Optimization of electron beam butt welding of 32 mm CLF-1 steel T-joints of Test Blanket Module (TBM) in ITER

Abstract

• Well-formed CLF-1 steel T-joints with no weld defects could be obtained by electron beam welding without a base plate. • The mean values of impact-absorbing energy of the welds were 296 J and 297 J, which higher than that of CLF-1 base metal. • The average tensile strengths of two joints at room temperature were 620 MPa, while those at 550℃ were 350 MPa. Reduced-Activation Ferrite/Martensitie (RAFM) CLF-1 steel thick plates currently exhibit extensive applications in helium cooled ceramic breeder Test Blanket Module (TBM) building. This study conducted electron beam welding (EBW) of T-joints without base plates, and achieved the single-pass welding and double-sided forming welding of CLF-1 thick steel plates via the optimization of technological parameters. Moreover, the microstructure and mechanical properties of two welded joints were systematically investigated. The welds in both joints were well-formed with no defects, such as pores, incomplete fusion, and cracks. The weld zone (WZ) was mainly composed of tempered martensite laths. No obvious δ-ferrites appeared in the weld, and numerous MX carbides were dispersed in the martensite laths. The coarse-grain heat-affected zone (CG-HAZ) near the fusion zone mainly included slender batten-shaped martensites and a few carbides. The fine-grain heat-affected zone (FG-HAZ) near the base metal (BM) mainly consisted of dual-phase mixed structures of tempered sorbite and martensite. The tensile strengths of the two welded joints at room temperature were 621 MPa and 623 MPa, while at 550℃, they were 350 MPa and 363 MPa, respectively. The fracture positions of both joints were located in the BM. The microhardness of the WZ slightly exceeded that of the BM, while HAZ exhibited no apparent softening. The mean impact absorbed energy values of the welds in two joints were 296 J and 297 J, being much higher than that of the BM. Accordingly, the welded joints fully satisfied the service requirements.