Liquid metal embrittlement of T91 martensitic steel evidenced by small punch test

In this work, the sensitivity of liquid metal embrittlement of the T91 martensitic steel is investigated with the small punch test (SPT). The material was studied in three tempering conditions (as quenched, tempered at 500 and 750 °C), at 300 °C in air and in the liquid lead bismuth eutectic (LBE). The load–displacement curves (four stages, low maximum force and large displacement to fracture) obtained for one test condition of the 750 °C tempered material is in general very different from those of the two other materials. An effect of LBE has been observed for the as quenched and 500 °C tempered steels. For these materials, the curves tend to be linear with a reduced displacement to fracture suggesting a brittle behavior. This ductile to brittle transition induced by liquid metal has been confirmed from the fracture surface analysis where cleavage was observed. In comparison with conventional tensile tests, small punch tests appear to be more sensitive to evidence liquid metal embrittlement.     

Influence of the Pb-Bi hydrodynamics on the corrosion of T91 martensitic steel and pure iron

The Pb-Bi eutectic liquid alloy is considered as spallation target material in hybrid systems due to its suitable nuclear and physical properties. One of the parameters which may have a significant influence on the corrosion of steels in contact with molten lead alloys is the hydrodynamic regime. Corrosion tests have been performed in the CICLAD device at 400 and 470 °C at low oxygen concentrations and for various cylinder rotating speeds with T91 martensitic steel. The results obtained show that increasing the rotating speed leads to an increase of the corrosion rate. Moreover, the need for controlling finely the Pb-Bi physico-chemistry as well as the surface state of the samples is also shown by these tests. Finally, a comparison is made between the experimental corrosion rates and calculated values obtained by using equations expressing the mass transfer coefficient.     

Bending tests on T91 steel in Pb-Bi eutectic, Bi and Pb-Li eutectic

The influence of specific liquid metals on the behaviour of the ferritic/martensitic steel T91 are investigated to understand better the processes taking place at the metal surface. Of special interest is particularly if there is a penetration of selected elements out of the melt along grain boundaries. Metallurgical investigation, SEM and EDX analyses analysis were performed on bent T91 specimens exposed to static LBE, Bi and Pb-17Li at 300 °C for 1000 h. Steel T91 was used in the standard and specially annealed conditions. It is shown that the heat treatment of the steel has a strong influence on the corrosion resistance. Additionally the strain affected on the component is responsible for the occurrence of LME.     

Effect of contact conditions on embrittlement of T91 steel by lead-bismuth

The T91 martensitic steel is a candidate structural material for the liquid lead-bismuth eutectic (LBE) MEGAPIE spallation target. This paper first reviews some results on Liquid Metal Embrittlement (LME) of martensitic steels by liquid metals. It appears that LME of steels can occur provided a few criteria are fulfilled simultaneously. Intimate contact between liquid metal and solid metal is the first one. Usually, it is impossible to avoid the oxide film formation on the steel surface even after short exposure to air. This explains the difficulty arising when one would like to determine the susceptibility to LME of T91 steel whilst put into contact with lead-bismuth. Later, we report on different methods of surface preparation in order to remove the oxide layer on the T91 steel (PVD, soft soldering fluxes) and the resulting susceptibility to LME.     

Heat treatment effect of T91 martensitic steel on liquid metal embrittlement

The sensitivity of liquid metal embrittlement of the T91 martensitic steel is investigated with small punch tests at 300 °C in air and in lead bismuth eutectic (LBE). The material was studied in six tempering conditions corresponding to different values of hardness. An effect of LBE has been observed for all the materials excepted for T91 steel tempered at 750 °C, the more ductile material. In high strength materials (T91 steel as quenched, tempered at 600 °C or 500 °C), a ductile to brittle transition is induced by liquid metal, confirmed by the observation of brittle fracture. In relative high strength materials (tempered at 650 °C and 700 °C), LBE promotes a decrease in mechanical properties and a reduction of the ductility of materials, with a mixed ductile and brittle fracture.     

Investigation of LBE embrittlement effects on the fracture properties of T91

The susceptibility to liquid metal embrittlement (LME) of the T91 steel was studied by performing 3-point bending tests in liquid lead-bismuth eutectic (LBE), and for comparison, in argon (Ar) atmosphere as well. The specimens of T91 with different heat treatments were tested to access the hardening effect on the fracture toughness of the steel after exposure in LBE. The results showed that the fracture toughness of steel was reduced by contacting with LBE. The susceptibility of T91 to LBE embrittlement increased with the hardening of the steel introduced by heat treatments.     

Liquid metal embrittlement of an austenitic 316L type and a ferritic-martensitic T91 type steel by mercury

The susceptibility to liquid metal embrittlement (LME) of 316L and T91 steels by mercury has been studied at room temperature. A dedicated experimental device using center crack tension (CCT) specimens was built. We developed a specimen preparation procedure that must be rigorously applied in order to investigate the embrittling effect of Hg. The high strength ferritic-martensitic steel of type T91 is embrittled by Hg at room temperature over a large range of crosshead speeds, between 6.67 × 10⁻⁷ and 6.67 × 10⁻³ m s⁻¹. More surprisingly, the austenitic steel of type 316L is also embrittled by Hg between 1.67 × 10⁻⁸ and 2.5 × 10⁻⁴ m s⁻¹. The fracture of the T91 and 316L CCT specimens in contact with Hg occurs by shear band decohesion over the above-mentioned range of crosshead speeds.     

Influence of Zn as a spallation product on the behaviour of martensitic steel T91 and austenitic steel 316L in liquid Pb-Bi

The liquid Pb-Bi alloy is proposed as material for the spallation target in hybrid systems. During the spallation process, several chemical elements are produced in the target which could generate specific liquid metal embrittlement phenomena. Among these species, zinc is known as an element which can promote LME (liquid metal embrittlement). Corrosion tests were carried out in liquid Pb-Bi in isothermal static conditions without and with 80 wppm of zinc at 150 °C, 350 °C and 600 °C up to 6000 h. No modification of the corrosion kinetics of T91 martensitic and 316L austenitic steels was observed for either unstressed or U-bend specimens with zinc in Pb-Bi. Moreover, no sign of embrittlement was observed for any of the samples with and without zinc.     

Creep-to-rupture tests of T91 steel in flowing Pb-Bi eutectic melt at 550 °C

Uniaxial creep-to-rupture tests were performed on T91 in air and in flowing lead-bismuth eutectic melts. Compared to specimens tested in air, the specimens tested in liquid-metal show: (i) strain and strain rate increase up to a factor of about 50 (strain rate); (ii) time-to-rupture decrease; (iii) rapid transition into the third creep stage at high stress (above 180 MPa). The analysis of the test results revealed several important surface phenomena, which lead to different behavior of the specimens tested in lead-bismuth eutectic melts compared to those tested in air. Under high stress, and therefore high strain, the crack propagation process is mostly controlled by the reduction of the surface energy due to Pb and Bi adsorption on the steel surface. Under low stress (140 and 160 MPa) and low strain, this process is delayed due to the competing mechanism of healing the oxide scale cracks.     

Liquid metal embrittlement of T91 and 316L steels by heavy liquid metals: A fracture mechanics assessment

LME of the martensitic T91 and the austenitic 316L steels have been investigated in the CCT geometry in the plane-stress condition. Using such a geometry, premature cracking induced by a liquid metal (PbBi and Hg) can be studied using a fracture mechanics approach based on CTOD, J-Δa and fracture assessment diagram. One is able to measure a reduction of the crack tip blunting and a reduction of the energy required for crack propagation induced by the liquid metal. In spite of some limitations, this qualitative evaluation shows that liquid metals do not induce strong embrittlement on steels in plane-stress condition. Rather, the effect of the liquid metal seems to promote a fracture mode by plastic collapse linked with strain localization. It indicates that the materials, in spite of a potential embrittlement, should still be acceptable in terms of safety criteria.     

Liquid metal embrittlement susceptibility of ferritic-martensitic steel in liquid lead alloys

The susceptibility of the ferritic-martensitic steels T91 and EUROFER97 to liquid metal embrittlement (LME) in lead alloys has been examined under various conditions. T91, which is currently the most promising candidate material for the high temperature components of the future accelerator driven system (ADS) was tested in liquid lead bismuth eutectic (LBE), whereas the reduced activation steel, EUROFER97 which is under consideration to be the structural steel for fusion reactors was tested in liquid lead lithium eutectic. These steels, similar in microstructure and mechanical properties in the unirradiated condition were tested for their susceptibility to LME as function of temperature (150-450 °C) and strain rate (1 × 10⁻³-1 × 10⁻⁶ s⁻¹). Also, the influence of pre-exposure and surface stress concentrators was evaluated for both steels in, respectively, liquid PbBi and PbLi environment. To assess the LME effect, results of the tests in liquid metal environment are compared with tests in air or inert gas environment. Although both unirradiated and irradiated smooth ferritic-martensitic steels do not show any or little deterioration of mechanical properties in liquid lead alloy environment compared to their mechanical properties in gas as function of temperature and strain rate, pre-exposure or the presence of surface stress concentrators does lead to a significant decrease in total elongation for certain test conditions depending on the type of liquid metal environment. The results are discussed in terms of wetting enhanced by liquid metal corrosion or crack initiation processes.     

Effects of pre-irradiation treatments and minor composition modification on neutron irradiation embrittlement of 316 stainless steel

The effects of the combination of heavy cold work and low temperature aging (873–1073 K, 100 h), and minor composition modification on the irradiation embrittlement of 316 stainless steel were investigated. The samples were irradiated by JMTR at JAERI to the dose level of 2.5 × 10²⁴ n/m²$\text{(E > 1 MeV)}$ at 823 and 923 K and tensile tested between R.T. and 1023 K. The embrittlement was compared from the standpoint of ductility survival ratio. The lowering of carbon content caused severer high temperature helium embrittlement in the solution treated condition. The heavy cold work and low temperature aging treatments could not improve the high temperature embrittlement compared with the solution treated condition. Titanium addition was beneficial especially for the reduction of the irradiation temperature sensitivity to the high temperature ductility.     

Experimental studies on steel corrosion in Pb-Bi with steam injection

Experimental studies on steel corrosion were performed in simulated PBWFR (Pb-Bi cooled direct contact boiling water fast reactor) coolant environment. Some candidate steels of high Cr contents were immersed in steam-injected liquid Pb-Bi pool to investigate how their Cr contents and oxygen potential in Pb-Bi or (P_H2/P_H2O) in the steam influence their corrosion behaviors at temperature range of the reactor operation. Test specimens were made from eight types of steel with Cr contents ranged from 8 to 18%. The experiments were conducted by exposing these specimens to Pb-Bi pool where steam was injected. (P_H2/P_H2O) ratios of the steam were employed as experimental parameter, ranged from < 3×10⁻⁷ to 1×10⁻⁵ to control oxygen potential of Pb-Bi. Exposure temperatures were studied of 400, 450 and 500°C. It was found that 12Cr steel (HCM12/HCM12A) was the most resistant to corrosion and therefore a candidate reactor material.     

Role of oxidation on LME of T91 steel studied by small punch test

A new technique for LME studies has been designed that makes use of the SPT coupled with a XPS/Auger spectroscopy analysis. The interface between the material (T91 steel) and the liquid metal (PbBi) can be varied to investigate the interplay between the oxide nature or the thickness and crack initiation induced by a liquid metal. It is shown in this work that LME can occur in some conditions on pre-oxidized surfaces indicating that there are other interfacial conditions than the oxide free intimate contact that could be detrimental to materials in contact with a liquid metal.     

Variation of martensite phase transformation mechanism in minor-stressed T91 ferritic steel

The effects of compressive stress applied at different temperatures on martensite transformation process of the T91 steel were studied by high-resolution differential dilatometer. The stress applied above 850 °C exhibits no influence on the martensite formation. The stress applied below 850 °C not only facilitates the formation of martensite, but also enhances the onset temperature of the martensite transformation. There are two different transformation mechanisms occurring: when the compressive stress is applied at high temperature, the mechanism of strain-induced martensite transformation takes place, as a result, the microstructure tends to be refined with irregular grain boundary. When the compressive stress is applied at low temperature, the stress-induced martensite transformation occurs, and its morphology is similar to that of thermal-activated martensite. In addition, it is summarized that 200 MPa is the critical stress and 440 °C is critical temperature for the onset of the stress-induced martensite transformation for the investigated T91 ferritic steel.     

Liquid metal embrittlement of the martensitic steel 91: influence of the chemical composition of the liquid metal.: Experiments and electronic structure calculations

In previous works [Scripta Mater. 43 (2000) 997; J. Nucl. Mater. 296 (2001) 256], we showed that the martensitic steel 91 is prone to liquid metal embrittlement (LME) by liquid lead provided that some metallurgical conditions are fulfilled. In this work, we report results of LME of the steel 91 in contact with Pb-Bi and other low melting temperature metals such as Sn and Hg. Our experimental results can be interpreted within the framework of the surface energy reduction models for LME. To account for the experimental observations, we performed electronic structure calculations to assess the chemical interaction between low melting temperature metal atoms and iron surfaces. Our results allow to establish a simple criterion that can give trends on the embrittlement power of a liquid metal in contact with iron.     

Neutron embrittlement surveillance of the garigliano reactor vessel steel

The effects of neutron radiation on the pressure vessel of the Garigliano Nuclear Power Station have been analyzed on the basis of results of a reactor vessel material surveillance program of the plant. A high radiation embrittlement sensitivity was determined for the weld metal and for the A336 forging steel of the ring forging course just above the level of the fuel core. Both showed high copper and phosphorus contents, which accounted for the embrittlement sensitivity. The ring forging opposite the fuel core had a low copper and phosphorus content and revealed relatively low embrittlement. A neutron fluence of 6.3 × 10¹⁹n/cm² > 1 MeV was determined for the peak flux plane for 40yr of operation. However, the 40yr fluence for the ring forging at the top of the core level (3.5 × 10¹⁹n/cm² > 1 MeV) resulted in the highest final transition temperature because of the sensitivity of this steel. The measured Charpy-V shelf energy absorption values were plotted against yield stress values for comparable irradiations on the ratio analysis diagram (RAD). The analysis revealed that the pressure vessel steel properties would continue to degrade toward a condition of possible frangibility at the end of its life. This projection is based on an assumption of uniform embrittlement throughout the vessel wall thickness. Such uniformity does not exist; in fact, a sharp gradient exists in the steel such that ductility rises rapidly in the steel toward the outside wall as well as above and below the fuel core. Hence, because of this strong ductility gradient, the Garigliano reactor vessel should be able to operate safely over its intended design lifetime.