Creep Behaviour of Thick-Wall Alloy 617 Seamless Pipes for 700 °C Power Plant Technology

The need for higher steam temperatures in the future high efficiency coal-based power plants (Advanced Ultra Super Critical (A-USC) power plants) has modified the customers’ materials requirements. At the steam temperatures up to 730 °C Ni-base alloys are needed for the construction of the critical components capable of operating under these more severe conditions. Additionally, the very high pressures lead to very thick steam piping connected to the steam turbine. These are very challenging tasks for both, the manufacturing technology and the material performance assurance, as the currently available data are mainly for thin walled components of gas turbines and the chemical industry. Vallourec & Mannesmann Tubes is since 1999 involved in several German and European projects concerning thick-walled Alloy 617 pipes fo 700 °C power plant technology. The seamless pipes, manufactured using Ehrhardt press and Pilger process, were su ject of extensive microstructural and mechanical investigations, including creep testing, as a part of qualification program. The material is tested in two conditions, as-delivered solution-annealed and after stabilization heat treatment. Creep testing programs for both conditions up to now include 108 testing specimens and a testing time period of 825 000 h in total. The results and creep performance, which is in excellent agreement with the existing results for Alloy 617, are going to be presented and discussed.