High Temperature Thermal-Elastic Analysis of Dissimilar Metal Transition Joints

Abstract

Thermal-elastic analyses of dissimilar metal transition weld joints of a 24″ sodium piping system were performed. This piping system is for Liquid Metal Fast Breeder Reactor application, operating at elevated temperature (965°F). These analyses form a basis for the selection of the material combination and weld preparation of the transition joints. Two material combinations were selected for weld joint thermal-elastic analysis: 2 1/4 Cr - 1 Mo ferritic steel to 316 stainless steel, and 2 1/4 Cr - 1 Mo steel to Incoloy 800, with Inconel 82 as the welding metal in both cases. Weld preparations with various geometries were assumed for each material combination. The transition joints were evaluated for thermal loadings due to the changes in sodium temperatures during anticipated operating conditions of the breeder reactor. Thermal analyses were performed to define the temperature time history in the metals; the temperature gradient across the wall thickness; and especially, the temperature distribution near the material interfaces. The magnitude of the temperature gradients and the temperature distribution as affected by the heat transfer characteristics of each material were of particular interest. Stresses created due to the differences of thermal expansion of the materials, radial and axial temperature gradients, and applied internal pressure were evaluated using finite element analysis methods. In this investigation, the materials were treated as elastic and isotropic. The contributions from the applied pressure and thermal loading were separated from the total stresses and the most important contributor was identified. The elastic analyses served for a preliminary evaluation of the transition joint selection. Based on the information obtained in stress versus material combination and the stress variation as a function of the geometry of the weld preparation, a weld design (material combination and geometry) was selected.

Austenitic-ferritic transition joint cracking under thermal fatigue loading

“…In order to minimize the construction cost of these plants, austenitic stainless steels were chosen as the appropriate material to operate successfully in elevated temperature without oxidation problems, only in the parts with the greatest thermal strain and temperature. Low-alloy ferritic steels were used in any other parts of the fossil-fuel power plants [1][2][3][4][5]. This use of two materials within one system necessitated a transition joint.…”