Low cycle fatigue and thermo-mechanical fatigue behavior of modified 9Cr–1Mo ferritic steel at elevated temperatures

In-phase (IP) and out-of phase (OP) thermo-mechanical fatigue (TMF) experiments were performed in vacuum between 623 K and 823 K on modified 9Cr–1Mo ferritic steel. Few isothermal low cycle fatigue (IF) experiments including one at the peak temperature of TMF cycle (823 K) were also performed in vacuum. Isothermal LCF tests resulted in the lowest fatigue life, followed by IP-TMF and OP-TMF respectively. In modified 9Cr–1Mo steel, oxidation played a major role in limiting fatigue life at elevated temperatures. However in the absence of oxides, other damaging effects such as creep influenced the fatigue life. In this paper, the deleterious effect of oxidation on fatigue life has been evaluated by conducting tension hold and compression hold experiments in air at 873 K. Isothermal and TMF experiment conducted in vacuum eliminated the deleterious effects of oxidation and hence creep effects alone were evident. The lower TMF life under IP condition was attributed to the microstructural rearrangements and cavity formation around the coarsened carbides that resulted in a mixed mode of failure.     

Influence of microstructural inhomogeneities on the fracture toughness of modified 9Cr–1Mo steel at 298–823 K

Quasistatic fracture behaviour of two heats of modified 9Cr–1Mo steel for steam generator applications have been assessed at 298, 653 and 823 K. J–R curves were established and the elastic–plastic fracture toughness values at 0.2 mm crack extension (J_0.2) were determined. The fracture mechanisms were entirely different for the two steels at 298 K, with brittle fracture controlled by cleavage crack initiation in one and ductile fracture in the other by void nucleation and growth. At 653 and 823 K, fracture in both materials was by ductile crack growth. The difference in behaviour between the two steels at 298 K was attributed to the differences in microstructure, distribution and density of inclusions as well as phosphorous contents.     

Preparation and investigation of aluminized coating and subsequent heat treatment on 9Cr–1Mo Grade 91 steel

Iron aluminide inner coating with alumina top layer is being considered as a potential solution for tritium permeation barrier and mitigating MHD pressure drop for liquid metal blanket concepts in the fusion reactor systems. Hot-dip aluminizing with subsequent heat treatment seems to offer a good possibility to produce aluminized coating with alumina top layer. 9Cr–1Mo Grade 91 steel samples were hot dipped in Al melt containing 2.25 wt% of Si at 750 °C for 3 min. Heat treatment was performed at 650, 750 and 950 °C for 5 h; samples were either air cooled or furnace cooled. Coatings have been evaluated by SEM, EDX, X-ray diffraction, microhardness, scratch adhesion and Raman spectroscopy. The thickness of the layers and phases formed were influenced by the heat treatment adopted. Fe₂Al₅ was the major phase present in the samples heat treated at 650/750 °C, whereas FeAl and α-Fe(Al) primarily made up the outer and inner layers respectively in the samples heat treated at 950 °C. Cooling method deployed affected the hardness. Air cooled samples had comparatively higher hardness than furnace cooled samples. The scratch test showed the adhesion for the samples heat treated at 950 °C was much better as compared to the samples heat treated at 650/750 °C. Raman spectroscopy analysis showed the presence of both α-Al₂O₃ and γ-Al₂O₃ on the surface of the samples heat treated at 950 °C, while Fe₃O₄ was present in the furnace cooled sample only.     

Effect of hot rolling process on the mechanical and microstructural property of the 9Cr–1Mo steel

The effect of hot rolling on the mechanical and microstructural property has been investigated to simulate the effect of hot extrusion during the manufacturing process of the fuel cladding for sodium cooled fast reactors (SFRs). Hot rolling of modified 9Cr–1Mo steel was carried out either at 1050 °C or 950 °C upon cooling after normalizing. Continuous annealing right after the hot rolling at 950 °C for 1 h has been carried out followed by the mechanical testing and microstructural analysis. The results showed that hot rolling without any annealing or tempering treatment leaves residual stress so that it leads to the abrupt increase of material strength that would affect cladding formability. Continuous annealing right after the hot rolling process can alleviate residual stress without decreasing too much of material strength. Hot rolling either at 1050 °C or 950 °C increases the number density of the remained precipitate which leads to the precipitation hardening. Introduction of continuous annealing results in an increase in the fraction of secondary V-rich MX precipitate that leads to an increase in the stability at high temperature mechanical property.     

Corrosion of Fe9Cr1Mo steel in stagnant liquid lead–lithium eutectic

The corrosion behaviour of Fe9Cr1Mo steel samples in a static Pb₈₃Li₁₇ eutectic melt at 823 K in a specially designed necked quartz capsule is investigated. The samples are kept isothermally for 500, 1000, 1500 and 2000 h. The changes in microstructure and depletion of alloying components of the sample kept for 2000 h have been studied using scanning electron microscope (SEM), energy disperses X-rays (EDS) and electron probe beam microanalysis (EPMA). Weight loss and thinning of the walls has been calculated by gravimetric analysis method for all the samples. Using this data the change in thickness per year is calculated. Preferential dissolution of major constituent elements namely iron and chromium from the samples with the formation of a ferritic–martenstic layer is observed. Percentage mass loss with respect to time follows a parabolic curve which indicates non linear mechanism of corrosion.     

Studies on hydrogen permeability of 2.25% Cr–1% Mo ferritic steel: correlation with microstructure

The influence of microstructure on the hydrogen permeability, diffusivity and solubility in 2.25% Cr–1% Mo ferritic steel was investigated using electrochemical permeation technique. Varieties of microstructures ranging from martensite in water-quenched (WQ) steel to a predominant ferrite structure in annealed steel were characterised using analytical transmission electron microscopy. In the tempered structures, continuous precipitation of a variety of carbides of different morphologies and sizes was also characterised. The hydrogen diffusivity showed a continuous increase as the structure changed from martensite to ferrite and also with increasing extent of tempering. Solubility showed a corresponding decrease. The trends have been understood in terms of the number of reversible traps available for hydrogen in these different structures. Accordingly, martensite structure offered the maximum resistance to hydrogen diffusivity and tempered structure the least resistance due to the annihilation of defects during tempering and reduction in the solute content of the matrix due to precipitation.     

Characterizing microstructural changes in ferritic steels by positron annihilation spectroscopy: Studies on modified 9Cr–1Mo steel

Applicability of positron annihilation spectroscopy in probing the microstructural changes in ferritic steels has been investigated with thermal treatment studies on modified 9Cr–1Mo steel, during 300–1273 K. Positron lifetime results are compared with those of ultrasonic velocity and hardness techniques with two initial microstructural conditions i.e., normalized and tempered condition as well as only normalized condition. In first case, positron lifetime is found to be sensitive to small changes in metal carbide precipitation which could not be probed by other two techniques. In later case, positron lifetime is found to be sensitive to defect annealing until 673 K and in distinguishing the growth and coarsening of metal carbide precipitation stages during 773–1073 K. The present study suggests that by combining positron lifetime, ultrasonic velocity and hardness measurements, it is possible to distinguish distinct microstructures occurring at different stages.     

Influence of microstructure on the hydrogen permeability of 9%Cr–1%Mo ferritic steel

The influence of microstructure of 9%Cr–1%Mo steel on the hydrogen diffusivity, solubility and hence the permeability was investigated using electrochemical permeation technique. This steel was austenitised and cooled at various cooling rates to produce different microstructures. Tempering behaviour was also studied by heat treating for different durations at 1023 K. Characterisation of microstructures was carried out using scanning electron microscopy and analytical transmission electron microscopy. A fully martensitic product was obtained during fast cooling and a mixture of proeutectoid ferrite and martensite during slow cooling. Tempering the normalised steel resulted in the formation of fine intragranular M₂X precipitates and M₂₃C₆ on the boundaries. The hydrogen diffusivity and solubility showed a regular trend with the amount of strain in the lattice. Lattice defects and precipitates act as trap sites for hydrogen. Increase in lattice strain either due to increase in defect density, substructure or coherent precipitates resulted in decrease in diffusivity due to increase in trap sites. Martensite structure offered the maximum resistance to hydrogen diffusivity and tempered martensite the least resistance due to the annihilation of defects during tempering.     

Kinetics of dynamic recrystallization in Ti-modified 15Cr–15Ni–2Mo austenitic stainless steel

The hot working behaviour of Ti-modified 15Cr–15Ni–2Mo austenitic stainless steel (Alloy D9) was studied by hot compression test at temperatures of 900–1200 °C and at strain rates of 0.002–1 s^?1. The progress of dynamic recrystallization (DRX) was modelled by the Avarmi kinetics equation. The flow softening was directly related to the DRX volume fraction and the DRX time was determined by strain rate. For quantification of recrystallization rate, the time corresponding to the DRX fraction of 50% was used. Analysis of the sigmoid-shaped recrystallization curves revealed that the rate of DRX increases with increasing deformation temperature and strain rate. Results of the Avrami analysis were used to predict the DRX flow curves.     

The effects of tungsten addition on the microstructural stability of 9Cr–Mo Steels

The effects of tungsten addition on the microstructure and high-temperature tensile strength of 9Cr–Mo steels have been investigated by using three different steels: M10 (9Cr–1Mo), W18 (9Cr–0.5Mo–1.8W), and W27 (9Cr–0.1Mo–2.7W) steels. The tungsten-added 9Cr steels have revealed better high-temperature tensile strength. Microchemical analysis for (Cr,Fe)₂ (C,N) revealed that the tungsten addition increased the Cr/Fe ratio, which resulted in the lattice expansion of (Cr,Fe)₂ (C,N), and then the enhanced pinning effect on the glide of dislocation. In addition, in M10 steel, the M₂₃C₆ carbides quickly grew and agglomerated, while the tungsten-added 9Cr steels revealed a fine and uniform distribution of M₂₃C₆ carbides. Dislocation recovery during tempering treatments was delayed in tungsten-added 9Cr steels, which was correlated with the stabilized precipitates and the decreased self-diffusivity of iron. It is, thus, believed that the excellent high-temperature tensile strength of tungsten-added 9Cr steels is attributed to the stabilized M₂X carbo-nitrides and M₂₃C₆ carbides and the decreased self-diffusivity of iron.     

Wear mechanism and microstructures of Mo-implanted 4Cr5MoV1Si steel

tance and microstructure of implanted specimen were1 Introduction discussed according to the results of RBS, GXRD and The influence of pulsed beam in ion implantation XPS.is a topic of interest in both ion beam modification 2 Experimentaland radiation effect research.[1-3] Currently, a majorapplication of ion implantation is to improve the cor- Square steel samples (0.36wt.%…     

Tensile and Charpy impact properties of an ODS ferritic/martensitic steel 9Cr–1.8W–0.5Ti–0.35Y2O3

A 9Cr-ODS ferritic/martensitic steel with a composition of 9Cr–1.8W–0.5Ti–0.35Y₂O₃ was fabricated by mechanical alloying and hot isostatic pressing, followed by hot rolling. Tensile properties were measured at room temperature (23 °C) and 700 °C in the rolling direction (LT) and the transverse direction (TL). The ultimate tensile strength (UTS) of the as-rolled samples in both directions reached 990 MPa at 23 °C, and still maintained at 260 MPa at 700 °C. The tensile strength and elongation of the rolling direction was greater than that of the transverse direction. The Charpy impact was tested from −100 to 100 °C in the LT direction. The lower shelf energy (LSE) was more than 65% of the upper shelf energy (USE). The total absorbed energy was separated into the energies for crack initiation and propagation. The propagation energy was always higher than the initiation energy in the range of temperatures tested. The ductile-to-brittle transition temperature (DBTT) of the rolled 9Cr ODS evaluated by an absorbed energy curve was about 0 °C. However, the high LSE and the fracture surface that still contained dimples at lower shelf indicated good toughness of the as-rolled 9Cr ODS steels at temperature of −60 °C.     

Microstructure and oxidation properties of 16Cr–5Al–ODS steel prepared by sol–gel and spark plasma sintering methods

The 16Cr–5Al oxide dispersion strengthened (ODS) ferritic steel was fabricated by sol–gel method in combination with hydrogen reduction, mechanical alloying (MA) and spark plasma sintering (SPS) techniques. The phase characterization, microstructure and oxidation resistance of the 16Cr–5Al–ODS steel were investigated in comparison with the Al free 16Cr–ODS steel. X-ray diffraction (XRD) patterns showed that the Al free and Al added 16Cr–ODS steels exhibited typical ferritic characteristic structure. The microstructure analysis investigated by transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS) revealed that Y–Ti–O complexes with particle size of 10–30 nm were formed in the Al free matrix and Y–Al–O complexes with particle size of 20–100 nm were formed in the Al contained high-Cr ODS steel matrix. These complexes are homogeneously distributed in the matrices. The fabricated 16Cr–5Al–ODS steel exhibited superior oxidation resistance compared with the Al free 16Cr–ODS steel and the commercial 304 stainless steel owing to the formation of continuous and dense Al₂O₃ film on the surface.     

Resistance spot weldability of 11Cr–ferritic/martensitic steel sheets

Resistance spot welding of 11Cr–0.4Mo–2W, V, Nb ferritic/martensitic steel sheets with different thicknesses was examined to develop a manufacturing technology for a fast reactor fuel subassembly with an inner duct structure. In the spot welding, welding current, electrode force, welding time and holding time were varied as welding parameters to investigate the appropriate welding conditions. Welding conditions under which spot weld joints did not have either crack or void defects in the nugget could be found when the electrode force was increased to 9.8 kN. It was also found that the electrode cap with a longer tip end length was effective for preventing weld defect formations. Strength of the spot welded joint was characterized from micro hardness and shear tension tests. In addition, the ductile-to-brittle transition temperature of the spot welded joint was measured by Charpy impact tests with specimens that had notches in the welded zone.     

Spherical nanoindentation stress–strain responses of SIMP steel to synergistic effects of irradiation by H and He ions

A novel Fe-10Cr ferritic/martensitic steel called SIMP was chosen to investigate synergistic effects of H and He on the mechanical properties of structural materials for innovative nuclear energy systems. Sequential and separate irradiation experiments on SIMP steel specimens at room temperature using H and He ions with various energy levels were conducted to produce an ion deposition plateau at 300-650 nm. The indentation stress-strain responses were examined using spherical nanoindentation tes...     

Effect of thermal exposure time on tellurium-induced embrittlement of Ni–16Mo–7Cr–4Fe alloy

The embrittlement of nickel-based structural alloys by fission-produced tellurium(Te) is a major challenge for molten salt reactors(MSR). In this study, the effects of thermal exposure time on tellurium diffusion in a candidate MSR structural alloy(Ni–16 Mo–7 Cr–4 Fe) and the consequent mechanical property degradation of the alloy were investigated through surrogate diffusion experiments at 700 °C. The results show that some tellurium reacted with the alloy to form tellurides on the surface,while some tellurium diffused into the alloy along grain boundaries. Ni_3Te_2 and CrTe were the most stable reaction products at the tested temperature, and the formation of CrTe on the surface induced the Cr depletion at grain boundaries of the alloy. The diffusion depth of Te increased gradually with thermal exposure time, and thediffusion rate kept stable within the test duration of up to3000 h. The Te diffusion in the alloy caused the embrittlement of grain boundaries, inducing crack formation and strength degradation in tensile test at room temperature.     

Cs–Te corrosion depth dependence on distribution of chromium carbide precipitation in high chromium steel

In an attempt to investigate Cs–Te corrosion depth dependence on distribution of chromium carbide precipitation in high chromium steel, Cs–Te corrosion out-pile tests of two 9Cr steels with different distributions of chromium carbide were carried out at 975 K for 100 h, and their corrosion depths were compared. The corrosion is obviously more advanced in a specimen which has grain boundary carbide than in the one that does not. A considerable reason of the result is that the carbide distributed at grain boundaries promoted the corrosion reaction and the corrosion extended along the grain boundary. This is the first case in which the Cs–Te corrosion depth dependence on distribution of chromium carbide precipitation in Fe–Cr steel is clarified experimentally.