[Kovove materialy - Metallic materials]
    Fri - October 20, 2017 - 16:09 No. of hits : 863196 ISSN 1338-4252 (online) ISSN 0023-432X (printed)
© Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Bratislava, Slovak Republic

VOLUME 55 (2017), Issue 4

Microscopic analysis of the stainless steel weld metal after long-term thermal exposition in the primary circuit of the nuclear power plant
vol. 55 (2017), no. 4, pp. 279 - 289

In the paper, results of microscopic analyses of the weld metal taken from weld joint of the thick-walled austenitic stainless steel pipeline of the primary circuit of the nuclear power plant WWER-440 are presented. The weld joint has been exposed in the main circulation pipeline approximately 180,000 h at temperatures approaching 300 °C. A sample taken from a top part of the multi-pass weld joint has been analyzed by scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy and high-resolution transmission electron microscopy. The analyzed weld metal exhibited a duplex microstructure consisting of the austenite phase and a semi-continual network of δ-ferrite phase. The mean hardness of the weld metal was 195 HV1. In the weld metal, globular non-metallic inclusions, as well as carbonitride particles of the MX type and chromium based carbide particles of M23C6 type, were identified. At the phase interfaces of δ-ferrite/austenite, the presence of particles of σ-phase placed in the volumes of δ-ferrite network was detected. The presence of σ-phase and M23C6 carbide particles in the top part of the weld can be explained as a consequence of the short-term expositions of the weld metal in the temperature range from about 900 to 600 °C during the multi-pass welding process of the primary circuit of the nuclear power plant. High-resolution transmission electron microscopy analysis of the ferrite phase regions revealed in thin foils the presence of mottled contrast indicating the spinodal decomposition of the δ-ferrite to Cr-rich α’- and Fe-rich α-phases. It is supposed that the decomposition of the δ-ferrite phase started during final stages of the multi-pass welding process at thermal expositions around 475 °C and continued during long-term exposition at the operation temperatures of the primary circuit.

Key words
stainless steel, weld metal, low-temperature thermal aging, σ-phase, carbide phase, α’-phase

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Full title of this journal is bilingual: Kovové materiály - Metallic Materials.
The official abbreviation in accordance with JCR ISI is Kovove Mater.

Article abstracts updated: 2017-09-26 11:15     
Articles in press revised: 2017-10-18 14:20    
Full text pdf uploaded: 2017-08-18 08:43   
Information updated: 2017-05-10 17:18  

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