Crack growth kinetic and fracture toughness of EUROFER steel in presence of hydrogen at room and higher temperature

This report summarizes the results of investigations of crack growth kinetic and fracture toughness on EUROFER at room and higher temperature in presence of hydrogen that were carried out in the frame of EFDA projects. Available literature data are summarized regarding the effect of hydrogen on fracture mechanical properties and crack growth of ferritic-martensitic steels for components of the HCLL fusion reactor prototype. Hydrogen will influence fracture/mechanical properties of martensitic steel particularly during RT testing. The J0.2 integral values will decrease depending on the hydrogen content in the range of 2 to 4 wppm. The hydrogen content of about 4 wppm decreases fracture toughness both of base and weld metals to the level of 30 KJm-2 or below, i.e. approx. by 90 % of original values. Hydrogen containing more than 2 wppm manifests itself significantly only in base metal. The corresponding degradation of fracture toughness in weld metal is insignificant and corresponds to the failure mechanism, involving, in case of base metal brittle fracture and in case of weld metal ductile dimple fracture only. At the 120 deg C testing the hydrogen effect manifests itself in base metal specimens only. The J0.2 value decreases by about 30 % in base metal with hydrogen containing about 1.6 wppm, while in weld metal with hydrogen containing up to about 1.3 wppm it does not change. The mechanism of hydrogen embrittlement has been inter- and trans-granular fracture controlled by the hydrogen content, testing temperature and type of material. Brittle fracture manifests itself in base and weld metal specimens with higher content of hydrogen of about 4 wppm tested at room temperature. At lower hydrogen content of about 2 wppm the base metal specimen breaks by mixed inter-granular and ductile mechanism while the weld metal specimens exhibit only ductile and dimple fracture. The following mechanism is probably operating: (i) The same hydrogen content in weld metal and base metal manifests itself by lower hydrogen partial pressure in trapping sites of weld metal as a consequence of a higher number of microstructural defects in comparison to base metals. (ii) The same hydrogen embrittlement for both metals requires higher hydrogen content in weld metal. This enables achievement of the same hydrogen partial pressure in weld trapping sites as is required for brittle fracture and consequently the same damage in base metals. (iii) Different values of hydrogen partial pressure can therefore impact the presence of both inter-granular and trans-granular damage and ductile and dimple fracture in specimens at which hydrogen critical content was not attained in all trapping sites. The crack growth testing did not show any hydrogen embrittlement effect on crack initiation and growth under the long-term exposure at room temperature and 250 deg C. Both hydrogen charged and uncharged specimens of base and weld metals have shown identical characteristics of stress-strain diagrams, which corresponds to the ductile and dimple fracture mechanism. The occurrence of brittle fracture at RT has not been affected by hydrogen contents of up to 1.7 wppm and 2.2 wppm in basic metal and weld metal, respectively. Similarly, the hydrogen contents of 1.3 wppm and 1.7 wppm in base metal and weld metal, respectively, have not manifested themselves during the 250 deg C testing. (author)