Abstract
Gas turbine parts defining the flow-path of the turbine section are normally made of high-grade material due to its severe operating conditions. Transition duct which provides aerodynamic coupling between high-pressure and low-pressure turbine modules define the flow-path and thus are made of high-grade material fabrication. This part is typically subjected to huge thermal and mechanical loads requiring a thorough design assessment to verify the fulfillment with respect to the product requirement. Careful consideration of various design requirements in selection of the material is very vital in keeping the cost of the gas turbine under control while achieving the durability requirements. These parts defining the flow-path are designed in such a way that the thermal induced stress is within the material capability. To achieve this requirement, the part is restrained in such a way that it remains at the desired position during the operation. Despite this consideration, thermal induced stress is unavoidable due the internal constraints wherein one side is exposed to the flue-gas and the other side is exposed to the cooling medium to keep the part’s operating temperature within the material capability. This causes the thermal gradient across the part cross-section resulting in thermal induced stress. Adding to this, the fabrication requires material selection from wrought and cast form to minimize the cost while fulfilling the product requirements. Material capability of these two forms of the same material possess marginally differing material characteristics. Due to these dissimilar material characteristics and the presence of weld material is expected to result in differential thermal expansion and thus significant thermal induced stress at the interface. The stress induced thus poses a limitation and scope of material separation under the extended operation of the gas turbine. Sample case studies using finite element analysis are performed to understand the creep behavior at the interface before application into the finite element analysis of the design under consideration. All the probable combinations of materials and numerical simulations are verified in this case study for a better understanding. The understanding from this case study is applied into the finite element analysis of the transition duct with all the probable material combinations and studied its creep behavior. This structured approach facilitated in completing the design evaluation within the project schedule and keep the cost within the target. This paper is intended to describe the steps followed in studying the creep behavior of the gas turbine transition duct made in dissimilar material.
