T91 Steel Research Articles

Study on Type IV Cracking Behavior of T92/Super304H Dissimilar Steel Welded Joints in Long-Term Service

The microstructure and residual mechanical properties of several groups of T92/Super304H dissimilar steel welded joints (hereinafter referred to as welded joints) in service for 70,000~85,000 h were analyzed. The results show that the early service history of the welded joint results in the polygonization of the martensite lath and the coarsening of the precipitated phase on the side of T92 steel. In the further creep process, the cavities nucleate along the precipitated phase interface and the triple junction grain boundary. Under the same load, the creep life of the joint decreases rapidly with the increase in service time and, finally, type IV cracking occurs. Type IV cracking needs to meet two conditions: 1. a large degree of precipitated phase coarsening and cavity nucleation in the fine grain heat-affected zone (FGHAZ) and 2. a much lower loading stress than the yield strength.

Corrosion analysis of 316L and T91 steels exposed to liquid lead-bismuth eutectic using X-ray CT

Corrosion analysis of 316L and T91 steels exposed to liquid lead-bismuth eutectic using X-ray CT

Failure behavior analysis of oxide film formed on super-heater tubes of boiler during deep peak process

High-temperature components in the supercritical power plant experienced alternating thermal stresses under deep peak regulation, accelerating the cracking and spalling of the oxide film formed on the steam-side of super-heaters and re-heaters. This study employed the corrosion testing and finite element simulations to investigate the failure behavior of oxide films on super-heaters made by heat-resistant steelsT91 and austenitic HR3C during deep peak conditions. The experimental results indicate that the oxide film on T91was a double layer film containing Fe3O4 and Fe-Cr oxides, which was much thicker than that on HR3C. The spalling of the outer oxide film on T91 was more severe than that of HR3C. The analysis of thermal stress-strain under various loads and oxide film thicknesses revealed the significant shear stress between Fe3O4 and Fe-Cr within the oxide film on T91 steel. The significant shear stress caused the bigger strain within the oxide film on T91 steel, causing the spalling of the oxide film. Compared to the strain within the oxide film on T91 steel, the stress-strain within Cr2O3 oxide film on HR3C steel was notably lower than that on T91 steel. The failure behaviors of the oxide film on T91 and HR3C were quantitatively explained through experimental result and stress-strain simulation with the oxide film. The failure models of the oxide film on investigated steels were put forward.

Assessment of the impact behavior of CLF-1 steel with Charpy V-notch testing and miniature Charpy V-notch testing

China Low-activation Ferrite steel (CLF-1), as one of the structural materials developed in China for fusion reactors, is a Reduced activation Ferritic/Martensitic (RAFM) steel modeled after the traditional T91 steel (9Cr-1Mo-0.2V-0.08Nb). For fusion reactor materials, the Charpy impact testing is an important method to assess their notch sensitivity. In this study, Charpy impact tests were conducted on CLF-1 steel using Charpy V-notch specimens (CVN) and Miniature Charpy V-notch specimens (Kleinstprobe, KLST) to assess the impact behavior of CLF-1 steel, and data normalization was performed on the KLST specimen. The results show that as the specimen size of CLF-1 steel decreased, the curves of impact absorbed energy, lateral expansion, and shear fracture appearance shifted towards lower temperatures, and the impact absorbed energy significantly decreased. Both the macroscopic and microscopic characteristics of the specimens obtained from the experiments indicated that KLST specimens could achieve the same fracture characteristics as CVN specimens. Additionally, at the same experimental temperature, KLST specimens exhibited a higher proportion of ductile fracture regions.

Material volume reduction for creep testing using composite cantilevers and its application for residual life assessment

Digital image correlation (DIC) and finite element analysis (FEA) demonstrate that creep deformation in bending occurs primarily in ≈30 % of the cantilever volume near the fixed end, especially when the creep stress exponent ranges from 5 to 7. As an alternative approach to minimize the material volume required for testing, the concept of fabricating composite cantilevers is proposed and validated in this study. A composite cantilever sample consists of an “active” creeping portion (e.g., T22 boiler steel) and an additively extended “passive” non-creeping portion (e.g., IN718). The volume reduction process involved varying the length of the “active” section, a, while keeping the total length of the cantilever, L, constant. The DIC measurements conducted at 600 °C to assess the creep behavior of T22 steel revealed that analytical expressions for monolithic cantilevers could aptly predict the constitutive steady-state creep laws from the composite cantilevers if measurements are made in a region at a critical distance away from the interface. FEA indicates that accurate stress estimation enables predicting monolithic creep behavior using composite samples with “a/L” ratios of as small as 5 %. Using the developed approach, the loss of creep resistance of T11 boiler steel that was in service for ∼ 240,000 h was ascertained in high throughput fashion using a composite cantilever having only 30 vol % of the “active” material. Guidelines to minimize the volume fraction of the “active” portion in the composite cantilever and the implications of the observations for estimating the residual life of in-service high-temperature components are discussed.

Creep-to-rupture of T91 steel in static liquid lead-bismuth eutectic: Effects of cyclic temperature and oxygen environment

The creep-to-rupture behavior of T91 steel in static liquid lead–bismuth eutectic (LBE) is investigated, focusing on the impacts of cyclic temperature and oxygen condition. The results indicate that thermal cycling, oxygen deficiency, and high applied stress levels significantly accelerate creep deformation and reduce the creep-to-rupture lifetime of T91 steel in LBE. Surface oxide scales on T91 steel and their self-healing mechanisms play a crucial role in enhancing the creep resistance by isolating the LBE contact and delaying crack initiation and propagation. However, the integrity of this oxide scale is compromised under cyclic thermal conditions and low oxygen levels, leading to premature failure. Microstructural examinations reveal the evolution of oxide scale damage and self-healing mechanisms. The findings suggest that oxide scale failure mechanisms should be considered when designing the long-term operational performance of advanced LBE-based reactors.

Enhancing corrosion resistance of T91 F/M steel in liquid lead-bismuth (LBE) by slurry FeAl coating

A FeAl coating was fabricated on T91 steel via slurry aluminizing. The corrosion behavior of coated and uncoated samples was assessed in static LBE (lead-bismuth eutectic) with two both high and low dissolved oxygen concentrations at 550 °C. The coating was mostly intact and exhibited great corrosion resistance compared to the uncoated specimens regardless of the oxygen concentration. That is because the coating can form a protective alumina film on the surface at extremely low dissolved oxygen level. This coating is of significant engineering value in enhancing the corrosion resistance of Fe base alloy in LBE.

Improving hot corrosion behaviour of Cr3C2-25NiCr coatings by reinforcing nano yttria stabilized zirconia at 850 °C

Yttria-Stabilized Zirconia (YSZ)-enhanced Cr3C2-25NiCr coatings were applied to T22 boiler tube steel using the High Velocity Oxy Fuel (HVOF) method. Conventional Ni-20Cr, 5 wt.% YSZ- Cr3C2-25NiCr, and 10 wt.% YSZ- Cr3C2-25NiCr composite coatings were prepared. High-temperature corrosion tests were conducted on both uncoated and coated samples in a Na2SO4-60%V2O5 environment at 850°C under fluctuating thermal conditions. These experiments were carried out in a silicon tube furnace at high temperatures, with each sample subjected to 50 cycles of exposure. Each cycle consisted of 1 hour in the corrosive environment followed by 20 minutes of cooling at room temperature. Corrosion products were analyzed using energy-dispersive x-ray analysis (EDS) and scanning electron microscopy (SEM). The addition of YSZ to the Cr3C2-25NiCr coatings significantly improved corrosion resistance, with the Ni-20Cr, 5 wt.% YSZ-Ni-20Cr, and 10 wt.% Cr3C2-25NiCr coatings reducing overall weight gain by 73.08%, 84.70%, and 89.96%, respectively, compared to uncoated T22 steel.

New insights into oxidation and creep characteristics of T91 heat-resistant steel in martensitic and ferritic states

This study systematically compared the oxidation and creep characteristics of T91 steel in both martensitic and ferritic states. Findings indicate the formation of mixed oxides, including Fe3O4, (Cr, Mn)2O3, and MnCr2O4, during the oxidation process. In the ferritic specimens, the oxygen diffusion channel is restricted due to the transition of M23C6 carbide. Moreover, the ferritic steel exhibits poor creep life, and a strong α fibre texture was observed. For T91 steel, appropriate thermomechanical treatment can lead to uniform distribution of M23C6 carbides within the matrix, achieving the optimal precipitation strengthening effect and resulting in improved high-temperature properties.

Microstructural Evolution of Pre-oxidized T91 Steel During LBE Dissolution Corrosion

Microstructural Evolution of Pre-oxidized T91 Steel During LBE Dissolution Corrosion

LBE corrosion behavior of helium pre-irradiated martensitic steel

In the design of the spallation target of CiADS, T91 steel was selected for the most critical component, the beam window. A Si-modified martensitic steel (SIMP) has been currently developed, which has better corrosion resistance and is planning to replace T91 as a material of beam window in the future. In order to assess the effect of irradiation damage on the microstructure evolution and corrosion behavior of the SIMP, the pre-irradiated samples with irradiation doses from 5 × 1015 to 3 × 1017 He/cm2 exposed in static lead-bismuth eutectic (LBE) with saturated oxygen at 350 °C for 4000 h were investigated. Results show that pre-irradiation neither change the double-layer structure of the oxide layer nor significantly affect the corrosion rate. This may be due to the slow diffusion of elements at low temperatures, and the low irradiation dose not reaching the threshold for triggering accelerated corrosion.

Thermally-induced fracture in the oxide scale of T91 ferritic/martensitic steel after exposure to oxygen-saturated liquid lead–bismuth eutectic

Ensuring the continuity and stability of the oxide scale to separate T91 steel from lead-bismuth eutectic (LBE) is an effective method for limiting corrosion in the lead-based fast reactor system. In this study, we specifically investigated the impact of cooling thermal stress on the stability and damage of the oxide scales of T91 steel when exposed to oxygen-saturated liquid lead–bismuth eutectic at high temperatures. A corrosion test was conducted on T91 steels exposed to stagnant oxygen-saturated LBE at 500 °C. Experimental results indicated that oxide damage exists without any external force, including interface cracks and through-scale cracks perpendicular to the interface. We developed a three-layer peridynamic model of T91-(Fe,Cr)3O4-Fe3O4 to simulate the deformation and failure of oxides under a cooling process from high temperature to room temperature. The simulation results show that a temperature drop of about 200 °C from 500 °C can cause through-oxide cracks in the oxide scale, and subsequent cooling can lead to the propagation of interfacial cracks. Additional modifications were introduced in the model to account for the porosity of the oxide scale. Parametric investigations illustrate the effects of oxide scale thickness and porosity on the resulting crack patterns.

Effect of Grain Boundary Character Distribution on the Precipitation Behavior: A Comparative Study for 304 Steel and T91 Steel

Effect of Grain Boundary Character Distribution on the Precipitation Behavior: A Comparative Study for 304 Steel and T91 Steel

EFFECT OF SEVERE PLASTIC DEFORMATION AT HIGH TEMPERATURE ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF FERRITIC-MARTENSITIC STEEL T91

The microstructure and mechanical properties of ferritic-martensitic steel T91 after severe plastic deformation (SPD) and subsequent annealing (tempering) have been studied. The SPD of steel T91 was carried out by the method of multiple “upsetting-extrusion” at a temperature of 875 °C, subsequent annealing was carried out in the temperature range of 550...650 °C for 1…100 h. It has been found that the multiple “upsetting-extrusion” process allows the formation of an ultra-fine grained structure with an average grain size of 110 nm, which remains stable at the annealing temperatures of 550 and 600 °C for up to 100 h of exposure. After annealing at these temperatures, the microhardness remains at the level 3100 and 2780 MPa, respectively, which is noticeably higher than with standard N&T treatment of this steel (2480 MPa). The refining of grain and MX-type carbide precipitates, due to severe plastic deformation, also leads to an increase in strength characteristics during tensile tests at temperatures of 20 and 600 °C which are significantly higher than after standard N&T treatment.

Insights into fatigue crack propagation mechanism of T91 steel in liquid lead-bismuth eutectic at 150–450 °C

The fatigue crack growth (FCG) behaviors of T91 steel in oxygen-saturated liquid lead–bismuth eutectic (LBE) at 150–450 °C were investigated. The FCG rate increased gradually with the increase of temperature at 150–350 °C, is firstly low at 450 °C and then comparable with that at 350 °C. The macroscopic trans-granular cracks preferentially propagated along the deformation-induced low-angle grain boundaries (LAGBs) near the crack tip at the microscopic scale. Intergranular precipitation-enhanced wetting of matrix by liquid LBE results in the reduction of atomic bonding force and formation of micro-cracks along the LAGBs, which triggers brittle cracking.

Enhancing Corrosion Behavior of T91 Steel in 3.5% NaCl through Graphene Oxide Nanocomposite Coatings

Enhancing Corrosion Behavior of T91 Steel in 3.5% NaCl through Graphene Oxide Nanocomposite Coatings

Comparison of hardening and microstructures of ferritic/martensitic steels irradiated with fast neutrons and dual ions

Ferritic/martensitic steels T91 and HT9 were irradiated with neutrons (BOR-60 reactor) and dual ions (9 MeV Fe3+ and 3.42 MeV energy degraded He2+) from 369 to 520 °C and damage levels of 16.6 to 72 dpa to quantify the possibility of using ion irradiation to simulate neutron irradiation in terms of microstructures and mechanical properties. Nanoindentation testing was performed to obtain the bulk equivalent hardness of the dual-ion irradiated samples. For the neutron irradiated samples, both nanoindentation and Vickers hardness testing were conducted. Transmission Electron Microscopy (TEM) characterizations of the cavities, dislocation loops and precipitates were conducted to account for the strengthening contribution of each microstructure element. The good agreement between the microstructure-predicted (dispersed barrier hardening) and measured strength of the irradiated specimens demonstrated the accuracy of the strengthening model and the nanoindentation tests. The comparison of mechanical property and microstructure changes in ion and neutron irradiated structural materials indicated that ion irradiation replicated many neutron irradiation features. However, a single 70 °C temperature shift is insufficient to match all complex microstructures of neutron vs. ion irradiation over the irradiation temperature range of 369–520 °C.

New insights into oxidation mechanism and kinetics of 9Cr–1Mo ferritic-martensitic steel in oxygen-saturated liquid lead-bismuth eutectic

The evolution of oxide scales on a 9Cr–1Mo ferritic-martensitic steel (T91) in oxygen-saturated liquid Lead-Bismuth Eutectic (LBE) at 500 °C is investigated. While taken out at around 500 °C, the T91 steel, after corrosion, shows clean surface with little LBE adhesion due to the poor wettability induced by surface oxides. The results reveal the evolution of surface oxides from irregular strip shapes to regular crystalline grains, and stack-packed equiaxed crystals in the outer magnetite layer. Thermally-induced cracks are captured in the oxide scale on T91 steel after 1000 h exposure to LBE. A new oxidation mechanism is proposed, with consideration of the formation of the outer layer by metal dissolution/oxide precipitation, and a thin film of chromium oxide at the inner/outer layers interface. The dependency of temperature and oxygen concentration on the oxidation kinetics of T91 steel in LBE is discussed.

Investigation on the correlation between grain boundary character distribution and precipitate behavior in T91 ferritic/martensitic steel

In the present study, the relationship between microstructure, grain boundary character distribution (GBCD) and precipitate behavior in T91 steel was investigated via a series of thermomechanical treatments (TMTs). The experimental results revealed that subjecting the steel to cold-rolling (CR) with a deformation of 15–30% followed by tempering at 750 °C for 1 h caused the microstructure of T91 steel to transition from the typical lath martensite structure to a uniform polygonal ferrite, resulting in a change in the GBCD. Specifically, the proportion of random grain boundaries for the ferritic structure was significantly higher than that for the martensitic structure. The M23C6 carbide appeared as irregular polygons and precipitated within the ferrite grains, while it exhibited a rod-like shape and precipitated at the martensitic boundaries. Additionally, the orientation relationship (OR) between M23C6 carbide and the adjoining matrix transformed from the double Kurdjumov-Sachs (DKS) relation to (112) M23C6 // (110) ferrite and 201¯M23C6 // [012] ferrite. The change of OR resulted in a higher coarsening rate of M23C6 carbides precipitated in ferrite compared to those precipitated at martensitic boundaries.

Corrosion kinetics of T91 steel under low oxygen conditions (10−7 wt% O) in lead-bismuth eutectic at 500 °C

Among the advanced reactor designs of the fourth generation, the Lead-cooled Fast Reactor (LFR) stands out as one of six distinct types. The corrosion caused by the liquid Lead-Bismuth Eutectic (LBE) on structural materials is recognized as a significant obstacle hindering the development of LFR. The corrosion kinetics of T91 steel in static LBE with 10−7 wt% oxygen at 500 ºC is analyzed in this study. The formed oxide scale consists of an outer (Fe, Cr)3O4 layer with a Fe-Cr spinel structure and an inner internal oxidation zone (IOZ). The thickness evolution of the oxide layers follows a parabolic rule, with rate constants determined for different layers. The dissolution rate of metals into the LBE decreases over time, indicating diminishing the concentration gradient of Fe across the oxide layer-LBE interface. The observed deviations from expected norms may be attributed to dynamic flow conditions and non-isothermal loop experiments, which accelerate Fe diffusion into LBE.