The Development of Clean Steels for Steam Turbine Applications: Their Demand and Use

The failures of discs on large steam turbines that occurred in the 1960s presented the electric power generation industry with a formidable problem. These fractures were intergranular and often appeared to initiate in the keyway of the disc as a result of steam condensation and subsequent corrosion. The steels that were used to make these discs were usually of the Cr-Mo type and had the required room temperature and elevated temperature strength. However, in some cases they were also found to be brittle at or even above room temperature. This brittleness was a result of temper embrittlement which in turn was caused by impurity segregation to grain boundaries. Early results also suggested that this impurity segregation might enhance the stress corrosion process as well. Consequently, great advances were made in steelmaking to provide steels of extremely high purity. While these steels did become available, several factors limited their use. These included their cost, the fact that the overall purity of conventional steels improved through appropriate scrap selection and other processing improvements, better control of steam chemistry, designs that eliminated the keyways, and improved plant practice. This chapter reviews the history of this problem, the development of the clean steels, and the ultimate combination of factors that were brought together to solve this problem. In considering such a complex problem, it is useful to think in terms of technology interaction spheres, that is, interactions between technologies brought about by many individuals, that help solve such a problem.