Upper-shelf Energy Research Articles

Influence of hydrogen content on impact toughness of Zr–2.5Nb pressure tube alloy

Influence of hydrogen content on the impact toughness of Zr–2.5% Nb alloy was examined by carrying out instrumented drop weight tests in the temperature range of 25–250 °C using curved Charpy specimens fabricated from unirradiated pressure tubes of Indian Pressurized Heavy Water Reactor (IPHWR). Hydrogen content of the samples was between 10 and 170 ppm by weight (wppm). Sharp ductile-to-brittle-transition behaviour was demonstrated by hydrided materials. The temperature for the onset of transition increased with the increase in the hydrogen content of the specimens. The fracture surfaces of unhydrided specimen exhibited ductile fracture caused by micro void coalescence and tear ridges at lower temperatures and by fibrous fracture at intermediate and at higher temperatures. Except for the samples tested at the upper shelf energy levels, the fracture surfaces of all hydrided samples were suggestive of hydride assisted failure. In most cases the transverse cracks observed in the fracture path matched well with the hydride precipitate distribution and orientation.

Effects of Specimen Thickness and Notch Shape on Fracture Modes in the Drop Weight Tear Test of API X70 and X80 Linepipe Steels

This study is concerned with effects of specimen thickness and notch shape on drop weight tear test (DWTT) properties and fracture modes of API X70 and API X80 low-carbon microalloyed linepipe steels. Detailed fractographic analysis of broken DWTT specimens showed that the fracture initiated in an initial cleavage mode near the specimen notch and that some delaminations occurred at the center of the fracture surface. The chevron notch (CN) DWTT specimens had broader initial cleavage areas than the pressed notch (PN) DWTT specimens. The larger inverse fracture areas (i.e., cleavage areas close the hammer impact side) appeared in the PN DWTT specimens, because their higher fracture initiation energy at the notch allowed a higher strain hardening in the hammer-impacted region. The number and length of delaminations were larger in the CN DWTT specimens than in the PN DWTT specimens, and increased with increasing specimen thickness due to the plane strain condition effect. As the test temperature decreased, the tendency of delaminations increased, but delaminations were not found when the cleavage fracture prevailed at very low temperatures. The DWTT test results such as upper shelf energy (USE) and energy transition temperature (ETT) were discussed with relation to microstructures and fracture modes including initial cleavage fracture, ductile fracture, inverse fracture, and delaminations.

Effect of thermo-mechanical treatments on the microstructure and mechanical properties of an ODS ferritic steel

The Fe–14Cr–2W–0.3Ti–0.3Y 2O 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).

Solid-state diffusion bonding of high-Cr ODS ferritic steel

Solid-state diffusion bonding (SSDB) was employed to join high-Cr oxide dispersion strengthened (ODS) ferritic steel (Fe–15Cr–2W–0.2Ti–0.35Y 2O 3) blocks under uniaxial hydrostatic pressure using a high-vacuum hot press, and the microstructure and mechanical properties of the joints were investigated. High-Cr ODS ferritic steels were successfully diffusion bonded at 1200 °C for 1 h, without precipitates and microvoids at the bonding interface or degradation in the base materials. Transmission electron microscopic observation revealed that the nano-oxide particles near the bonding interface were uniformly distributed in the matrix and that the chemical composition across the bonding interface was virtually constant. At room temperature, the joint had nearly the same tensile properties and exhibited anisotropic behavior similar to that of the base material. The tensile strength of the joint region at elevated temperatures is nearly the same as that of the base material, with necking behavior at several micrometers from the bonding interface. The total elongation of the joint region decreased slightly at 700 °C, with an exfoliation fracture surface at the bonding interface. Although a small ductile–brittle transition temperature shift was observed in the joints, the upper shelf energy was fully reserved in the case of joints with L–R bonding orientation, for which cracks did not propagate on the bonding interface. Therefore, it is concluded that SSDB can be potentially employed as a joining method for high-Cr ODS ferritic steel owing to the excellent mechanical properties of the joints.

Influence of Y 2O 3 and Fe 2Y additions on the formation of nano-scale oxide particles and the mechanical properties of an ODS RAF steel

The main goal of this work was to manufacture an oxide dispersion strengthened (ODS) reduced activation ferritic steel from a pre-alloyed, gas atomised Fe–14Cr–2W–0.2Ti (in wt.%) powder mechanically alloyed with either 0.3%Y 2O 3 or 0.5%Fe 2Y particles and consolidated by hot isostatic pressing, and to investigate its microstructure, microhardness and Charpy impact properties. A lower oxygen content was measured in the ODS Fe 2Y steel than in the ODS Y 2O 3 steel. However, the mean size of nanoclusters in the ODS Fe 2Y steel was found larger, whereas density was smaller, than in the ODS Y 2O 3 steel. In addition, the nanoclusters in the ODS Fe 2Y steel appear less stable upon thermal annealing at 1350 °C for 1 h. Vickers microhardness measurements revealed that after HIPping the ODS Y 2O 3 is about 40% harder (366 HV 0.1) than the ODS Fe 2Y (260 HV 0.1). After heat treatment at 1350 °C the microhardness of both alloys was found smaller by about 30%. The ODS Fe 2Y steel was found to exhibit a much better Charpy impact behaviour, with an upper shelf energy of 8.8 J and a ductile-to-brittle transition temperature of −24 °C. The differences in mechanical properties were discussed in terms of the oxygen content as well as in the mean size, number density and crystallographic structure of the nanoclusters.

Effects of cold work and phosphorous on the ductile to brittle transition behavior of F82H steels

The effects of cold work and phosphorous on ductile to brittle transition behavior were investigated in order to evaluate the effects of hardening and intergranular degradation on the ductile to brittle transition temperature (DBTT) of F82H steel. Yield strength and ultimate tensile strength are increased by increasing the level of cold work. Phosphorous additions resulted in a small increase in the strength of F82H. The DBTT after cold work is shifted to high temperature and upper shelf energy (USE) is decreased. The shift of DBTT and the reduction of USE increase with the cold work level. F82H as-received and cold worked steel fractured in a cleavage mode at temperatures of lower shelf energy (LSE) region. The DBTT is shifted to higher temperatures with increasing phosphorus additions accompanied by the reduction of the USE. Although the hardening is small, a significant embrittlement was observed in the steels added with phosphorus, which was accompanied by intergranular cracking at temperatures in LSE region.

Influence of microstructure on impact properties of 9–18%Cr ODS steels for fusion/fission applications

The paper describes the influence of the microstructure (coming from the extrusion shape, the chemical composition and the thermo-mechanical treatments) of (9–18%)Cr–W–Ti–Y 2O 3 ODS steels on their impact fracture properties. The extrusion shape plays a major role on the impact properties, materials extruded as a rod present a higher upper shelf energy (USE) and a lower ductile to brittle transition temperature (DBTT) compared to materials extruded as plates. The DBTT for the non-recrystallized 14%Cr ferritic steels was shifted towards higher temperatures compared to the 9%Cr tempered ferritic–martensitic steel. Increasing the W and Ti content in 9%Cr tempered ferritic–martensitic ODS steel leads to a USE and a DBTT shifted towards higher energies and higher temperatures respectively. Increasing the yttria content leads to a drop of the impact energy and a shift of the DBTT of ferritic ODS steel towards higher temperatures. The present study highlights extensive splitting of the fracture surfaces and a dependency of the impact energy on the fracture plane orientation according to the microstructure anisotropy.

Effects of Rolling and Cooling Conditions on Microstructure and Tensile and Charpy Impact Properties of Ultra-Low-Carbon High-Strength Bainitic Steels

Six ultra-low-carbon high-strength bainitic steel plates were fabricated by controlling rolling and cooling conditions, and effects of bainitic microstructure on tensile and Charpy impact properties were investigated. The microstructural evolution was more critically affected by start cooling temperature and cooling rate than by finish rolling temperature. Bainitic microstructures such as granular bainites (GBs) and bainitic ferrites (BFs) were well developed as the start cooling temperature decreased or the cooling rate increased. When the steels cooled from 973 K or 873 K (700 °C or 600 °C) were compared under the same cooling rate of 10 K/s (10 °C/s), the steels cooled from 973 K (700 °C) consisted mainly of coarse GBs, while the steels cooled from 873 K (600 °C) contained a considerable amount of BFs having high strength, thereby resulting in the higher strength but the lower ductility and upper shelf energy (USE). When the steels cooled from 673 K (400 °C) at a cooling rate of 10 K/s (10 °C/s) or 0.1 K/s (0.1 °C/s) were compared under the same start cooling temperature of 873 K (600 °C), the fast cooled specimens were composed mainly of coarse GBs or BFs, while the slowly cooled specimens were composed mainly of acicular ferrites (AFs). Since AFs had small effective grain size and contained secondary phases finely distributed at grain boundaries, the slowly cooled specimens had a good combination of strength, ductility, and USE, together with very low energy transition temperature (ETT).

압력용기강재 SA508 class 3 용착금속의 조사거동

Irradiation behavior of the reactor pressure vessel SA508 class 3 steel weld metals was examined by Charpy V Notch impact specimens. The specimens were exposed to a fluence of <TEX>$2.8{\times}1019$</TEX> neutrons(n)/<TEX>$cm^2$</TEX>(E>1 MeV) at <TEX>$288^{\circ}C$</TEX>. The irradiation damage of weld metal was evaluated by comparison between unirradiated and irradiated specimens in terms of absorbed energy and lateral expansion. The specimens for neutron irradiation were welded by submerged arc welding process at a heat input of 3.2 kJ/mm which showed good toughness in terms of weld microstructure, absorbed energy and lateral expansion. The post-irradiation Charpy V notch 41J and 68J transition temperature elevation were <TEX>$65^{\circ}C$</TEX> and <TEX>$70^{\circ}C$</TEX>, respectively. This elevation was accompanied by a 20% reduction in Charpy V notch upper shelf energy level. The lateral expansion at 0.9mm irradiated Charpy specimens showed temperature elevation of <TEX>$65^{\circ}C$</TEX> and was greatly decreased due to radiation damage.

Influence of hot rolling and high speed hydrostatic extrusion on the microstructure and mechanical properties of an ODS RAF steel

An argon gas atomized, pre-alloyed Fe–14Cr–2W–0.3Ti (wt.%) reduced activation ferritic (RAF) steel powder was mechanically alloyed with 0.3wt.% Y 2O 3 nano-particles in an attritor ball mill and consolidated by hot isostatic pressing at 1150 °C under a pressure of 200 MPa for 3 h. In the aim to improve its mechanical properties the ODS steel was then submitted to a thermo-mechanical treatment (TMT): hot rolling (HR) at 850 °C or high speed hydrostatic extrusion (HSHE) at 900 °C, followed by heat treatment (HT). Transmission electron microscopy (TEM) observations of the ODS alloys after TMT and heat treatment revealed the presence of elongated grains in the longitudinal direction, with an average width of 8 μm and an average length of 75 μm, and equiaxed grains, a few microns in diameter, in the transverse direction. Two populations of oxide particles were observed by TEM: large Ti–Al–O particles, up to 250 nm in diameter, usually located at the grain boundaries and small Y–Ti–O nanoclusters, about 2.5 nm in diameter, uniformly distributed in the matrix. Charpy impact tests revealed that the HSHE material exhibits a larger upper shelf energy (5.8 J) than the HR material (2.9 J). The ductile-to-brittle transition temperature of both alloys is relatively high, in the range of 55–72 °C. Tensile mechanical properties of both ODS alloys were found satisfactory over the full range of investigated temperatures (23–750 °C). The HSHE material exhibits better tensile strength and ductility than the HR material. These results indicate that HSHE can be considered as a promising TMT method for improving the mechanical properties of ODS RAF steels.

Effects of Chemical Composition on the Impact Properties of A533B Steels

In order to estimate the long life integrity of vessel steels with considering various material compositions and irradiation conditions, it is necessary to understand physical mechanisms of the degradation of mechanical properties. In this research, chemical composition effects were investigated for Reactor Pressure Vessel Steels (RPVS) to apply small specimen test technique to surveillance test method. All specimens used in this study were machined from the A533B plate material, which are standard, low Mn, high Cu, high P, and high Cu and high P steels. Tensile strength is increased by phosphorous and copper additions. Charpy tests were carried out at temperature from 73 K to 473 K. The ductile to brittle transition temperature (DBTT) is shifted to higher temperatures with phosphorus additions accompanied by the reduction of the upper shelf energy (USE). The fracture mode of P-added A533B steels at temperatures in the lower shelf energy (LSE) region is intergranular cracking. Test results were discussed in view of the differences on elements of Cu, Mn and P.

Microstructure and mechanical properties of an oxide dispersion strengthened ferritic steel by a new fabrication route

A reduced activation oxide dispersion strengthened (ODS) ferritic steel with nominal composition of Fe–12Cr–2.5W–0.25Ti–0.2V–0.4Y 2O 3 (designated 12Cr-ODS) was produced by using EDTA–citrate complex method to synthesize and add Y 2O 3 particles to an argon atomized steel powder, followed by hot isostatic pressing at 1160 °C for 3 h under the pressure of 130 MPa, forging at 1150 °C, and heat treatment at 1050 °C for 2 h. The microstructure, tensile, and Charpy impact properties of the 12Cr-ODS steel were investigated. Transmission electron microscopy studies indicate that the 12Cr-ODS steel exhibits the characteristic ferritic structure containing few dislocations. Tensile characterization has shown that the 12Cr-ODS steel has superior tensile strength accompanied by good elongation at room temperature and 550 °C. The material exhibits very attractive Charpy impact properties with upper shelf energy of 22 J and a ductile-to-brittle transition temperature (DBTT) of about −15 °C. The formation of small, equiaxed grains and fine dispersion of oxide particles are the main reasons for the good compromise between tensile strength and impact properties.

Reduced activation ODS ferritic steel – recent development in high speed hot extrusion processing

AbstractThe paper presents the microstructure and mechanical properties of an oxide dispersion strengthened (ODS), reduced activation, ferritic steel, namely the Fe‐14Cr‐2W‐0.3Ti‐0.3Y2O3 alloy, which was fabricated by hot isostatic pressing followed by high speed hydrostatic extrusion (HSHE) and heat treatment HT at 1050 °C. Transmission electron microscopy (TEM) observations revealed significant differences in the grain size and dislocation density between the as‐HIPped and as‐HSHE materials. It was also found that the microstructure of the steel is stable after HT. The HSHE process improves significantly the tensile and Charpy impact properties of the as‐HIPped steel. The ultimate tensile strength at room temperature increases from 950 up to 1350 MPa, while the upper shelf energy increases from 3.0 up to 6.0 J. However, the ductile‐to‐brittle transition temperature (DBTT) remains relatively high (about 75 °C).These results indicate that HSHE is a promising method for achieving grain refinement and thus improving the mechanical properties of ODS ferritic steels.

Relation between microstructure and Charpy impact properties of an elemental and pre-alloyed 14Cr ODS ferritic steel powder after hot isostatic pressing

This article describes the microstructure and Charpy impact properties of an Fe–14Cr–2W–0.3Ti–0.3Y2O3 oxide dispersion strengthened (ODS)-reduced activation ferritic (RAF) steel, manufactured either from elemental powders or from an Fe–14Cr–2W–0.3Ti pre-alloyed powder. ODS RAF steels have been produced by mechanical alloying of powders with 0.3 wt% Y2O3 nanoparticles in either a planetary ball mill or an attritor ball mill, for 45 and 20 h, respectively, followed by hot isostatic pressing (HIPping) at 1,150 °C under a pressure of 200 MPa for 4 h and heat treatment at 850 °C for 1 h. It was found that the elemental ODS steel powder contains smaller particles with a higher specific surface area and a two times higher oxygen amount than the pre-alloyed ODS steel powder. After HIPping both materials exhibit a density higher than 99%. However, the pre-alloyed ODS steel exhibits a slightly better density than the elemental ODS steel, due to the reduced oxygen content in the former material. Charpy impact experiment revealed that the pre-alloyed ODS steel has a much larger ductile-to-brittle transition temperature (DBTT) (about 140 °C) than the elemental ODS steel (about 25 °C). However, no significant difference in the upper shelf energy (about 3.0 J) was measured. TEM and SEM–EBSD analyses revealed that the microstructure of the elemental ODS steel is composed of smaller grains with a larger fraction of high-angle grains (>15°) and a lower dislocation density than the pre-alloyed ODS steel, which explains the lower DBTT value obtained for the elemental ODS steel.

Effect of Oxygen Content on Toughness in High Strength Weld Metal

The effect of oxygen content on toughness in the high strength weld metals with full martensitic microstructures was investigated for Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW). Solid and Flux cored two types wires were examined for their influence on the resulting oxygen content in weld metals. It has succeeded in controlling the oxygen contents without changing welding processes. As expected, the increasing oxygen content obviously decreased the upper shelf energy (vEshelf). One of the primary reasons of this tendency is considered that the higher density of oxide makes dimples on the ductile fracture surface smaller. On the other hand, as unexpected, the oxygen content from 7 to 450 ppm had no impact on the fracture appearance transition temperature (FATT), and oxides at the brittle fracture initiation point have not been found. This result supports that the oxides in high strength full martensitic weld metals (Vickers hardness = 360 ~ 430) have no harmful effect on FATT.

Enhancement of Impact Toughness of 0.05% P Doped High Strength Steel through Formation of an Ultrafine Elongated Grain Structure

An ultrafine elongated grain (UFEG) structure with a strong ‹110›//rolling direction (RD) fibre deformation texture was produced by warm calibre-rolling at 773 K in 1200 MPa-class medium-carbon low-alloy steels with phosphorous (P) contents of 0.001 and 0.053 mass%. Charpy impact tests were performed at room temperature on the UFEG structure along with a conventional quenched and tempered (QT) structure. P segregation embrittlement completely disappeared in the UFEG structure. The UFEG structure exhibited the same high absorbed energy of 150 J regardless of P content, although the absorbed energy of the QT structure was significantly decreased from 93 J for 0.001% P steel to 23 J for 0.053% P steel due to occurrence of intergranular fracture. The high absorbed energy of 150 J for 0.053% P-doped UFEG structure was believed to be due to both delamination fracture and fine/deep ductile dimples. The present results emphasized that the detrimental effects of P grain boundary segregation can be suppressed and the upper shelf energy can be increased through the formation of the UFEG structure.

Effects of Cooling Conditions on Tensile and Charpy Impact Properties of API X80 Linepipe Steels

In this study, four API X80 linepipe steel specimens were fabricated by varying the cooling rate and finish cooling temperature, and their microstructures and crystallographic orientations were analyzed to investigate the effects of the cooling conditions on the tensile and Charpy impact properties. All the specimens consisted of acicular ferrite (AF), granular bainite (GB), and martensite-austenite (MA) constituents. The volume fraction of MA increased with an increasing cooling rate, and the volume fraction and size of MA tended to decrease with an increasing finish cooling temperature. According to the crystallographic orientation analysis data, the effective grain size and unit crack path decreased as fine ACs having a large amount of high-angle grain boundaries were homogeneously formed, thereby leading to the improvement in the Charpy impact properties. The specimen fabricated with the higher cooling rate and lower finish cooling temperature had the highest upper-shelf energy (USE) and the lowest energy transition temperature (ETT), because it contained a large amount of MA homogeneously distributed inside fine AFs, while its tensile properties remained excellent.

Delamination Effect on Impact Properties of Ultrafine-Grained Low-Carbon Steel Processed by Warm Caliber Rolling

Bulk ultrafine-grained (UFG) low-carbon steel bars were produced by caliber rolling, and the impact and tensile properties were investigated. Initial samples with two different microstructures, ferrite-pearlite and martensite (or bainite), were prepared and then caliber rolling was conducted at 500 °C. The microstructures in the rolled bars consisted of an elongated UFG structure with a strong α-fiber texture. The rolled bar consisting of spheroidal cementite particles that distributed uniformly in the elongated ferrite matrix of transverse grain sizes 0.8 to 1.0 μm exhibited the best strength-ductility balance and impact properties. Although the yield strength in the rolled bar increased 2.4 times by grain refinement, the upper-shelf energy did not change, and its value was maintained from 100 °C to −40 °C. In the rolled bars, cracks during an impact test branched parallel to the longitudinal direction of the test samples as temperatures decreased. Delamination caused by such crack branching appeared, remarkably, near the ductile-to-brittle transition temperature (DBTT). The effect of delamination on the impact properties was associated with crack propagation on the basis of the microstructural features in the rolled bars. In conclusion, the strength-toughness balance is improved by refining crystal grains and controlling their shape and orientation; in addition, delamination effectively enhances the low-temperature toughness.

Correlation of Microstructure and Mechanical Properties of Thermomechanically Processed Low-Carbon Steels Containing Boron and Copper

The correlation of the microstructure and mechanical properties of thermomechanically processed low-carbon steels containing B and Cu was investigated in this study. Eighteen kinds of steel specimens were fabricated by varying B and Cu contents and finish cooling temperatures (FCTs) after controlled rolling, and then tensile and Charpy impact tests were conducted on them. Continuous cooling transformation (CCT) diagrams of the B-free and B-added steel specimens under nondeformed and deformed conditions were constructed by a combination of deformation dilatometry and metallographic methods. The addition of a very small amount of B remarkably decreased the transformation start temperatures near a bainite start temperature (Bs) and thus expanded the formation region of low-temperature transformation phases such as degenerate upper bainite (DUB) and lower bainite (LB) to slower cooling rates. On the other hand, a deformation in the austenite region promoted the formation of quasipolygonal ferrite (QPF) and granular bainite (GB) with an increase in transformation start temperatures. The tensile test results indicated that tensile strength primarily increased with decreasing FCT, while the yield strength did not vary much, except in some specimens. The addition of B and Cu, however, increased the tensile and yield strengths simultaneously because of the significant microstructural change occasionally affected by the FCT. The Charpy impact test results indicated that the steel specimens predominantly composed of LB and lath martensite (LM) had lower upper-shelf energy (USE) than those consisting of GB or DUB, but had nearly equivalent or rather lower ductile-to-brittle transition temperature (DBTT) in spite of the increased strength. According to the electron backscatter diffraction (EBSD) analysis data, it was confirmed that LB and LM microstructures had a relatively smaller effective grain size than GB or DUB microstructures, which enhanced the tortuosity of cleavage crack propagation, thereby resulting in a decrease in DBTT.

Reduction method of DBTT shift due to irradiation for reduced-activation ferritic/martensitic steels

The method for reducing irradiation-induced DBTT shift of reduced-activation ferritic/martensitic steels was examined. F82H-LN (low nitrogen, 20 ppm), F82H+60 ppm 11B+200 ppmN and F82H+60 ppm 10B+200 ppmN steels tempered at 780 °C for 0.5 h were irradiated at 250 °C to 2 dpa, and the results for Charpy impact tests were analyzed. The upper shelf energy of F82H+ 11B+N steel was hardly changed by the irradiation, and DBTT shift was very small. From our research, DBTT shift due to irradiation can be reduced by the control of tempered conditions before irradiation, and it is found to be furthermore reduced by impurity doping with 60 ppm 11B and 200 ppmN to F82H steel.