T91 Steel Research Articles

An Electrochemical Study of the Corrosion Behaviour of T91 Steel in Molten Nitrates

A study of the corrosion behaviour of T91 steel in molten 60 wt% NaNO3-40%KNO3 has been carried out at 300, 400 and 500 °C during 1000 h. Employed techniques included potentiodynamic polarization tests, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) measurements. Experiments were complemented by detailed scanning electronic measurements and X-ray diffraction studies. Polarization curves revealed the existence of a passive layer formed onto the steel, composed mainly of Cr2O3, FeCr2O4, NaCrO4 and K2Fe2O4. Corrosion current density values increased, whereas the polarization resistance value decreased more than one order of magnitude as the testing temperature increased. EIS tests indicated a charge transfer controlled corrosion process, regardless of the testing temperature, and that the double electrochemical layer resistance decreased with the temperature.

Molecular dynamics, thermodynamics and experimental studies on the corrosion mechanism of T92 and TP347H steels in high-pressure CO2 and H2O at 600 °C

The corrosion mechanisms of T92 and TP347H heat-resistance steels in high-temperature and high-pressure (HTHP) CO2 and H2O environments were investigated by molecular dynamics, thermodynamics and experiments. In the initial corrosion stage, the combination ability of Cr with O and C was stronger than that of Fe with O and C, which caused oxygen and carbon absorption capacity of TP347H steel was higher than that of T92 steel. In HTHP H2O environment, the Fe3O4 oxides on T92 steel were coarsened and pulverized, which made the Fe3O4 oxides become loose. The volatile CrO2(OH)2 were formed on the surface of TP347H steel, which caused the coarsening of Cr2O3 oxides.

Influence of Martensite–Austenite Constituents and Precipitates on Reheat Cracking Susceptibility of Coarse-Grained Heat-Affected Zone in T23 and Modified T23 Steels

Influence of Martensite–Austenite Constituents and Precipitates on Reheat Cracking Susceptibility of Coarse-Grained Heat-Affected Zone in T23 and Modified T23 Steels

Oxidation behavior of the T91 ferritic/martensitic steel and 304 austenitic stainless steel in supercritical water

PurposeThis study aims to report the oxidation behaviors of the T91 ferritic/martensitic steel (T91 steel) and 304 austenitic stainless steel (304 steel) in supercritical water (SCW) at 600°C.Design/methodology/approachThe microstructure, elemental distribution and phase structure of the oxidation layers derived from the corrosion of the T91 steel and 304 steel were analyzed by scanning electron microscopy, Oxford Instrument X-ray spectroscopy, electron scattered diffraction and transmission electron microscopy.FindingsThe oxidation layers on the T91 steel and 304 steel have duplex structure. The two steels all suffer internal oxidation, and the phase of the internal oxidation layers are indexed as Fe-Cr spinel, although their morphologies are different. The formation of a continuous Cr-rich layer is not detected because of the relatively low Cr content of the steels, which is attributed to the corrosion property.Originality/valueThe accelerated corrosion and corrosion mechanism of the T91 steel and 304 steel with low Cr occurring in SCW at 600°C was clarified.

Enhanced Corrosion−Resistance of AlTiCrFeMoSi High−Entropy Alloy Coating by Magnetron Sputtering

The amorphous AlTiCrFeMoSi high entropy alloy (HEA) coating with high hardness (11.88 GPa) is successfully deposited on T91 substrate by the magnetron sputtering method. Both T91 steel and as−deposited AlTiCrFeMoSi coating samples are exposed to a static liquid lead−bismuth eutectic (LBE) at 550 °C for up to 2000 h. The coating exhibits excellent corrosion resistance against lead−bismuth eutectic (LBE) compared with the uncoated T91 steel. The results show that the AlTiCrFeMoSi HEA coating has great potential in LBE−cooled fast reactor application.

Improving the Surface Performance of Discarded AISI T1 Steel by Cathodic Reduction and Thermal Diffusion-Based Boriding

Improving the Surface Performance of Discarded AISI T1 Steel by Cathodic Reduction and Thermal Diffusion-Based Boriding

Magnetic pulse welding of thin walled ferritic martensitic steel T91 tube to T91 rod

Magnetic pulse welding of thin walled ferritic martensitic steel T91 tube to T91 rod

Mechanism of accelerating creep rupture under high stress level of pack cementation aluminide coatings deposited onto T92 steel tube

An aluminide coating was deposited on commercial T92 steel by pack cementation. The coating exhibits a typical bilayer structure composed of an outer FeAl layer and an inner interdiffusion layer. The creep rupture life of the coated and uncoated T92 was investigated. Results show that the rupture life of the coated T92 decreases significantly, i.e., decreases by 46 % at the creep stress of 110 MPa compared with the uncoated T92. By examining the microstructure and fractography of the coated specimen, the mechanism of aluminide coating deteriorating creep rupture life is clarified. The main fracture mode of T92 substrate with and without coating is microvoid accumulation fracture. The characteristic microstructure of the aluminide coating is responsible for the premature creep rupture. The formation of columnar grains and brittle nature of FeAl phase make it tend to crack during creep. The phase transformation and needle-like AlN precipitates in the inner interdiffusion layer contribute to the inward propagation of cracks and accumulation of creep damage.

Research on the Fracture Behavior and Microstructure of T91 Steel at Ultrahigh Creep Temperatures

The creep behavior of T91 steel at ultrahigh creep temperatures (650–750 °C) at 100 MPa was investigated using small-size specimens, and the related microstructures were characterized. The results showed that the creep failure susceptibility of T91 steel increases significantly as the creep temperature increases from 650 °C to 675 °C. The creep life rapidly decreased from 7880 h to 224 h, and the fracture surface shrinkage increased from 61.9% to 86.5%. Thermodynamic calculations and microstructure observations showed that the increase in creep temperature has a significant effect on the morphology of M23C6-type carbides, which were distributed around grain boundaries, significantly pinning and hindering the movement of dislocations and the grain boundaries. As the creep temperature increases, the subcrystalline and M23C6-type carbides significantly coarsen, which is the main reason for the rapidly shortening creep life of T91 steel. This research provides guidance for the material design and microstructure regulation of T91 steel in service at ultrahigh environmental temperatures.

Effect of silicon on ductility of T91 steel in liquid lead–bismuth eutectic environment

Effect of silicon on ductility of T91 steel in liquid lead–bismuth eutectic environment

Estimation of stress redistribution for the extraction of power law creep parameters from bending

A numerical method is developed to calculate stress distribution and its evolution throughout the beam during power-law creep deformation. A constitutive law containing primary and secondary terms is considered and strain hardening approach of stress redistribution is used. Time for stress redistribution to saturate is defined in terms of a parameter, stress saturation time (SST), and its dependence on loading and material properties is studied for ferritic steels P91 and T22 using the model. It is observed that SST has a power law dependence on the applied moment and it can be as high as 3300 h at 0.33σYS (σYS is yield strength) for P91 steel leading to misinterpreted results at low loads if strain rate is measured much earlier. In contrast, at high loads, as shown for T22 steel, the loading must be redesigned from a continuous ramp to step-loading in order to ensure that the maximum stress of the extreme locations do not exceed yield strength at any instant. Therefore, testing methodologies are developed using the model to aid in the efficient design of the testing procedure for a broadened testing range with a meaningful interpretation of the test results.

Point Defect Model for the kinetics of oxide film growth on the surface of T91 steel in contact with lead–bismuth eutectic

Point Defect Model (PDM) has been successfully employed for aqueous solutions to describe the rate of surface corrosion of structural materials. In this work, the adaptation of PDM for the case of lead–bismuth coolant is developed. The necessary modifications in the set of electrochemical reactions and in the model of electrostatic potential distribution in the oxide film are made. Parameters of the proposed model, PDM_PB, are optimized using the experimental data for T91 grade steel. The obtained adaptation of PDM shows applicability to the case of lead and lead–bismuth coolants which is of critical importance for the next-generation fast-neutron reactors.

Microstructural Evolution of T91 in High Temperature Conditions and under Combined Effect of Stress and Temperature

ASTM A213 T91 steel is widely used in power plants and petrochemical industry for long-term service components. Due to its high resistance to creep, thermomechanical fatigue and corrosion, the use of grade 91 steel allows usual plant service parameters to be raised up to ultra-supercritical conditions (600 °C, 300 bar) so that performances are remarkably increased. The strongest factors that affect performances are the time of exposure, the temperature and the applied stress: such parameters can dramatically decrease the service life of a plant component. The improved mechanical properties of grade 91 are strictly related to its specific microstructure: a tempered martensite matrix with fine precipitates embedded in. Two typologies of secondary phases are present: M23C6 carbides (where M = Cr/Fe/Mo/Mn) and finely dispersed MX-type carbonitrides (where M = V/Nb and X = C/N). This study is focused on the microstructure evolution of grade 91 steel under creep conditions. First, three sets of laboratory-aged specimens heated in oven at 550 °C, 600 °C and 650 °C were examined; the exposure time was up to 50,000 h. Furthermore, the influence of stress on the microstructure in two sets of samples was evaluated: a first batch of specimens cut from an ex-service tube of a petrochemical plant (over 100,000 h of service at 580 °C and 19–25 bar) and a second set of samples coming from another ex-service tube under ultra-supercritical conditions (605 °C, 252 bar) in a power plant. The microstructures were characterized by optical, scanning electron and transmission electron microscopy and the results were compared to the literature. Some interesting trends were evidenced, in terms of secondary phases precipitation and coarsening, as well as martensite recovery. Furthermore, the applied stress seems to influence size and number of Laves phase particles.

Effect of temperature on low cycle fatigue behavior of T91 steel in liquid lead-bismuth eutectic environment at 150–550 °C

The low cycle fatigue behavior of T91 steel in a lead–bismuth eutectic and an air environment was investigated at 150–550 °C. The temperature dependent “fatigue endurance peak and trough” was found in liquid LBE. Quasi-cleavage cracking was observed on the fracture surface at 150 °C and 350 °C, which indicates the occurrence of liquid metal embrittlement (LME). The plasticity recovery reduced LME susceptibility of T91 steel at 250 °C and 550 °C. The effects of LME, liquid LBE corrosion and dynamic strain aging on the mechanisms of fatigue crack initiation and propagation are discussed.

Performance of the new ferritic/martensitic steel SIMP against liquid lead‐bismuth eutectic corrosion: Comparison with T91 and 316L steels

AbstractThe degradation of structural materials caused by the corrosion of liquid lead‐bismuth eutectic (LBE), the coolant of lead‐cooled fast reactors (LFRs), has become one of the limiting factors for the development of LFRs. In this study, LFR structural candidates SIMP and T91 ferritic/martensitic steels and 316L austenitic stainless steel were selected to conduct corrosion experiments at different durations (120–1218 h) in oxygen‐saturated liquid LBE at 550°C. It was found that the austenitic stainless steel 316L has the best resistance to LBE corrosion due to its high Cr content, which allows dense Fe–Cr spinel with high Cr content to be generated in oxygen‐containing LBE, slowing down the oxidation rate. However, a certain degree of dissolution corrosion occurred in the early stages of corrosion in 316L steel, which disappeared with the increase of time. The ferritic/martensitic steel SIMP has better oxidation resistance than T91 due to its higher Si content, which results in a more compact inner oxide layer of SIMP steel, thus impeding the diffusion of Fe and O ions more effectively.

Discontinuous oxidation in wet air of T91 with a novel Al2O3-forming NiCrAl nanocomposite coating in as-deposited and pre-oxidized states

The 9 wt% Cr containing steel T91 was deposited with a Ni-16Cr-14Al nanocomposite coating by combining electrophoretic deposition and electrodeposition. After 500 h oxidation at 800 °C in 10 % H2O containing air, the bare T91 steel grew a triple-layered oxide scale consisting of an outer MnCr2O4 layer, an intermediate Cr2O3 layer and an inner SiO2 layer. However, the nanocomposite-coated T91 grew an oxidation resistant oxide scale in the wet air, which was double-layered and consisted of outer NiAl2O4 and inner α-Al2O3. Pre-oxidation at 1000 °C in dry air further reduced the oxidation rate of the coated T91, due to the formation of an exclusive α-Al2O3 scale. The oxidation-resistant mechanism of the nanocomposite-coated steel without and with the pre-oxidation treatment was discussed.

Effect of Carbon on Stress-Relief Cracking Susceptibility of T23 Steel

The coarse-grained heat-affected zone (CGHAZ) samples were prepared from T23 steel with three different carbon contents on a thermal simulator, then isothermal slow strain rate tensile tests were carried out at 500° ~ 750°C, and the effect of carbon content on the stress-relief cracking (SRC) sensitivity of T23 steel was evaluated according to fracture ductility. The microstructure evolution of the CGHAZs in the process of SRC was observed by optical microscopy, structural equation modeling energy-dispersive spectroscopy, and transmission electron microscopy energy-dispersive spectroscopy/selected area electron diffraction to reveal the mechanism of SRC. Finally, the feasibility of adjusting carbon content to prevent SRC was discussed. The results showed the classical theory of intragranular precipitation hardening cannot explain the cause of SRC, and a new viewpoint was put forward that the precipitation of M23C6 at the grain boundary was the main reason for SRC. The precipitation of M23C6 carbide cannot only provide a preferred site for void nucleation but also leads to the depletion of alloy elements in the matrix near the grain boundary. The interaction of these two aspects leads to the direct weakening of grain boundaries. Moreover, the SRC of T23 steel could be inhibited when the carbon content was determined to be close to the ASME standard lower limit of 0.04 wt-%. In summary, reducing the carbon content can inhibit the SRC sensitivity of T23 steel, which is not due to reducing the relative weakening of the grain boundary by reducing the precipitation hardening in the grain interior but to inhibiting the direct weakening of the grain boundary by reducing the precipitation of M23C6 at the grain boundary.

Microstructural study of the thermal stability of a thermomechanically treated T91 steel by TEM in situ annealing

Thermal stability of a conventional 9Cr-1Mo ferritic/martensitic steel submitted to a thermomechanical treatment has been investigated by means of an in-situ annealing experiment up to 700 °C. For comparison, same experiment was carried out in the as received material, before thermomechanical treatment. The results showed an evolution of the microstructure in both cases, more acute for as received material. Before thermomechanical treatment, during annealing small precipitates within grain grew significantly, new ones precipitated and dislocations started to move around 600 °C, finding no effective obstacles to their movement and disappearing at grain boundaries and the free surface. After thermomechanical treatment, the microstructure developed was much more stable. Only a slight increase in the original precipitates size was observed and the dislocation structure remained stable after annealing. To compare the results of in-situ experiments, short term heat treatments up to 700 °C were carried out for both materials on bulk pieces. The principal difference observed was the dislocation density in the case of as received material. After in-situ experiment, the analyzed area was free of dislocations while after heat treatment a certain density of dislocations remained within grains. This fact can be related to the proximity of the free surface in the case of thin foils.

Microstructure and its effect on high temperature tensile properties of T92/HR3C dissimilar weld joints

A T92/HR3C dissimilar metal weld joint, which is widely used in the boiler components of ultra-supercritical power plants, was fabricated by manual tungsten inert gas welding with Inconel 82 as the filler metal. This study reports the high-temperature tensile properties, deformation behavior, and fracture mechanism of dissimilar weld joints. The results of tensile tests at 640 °C show a severe strength mismatch between the weld metal and the heat-affected zone of the T92 steel (T92-HAZ). The serrated flow behavior of the T92/HR3C weld joint at different tensile rates was observed during plastic deformation. Based on the results of transmission electron microscopy, dynamic recrystallization and re-precipitation of Cr23C6 carbides both likely triggered the serrated flow behavior and enhanced the tensile strength of the weld metal. Additionally, the inter-critical T92-HAZ was the weakest zone in the weld joint because of the dissolution of a large amount of carbides and the transformation of martensite laths into equiaxed subgrains.

Fatigue fracture mechanism of T92/HR3C dissimilar metal weld joints at elevated temperature

The T92/HR3C dissimilar metal weld joints were fabricated by manual tungsten inert gas (TIG) welding with Inconel 82 as the filler metal. After fatigue tests at 640 °C, it was found that the location of the fatigue fracture shifted from the weld metal (WM) to the heat-affected zone of the T92 steel (T92-HAZ) with a decrease in stress amplitudes (171–189 MPa to 155–166.5 MPa). The fatigue crack propagation behavior of the two fracture modes mentioned above was monitored by an in-situ scanning electron microscopy (SEM) imaging test. In addition, the results of the transmission electron microscopy (TEM) analysis showed that under high-stress amplitudes, the initiation of cracks at the weld metal was caused by the interaction between dislocations with Nb(C,N) in the matrix and M23C6 on the grain boundary. At long-term high temperature, the carbides precipitated in the weld metal played a role in strengthening the grain boundaries, so the matrix structure still maintained high strength and stability. However, the dislocation movement in the T92-HAZ accelerated the transformation of lath martensite into equiaxed subgrains, and the coarsening and aggregation of Cr-rich M23C6 carbides resulted in the initiation of cracks in this region. Therefore, as the stress amplitude decreased, the fatigue fracture location of the DMWJs shifted from the weld metal to T92-HAZ.