Microstructural and mechanical property changes in HK40 reformer tubes after long term use

The microstructural change in the steam reformer tubes which had been used for approximately 60,000–90,000 h in a hydrogen manufacturing plant was investigated and their effect on the remaining creep-rupture life was studied by performing lifetime prediction using Larson–Miller Curves. In addition solutionization treatment at 1100–1200 °C was attempted on the used tubes in order to study the possibilities of extending the remaining lifetime and of improving the mechanical properties of the used tubes. The results showed that the remaining lifetime of the HK40 reformer tubes was largely dependent upon the precipitation of σ phase under the current operating conditions of 32–34 kgf/cm2 pressure at 800–1000 °C, showing extensive degradation of the mechanical properties. Observed was no major improvement in the creep-rupture properties of the used tubes treated by solutionizing treatment at 1100–1200 °C, but the tensile properties, especially elongation and impact resistance at room temperature, however were enhanced extensively. Also the σ phase was decomposed by the treatment, indicating that the materials reliability against the embrittlement at room temperature could be improved in a great deal.