Microstructural Evolution During Long Term Creep Tests of 9%Cr Steel Grades

In the last two decades the service pressure and temperature of components for advanced power plants increased significantly and more severe requirements on strength, corrosion resistance and creep properties were imposed on high temperature steels. To comply with these requirements, several new 9–12%Cr martensitic steels were developed and some of them, such as ASTM Grades 91, 911 and 92 are currently used in new high efficiency Ultra Super Critical power plants. The initial evaluation of their creep strength above 550°C was defined with relatively short term tests, but the long experience in service and long term creep laboratory tests showed that the original estimation of creep strength values were not reliable and a reduction of the creep resistance occurred at long service time. Short creep tests (elaborated with time-temperature-parameter methods, i.e. Larson Miller equation) usually give an over-estimation of the long-term creep properties of 9%Cr steels. The results of the creep assessments of Grade 92 (Japanese NF616) are an example of the significant lowering of the creep properties: the creep resistance of this grade was initially evaluated in 600°C/160MPa/105h by means extrapolation of short creep tests, within 103 hours; recently the creep strength was reduced down to 113MPa (ECCC assessment, 2005). Moreover some premature failures of Japanese Grade PI 22 took place and similar problems appeared on other 12%Cr steels. The lowering of creep strength in 9–12%Cr steels at long times is a consequence of the evolution of their microstructure during high temperature service. The causes of this phenomenon in Grades 91, 911 and 92 are examined in this article, paying special attention to the metallurgical explanation. The most evident changes in the microstructure of 9%Cr steels occur with the nucleation of Laves-phase as well as the nucleation of Z-phase at longer times. The precipitation of Laves phase has two relevant aspects by the creep strength point of view. On one hand, high amounts of Mo and W contents are incorporated in this phase, causing a depletion of these elements from the solid solution and thus a reduction of their contribution to the overall creep resistance. On the other hand, the increased volume fraction of secondary phases leads to a higher precipitation strengthening during the first precipitation phase: at the beginning, the precipitation of fine Laves phase increases the creep resistance; however if the coarsening rate is not taken under control, the mean diameter of these particles reaches micrometric dimensions with a detrimental effect on creep behaviour within 103 hours in the range 600°C–650°C. The high coarsening rate of Laves phase is therefore the major cause of the lowering of creep properties of Grades 91, 911 and 92. Coarsening of Laves phase particles over a critical size triggers the cavity formation and the consequent brittle intergranular fracture. Transition from ductile fracture to brittle intergranular fracture often occurs in long-term creep at the onset of coarsening of Laves particles, which result to be the preferential site for cavities nucleation in the 9%Cr steels. Z-phase was recognized in 9%Cr steels after long term exposure, but in far smaller amount than on 12%Cr steels: no dramatic drop in volume fraction of MX was observed in association to the nucleation of this phase, therefore it is believed that the modified Z-phase does not affect significantly the long term creep properties of Grades 91, 911 and 92. The dimple pattern is typical of ductile fracture, which occurs for short service period (hence highest stress). At low stresses, cavities are formed at the triple grain junctions at which Laves particles are often found, causing wedge crack, otherwise isolated cavities can form independently at coarse Laves phase particles (Figure 13). The latter type is often observed after long-term creep in the interganular fracture region. In both cases, brittle fracture occurs at the onset of coarsening of Laves particles, which result to be the preferential site for cavities nucleation in the 9%Cr steels.