Estimation of residual stresses developed in SS 304L / Zr-2.5%Nb joints

Abstract

Joints between stainless steel (SS) and zirconium based alloys have various applications in nuclear technology [1]. Since mechanical integrity of these joints under reactor operating conditions for the reactor life time viz. 40-50 years is vital, the qualification criterion for these joints should be very critical. The large difference in thermal coefficient of expansion of Zr-based alloys and SS leads to generation of residual stresses at the joint interface, which can have deteriorating effect on the bond integrity and mechanical properties of the joint [2]. Therefore, a proper estimation of these stresses is essential for its qualification. One of the accepted ways of mitigating such high residual stresses is by using suitable ductile interlayers between the joining materials. In the present work, the magnitude, nature and distribution of the residual stresses have been estimated using finite element method (FEM). Simulations were performed using axisymmetric model to evaluate the residual stress field generated while cooling SS304L/Zr-2.5%Nb joint from 1273 K to ambient temperature. The stress fields were calculated assuming elasto-plastic behaviour of the joining materials. The SS part of the joint was found to experience a maximum radial tensile stress of about 260 MPa, and a maximum shear stress of 55 MPa at the SS/Zr-2.5%Nb interfacial region. The maximum compressive stress in axial direction was found to be 92 MPa. Similar simulations were also performed by introducing Ni and Ti interlayers of thickness 200 μm each between SS304L and Zr-2.5%Nb. The maximum radial tensile stress on the SS side was found to be 243 MPa and a maximum shear stress of about 74 MPa near SS/Ni interface, while the maximum compressive stress in axial direction was 55 MPa. Therefore, it can be seen that a reduction in residual stress levels can be achieved upon incorporation of Ti and Ni interlayers.