Tensile properties and deformation mechanisms of a 14Cr ODS ferritic steel (original) (raw)
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Development and characterisation of a new ODS ferritic steel for fusion reactor application
Journal of Nuclear Materials, 2009
This paper describes the microstructure, tensile properties and Charpy impact resistance of a reduced activation oxide dispersion strengthened ferritic steel Fe-14Cr-2W-0.3Ti-0.3Y 2 O 3 produced by mechanical alloying of a pre-alloyed, gas atomised steel powder with Y 2 O 3 particles, compaction by hot extrusion at 1100°C, hot rolling at 700°C and heat treatment at 1050°C for 1 h. At room temperature the material exhibits a high ultimate tensile strength of about 1420 MPa and high yield strength of about 1340 MPa in the transverse direction. In the longitudinal direction the values are about 10% lower, due to the anisotropy of the microstructure (elongated grains in the rolling direction). At 750°C the material still exhibits relatively high yield strengths of about 325 MPa and 305 MPa in the longitudinal and transverse directions, respectively. The material exhibits reasonable uniform and total elongation values over the temperature range 23-750°C, in both transverse and longitudinal directions. The material exhibits weak Charpy impact properties in the transverse direction. Charpy impact properties are slightly better in the longitudinal direction, with upper shelf energy of about 4.2 J and a ductile-to-brittle transition temperature of about 8.8°C.
Journal of Nuclear Materials, 2011
The Fe-14Cr-2W-0.3Ti-0.3Y 2 O 3 oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel was fabricated by mechanical alloying of a pre-alloyed, gas atomised powder with yttria nano-particles, followed by hot isostatic pressing and thermo-mechanical treatments (TMTs). Two kinds of TMT were applied: (i) hot pressing, or (ii) hot rolling, both followed by annealing in vacuum at 850°C. The use of a thermo-mechanical treatment was found to yield strong improvement in the microstructure and mechanical properties of the ODS RAF steel. In particular, hot pressing leads to microstructure refinement, equiaxed grains without texture, and an improvement in Charpy impact properties, especially in terms of the upper shelf energy (about 4.5 J). Hot rolling leads to elongated grains in the rolling direction, with a grain size ratio of 6:1, higher tensile strength and reasonable ductility up to 750°C, and better Charpy impact properties, especially in terms of the ductile-to-brittle transition temperature (about 55°C).
ODS steel as structural material for high temperature nuclear reactors
Oxide dispersed strengthened (ODS) ferritic-martensitic steels are investigated as possible structural material for the future generation of High Temperature Gas Cooled Nuclear Reactors. The Ni based austenitic ODS superalloys are not considered, because of the Ni presence, which is unfavorable under neutron irradiation. ODS-steels are considered to replace other high temperature materials for tubing or structural parts. Interestingly, ODS is also considered as material being used in future fusion applications. The oxide particles serve for interfacial pinning of moving dislocations. Therefore the creep resistance is improved. In case of the usage of these materials in reactor, the behavior under irradiation must be further clarified. In this paper the effects induced by He implantation are investigated. The induced swelling is measured and the mechanical behavior of the irradiated surface is investigated. These first tests are performed at room temperature, where a clear swelling and hardening could be observed.
Two different ferritic oxide-dispersion-strengthened (ODS) steels compositions Fe–16.5Cr–0.89Mn-0.3Y 2 O 3-2ZrO 2 (Sample1) and Fe–16.5Cr–0.89Mn-0.3Y 2 O 3-2ZrO 2-2Al (Sample2) were prepared through Mechanical Alloying (MA) and X-Ray Diffraction (XRD) pattern were taken at different time intervals for confirming the Nano-crystallite size. Milled powders were consolidated at 1170 O C at three different pressure levels of 30, 40 and 60 MPa respectively. Densification studies, microstructural examination ,and mechanical properties evaluation were analyzed and reported in the current study. MA of the hot pressed powder at 60 MPa obtained the highest density, hardness ,and tensile strength compared with another pressure compact irrespective of its composition. Aluminum addition in the ferritic ODS steel lowers the density, hardness ,and elongation of the ferritic ODS steel samples.
Assessment of ODS-14%Cr ferritic alloy for high temperature applications
Journal of Nuclear Materials, 2004
The present paper evaluates the mechanical behaviour of the ODS-ferritic alloy, Fe-14Cr-1Ti-0.25Y 2 O 3 (MA957), produced with two microstructures characterized by different grain sizes and degree of crystallographic texture. Tensile, creep and impact properties were found to be strongly dependent on the metallurgical condition. The neutron irradiation behaviour was also investigated. For this purpose, irradiation experiments were performed at 325°C for various dose levels. The specimens of ODS-ferritic steel irradiated up to 6 dpa exhibited a lower irradiation-induced hardening and higher ductility values compared to conventional martensitic steels irradiated in the same conditions. The occurrence of a 0 precipitation was detected using small angle neutron scattering techniques.
Microstructural and Cold Workability Assessment of a New ODS Ferritic Steel
Advanced Materials Research, 2008
A new ODS composition (Fe-14Cr-2W-0.3Ti-0.3Y 2 O 3) developed in the ExtreMat integrated Project has been produced by mechanical alloying techniques and consolidated by hot extrusion. This study summarizes some results of characterization and cold workability tests carried out at CEA and EPFL. It appears that the microstructure is fine and uniform after hot extrusion. According to microprobe analysis, solute elements are homogenously distributed in the matrix. However, the relatively high hardness level measured after hot extrusion and heat treatment may be detrimental in case of additional cold processing which is required to produce final shape like thin plates or cladding tubes. An assessment of the cold workability and the effect of the degree of cold work by rolling on recrystallisation temperature are addressed here. It is found that this material can be successfully cold rolled with a high degree of cold work (up to 60% of thickness reduction) without any damage. According to optical micrographs and Differential Scanning Calorimetry (DSC) measurements, it seems that the recrystallisation temperature remains always very high (above 1400°C) even though cold work level increases (up to 66% of thickness reduction). However, the hardness values begin to decrease for heat treatment temperatures above 1200°C for hot worked conditions and below 1000°C for cold worked conditions, respectively.
Role of Cr and Ti Contents on the Microstructure and Mechanical Properties of ODS Ferritic Steels
Advanced Materials Research, 2009
Six oxide dispersion strengthened (ODS) ferritic steels, with the composition of Fe-(12-14)Cr-2W-(0.1-0.3-0.5)Ti-0.3Y2O3 (wt.%), have been prepared by mechanically alloying elemental powders of Fe, Cr, W, and Ti with Y2O3 nano-particles followed by hot isostatic pressing. The influence of the chemical composition on the microstructure and mechanical properties of various materials was studied. It was found that the chromium content has a significant influence on the microstructure and mechanical properties of the compacted ingots. The 14Cr ODS steel exhibits slightly higher ultimate tensile strength and yield strength values than the 12Cr ODS steel. The total elongation and uniform elongation of both materials, in general, decrease with raising the test temperature, although in the case of the 12Cr ODS steel the elongation is about 30% higher than that of the 14Cr ODS material. In what concerns the effect of titanium content it can be concluded that variations between 0.1 and 0.3% h...
International Journal of Pressure Vessels and Piping, 2020
This paper presents the results of an experimental study aimed to obtain an ultrafine equiaxed grain distribution in 18Cr oxide dispersion strengthened (ODS) ferritic steel through cold working and annealing starting with an initial columnar grain structure with a predominant α-fibre texture in a product consolidated from the alloy powders during extrusion at high temperatures. Deformation along the extruded direction (ED) resulted in the retention of α-fibre texture, while deformation in the transverse direction (TD) showed a shear banded structure with a reduced percentage of α-fibre texture. Differential Scanning Calorimetry (DSC) analysis of the deformed steel established the occurrence of two significant events during heating namely recovery and recrystallization, whose temperatures were influenced by the heating rate. The recovery and recrystallization domains have been distinctly observed at 1350 K and 1420 K respectively at a low heating rate of 7 K min À 1. The resultant microstructure showed very coarse elongated grains interspersed with regions of ultrafine (<1 μm) equiaxed grains, due to the incomplete recovery. The deformed steel was subjected to a two step heat treatment designed based on the above inputs with an aim to reduce the microstructural anisotropy in longitudinal direction. The microstructure of the heat treated steel showed randomization of the initial <1 1 0>//ED α-fibre texture, which improved further with repeated deformation and two step heat treatment cycles. A gradual increase in hardness during the above cycles was observed reflecting the increase in dislocation density which offers the propensity to achieve an ultrafine grained microstructure.
High heat flux testing of 12–14Cr ODS ferritic steels
Journal of Nuclear Materials, 2010
The thermal performance of Fe-(12-14)Cr-2W-0.3Ti-0.3Y 2 O 3 ODS reduced activation ferritic steels, which are considered as candidate first wall materials for the future fusion power reactors and were manufactured by mechanical alloying in hydrogen and hot isostatic pressing, was assessed by high heat flux (HHF) testing with the electron beam JUDITH facility at the Forschungszentrum Jülich (FZJ), Germany. An analysis of the microhardness and microstructure of the specimens was done before and after HHF tests. In general, both materials present a ferritic (a-Fe, bcc) microstructure with a wide range of grain sizes from 100 to 500 nm up to a few micrometers. The coarse grains are almost dislocation-free, while the smaller ones are surrounded by tangles of dislocations. Oxide and carbide impurities (about a few hundreds nm in size) and a high density of Y-Ti-O nano-clusters, with a mean size of about 5 nm, are also present. The microhardness, density and tensile strength of the 14Cr material are slightly larger than those of the 12Cr material. HHF tests revealed that there is no difference in thermal performance, level of degradation and erosion behaviour of 12Cr and 14Cr ODS steels. The onset of melting of the materials occurs for an energy density between 1 and 1.5 MJ/m 2. Below this value only some kind of thermal etching takes place. This is a significant improvement compared to stainless steel, for which severe plastic deformation at the material surface was observed.
Metallurgical and Materials Transactions A, 2013
Oxide-dispersion strengthened (ODS) ferritic steels were produced by mechanical alloying and subsequent spark plasma sintering. Very fast heating rates were used to minimize porosity when controlling grain size and precipitation of dispersoids within a compacted material. Sintering cycles performed at 1373 K (1100°C) induced heterogeneous, but fine grain size distribution and high density of nano-oxides. Yield strengths at room temperature and at 923 K (650°C) are 975 MPa and 298 MPa, respectively. Furthermore, high-temperature ductility is much increased: total strain of 28 pct at 923 K (650°C).