13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Creep-Fatigue Interactions in P91 Steel Magdalena Speicher1,*, Andreas Klenk1, Kent Coleman2 1 Materialpruefungsanstalt Universitaet Stuttgart, Stuttgart 70569, Germany 2 Electric Power Research Institute, Charlotte NC 28262, USA * Corresponding author: magdalena.speicher@mpa.uni-stuttgart.de Abstract To determine and quantify the influence of creep-fatigue interactions on the crack behaviour of P91 steel creep-fatigue crack growth (CFCG) tests were carried out with holding times up to 60 minutes and compared to results from fatigue crack growth (FCG) and creep crack growth (CCG) tests. It was found that the introduction of a holding time influences the crack initiation time but not the crack propagation. With holding times of 60 minutes, the difference between CCG and CFCG tests is very small. To characterise the fracture mode metallographic analyses were performed on selected specimens after CFCG tests. The crack path is inter-granular and an area of high cavity density was identified around the crack tip in all specimens. By comparing CFCG, FCG and CCG results, it can be finally noticed that specimens with a holding time of 6 and 60 min show a similar crack growth rate as samples under creep loading. Keywords martensitic steel, creep-fatigue, crack initiation, crack growth 1. Introduction In components operated at high temperature, there is always some amount of interaction between fatigue, creep and the environment. This is typically the case in power plants where steam generating service start-ups or load changes produce changes in steam conditions and transient temperatures. In the last decades, it has been proven that in the creep-fatigue range, the lifetime of components is significantly shorter, particularly when hold periods are introduced between strain cycles. These interactions can be enhanced due to environmental effects such as for instance internal or surface oxidation. They are finally more pronounced for some materials, can be neglected for others. In this work, the crack behaviour of the steel P91 was investigated in creep-fatigue tests. The aim of this analysis is to gain information about how the crack behaviour of this steel is influenced by creep-fatigue-environment interactions at high temperature. 2. Experimental Details 2.1. Material The base material P91 (X10CrMoVNb9-1) being under investigation, comes from a thick pipe with outer diameter of 492 mm and wall thickness of 85 mm. Its chemical composition is given in Table 1. These values are in accordance with the EN10216 requirements [1]. The P91 pipe was used in [2] for the investigation of similar welds under creep conditions. A classical Post Weld Heat Treatment (PWHT) was therefore used to relax the stresses that were induced by the welding process. Table 1. Chemical composition of P91 used in this work Chemical composition in mass (%) C Si Mn P S Cr Mo Ni Al Nb V Min 0,08 0,20 0,30 - - 8,0 0,85 - - 0,06 0,18 [1] Max 0,12 0,50 0,60 0,02 0,010 9,5 1,05 0,40 0,04 0,10 0,25 this work Meas. 0,104 0,27 0,46 0,014 0,003 8,17 0,9 0,16 0,02 0,064 0,194
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