Ambient, elevated temperature tensile properties and origin of strengthening in friction stir welded 6 mm thick reduced activation ferritic-martensitic steel plates in as-welded and post-weld normalised conditions

Abstract

Reduced Activation Ferritic-Martensitic (RAFM) steels are currently being considered for test blanket modules of International Thermonuclear Experimental Reactor. This study is aimed at understanding the microstructure and mechanical properties of Indian RAFM steel during friction stir welding (FSW) of 6 mm thick plates. The full penetration bead-on-plate welds were fabricated employing PcBN tool at a rotational speed of 200 rpm. The FSW joint consisted of stir zone, thermomechanically affected zone and BM. Microstructure, hardness and tensile properties were evaluated in as-received (base metal, BM), as-welded and post weld normalised + tempered states. The as-received microstructure was composed of Cr-rich M23C6 on prior austenite grain and tempered lath martensite boundaries with intra-lath V-and Ta -rich monocarbides. Substantial changes occurred during welding leading to destruction and dissolution of M23C6 and precipitation of Fe3C in SZ. The microstructure was restored to the BM level by giving a post weld normalising and tempering treatment. Comparative study on tensile properties has been carried out at room and elevated temperatures (up to 550 °C) in all the three states and the observed variations have been explained on the basis of initial microstructure and evolving substructures. The as-welded samples showed higher strength and low percentage elongation values. The ductility was minimum in all the states at an intermediate temperature and ascribed to the dynamic strain ageing. All the states revealed falling strength with increasing temperature up to 550 °C. Failure occurred in AW and PWNT states at the interface between TMAZ and BM. Irrespective of material state and tensile test temperature, the transgranular ductile fracture prevailed. The contribution of the different strengthening mechanisms to the yield strength of the SZ region has been estimated theoretically and it matches with the experimental results. The influence of low angle grain boundaries (LAGB) and high angle grain boundaries (HAGB) on the tensile behaviour of the SZ was well explained using the Electron Back Scattered Diffraction (EBSD) maps of these regions.