lower limit of the 20% scatter band around the standardized curve for the creep strength of the P23 steel 2 . The microstructure of the partly decarburized zone in the P23 (WM23) steel remained bainitic in all creep ruptured specimens. This was related to undissolved MX particles in the partly decarburized zone which slowed down recovery/recrystallization processes in the bainitic matrix. Results of microstructural investigations indicate that in the case of the P23 steel the SSOL database is more appropriate for microstructural modeling than the STEEL 16 database.