No abstract

No abstract

No abstract

The application of new heat resistant steels in power plants requires reliable long term creep rupture strength values as basis for design. Modern martensitic 9% Cr-steels have complex microstructures that change with service exposure. That is why extrapolations of long term strength properties will be most difficult. Due to new long term test results, re-assessments became necessary for grades 911 and 92. Different methods have been used. Good agreement was obtained between a graphical and the numerical ISO 6303 method. In both cases a two-step assessment procedure was used. First the raw data was prepared in a suitable way, which was followed by mathematical averaging procedures. For comparison a Larson-Miller analysis on the raw data was performed, too. The results turned out to be too optimistic at temperatures higher than 575°C (1050°F). It is shown that a suitable preparation of data can improve the Larson-Miller assessment. As a result of the new assessments the design values had to be reduced for both grades. With respect to previous assessments the new values are up to almost 10% lower. In the case of grade 92 the difference from the former ASME values are even higher. Consequences concerning design and service operation are discussed.

Improvement of boiler efficiency is reached by increasing the pressure and temperature of newly designed boilers. Recently developed steel grades such as T/P91 and T/P92 are used in the advanced power plants thanks to their high creep rupture strength resistance which enables maintaining acceptable thickness of the tubes and pipes. Nevertheless their operating temperature range is limited by their oxidation behavior which is lower than classical 12%Cr steels or austenitic steels. For these reasons we have developed a new steel grade which combines good creep resistance and high steam-side oxidation behavior. This new steel, based on chromium content of 12% and with other elements such as cobalt, tungsten and boron, is named VM12. Manufacturing of this grade has been proved by production of several laboratory and industrial heats and rolling of tubes and pipes in several dimensions by different rolling processes. In addition to base metal property investigations — including creep tests and high temperature oxidation behavior — welding, cold bending and hot induction bending qualifications took place. This paper summarizes the results of the investigations and presents the first findings for processing.