Compatibility of ferritic-martensitic steel T91 welds with liquid lead-bismuth eutectic: Comparison between TIG and EB welds

The 9 wt.% chromium ferritic-martensitic steel T91 is being considered as candidate structural material for a future experimental accelerator driven system (XT-ADS). This material and its welded connections would need to be used in contact with liquid lead-bismuth eutectic (LBE), under high irradiation doses. Both unirradiated tungsten inert gas (TIG) and electron beam (EB) welds of T91 have been examined by means of metallography, scanning electron microscopy (SEM-EDX), transmission electron microscopy (TEM), Vickers hardness measurements and tensile testing in both gas and liquid lead-bismuth environment. The TIG weld was commercially produced and post weld heat treated by a certified welding company while the post weld heat treatment of the experimental EB weld was optimized in terms of the Vickers hardness profile across the welded joint. The mechanical properties of the T91 TIG and EB welds in contact with LBE have been examined using slow strain rate tensile testing (SSRT) in LBE at 350 °C. All welds showed good mechanical behaviour in gas environment but total elongation was strongly reduced due to liquid metal embrittlement (LME) when tested in liquid lead-bismuth eutectic environment. The reduction in total elongation due to LME was larger for the commercially TIG welded joint than for the EB welded joint.     

Qualification of P91 welds through Small Punch creep testing

The deployment of the martensitic alloy P91 in future nuclear plant components operating in the creep regime depends very much on the performance of joints, found occasionally susceptible to failure in fossil fuel fired plant A completely novel approach is proposed to assess the integrity of P91 weldments which relies on the determination of the creep properties of the different zones of the weldment on a micro-scale based on the Small Punch (SP) testing methodology. Results are presented from the Small Punch creep tests carried out exactly following a CEN Code of Practice for the various constituent zones of the weldment and the potential weaknesses are identified within the heat affected zone. In addition, the SP test is applied to determine the fracture properties of the weld compared to the parent metal, showing the expected shift related to the test method in the ductile brittle transition temperature and the inferior performance of the weld metal.     

Improved quality of heavy steels and their weilds as related to the integrity of reactor pressure components

The improvements of heavy structural steels and their welds for reactor pressure components (RCPs) of LWRs achieved in the past decade are reviewed. These include the steady improvement of the quality of the following specific properties of heavy structural steels for RPCs: uniformity and isotropy of mechanical properties, including less mass effect in the mid-section; fracture toughness; internal defects; weldability; degradation by neutron irradiation. Further, requirements of the easier execution of inspections such as: fewer weld seams in RPCs; larger and more integral design of component materials; change of cast steels to forged steels. As regards main welds, the experience of underclad cracking (UCC) and high levels of Cu content in the weld of some RPCs were two of the incentives for reviewing the welding techniques for RPCs. This has led to a change of SA508, C1.2 steel to C1.3 (equivalent designation of steel, too) and a change of welding wire coating from Cu to Al or Cr. Recent progress was the application of narrow gap welding processes and control of welding residual stresses. UCCs of two different origins were basically solved by the improvement of base materials and proper welding sequences. The inspectability of RPCs was further secured by the improved internal quality of steels and welds.     

State of experience in the application of the steel grade 15 MnNi 6 3 for containments and components of nuclear power stations

Steel grade TSB 370/15 MnNi 6 3 presented first in 1976 as material for the erection of nuclear safety containments has been accepted very quickly in practice. The material of which yield point and high tensile values are in the range of the well-known steel StE 36, shows high toughness properties. This is valid either for the unaffected base material as well as for material conditions produced by fabrication as for instance in the HAZ formed by welding and after stress-relief annealing. There is no sign to expect crack formation in the HAZ after stress-relieving also in case of unfavourable process stress conditions. By means of wide plate tests with weld seams showing artificially introduced defects, the high safety in brittle fracture of this material in the range of the application temperatures could be shown. The concept of transition temperature (RT-NDT concept) as specified in the ASME rules and the German rules for materials for the construction of nuclear power stations can be met with great safety by application of material TSB 370/15 MnNi 6 3. The favourable properties of the base material and its good behaviour during fabrication have led to the fact that steel grade TSB 370/15 MnNi 6 3 has a constant place in the election of material for nuclear power station components.     

Compatibility of martensitic/austenitic steel welds with liquid lead bismuth eutectic environment

The high-chromium ferritic/martensitic steel T91 and the austenitic stainless steel 316L are to be used in contact with liquid lead-bismuth eutectic (LBE), under high irradiation doses. Both tungsten inert gas (TIG) and electron beam (EB) T91/316L welds have been examined by means of metallography, scanning electron microscopy (SEM-EDX), Vickers hardness measurements and tensile testing both in inert gas and in LBE. Although the T91/316L TIG weld has very good mechanical properties when tested in air, its properties decline sharply when tested in LBE. This degradation in mechanical properties is attributed to the liquid metal embrittlement of the 309 buttering used in TIG welding of T91/316L welds. In contrast to mixed T91/316L TIG welding, the mixed T91/316L EB weld was performed without buttering. The mechanical behaviour of the T91/316L EB weld was very good in air after post weld heat treatment but deteriorated when tested in LBE.     

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.     

Stress corrosion cracking in the heat-affected zone of A508 steel welds under high-temperature water

The influence of heat input on stress corrosion cracking (SCC) in the heat-affected zone (HAZ) of A508 steel welds was investigated. Constant extension rate tensile tests were conducted on notched round-bar specimens in simulated reactor coolant conditions to assess SCC performance. In multi-pass welds, the use of a low heat input resulted in a better SCC resistance than that of a high heat input due to the existence of a more refined microstructure.     

Current in-service inspections of austenitic stainless steel and dissimilar metal welds in light water nuclear power plants

A tendency towards growing requirements for the inspection of austenitic piping can be observed in several countries. In Germany the revised KTA rule demands the UT inspection of austenitic and dissimilar metal welds in piping with diameters of 200 mm or more.On the basis of experience gained from austenitic piping with integranular stress corrosion cracking (IGSCC), longitudinal waves and mode conversion techniques are used. Depending on the geometry, material and grain orientation, spurious signals can be observed which require additional evaluation or analysis measurements.A promising new technique is based on horizontally polarized shear (SH) waves generated by electromagnetic acoustic transducers (EMATs). Investigations in the laboratory and field inspections showed that SH waves are well suited for the detection of longitudinal flaws, especially where the weld can be examined from one side only.For the complete solution of a given inspection problem SH waves can be combined with well-known standard techniques in order to provide redundant information for the characterization and sizing of indications.The investigation of possibilities of SH waves showed that the problem of cast austenitic steel inspection might not be solved using this technique. However, measurements using low frequency UT transducers showed promising results.     

Probabilistic assessment of creep crack growth rate for Gr. 91 steel

This paper focuses on a probabilistic assessment of creep crack growth rate (CCGR) for Gr. 91 steel which is regarded as one of major structural materials of Gen-IV reactors. A series of creep creak growth (CCG) data was obtained from the CCG tests under various applied loads at 600 °C. Using the experimental CCG data, four methods such as a least square fitting method (LSFM), mean value method (MVM), probabilistic distribution method (PDM), and the Monte Carlo method (MCM) were used to determine the parameters B and q for a power law equation between CCGR and C^* integral. The commonly used LSFM revealed a considerable difference in the CCGR lines compared with the MVM and PDM. The PDM was found to be more useful than the LSFM, because it can assess the CCGR lines from the probabilistic viewpoints. It was verified that the two parameters B and q followed a lognormal distribution well. From the lognormal distribution, a number of random variables for the B and q parameters were successfully generated by the Monte Carlo Simulation (MCS) technique. The CCGR lines for the 10% and 90% probabilities were predicted by the PDM and MCM, and the MCM result was compared with the PDM one.     

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.     

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.     

Probing the deterioration of 316L stainless steel welds due to ageing and creep by indentation creep tests

Authors have probed into the creep behaviour of AISI 316L stainless steel welds through the indentation creep test methodology and assessed the deterioration effects of these welds under different ageing conditions subjecting them to different test conditions. Comparison is made between the parent metal and the weld metal for integrity at different levels of ageing and test loads. It is concluded that although the aged weld's deteriorated status is not revealed at low temperature and low load test conditions, it is explicitly revealed when tested at higher temperature and higher loads. Microstructural evidences have been given by the authors and they have suggested mechanisms of creep at different test conditions.     

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.     

Laser cutting of thick steel plates and simulated steel components using a 30 kW fiber laser

Laser cutting of thick steel plates and simulated steel components using a 30 kW fiber laser was studied for application to nuclear decommissioning. Successful cutting of carbon steel and stainless steel plates up to 300 mm in thickness was demonstrated, as was that of thick steel components such as simulated reactor vessel walls, a large pipe, and a gate valve. The results indicate that laser cutting applied to nuclear decommissioning is a promising technology.     

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.     

Investigation of the ductile fracture properties of type 304 stainless steel plate, welds, and 4-inch pipe

J-integral fracture toughness tests were performed on welded 304 stainless steel 2-inch plate and 4-inch diameter pipe. The 2-inch plate was welded using a hot-wire automatic gas tungsten arc process. This weldment was machined into 1T and 2T compact specimens for single specimen unloading compliance J-integral tests. The specimens were cut to measure the fracure toughness of the base metal, weld metal and the heat affected zone (HAZ). The tests were performed at 550°F, 300°F and room temperature. The results of the J-integral tests indicate that the J_Ic of the base plate ranged from 4400 to 6100 in lbs/in² at 550°F. The J_Ic values for the tests performed at 300°F and room temperature were beyond the measurement capacity of the specimens and appear to indicate that J_Ic was greater than 8000 in lb/in². The J-integral tests performed on the weld metal specimens indicate that the J_Ic values ranged from 930 to 2150 in lbs/in² at 550°F. The J_Ic values of the weld metal specimens tested at 300°F and room temperature were 2300 and 3000 in lbs/in² respectively. One HAZ specimen was tested at 550°F and found to have a J_Ic value of 2980 in lbs/in² which indicates that the HAZ is an average of the base metal and weld metal thoughness. These test results indicate that there is a significant reduction in the initiation fracture toughness as a result of welding.The second phase of this task dealt with the fracture toughness testing of 4-inch diameter 304 stainless steel pipes containing a gas tungsten arc weld. The pipes were tested at 550°F in four point bending. Three tests were performed, two with a through wall flaw growing circumferentially and the third pipe had a part through radial flaw in combination with the circumferential flaw. These tests were performed using unloading compliance and d.c. potential drop crack length estimate methods. The results of these test indicate that the presence of a complex crack (radial and circumferential) reduces in the initiation toughness and the tearing modulus of the pipe material compared to a pipe with only a circumferentially growing crack.     

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.     

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.     

Mechanical behaviour of the T91 martensitic steel under monotonic and cyclic loadings in liquid metals

The paper deals with the mechanical properties in liquid metals of the T91 martensitic steel, a candidate material for the window of an accelerating driven system (ADS). Two main questions are examined, the risk of liquid metal embrittlement and the accelerated fatigue damage by a liquid metal. It is found that the transition from ductile to brittle behaviour induced by a liquid metal is possible as a result of a decrease in surface energy caused by the adsorbed liquid metal. The embrittlement can occur only with a hard microstructure and a nucleation of very sharp defects inside the liquid metal. Under cycling straining, the fatigue resistance of the standard T91 steel is decreased by a factor of about 2 in the liquid metal as compared to air. It is proposed that short crack growth is promoted by the liquid metal which weakens the microstructural grain boundary barriers and skip the microcrack coalescence stage.