The influence of heavy liquid metal environment on mechanical and corrosion properties of austenitic stainless steel welds

Lescur Amke

Promoter

Petrov Roumen, roumen.petrov@ugent.be

SCK•CEN Mentor

Lim Jun
jun.lim@sckcen.be
+32 14 33 80 15

SCK•CEN Co-mentor

Stergar Erich
erich.stergar@sckcen.be
+32 14 33 31 80

Expert group

Conditioning and Chemistry Programme

PhD started

2019-01-01

Short project description

ASTRID, MYRRHA, and ALFRED will most probably use austenitic stainless steels. This means that also large welded parts like the vessel will be made of this material. Therefore, understanding the influence of heavy liquid metal environment on welded materials is an important part in the qualification procedure of these reactors.
During welding, the temperatures of the base metal around the weld reach levels where different microstructural transformations occur. To which extend these changes occur and their effect on the final properties of the weld (including melting and heat effected zone(s)) depend on parameters like alloy content, material thickness, filler metal, joint design etc. Nevertheless, regardless of all the influencing factors the ultimate objective of welding is to provide a sound joint with qualities equal or better than the base metal [1]. In the current case the preservation of corrosion resistance, prevention of cracking and retaining the mechanical properties of the weld  in heavy liquid metal environment is of high importance.
For ensuring the performance of the weld the main concerns are avoidance of Cr depletion on grain boundaries due to carbide formation to maintain corrosion resistance as well as formation of delta ferrite and its influence on the mechanical properties. In the current context the influence of δ-ferrite is of special interest as it acts in welding of austenitic steels as a collection site for steel impurities such as S and P. Therefore, a typical 316L welding is produced with filler material that guarantees a certain minimum δ-ferrite content in the final weld. While the formed delta ferrite is beneficial for welding chemistry and the final quality of the weld it has a detrimental effect on the irradiation properties. Due to long term annealing (days to months) at temperatures in the range of 300 °C to 500 °C and neutron irradiation δ-ferrite shows a significant higher hardening than the surrounding matrix [2]. This is related to irradiation effects (radiation induced segregation, dislocation loops etc.) as well as thermally activated precipitation of carbide and intermetallic phases and Cr-rich α-prime due to spinodal decomposition [3, 4]. So far it is not clear in which ratio annealing and irradiation effects are responsible for the hardening of δ-ferrite. Therefore, it is proposed to study the microstructural changes and the consequences regarding fracture toughness, crack initiation behaviour, and corrosion properties of austenitic stainless steel (in particular 316L and 316L(N)) in the welded and long term annealed state. These investigations will lay a sound basis for future investigations on irradiated materials.
 

Objective

The activities planned within WP4 of the H2020-GEMMA project will comprise corrosion/erosion tests to generate data for establishing corrosion correlations for design and safety assessments. Beside corrosion testing also mechanical tests and microstructural investigations are foreseen. The first batch of material is expected in the middle of 2018 which means that material is readily available when this PhD topic will start. The GEMMA project is mainly aimed towards practical application. While taking advantage of the possibility to get industrial relevant material, this PhD topic should go a step further and aim to identify and explain underlying mechanisms. During this PhD the welded material, including the heat effected zone and the base material should be characterized by mechanical testing methods like fracture toughness-, tensile- and hardness tests in different environments. Combining this with different microscopy techniques (Optical-, focused ion beam-, scanning electron, electron back scatter, and transmission electron microscopy) as well as X-ray diffraction will lead to a thorough understanding of the influence of delta ferrite, precipitates and segregations on the properties of austenitic stainless steel in heavy liquid metal environment.
For the investigations at SCK•CEN readily available testing equipment for liquid metal testing and microstructure facilities will be used.

Bibliography
[1] G. A. Young, M. J. Hackett, J. D. Tucker, and T.E. Capobianco, “Welds of Nuclear Systems” Comprehensive Nuclear Materials, 273-296, 2012
[2]  A. Tavassoli, C. Picker and J. Wareing, "Data Collection on the Effect of Irradiation on the Mechanical Properties of Austenitic Stainless Steels and Weld Metals," Effects of Radiation On Materials : 17th Volume, pp. 995-1009, 1996.
[3]  O. Chopra, "Degradation of LWR Core Internal Materials due to Neutron Irradiation," NUREG/CR-7072, p. 121, 2010.
[4]  K. Chandra, V. Kain, V. Bhutani, V. S. Raja, R. Tewari, G. K. Dey and J. K. Chakravartty, "Low temperature thermal aging of austenitic stainless steel welds: Kinetics and effects on mechanical properties,"  Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, vol. 534, pp. 163-175, 2012.