T91 Steel Research Articles

Oxide Scale Characterization and Study of Oxidation Kinetics in T91 Steel Exposed to Dry Air at High Temperatures (873–1073 K)

Oxide Scale Characterization and Study of Oxidation Kinetics in T91 Steel Exposed to Dry Air at High Temperatures (873–1073 K)

Effect of alloying elements on stacking fault energy and ductility of iron

First principles calculation has been directed to calculate the surface energy, stacking fault energy, and ductility parameter of the {110}<111> and {112‾}<111> slip systems of BCC Fe and Fe47X (X = Cr, Mo, Pb, and Bi). It is found that Cr and Mo should have a negligible effect on surface energy, unstable stacking fault energy, and ductility parameter of the two slip systems of BCC Fe. Calculation also reveals that both Pb and Bi could considerably reduce the ductility, surface energy, and unstable stacking fault energy of BCC Fe, and that such a decrease of the {110}<111> slip system is much greater than that of the {112‾}<111> slip system. In addition, the decrease of the surface energy of both slip systems of Fe47Pb and Fe47Bi is much more remarkable than that of the unstable stacking fault energy, i.e., the easier cleavage because of the weaker cohesion with a lower surface energy intrinsically brings about the brittleness of Fe47Pb and Fe47Bi. The obtained results are in nice agreement with experimental and theoretical evidence in the literature, and could offer a deep understanding of the intrinsic mechanism of liquid metal embrittlement of the T91 steel.

Effect of Shot Peening on the Evolution of Scale on T91 Steel Exposed to Steam

Shot peening can be an effective solution for the prevention or retardation of scale formation, and subsequent exfoliation, upon exposure of the inner tube to steam in coal-fired power plants. In this study, specimens of T91 tubes were shot peened and then exposed to 1-bar steam for 100–1000 h at 650 °C, and were then analyzed using Vickers hardness test and microscopic techniques OM, SEM, TEM, etc. The analysis indicates that the oxide scales are typically Fe2O3 on the topmost layer, Fe3O4 below, and a FeCr2O4 spinel on the bottom in both shot peening treated and untreated specimens. However, the oxide scale thicknesses of shot peened specimens are thinner, indicating that shot peened specimens have better oxidation resistance. In addition, numerous defects, such as voids and micro-cracks, were found in the untreated specimens, which are believed to cause exfoliation of the uppermost Fe2O3 layers of the specimens exposed to steam for 800 and 1000 h. By contrast, the shot peened specimens maintained a dense contact oxide scale with fewer defects.

Liquid lead–bismuth eutectic embrittlement effect on the tensile properties of T91 steel at 350°C

AbstractThe effects of pre‐exposure and surface notch on the tensile properties of T91 steel in oxygen saturation lead–bismuth eutectic (LBE) have been investigated by slow strain rate tensile tests at 350°C with a strain rate of 1 × 10−5 s−1, and the reference tests were performed in Ar gas. The pre‐exposure was conducted in liquid LBE at 500°C with an oxygen concentration of 10−8 wt% for 500 h. It is shown that the tensile properties of the nonpre‐exposed specimens whether with or without a notch were not affected by LBE, but obvious liquid metal embrittlement (LME) was observed on the pre‐exposure specimens. After being pre‐exposed, the notched one was more prone to liquid LBE embrittlement compared to the unnotched specimen. Morphology similar to fatigue striations was observed on the fracture surface of the notched specimen with pre‐exposure, which was probably attributed to the rapid growth of the crack. The results revealed that pre‐exposure had a critical effect on LME, and surface notch intensified this phenomenon when pre‐exposure was present.

Corrosion characteristics of T91 steel in lead–bismuth eutectic with different oxygen concentrations at 500°C

AbstractCorrosion tests of T91 were carried out in the stagnant lead–bismuth eutectic (LBE) containing 10−6, 10−7, and 10−8 wt% oxygen at 500°C for up to 2000 h, respectively. The dependence of corrosion characteristics on oxygen concentrations in LBE was obtained. The results show that the oxide‐scale structure changes from a three‐layer magnetite/spinel/internal oxidation zone (IOZ) scale under the oxygen concentration of 10−6 wt%, to the formation of a two‐layer spinel/IOZ scale under the oxygen concentrations of 10−7 and 10−8 wt%. In addition, with the decrease of the oxygen concentration, the Cr/Fe ratio in the Fe–Cr spinel slightly increases in a linear rule while the thicknesses of the oxide layers gradually decrease.

Designing micro-nano structure of anodized iron oxide films by metallographic adjustment on T8 steel

In this study, the anodized iron oxide films with micro-nano structure were prepared by a combination of heat treatment and anodization. The adjustment of these oxide films at the micron-scale was achieved by heat treatment on substrate to modify the position of the cementite and ferrite. The distribution and existing forms of carbon on cementite and ferrite determine the morphology of their respective anodization. It was found annealing at 300 °C can improve the crystallinity of iron oxide and maintain the original micro-nano structure. Although the crystal quality of iron oxide can be further improved with higher annealing temperature, it would result in the collapse of nanoporous structures and the reconstruction of the surface morphology. Furthermore, anodized samples made from those with lamellar ferrite and cementite show better specific capacitance. The iron oxide film on the annealed substrate obtained the highest specific capacitance of 35.3 mF/cm2 at a scan rate of 20 mV/s in our samples.

Electrochemical studies of heat‐treated NiCrMoFeCoAl and NiCrMoFeCoAl‐30%Cr3C2 HVOF coatings on T22 boiler steel

AbstractHerein, we present the corrosion behavior of NiCrMoFeCoAl and NiCrMoFeCoAl‐30%Cr3C2 high‐velocity oxy fuel (HVOF) coatings on the ASTM‐SAE213‐T22 boiler tube steel alloy. The samples exposed to molten salt (Na2SO4–60%V2O5) environment at 700℃ under thermocyclic conditions were investigated in conjunction with electrochemical techniques. The surface structures and morphologies of heat‐treated samples suggest the extent of corrosion is least for the NiCrMoFeCoAl‐30%Cr3C2‐coated steel. Room‐temperature electrochemical analysis of heat‐treated samples demonstrate the good stability with NiCrMoFeCoAl‐30%Cr3C2 layers on T22 steel at solid/liquid interface under neutral pH conditions. The potentiodynamic and impedance analyses reveal that the higher charge transfer resistance was observed for NiCrMoFeCoAl‐30%Cr3C2‐coated steel, followed by NiCrMoFeCoAl‐coated steel and bare T22 steel. The chromium carbide containing the HVOF coating exhibit a highly dense layer and the metal chromites/chromates on the post‐heat‐treated coatings provide poor access for the electrolyte to base alloy and subsequently improve corrosion stability.

Hot corrosion behavior of HVOF-sprayed carbide based composite coatings for boiler steel in Na2SO4–60 % V2O5 environment at 900 °C under cyclic conditions

Hot corrosion behavior of HVOF sprayed coatings, namely NiCrAlY-SiC and NiCrAlY-B4C deposited on T22 steel were studied for 50 cycles at 900 °C in an Na2SO4-60 wt%V2O5 molten-salt environment. Compound formations and microstructure were analyzed using XRD, SEM-EDS and EDS mapping techniques. Uncoated T22 steel suffered an accelerated corrosion attack due to the formation of unprotective FeO, Fe2O3, FeS and FeSO4 phases. NiCrAlY-SiC coatings showed formation of a protective layer containing SiO2, Al2O3, Cr2O3, NiO with spinals phases, whereas NiCrAlY-B4C coatings featured layers comprising B2O3, Al3BC, Al2O3, Cr2O3 and spinals phases. NiCrAlY-SiC coatings delivered greater hot corrosion resistance than NiCrAlY-B4C coatings.

A new approach to thermo-mechanical treatment of steel T91 by multiple upsetting-extrusion in a ferritic range

The microstructure and mechanical properties of the ferritic-martensitic steel T91 after severe plastic deformation (SPD) and subsequent thermal treatment are investigated. SPD was realized by the method of multiple “upsetting-extrusion” (MUE) in temperature range of stability of ferritic phase (750-575 °C), thermal treatment of SPD specimens was carried out at temperatures 550–730 °C during 1–100 h. It is revealed that decrease of temperature of “upsetting-extrusion” and increase of cycle number to 5 induces the decrease of mean grain size to 145 nm and increase of degree of microstructure homogeneity. At the same time microhardness increases from 1800 MPa to 2850 MPa. Composition, shape, size and density of carbide precipitates in steel after SPD and thermal treatment are determined. Obtained submicron grain ferritic microstructure with high value of microhardness is stable at exposure temperature of 550 °C. It is shown that temperature variation of thermal treatment induces the considerable change in grain size and mechanical characteristics.

Effect of Nd: YAG pulsed laser radiation on oxidation and segregation processes in the surface layers of T8 high speed tool steel: Tribological consequences

The paper takes a new look at studying the phase composition and structure of oxide films of the T8 tool induced by Nd: YAG laser radiation in air. It was decided that the best method for this investigation is the XPS method combined with ion depth profiling of the T8 tool steel sample by the SPECS Surface Analysis System. The maximum thickness of the oxidized FeO surface layer is 126 nm; the thicknesses of WO 3 , V 2O 5 , and V 2O 3 oxide layers of the alloying elements of T8 steel are 97 nm, 90 nm, and 126 nm, respectively. We have provided further evidence that pulsed laser radiation contributes to selective enrichment of the steel surface with W and Mo alloying elements, whose maximum is located at a depth of 90 nm. The segregation theory of Guttman ternary systems was employed to prove that the enrichment of the T8 steel surface layer is influenced by oxidative processes occurring during laser irradiation in air. In addition, we surveyed the impact of the phase composition of T8 surface oxides on their tribological properties under environmental conditions.

Microstructure evolution of T91 steel after heavy ion irradiation at 550 °C* *Project supported by Guangdong Major Project of Basic and Applied Basic Research (Grant No. 2019B030302011), the National Natural Science Foundation of China (Grant Nos. U2032143, 11902370, and 52005523), the International Science and Technology Cooperation Program of Guangdong Province, China (Grant No. 2019A050510022), the China Postdoctoral Science Foundation (Grant

Fe-Cr ferritic/martensitic (F/M) steels have been proposed as one of the candidate materials for the Generation IV nuclear technologies. In this study, a widely-used ferritic/martensitic steel, T91 steel, was irradiated by 196-MeV Kr+ ions at 550 °C. To reveal the irradiation mechanism, the microstructure evolution of irradiated T91 steel was studied in details by transmission electron microscope (TEM). With increasing dose, the defects gradually changed from black dots to dislocation loops, and further to form dislocation walls near grain boundaries due to the production of a large number of dislocations. When many dislocation loops of primary a 0/2〈 111 〉 type with high migration interacted with other defects or carbon atoms, it led to the production of dislocation segments and other dislocation loops of a 0 〈 100 〉 type. Lots of defects accumulated near grain boundaries in the irradiated area, especially in the high-dose area. The grain boundaries of martensite laths acted as important sinks of irradiation defects in T91. Elevated temperature facilitated the migration of defects, leading to the accumulation of defects near the grain boundaries of martensite laths.

Effects of imposed stresses on high temperature corrosion behaviour of T91

T91 steel specimens were tested at a high temperature with and without Na2SO4-NaCl molten salt under different stresses to investigate the effect of imposed stress on the corrosion behaviour. The imposed tensile stress facilitated chromium-oxide growth near the substrate and strengthened the corrosion resistance. The mechanism of the stress-salt-gas coupling effect on the high temperature corrosion of alloy T91 was discussed. The “optimal stress” of 20 MPa provided T91 with the best corrosion resistance.

Fusion Welding of T23 and T91 Steel Joints for Advanced Thermal Power Plant Application

Fusion Welding of T23 and T91 Steel Joints for Advanced Thermal Power Plant Application

Comparative hot corrosion performance of APS and Detonation sprayed CoCrAlY, NiCoCrAlY and NiCr coatings on T91 boiler steel

Hot corrosion performance of Atmospheric Plasma Spray (APS) and Detonation spray (DSC) CoCrAlY, NiCoCrAlY, and NiCr coatings on T91 steel were investigated at 650 °C for 100 cycles under Na2SO4-K2SO4-Fe2O3 mixed salt deposit in ambient air. The hot corrosion resistance of DSC coatings was found to be superior to their APS counterparts. Chromia and spinel oxides provided excellent corrosion resistance while no α-Al2O3 was observed. DSC NiCr and APS NiCoCrAlY offered the most and the least corrosion resistance, respectively. DFT calculation was performed to validate the thermodynamic stability of each oxide and identify the precise oxide formation.

Experimental Analysis of T23–T91 Dissimilar Steel Joining in Supercritical Boiler by a Cost‐Effective Fusion Welding Route

The critical aspect of joining T23 steel to T91 steel in a supercritical boiler is studied by a low‐cost gas tungsten arc welding (GTAW) process using an iron‐based filler wire (ER90S‐G). The hardness variation across the weldment does not reflect any drastic fall in hardness, thereby indicating an absence of “soft zone.” A significant joint strength (UTS = 318 MPa) together with a modest ductility (%Elongation = 6) is established by carrying out tensile test at boiler operation temperature (571 °C) and thereafter carrying out advanced characterization keeping all different regions of the weldment intact. During high‐temperature tensile deformation, Cr23C6 particles are found to be pushed along with grain boundary sliding and finally are clustered around the cavity formed thereby. The presence of Cr23C6 particles surrounding cavity promotes a cleavage mode of failure so as to negate a high ductility to be achieved. Yet, a modest ductility is still retained by virtue of soft α‐ferrite matrix promoting a concomitant dimple mode of failure. Furthermore, these Cr23C6 particles are also found to be responsible for an accumulation of dislocation loops around them, thereby providing quite significant high‐temperature strength with 88% joint efficiency.

Kinetics of oxidation and surface oxide cracking behaviours of weld metal and heat affected-zones of SMA weldment in thermal cyclic environment

Overheating is the main concern in steam generating system because of the deposition of oxalates on the water side resultant of high-temperature which creates localized spallation/cracking in normally protective oxide scale, leads to catastrophic failures. This research article, therefore, focused on oxidation kinetics, oxide cracking of HAZs and weld-metal of Shielded Metal Arc (SMA) weldment in T22 steel after exposure to thermal-cyclic conditions at 900 °C in air. Thermogravimetry method was applied to examine the kinetics-of oxidation. The scale thickness on HAZs was merely higher than weld-metal because the formation of higher extent of surface cracks, and non-availability of Cr-in inner-scale as confirmed by EDX, provides the easy diffusion path for ionic-transport. Oxide-scale of these regions of weldment obeys the parabolic-kinetics.

Investigation of reheat cracking susceptibility in CGHAZ of modified T23 steel

T23 steel (2.25Cr–1Mo–1.6W–0.24V–0.05Nb, in wt.%) has been widely used as water walls in ultra-supercritical (USC) power plants. However, high reheat cracking susceptibility of T23 steel hazarded the safety of USC power plants. It is aimed to improve the reheat cracking susceptibility of T23 steel from the perspective of modifying chemical composition. Gleeble-3800 thermal simulator was used to simulate the coarse-grained heat-affected zone (CGHAZ) in T23 and modified T23 steels via thermal simulation of welding, and then evaluate the reheat cracking susceptibility in CGHAZ of T23 and modified T23 steels by the isothermal slow strain rate tensile test. The microstructure was systematically investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show that the reheat cracking susceptibility of modified T23 steel is extremely improved. Compared with T23 steel, there are fewer M23C6 particles at the grain boundaries in modified T23 steel, and the degree of intergranular weakening is smaller. At the same time, not only MX in the grain interiors but also the solid-solutioned C and W in the matrix decrease in modified T23 steel, leading to the decline of intragranular strengthening. In addition, small grain size is beneficial to the improvement of the reheat cracking susceptibility of modified T23 steel. The decreased number of M23C6 at grain boundaries helps to retard the formation and propagation of reheat cracks because of decreased denuded zones.

Corrosion behavior of ferritic–martensitic steels SIMP and T91 in fast-flowing steam

The oxidation behavior of ferritic–martensitic steels was investigated by exposure to 500 °C steam with a flow rate of 5 m/s. The oxidation kinetic curves followed near-parabolic rate equation and a type of Si modified steel SIMP displayed better oxidation resistance than T91. In the 5 m/s steam, the oxidation rate was significantly enhanced and the formation of hematite was also promoted at relatively low dissolved oxygen of 100 ppb. The results indicated that not only the alloy composition and dissolved oxygen but the flow rate played an important role in the corrosion behavior of ferritic–martensitic steels.

Analysis of thermo-physical properties of NiCr HVOF coatings on T24, T92, VM12 and AISI 304 steels

Abstract This investigation reports on the thermal diffusivity of NiCr HVOF coatings on three ferritic steels (T24, T92 and VM12) and one austenitic steel (AISI 304) determined by laser-flash technique from room temperature up to 800 °C. The NiCr HVOF coatings were compact, dense, and well-adhered to the substrates with an average thickness of 194 ± 51 μm. The thermal diffusivity of AISI 304 austenitic steel increases steadily with temperature. In contrast, the thermal diffusivity decreases from room temperature till the Curie's temperature and then increases up to 800 °C in the T24, T92 and VM12 ferritic steels. It is also shown that the substrate has a greater influence on the thermal diffusivity at lower temperatures while the coating increases thermal diffusivity of the overall sample (substrate + coating) at temperatures above 400 °C. Therefore, the intrinsic thermal diffusivity of NiCr, coating calculated with a 2-layer model, evolves similarly than the 304 steel with temperature.

Oxidation damage and interfacial failure of dissimilar metal welds containing ferritic heat resistant steels

The ex-service steam tubes containing dissimilar metal weld (DMW) between high Cr ferritic steel T91 and austenitic stainless steel TP347H and the ex-service steam tubes containing DMW between low Cr ferritic steel G102 and austenitic stainless steel TP347H were obtained from coal-fired thermal power plants in China, and their microstructures at the nickel-based weld metal (WM)/ferritic steel interfaces and oxidation characteristics were investigated. After operating for 15,000 h at steam temperature of 541 °C and steam pressure of 17.5 MPa, a G102/TP347H DMW failed along the WM/G102 steel interface, which was a dangerous premature failure mode without obvious plastic deformation. This interfacial failure was attributed to the interaction between oxidation and cracking along the interface, where fracture appeared to be related with the strain concentration at the interface. Oxide notch along the WM/G102 steel interface was the precursor of premature interfacial failure of DMW involving G102. For the DMW involving high Cr ferritic steel T91, ferritic steel side could form a Cr-rich passive film during service and thus would not be further oxidized after operating for 67,000 h at steam temperature of 541 °C and steam pressure of 3.5 MPa. It was concluded that oxidation played a more important role in failure of these DMWs, and retarding the development of oxidation and avoiding the interfacial oxide notch would dramatically improve the service performance of steam tubes containing DMWs.