9Cr-ODS Steel Research Articles

Characterization of a 14Cr ODS steel by means of small punch and uniaxial testing with regard to creep and fatigue at elevated temperatures

A 14Cr ODS steel was characterized at elevated temperatures with regard to its behavior in small punch and uniaxial creep tests and in low cycle fatigue tests. A comparison of small punch and uniaxial creep tests at 650°C revealed a strong anisotropy of the material when strained parallel and perpendicular to the extrusion direction with rupture times being several orders of magnitude lower for the perpendicular direction. The stress-rupture and Larson–Miller plots show a very large scatter of the creep data. This scatter is strongly reduced when rupture time is plotted against minimum deflection rate or minimum creep rate (Monkman–Grant plot). Fatigue tests have been carried out at 650°C and 750°C. The alloy is cyclically very stable with practically no hardening/softening. Results from the tests at both temperatures can be described by a common power law. An increase in the test temperature has little influence on the fatigue ductility exponent. For a given total strain level, the fatigue life of the alloy is reduced with increasing temperature.

Corrosion Resistance of 9 - 15% Cr ODS Steels and its Comparison with Austenitic Stainless Steel

An oxide dispersion strengthened steels are one of the most promising high temperatures, and high performance advanced structural material being developed for future fast reactors and high-temperature Generation IV reactors. In the present work, the corrosion resistance and its correlation with the passive film compositions of 11% Cr F/M and 9-15% Cr (with Zr or Hf) ODS steels is examined and compared with AISI type 304L stainless steel in boiling 60 - 62% (~13 M) HNO3. The corrosion rate measured in 62% HNO3 for 240 h of 11% Cr F/M, 9% Cr and 15% Cr (Zr) ODS steels show high corrosion rate. On the other hand, low corrosion rate was observed in 304L stainless steel (0. 21 to 23 mm y-1). However, severe intergranular corrosion attack was revealed in type 304L SS after 240 h exposure, but none in ODS steels. Such an intergranular corrosion attack seen in type 304L stainless steel is undesirable. On the contrary, low corrosion rate (0.04 0.15 mm y-1) of 15% Cr (Hf) ODS steel in 3 M, 6 M and 9 M HNO3, comparable to that of type 304L stainless steel was observed. The improved corrosion resistance of 15% Cr (Hf) ODS steel was attributed to enrich (20 at. %) and protective Al2O3 layer formation in addition to Cr2O3 in the passive film.

Corrosion Study and Passive Film Characterization of 11% Cr F/M and 15% Cr ODS Steels

Corrosion Study and Passive Film Characterization of 11% Cr F/M and 15% Cr ODS Steels

Helium Ion Implantation Effects of 9Cr-ODS (Oxide Dispersion Strengthened) Steel

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Corrosion behavior of 9Cr-ODS steel in stagnant liquid lithium and lead–lithium at 873 K

Corrosion experiments of 9Cr-ODS steel were carried out in static Li at 873K for 250h and compared with those of 9Cr-ODS in Pb–Li at the same exposure conditions. After exposure to liquid Li, 9Cr-ODS showed slight weight loss and decrease in hardness near surface. The tensile property exhibited a negligible change at 973K and the creep property degraded at 973K. By metallurgical analyses, 9Cr-ODS demonstrated a non-uniform corrosion behavior by preferential grain boundary attack and pronounced nodule-like morphology. The slight depletion of Cr was detected to about 15μm in depth by chemical composition analyses. According to the calculation of free energy of formation, the nano-scale oxide particles of TiO2 in 9Cr-ODS were thermodynamically less stable while Y2O3 were more stable compared with those of Li2O in Li environment. However, Y2O3 may be reacted with Li to form YLiO2. The mechanism of corrosion was proposed as the slight dissolution of Cr and Fe in matrix into liquid Li, chemical interaction of nano-scale oxide particles with Li, preferential grain boundary attack and penetration of Li, and finally development of extreme nodule-like structure during cleaning of Li. On the contrary, 9Cr-ODS showed much larger weight loss and depletion of Cr near surface but less marked corroded morphology in Pb–Li than in Li, which may be derived from higher solubility of Cr and Fe and lower activity of Li in Pb–Li than in Li.

Microstructure and high-temperature strength of high Cr ODS tempered martensitic steels

11-12Cr oxide dispersion strengthened (ODS) tempered martensitic steels underwent manufacturing tests and their ferritic–martensitic duplex structures were quantitatively evaluated by three methods: high-temperature X-ray diffraction (XRD), electron probe microanalyzer (EPMA), and metallography. It was demonstrated that excessive formation of residual-α ferrite, due to increasing Cr content, could be suppressed by appropriately controlling the concentration of the ferrite-forming and austenite-forming elements on the basis of the parameter “chemical driving force of α to γ reverse transformation. 11Cr-ODS steel containing a small portion of residual-α ferrite was successfully manufactured. In the as-received condition, this 11Cr-ODS steel was shown to have satisfactory creep strength and ductility, both as high as those of the 9Cr-ODS steel, while its 0.2% proof strength at 973K was lower than in the 9Cr-ODS steel.

Charpy impact properties of 9CrODS ferritic steels

The toughness of 9CrODS steels was evaluated by using an instrumented Charpy impact test machine, focusing on various processes such as hot-forging and hot-rolling. The upper-shelf energy (USE) of the hot-forged specimens is superior; their full-size USE converted from the miniaturized specimen lies within the published data for ODS steels, whereas hot-rolling leads to poor toughness. The total absorbed energy was separated into the energies for crack initiation and propagation. The hot-forged 9CrODS steels provide similar crack initiation energy to ferritic/martensitic steels (FMSs); their low absorbed energy is attributed to a limited amount of crack propagation energy induced by plastic deformation.

Strength correlation with residual ferrite fraction in 9CrODS ferritic steel

The microstructure and tensile properties at 973K in 9CrODS steels were investigated with respect to various fractions of the residual ferrite from zero to 47vol.%. The formation of the residual ferrite was discussed from a balance between a chemical driving force for α to γ reverse transformation and the oxide particle pinning force, while 0.04mass% carbon sample could contain an equilibrium δ-ferrite. With increasing volume fraction of the residual ferrite, strength at 973K increases but ductility decreases, which is attributed to the fact that the residual ferrite is harder than the tempered martensite.

Development of 15CrODS ferritic steels for over 1273 K service

The 15wt% CrODS steels (15CrODS steels) were manufactured as the high-temperature fusion reactor materials and their tensile properties at 1273K were correlated with the oxide dispersion strengthening before and after aging. Fine dispersion of the oxide particles at initial condition in 15CrODS steels gives significantly higher strength compared with commercial MA956. The strength decrease in 15CrODS steels after aging at 1473K for 100h is attributed to coarsening of oxide particles, thus weakening of dispersion strength.

Microstructure refinement and strengthening mechanisms of a 12Cr ODS steel processed by equal channel angular extrusion

• The first successful severe plastic deformation of 12Cr ODS steels. • Significantly refined and homogeneous microstructure in ODS steels. • Shear stress induced drastic variation of geometry of Y 2 O 3 nanoparticles. • Strengthening arising primarily from grain refinement. Multiscale microstructure refinement of a 12Cr-ODS steel by equal channel angular extrusion (ECAE) was investigated by numerous microscopy techniques. The as-HIPped material showed a bimodal microstructure. Three types of second phase particles, including Y 2 O 3 , (Cr, Fe) 23 C 6 and Cr oxide, were observed. After ECAE, the large matrix grains and Cr rich particles were effectively refined, and yttrium oxide nanoparticles surrounding ultrafine grain boundaries were redistributed. Geometry change of nanoscale yttrium oxide particles was also observed. An increased hardening by ∼35% was related to ECAE induced refinement of microstructures.

Investigation of the cause of peculiar irradiation behavior of 9Cr-ODS steel in BOR-60 irradiation tests

Four experimental fuel assemblies (EFAs) containing 9Cr-ODS steel cladding fuel pins were previously irradiated in the BOR-60 to demonstrate the in-reactor performance of 9Cr-ODS steel for use as fuel cladding tubes. One of the EFAs achieved the best data, a peak burn-up of 11.9at% and a neutron dose of 51 dpa, without any microstructure instability or any fuel pin rupture. On the other hand, in another EFA (peak burn-up, 10.5at%; peak neutron dose, 44 dpa), peculiar irradiation behaviors, such as microstructure instability and fuel pin rupture, occurred. Investigations of the cause of these peculiar irradiation behaviors were carried out. The detection sensitivity in an ultrasonic inspection test was shown to be low for the metallic Cr and metallic Fe inclusions. The peculiar microstructure change reappeared with high-temperature thermal-aging of the 9Cr-ODS steel containing metallic Cr inclusions. The strength and ductility of the defective part containing metallic Cr inclusions were appreciably lower than those of a standard part without the inclusions. The combined effects of matrix Cr heterogeneity (presence of metallic Cr inclusions) and high-temperature irradiation were concluded to be the main cause of the peculiar microstructure change in 9Cr-ODS steel cladding tubes in the BOR-60 irradiation tests. They contributed to the fuel pin rupture.

Corrosion behavior of 9CrODS steel by simulated fission product cesium and tellurium

Abstract Out-of-pile FCCI tests for 9CrODS steel were performed at 973 K by using simulated fission products Cs and Te under the oxygen potential in equilibrium with Fe/FeO and Cr/Cr2O3. Al2O3 powder were inserted to reduce a concentration of the Cs and Te in the system; its molar fraction is Cs:Te:Al2O3 = 1:1:1000. From EPMA and XRD analyses, Cr2O3 was formed at the most outer layer, which significantly suppressed the fission product corrosion. Cr2Te3 was also produced at the outer layer and interior of 9CrODS steel through liquid Te migration along grain boundaries. It was demonstrated the corrosion depth of 9CrODS steel is between PNC-FMS and PNC316, which were tested as reference. The Cs and Te assisted corrosion of 9CrODS steel was thermodynamically analyzed through the formation of Cs2O, Cs3CrO4, Cr2O3 and Cr2Te3.

Evaluation of mechanical properties and nano-meso structures of 9–11%Cr ODS steels

Abstract This study carried out mechanical tests and microstructural characterizations of several 9Cr and 11Cr-ODS tempered martensitic steels. From those results, the appropriate chemical composition range of 11Cr-ODS tempered martensitic steel was discussed from the viewpoint of high temperature strength improvement. It was shown that the residual α-ferrite fraction in 11Cr-ODS steel was successfully controlled to the same level as the 9Cr-ODS steel, which has excellent high temperature strength, by selecting the chemical compositions on the basis of the multi-component phase diagram. The tensile strength decreased with decreasing W content from 2.0 to 1.4 wt%. On the other hand, creep strength at 973 K did not degrade by the decreasing W content. Both tensile strength and creep strength increased with increasing population of the nano-sized oxide particles. Small angle X-ray scattering analysis revealed that titanium and excess oxygen contents were key parameters in order to improve the dispersion conditions of nano-sized oxide particles.

Controlling the ductile to brittle transition in Fe–9%Cr ODS steels

Probably the most important range of materials for consideration as the blanket material for the tokamak design for fusion reactors ITER and DEMO is the high alloy Fe–9Cr oxide dispersion strengthened ferritic steels. Ferritic steels possess exceptional thermal conductivity and low thermal expansion and are resistant to void swelling. Their main drawback is high ductile to brittle transition temperatures, particularly in the oxide dispersion strengthened versions. This paper describes attempts to reduce the DBTT in an un-irradiated ferritic steel by a novel heat treatment procedure. New batches of high alloy Fe–9Cr oxide dispersion strengthened (Eurofer) ferritic steel have been produced by a powder metallurgy route, and relatively homogeneous material has been produced by hot isostatic pressing (HIP). Mini-Charpy test specimens were made from materials which had been subjected to a matrix of heat treatments with varying solution treatment temperature (ST), cooling rate from the ST temperature, and tempering treatment. The initial DBTT was in the range of 150–200°C (423–473K). Downward shifts of up to approximately 200°C (473K) have been observed after solution treatment at 1300°C (1573K) followed by slow cooling. This paper describes the microstructure of this material, and discussion is made of the likely microstructural factors needed to produce these DBTT downward shifts.

Development of oxide dispersion strengthened ferritic steel prepared by chemical reduction and mechanical milling

The oxide dispersion strengthened ferritic steel with a nominal composition of Fe–14Cr–2W–0.5Ti–0.06Si–0.2V–0.1Mn–0.05Ta–0.03C–0.3Y2O3 (14Cr–ODS) was fabricated by sol–gel method in combination with hydrogen reduction, mechanical alloying (MA) and hot isostatic pressing (HIP) techniques. Pure Fe–1.5Y2O3 precursor was obtained by a sol–gel process and a reduction process at 650°C for 3h and pure 14Cr–ODS alloy powders were obtained from this precursor and the alloying metallic powders by mechanical alloying. The microstructure analysis investigated by transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS) reveal that Y–Ti–O complexes and V–Ti–O complexes with a main particle size of 8nm are formed in the 14Cr–ODS steel matrix. After HIP sintering the weight and the relative density of the compacted ingots are about 0.8kg and 99.7%. The uniform elongation and ultimate tensile strength of the ODS steel obtained by HIP after annealing at 1100°C for 5h are about 13% and 840MPa, respectively.

Fabrication and mechanical properties of a 14Cr-ODS steel

In this paper, the fabrication method and the mechanical properties of a 14Cr-ODS steel are investigated. The pre-alloyed steel powders and oxide powders were mechanical alloyed after milling for 30 h under a rotation of 300 rpm. Y-Ti-O phase was obtained after annealing the MA powders at 1100 °C. The tensile strength of the HIPed 14Cr-ODS steels at room temperature was about 1120 MPa. Micro structure observation showed that there were amount of nano dispersion particles in the base alloy which resulted in the high hardness and tensile strength.

Ferrite Grain Coarsening from Hot Rolled Austenite in ODS Steels

The hot rolling at temperature range of 1100 °C to 862 °C and subsequent air-cooling induce a formation of the coarse ferrite grains in the 9CrODS steels. This coarse ferrite is produced by transformation from the severely hot rolled γ-grains to ferrite. Formation process and mechanism of the transformed coarse ferrite are interpreted in terms of a nucleation, growth and coalescence of the same variant ferrite grains under a variant restriction rule.

Effect of hot-rolling and cooling rate on microstructure and high-temperature strength in 9CrODS steel

Abstract The 9CrODS steel specimens were prepared by different processing with hot-rolling and different cooling rate. The hardness and high-temperature tensile properties were measured. Microstructure was analyzed by means of EBSD inverse pole figure and kernel average miss-orientation angles. The hot-rolled and then air-cooled specimen has the highest tensile strength. The furnace-cooled specimen also has better tensile strength at 700 °C than air-cooled specimen at normalized condition. The high-temperature strength of 9CrODS steel is significantly improved with increasing grain size that can be induced by hot-rolling or furnace-slow cooling, where the localized grain boundary deformation can be suppressed.

Corrosion behavior of a 14Cr-ODS steel in supercritical water

The corrosion behavior of a 14Cr-ODS steel in the supercritical water was investigated using a variety of characterization techniques. Compared with 316L austenitic steel, the 14Cr-ODS steel had better corrosion resistant property. As the increasing of the exposure time, the weight gain increased, but the corrosion rates decreased. The curve of weight gain as a function of time followed a parabolic law. The general weight gain was 0.3476mg/(dm2h). A triple layer was observed which consisted of an outer layer, an inner layer and a diffusion layer. The outer layer was iron rich and contained Fe3O4, on which pores were observed. The inner layer and diffusion layer contained mainly (Fe,Cr2)O4. The oxidation mechanism was also discussed.

Corrosion Characteristics of RAFM Steels and Unalloyed Metals in Static Pb-17Li

The corrosion characteristics of reduced activation ferritic martensitic steels, JLF-1 (Fe-9Cr-1.94W-0.09C) and 9Cr-ODS (Fe-9Cr-1.97W-0.14C-0.29Y-0.23Ti), and unalloyed metals of Cr, W and Mo in liquid Pb-17Li were investigated by means of static corrosion tests at 600 ◦ C for 500 hours. The corrosion of the JLF-1 steel was based on the dissolution of Fe and Cr from the steel surfaces in the Pb-17Li. The dissolution type corrosion of the 9Cr-ODS steel was based on the strong depletion of Cr, W and O at the steel surface. The corrosion of the unalloyed Cr specimen in the Pb-17Li was much larger than that based on the Cr solubility in the Pb-17Li when a Mo crucible was used in the corrosion test. The corrosion was promoted under an unsaturated condition because the wetted surface of the Mo crucible trapped the dissolved Cr in the Pb-17Li by the alloying process. The dissolution of Mo in the Pb-17Li was also promoted by the alloying with the dissolved Cr though the solubility of Mo in the Pb-17Li was quite low. The weight loss of the unalloyed W specimen exposed in the Pb-17Li was larger than that estimated from the solubility in the Pb-17Li. Some oxide particles were detected on the surface of the W specimen. The reasonable mechanism on the large weight loss of the W specimen was the formation and detachment of the oxide particles in the Pb-17Li. c