Mechanical and microstructural characterization of electron beam welded reduced activation oxide dispersion strengthened – Eurofer steel

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Abstract

Electron beam welding has been considered as a potential process to join divertor structures made of reduced activation ferritic martensitic 9Cr% ODS-Eurofer steel. Strips of this ODS steel were welded under well defined conditions.

Two different post-weld heat treatments were applied to investigate their impact on the mechanical and microstructural properties of the welded joints. Miniaturised tensile specimens were used to determine the tensile behaviour in the temperature range between RT and 773 K. KLST specimens were used for Charpy impact tests. The tensile strength decreases to values similar to Eurofer steel without ODS strengthening. The impact behaviour is far worse than both, of ODS-Eurofer and Eurofer steel.

The microstructure of the weld and heat-affected zone as well as the fracture surface of the samples were examined using optical and scanning electron microscopy, dual-beam SEM/FIB, low magnification and analytical transmission electron microscopy. The changes of the mechanical properties can be well correlated with the detected changes of micro- and nano-structure.

Introduction

For specific blanket and divertor applications in future fusion power reactors a replacement of presently considered reduced activation ferritic martensitic (RAFM) steels as structural material by suitable oxide dispersion strengthened (ODS) ferritic martensitic or ferritic steels would allow a substantial increase of the operating temperature from ∼823 K to about 923 K [1], [2]. In all cases appropriate joining technologies have to be developed. Diffusion welding techniques to perform similar and dissimilar joints have been studied successfully [3]. Friction Stir Welding (FSW) has shown a good potential but application is limited due to geometrical restrictions and needs further development [4].

For the advanced helium-cooled modular divertor concept, various joining techniques are required for joining the complex structural parts made of different materials [5]. The electron beam welding process, with its highly concentrated energy input, has been investigated as a potential process to join divertor structures made of ODS-Eurofer, an ODS steel based on the 9%CrWVTA-RAFM steel Eurofer.

The microstructure of the weld and heat-affected zone as well as the fracture surface of the samples were examined using optical and scanning electron microscopy (SEM), Dual-Beam SEM/FIB, low magnification and analytical transmission electron microscopy (TEM). The changes of the mechanical properties can be well correlated with the detected changes of micro- and nano-structure. SEM and low magnification TEM analyses show significant changes of the microstructure in the welded area. In all specimens a grain coarsening and changes of the distribution and morphology of the carbide precipitates and ODS particles has been observed. The formerly nano-sized ODS particles agglomerate to complex structured yttria-containing larger particles.

Section snippets

Experimental

The material used in these investigations, is a reduced activation ferritic martensitic ODS steel on the basis of the 9% Cr RAFM steel Eurofer. The chemical analysis is given in Table 1. Oxide dispersion strengthening was achieved by addition of nominally 0.3 wt.%Y2O3 in the mechanical alloying process of the argon gas-atomised Eurofer basic powder [1], [2].

To study the EB-welding, strips of 212 × 20 × 6 mm of this ODS steel were welded under well defined conditions (150 kV, 8.5 mA) using a PTR 150 kV/15 

Metallographical examinations

Prior to the mechanical testing, metallographic examinations of the weld in the as-welded and post-weld heat-treated conditions were performed. Microhardness measurements were performed at three different positions, the weld nugget, heat-affected zone (HAZ) and in some distance, the base material. The results as well as the different PWHT conditions are given in Table 2. In the as-welded condition the hardness in the weld and the heat-affected zone are increased to values of 352 and 431,

Summary and conclusions

EB-welding was investigated as a potential process to join divertor structures made of ODS-Eurofer. Similar ODS/ODS joints were fabricated and the tensile and impact properties were investigated in dependency of different post-weld heat treatments. Dissimilar ODS-Eurofer/Eurofer 97 joints were fabricated and tested for comparison. The microstructure of the different welds was investigated to explain the mechanical behaviour.

EB-welding of similar ODS-Eurofer joints leads to weak weld seams. The

Acknowledgements

The authors would like to thank Mr. S. Baumgärtner and Mr. B. Dafferner for the execution of the tensile and impact bending tests and Mr. H. Zimmermann for the metallographic examinations.

“This work, supported by the European Communities under the contract of Association between EURATOM and Karlsruhe Institute of Technology, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European

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