Differential thermal expansion characteristics of a dissimilar fusion joint in the electromagnet of diverse safety rod drive mechanism (DSRDM)

The electromagnet of Diverse Safety Rod Drive Mechanism (DSRDM) of Prototype First Breeder Reactor contains a dissimilar joint between ASTM A848-87 Alloy-1 (magnetic soft iron alloy) and Inconel-82. The lattice thermal expansion behaviour of the constituents of this dissimilar fusion joint has been investigated by high temperature X-Ray Diffraction, as the mismatch in thermal expansivity may give rise to thermal stress at the weld interface. It was revealed that alloy-l does not experience any significant thermal expansion. But in the Inconel side of the weld experiences certain thermal expansion, which is less than that expected for pure Inconel-82. These two facts namely, a very low thermal expansion on the part of alloy-l, and the reduced thermal expansivity of Inconel in weldment, imply that thermal expansivity mismatch is not a serious issue in the DSRDM electromagnet. The microstructural characterisation performed on aged samples demonstrate that, a major compositional restructuring is taking place at the weld interface, which address the reduced thermal expansivity of Inconel phase in the weldment. (author)