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Reduced Activation Ferritic Martensitic Steel and Fabrication Technologies for the Indian Test Blanket Module in ITER

T. Jayakumar, M. D. Mathew, K. Laha, S. K. Albert, S. Saroja, E. Rajendra Kumar, C. V. S. Murthy, G. Padmanabham, G. Appa Rao, S. Narahari Prasad

Fusion Science and Technology / Volume 65 / Number 2 / March 2014 / Pages 171-185

Technical Paper / dx.doi.org/10.13182/FST13-690

India is one of the countries associated with the development and testing of test blanket modules (TBMs) in ITER. Accordingly, India has taken up development of 9Cr-W-Ta reduced activation ferritic martensitic (RAFM) steel, which is the structural material chosen for TBMs, together with the associated manufacturing technologies required for TBM fabrication. With the objective of developing an India-specific RAFM steel, four heats of RAFM steel with tungsten and tantalum contents varying in the ranges 1 to 2 wt% and 0.06 to 0.014 wt%, respectively, were melted. The steel was melted through vacuum induction melting and vacuum arc refining routes with strict control over the amounts of elements that induce radioactivity (Mo, Nb, B, Cu, Ni, Al, Co, and Ti) and the elements that promote embrittlement (S, P, As, Sb, Sn, Zr, and O). Extensive characterization of the microstructure and mechanical properties of the steel was carried out. The ductile-to-brittle transition temperature of the steel increased slightly with increasing tungsten and tantalum content. The tensile strength of the steel was found not to change significantly with increasing tungsten content; however, it decreased marginally with increasing tantalum content, with a consequent increase in ductility. The creep rupture strength of the steel at 823 K was found to increase significantly with increasing tungsten content, whereas it decreased with increasing tantalum content. The low-cycle fatigue life of the steel at 823 K was found to increase with increasing tungsten and tantalum content; however, extensive cyclic softening was exhibited when the tungsten content was >1.4 wt%. RAFM steel containing 1.4 wt% tungsten and 0.06 wt% tantalum was found to have a better combination of strength and toughness and is specified as Indian RAFM (INRAFM) steel. The joining technologies adopted for the fabrication of a TBM are hot isostatic pressing to produce the first wall, followed by gas tungsten arc (GTA), electron beam (EB), laser, and laser hybrid welding for joining the rest of the TBM. Welding techniques for joining RAFM steel have been developed and characterized. The properties of the GTA welds met the full specifications of the requirement and were comparable to the properties of the base metal. This consumable has also been used to carry out hybrid laser welding successfully. A procedure for using EB welding to join plates of thicknesses up to 12 mm has been developed. Impact tests conducted on EB welds showed that the toughness of the weld metal in the as-welded condition is comparable to that of the base metal. A box structure that simulates one of the components of a TBM has been fabricated using EB welding to demonstrate the applicability of the process to component fabrication. Laser welding of 6-mm-thick plates of RAFM steel has also been carried out successfully, and the properties of the weld joints have been found to be satisfactory. This paper discusses the development of INRAFM steel and its properties and the current status of the fabrication technologies being developed for fabrication of the Indian TBM to be tested in ITER.

 
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