Lifetime prediction of 9–12%Cr martensitic steels subjected to creep–fatigue at high temperature
Section snippets
Introduction and literature review
The 9–12%Cr martensitic steels are under study for several components of the generation IV nuclear reactors and fusion reactors. In these future applications, in addition to long holding periods (typically 1 month), cyclic loadings corresponding to start and stop-operations and maintenance must also be taken into account. Creep–fatigue lifetime must therefore be considered to design the corresponding components. As creep–fatigue tests with such long holding periods cannot be carried out in
Materials and experimental procedures
Three different steels are tested in the present study. Firstly a commercial martensitic P92 steel produced by Vallourec Mannesmann. Secondly two laboratory martensitic steels reinforced with either 0.007% of boron (VY2 steel) or 0.2% of titanium (Ti1) steel (to improve their long term creep strength) were produced by Aubert & Duval. Both materials were austenitized at a temperature larger than 1150 °C for 30 min and tempered at 720 °C for 10 h. Fig. 2 presents optical observations of P91, P92,
Cyclic behaviour
Martensitic steels are known to experience cyclic softening effect [8], [7], [28], [29], [30] due to the coarsening of laths and sub-grains. On the one hand the cyclic behaviour of P92 is very similar to that of P91, except that the amount of plastic strain applied per cycle is slightly lower. On the other hand, VY2 and Ti1 experience a faster and even more pronounced cyclic softening effect as illustrated in Fig. 3. This explains why the plastic strain range per cycle experienced by VY2 and
Conclusions
The present study reported the fatigue and creep–fatigue lifetimes measured on P92, VY2, and Ti1 9%Cr steels at 550 °C. They were compared to those of P91. P92 presents a very similar fatigue strength than P91, whereas VY2 and Ti1 are both less resistant to cyclic loadings.
The samples observations revealed that the damage mechanisms identified for P91 remain valid for these three steels. The only difference lies in the higher density of cracks observed after pure fatigue loading on VY2 and Ti1.
Acknowledgements
Direction of the Nuclear Energy of the CEA is acknowledged for financial support through the DDIN/RSP project. The authors are also grateful to E. Cini and J. Gabrel from Vallourec for fruitful discussions and material supply.
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