Component test of nickel-based alloys and their welded joints under long-term creep loading

Abstract

The aim of the paper is to investigate experimentally the creep behavior of improved materials and welded joints at component scale under near-to-service operating conditions, which are promising for the application in highly efficient and flexible future power plants.It focusses on the creep behavior under multiaxial loading of similar and dissimilar welded joints of high temperature resistant nickel-based alloys (A740H, A617B and HR6W) in the temperature range between 650 °C and 750 °C by means of thick-walled-component tests. In this case, the temperature, the external axial load and the internal pressure remain constant during testing.Numerical simulations are used for the estimation of the component axial load in order to induce a meaningful damage evolution without causing a total component failure. The metallographic investigation on the welds A740H/A740H and A617B/A740H indicated no test-related damage. Small cracks in circumferential direction were found on the welds HR6W/HR6W and HR6W/A617B after having reaching 2% local creep strain. The damage mechanism could be classified as a typical creep damage with carbides formation in the grain boundaries and thus grain boundary separations occur, which ultimately lead to the crack formation.In general, the FEM calculation underestimates the axial creep strain on the investigated welds, especially the initial creep strain evolution is not accurately predicted. However, the long-term creep strain evolution could be well predicted by FEM. In this case, the material parameters for the creep law should be further optimized in order to minimize the underestimation.