ABSTRACT

The service pressure and temperature of advanced power plants components increased significantly in the last 20 years, from sub-critical (540°C 25MPa) to ultra-super-critical (620°C-28MPa) operating conditions. Consequently more severe requirements on strength, corrosion resistance and creep properties were imposed on high temperature steels. To comply with these requirements the 9%Cr martensitic steels were developed in the year ’80-‘90 and now the ASTM Grades 91, 911 and 92 are currently used in USC plants. The initial evaluation of their creep strength was defined with relatively short term tests with time-temperature-parameter methods (i.e. Larson-Miller equation) giving an overestimation of the long-term creep properties: but the long experience in service, long term creep laboratory tests and the assessment test methodologies showed that the original estimation of creep strength values were not reliable and a reduction of the creep resistance occurred at long service time. For instance the Grade 92 creep strength (NF616), initially evaluated in 600°C/160MPa/100,000h, recently was reduced down to 113MPa (ECCC(*) assessment, 2005). Moreover some premature failures of Japanese Grade P122 took place and similar problems appeared on other 12%Cr steels. The reduction of creep behaviours in 9-12%Cr steels at long times is a consequence of the microstructural evolution during high temperature service: coarsening of M23C6 carbides and the nucleation of Laves-phase. The Laves-phase precipitation generate a depletion of Mo and W with consequent reduction of their contribution to the overall creep resistance. Otherwise the increased volume fraction of secondary phases produces an initial strengthening effect, but if the coarsening rate is not under control, the mean diameter of these particle quickly reaches micrometric dimensions with a detrimental effect on creep behaviour. In 9%Cr steels also Z-phase was recognized after long term exposure, but in smaller amount than on 12%Cr steels: consequently no dramatic drop in volume fraction of MX was observed, therefore Z-phase does not affect significantly the long term creep properties of 9%Cr Grades.