Deformation and damage of martensitic steels tempered at high temperature: fatigue, creep and creep/fatigue

Martensitic steels are commonly used in power plants as circuit components. Research into these materials is also being carried out as part of the design of future nuclear reactors, whether fast-neutron sodium-cooled (Astrid) or gas-cooled, or the ITER (International Thermonuclear Experimental Reactor) fusion reactor. These reactors have different characteristics in terms of technological maturity and environmental friendliness. The components considered are often circuitry or steam generator elements. Operating temperatures range from 450 to 650°C. Higher operating temperatures enable higher energy efficiency. Depending on the type of power plant, the loads to be considered for structural design include fatigue, fatigue-relaxation (or fatigue-flow) and creep. Martensitic steels tempered to 9-12%Cr have certain advantages that may make them preferable to some of their competitors, such as austenitic stainless steels like AISI 316L. Indeed, they have a low coefficient of thermal expansion and, on the contrary, a high thermal conductivity, which can help reduce mechanical loads induced by thermal fatigue. They are also less prone to swelling than austenitic steels in the event of irradiation, and finally their price can be an advantage. Their mechanical behavior under fatigue and creep conditions has therefore been extensively studied in order to guarantee their use.

A noticeable softening is observed during fatigue and/or creep tests at high temperatures (450-700°C), which can constitute a weakness, with, for example, greatly increased creep speeds. Research has been carried out on a large number of increasingly sophisticated grades. The aim of these new grades is to limit deformation rates and microstructural changes during high-temperature deformation. Numerous laboratories around the world are involved in this research, whether in academia, research institutes or industry. In the 1980s, work focused mainly on high-amplitude fatigue and creep loading, enabling results to be obtained from short-duration tests. A noticeable cyclic softening and creep acceleration were observed, linked to various microstructural evolutions operating during loading (coarsening of subgrain size, decrease in dislocation density, precipitation). Very early on, mechanical behavior was compared with microstructural observations, mainly by Transmission Electron Microscopy (TEM).

Work then focused on improved grades, more resistant to creep deformation, and on the study of low-amplitude fatigue and/or creep loads, which are much closer to in-service conditions than the high-amplitude loads previously studied. At the same time, structural design is based on codes (RCC-MRx in France, R5 in Great Britain, ASME in the USA) whose laws and criteria are based on test results and can be refined thanks...