Showing papers by "Fujio Abe published in 2005"
TL;DR: In this paper, the effect of fine precipitation and subsequent coarsening of Fe2W Laves phase on the creep deformation behavior was investigated for simple 9Cr-W steels containing 0, 1, 2, and 4 wt pct W.
Abstract: The effect of fine precipitation and subsequent coarsening of Fe2W Laves phase on the creep deformation behavior was investigated for simple 9Cr-W steels containing 0, 1, 2, and 4 wt pct W. After tempering, the specimens were subjected to creep tests at 823, 873, and 923 K for up to 15,000 hours. The precipitation of Fe2W Laves phase takes place during creep at boundaries from the supersaturated solid solution of the high-W steels, the 9Cr-2W and 9Cr-4W steels, but not in the low-W steels, the 9Cr-0W and 9Cr-1W steels. The fine precipitation of Fe2W Laves phase decreases the creep rate in the primary or transient creep region, while the subsequent large coarsening of Fe2W Laves phase reduces the precipitation strengthening and promotes the acceleration of creep rate in the tertiary or acceleration creep region after reaching a minimum creep rate. The change in shape of creep rate curves with stress and temperature is explained by taking fine precipitation and subsequent coarsening of Fe2W Laves phase into account.
134 citations
TL;DR: In this article, the creep properties of 9Cr-3W-3Co-NbV steels with boron contents varying from 47 to 180 ppm and of their weld joints have been investigated.
Abstract: New ferritic steels with a controlled addition of boron have been developed recently for ultrasuper-critical fossil power plants. These steels possess excellent creep resistance compared to conventional steels like P91, P92, P122, etc., and this has been attributed to the delay in coarsening of the carbides during creep owing to partial replacement of carbon by boron in these carbides. However, the susceptibility of the weld joints of the boron-containing ferritic steels to type IV cracking, which significantly brings down the rupture life of the weld joints, has not been investigated so far. In the present work, the creep properties of recently developed 9Cr-3W-3Co-NbV steels with boron contents varying from 47 to 180 ppm and of their weld joints have been studied. Creep tests were carried out at 923 K in the stress range of 140 to 80 MPa. Specimens were examined for particle coarsening using field-emission scanning electron microscopy, and the boron content in the precipitates was estimated using field-emission auger electron spectroscopy (FE-AES). The grain size of the parent metal and the heat-affected zone (HAZ) were estimated using electron backscattered pattern (EBSP) imaging. Results showed that the creep properties of the steels with 90 and 130 ppm boron and of their weld joints are superior to those of the P92 steels and its weld joints. Further, no weld joints exhibited type IV cracking. No significant coarsening of the carbides was observed, not only in the parent metal but also in the HAZ of the steels with ≥90 ppm of boron. In addition to the delay in carbide coarsening, the large prior-austenite grain size of the parent metal and the absence of a conventional fine-grained HAZ (FGHAZ) in the weld joints also seem to have a beneficial effect on improving the creep properties of these steels and their weld joints.
59 citations
TL;DR: In this article, Atmospheric plasma spray (APS) coatings of 50Ni-50Cr as undercoat and Al topcoat were attempted with the aim that the pores produced by 50Ni−50Cr undercoat can be filled with Al topcoating during the steam oxidation.
22 citations
20 citations
15 Oct 2005-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, a new attempt has been demonstrated using carbon free Fe-Ni-Co martensitic alloys strengthened by Laves phase, such as Fe 2 W or Fe 2 Mo, and μ phase, and Fe 7 W 6 or Fe 7 Mo 3, to achieve homogeneous creep deformation at high temperatures under low stress levels.
Abstract: A new attempt has been demonstrated using carbon free Fe–Ni–Co martensitic alloys strengthened by Laves phase, such as Fe 2 W or Fe 2 Mo, and μ phase, such as Fe 7 W 6 or Fe 7 Mo 3 , to achieve homogeneous creep deformation at high temperatures under low stress levels. Creep behavior of the alloys is found to be completely different from that of the conventional high-Cr ferritic steels. The alloys exhibit gradual change in the creep rate with strain both in the transient and acceleration creep regions, and give a larger strain for the minimum creep rate. In these alloys the creep deformation takes place very homogeneously and no heterogeneous creep deformation is enhanced even at low stress levels. The fine precipitates are found to be stable even at 973 K and effectively decrease the minimum creep rate much lower than that of the conventional high-Cr ferritic steels. Moreover, superior creep resistance is achieved by addition of Cr to the alloy.
18 citations
TL;DR: In this article, the authors investigated the impact of chemical compositions, heat treatment and microstructure on impact toughness of 15Cr ferritic steel and found that the improvement in impact toughness was achieved by controlling carbon and nitrogen contents, by the addition of nickel and by the increase in cooling rate after annealing.
Abstract: Influences of chemical compositions, heat treatment and microstructure on impact toughness of 15Cr ferritic steel have been investigated. Charpy impact values of the furnace cooled steels were lower than 15J/cm2 at room temperature independent of chemical compositions. Drastic improvement in impact toughness has been attained by controlling the carbon and nitrogen contents, by the addition of nickel and by the increase in cooling rate after annealing. However, the effect of nickel on impact toughness strongly depends on carbon and nitrogen contents. Improvement in impact toughness of the 15Cr ferritic steel has not been explained by individual microstructural factors of grain size, distribution of precipitates, volume fraction of martensitic phase. It has been supposed that the increase in Charpy impact toughness of the 15Cr ferritic steel was attained by improvement in toughness of ferrite matrix itself.
18 citations
TL;DR: In this article, the steam oxidation resistance of 80Ni-20Cr coatings on modified 9Cr-1Mo steel has been evaluated in the temperature range of 600 °C to 750 °C.
Abstract: The steam oxidation resistance of 80Ni-20Cr coatings on modified 9Cr-1Mo steel has been evaluated in the temperature range of 600 °C to 750 °C. High-velocity oxyfuel (HVOF) spray was used for the coating process. The steam-oxidized specimens exhibited the diffusion of iron from the substrate to the coating and diffusion of nickel from the coating to the substrate. Nickel diffusion to the substrate adjacent to the coating induced the microstructural changes. Micro X-ray diffraction (XRD) studies and energy-dispersive spectroscopy (EDS) point analysis on these transformed zones revealed that the newly formed phases are the austenite in the high-Ni-concentration region and the martensite in the low-Ni-concentration region. Microhardness studies showed higher hardness values in the martensite zones.
17 citations
TL;DR: In this article, the effect of thermal cycling on the adhesive strength has been evaluated for the 80Ni-20Cr and 50Ni-50Cr coatings sprayed by a high-velocity oxyfuel (HVOF) spraying process on modified 9Cr-1Mo steel substrate.
Abstract: In our previous study, we reported the adhesive strength of thermal sprayed Ni–Cr coatings on modified 9Cr–1Mo steel after a long-term aging at 750 °C for 3000 h In this paper, the effect of thermal cycling on the adhesive strength has been evaluated for the 80Ni–20Cr and 50Ni–50Cr coatings sprayed by a high-velocity oxyfuel (HVOF) spraying process on modified 9Cr–1Mo steel substrate The rapid heating and air-cooling were performed to simulate the thermal cycling condition of the thermal power plants where the coatings are targeted to use ASTM C-633 type tensile test was adopted to evaluate the adhesive strength of the coated and thermal-cycled specimens The specimens exposed to thermal cycling test for 100 and 300 cycles did not show significant change in their adhesive strength values compared to as-coated condition The 80Ni–20Cr specimens aged for 3000 h at 750 °C exposed to thermal cycling test for 300 cycles also showed similar performance; the tensile specimen broke within the resin The 50Ni–50Cr coatings aged for 3000 h showed a significant reduction in adhesive strength The coatings were delaminated from the substrate during the tensile test Thermal cycling on aged 50Ni–50Cr specimens resulted in similar performance of coating delamination under the tensile test and the adhesive strength values remained unchanged The results suggest that there is only limited probability for the coating exfoliation due to thermal cycling when Ni–Cr coatings are used to protect the boiler component against steam oxidation
17 citations
TL;DR: In this paper, the effects of nickel content and heat treatment conditions on the creep strength of precipitation-strengthened 15Cr ferritic steel were investigated, and it was shown that the volume fraction of martensite phase increased with increased nickel content in both the furnace-cooled and water-quenched steels.
Abstract: The effects of nickel content and heat treatment conditions on the creep strength of precipitation-strengthened 15Cr ferritic steel were investigated. The creep strength of the 15Cr ferritic steel was drastically improved by solution treatment and water quenching. However, over the long term, the detrimental effect of nickel on the creep strength was pronounced for water-quenched steels. The volume fraction of martensite phase increased with increased nickel content in both the furnace-cooled and water-quenched steels. The volume fraction of martensite phase in the water-quenched steel was smaller than that in the furnace-cooled type, even for the same nickel content. Fine particles, smaller than 500 nm, were precipitated homogeneously within the ferrite phase of the water-quenched steel. On the other hand, coarse block-like particles 1 μm in size were precipitated sparsely within the martensite phase, and the recovery of martensitic microstructure was accelerated. The creep strength of the steels decreased with increased volume fraction of the martensite phase caused by furnace cooling and nickel addition. The lower creep strength and microstructural stability of the martensite phase is attributable to less precipitation strengthening. To enable this steel to be put to practical use, it will be necessary to suppress the formation of the martensite phase caused by addition of nickel by optimizing the chemical composition and heat treatment conditions.
15 citations
TL;DR: In this article, the effects of gas contamination from milling atmospheres of mechanical alloying (MA) on mechanical properties were investigated, and it was concluded that nitrogen is the most suitable milling atmosphere to be applied as high-temperature materials.
Abstract: This study seeks to clarify the effects of gas contamination from milling atmospheres of mechanical alloying (MA) on mechanical properties. An iron-based dispersion alloy of Fe- 13Cr-3W-0.5Ti-0.5Y 2 O 3 (mass%) was selected as the experiment material. We prepared MA powders by milling mixed powders in atmospheres of argon, helium, hydrogen, nitrogen, and vacuum; then we made bulk alloys by groove rolling the MA powders. We then examined atmospheric elements trapped in the MA powders and their releasing processes with heat treatment in vacuum. For bulk alloys, we also examined the high-temperature behavior of residual atmospheric elements and their effects on impact strength as a function of heat-treatment time at 923 K. Experiment results demonstrated that some of the atmospheric element trapped by MA powder was unexpectedly difficult to remove with heat treatment. The content of widely used argon in MA powder, for example, was 0.013 mass%, and the argon was difficult to remove even with treatment at 1473 K, which is considered the maximum allowable temperature. Therefore, most of the argon was introduced into bulk alloy as gas contamination. Nitrogen was effectively reduced with treatment at 1323 K; however, hydrogen could not be sufficiently removed with this treatment. Residual argon and helium in bulk alloys formed bubbles at elevated temperatures and caused density decrease (swelling). Impact strengths of the bulk alloys obtained through milling in argon, hydrogen nitrogen, and vacuum decreased with increased treatment time; more remarkable decreases were observed in the alloys including argon and hydrogen. We concluded that nitrogen is the most suitable milling atmosphere to be applied as high-temperature materials.
14 citations
TL;DR: In this article, the authors discuss the effects of Type IV and Type IV types on the performance of the K-means and Type II-type models, and show that Type IV is more effective than Type II.
Abstract: 温室効果ガスの削減や省エネルギーの観点から,火力 発電のボイラ蒸気温度・圧力を向上させる目的で,高 Cr フェライト系耐熱鋼の研究開発が行われている.Wで強 化された P92鋼や P122鋼は,625°Cで実用化されてい る.現在は,650°C級超々臨界圧 (USC) 発電プラント用 ボイラ鋼の研究開発プロジェクトが進められている.1)~ 3) 高 Crフェライト系耐熱鋼では,溶接熱影響部 (HAZ) の細粒域に Type IVと呼ばれるクリープ損傷が生じ,溶 接部のクリープ寿命が母材よりも低下することが問題と なっている.4), 5)HAZ細粒域のクリープ強度が母材よりも 低いことが,Type IV破壊の根本的な原因と考えられ る.6), 7)細粒組織では,クリープ中の粒界炭化物の成長 や,マルテンサイト組織の回復が速いことが示されてい る.8), 9)従って,溶接継手のクリープ寿命を改善するに は,粒界を強化することが有効であると考えられる. 適量のボロン添加は,粒界炭化物の成長やクリープボ イドの成長を抑制するなど,粒界強化によってクリープ 強度を向上すると考えられている.10)最近のアトムプロー ブ電界イオン顕微鏡 (AP-FIM) や電界放射型オージェ電 子分光分析装置 (FE-AES) を用いた研究でも,ボロンは 炭化物中に存在し,炭化物を微細分散させると報告され ている.11)~ 13)物質・材料研究機構では,ボロンを添加し 窒素含有量を低くした 9Cr-3W-3Co-VNb鋼を開発し,長 時間クリープ特性が優れていることを報告している.13)こ れは,ボロンが粒界のM23C6を安定化し,粒界における 転位組織の回復を遅らせるためであると考えている.ボロ ンの効果をうまく利用できれば,HAZ細粒域のクリープ 強度が改善でき,溶接継手のクリープ寿命を改善できる と期待される.そこで本研究では,ボロン添加量を変え 窒素含有量を低くした 4種類の 9Cr-3W-3Co-VNb鋼の溶 接継手を作製し,クリープ強度とHAZ組織を調査した. 2 実 験 方 法 供試材は 9Cr-3W-3Co-VNbをベースにボロン添加量の みを 47,90,130 および 180ppmに変化させた 4種類の 鋼板(板厚 25mm)である.これら 4種類の供試材およ び比較材として用いた P92鋼の化学成分を TableIに示 す.ボロン窒化物 (BN) の析出を減らし,粒界強化に有 効なボロン量を増やすために,窒素含有量を既存鋼より 低い 20ppm程度とした.1353Kで 1時間の焼きならし, 1073Kで 1時間の焼き戻しを行った. 溶接継手は,ガスタングステンアーク (GTA) 溶接に より作製した.HAZにおける Type IV破壊特性を調べ るため,ここではインコネル系の溶接ワイヤ(AWS ER NiCr-3相当品)を用い,溶接金属が母材よりも高強度な オーバーマッチングな溶接継手を作製した.溶接施工条 件を TableIIに示す.溶接後熱処理 (PWHT) は 1013K で 4時間行った.溶接継手の側曲げ試験,引張試験およ びシャルピー衝撃試験を行った.クリープ試験は,母材 は 6mmφ,30mm G.L.の丸棒試験片を用い,溶接継手は 「材料」(J. Soc. Mat. Sci., Japan), Vol. 54, No. 2, pp. 162-167, Feb. 2005
TL;DR: A phenomenological calculation of phase equilibria by combining the cluster variation method (CVM) with the Lennard-Jones-type (L-J-type) pairwise atomic interaction energies has been attempted for the Fe-Pd-Ni ternary system in order to examine the possibility of Ni as the substituting element for Pd as mentioned in this paper.
Abstract: A phenomenological calculation of phase equilibria by combining the cluster variation method (CVM) with the Lennard-Jones-type (L-J-type) pairwise atomic interaction energies has been attempted for the Fe-Pd-Ni ternary system in order to examine the possibility of Ni as the substituting element for Pd. The calculated Ni content in an Fe-Pd-based L10 ordered phase at 863 K shows fairly good agreement with the experimental analysis performed for Fe-5Pd-5Ni alloy by energy-dispersive X-ray spectroscopy of a transmission electron microscope. The detailed calculation indicates that Ni is preferentially substituted in a Pd site in the L10-ordered phase, which can be interpreted by the Lennard-Jones-type pairwise potentials employed for the present study. It is concluded from the thermodynamic point of view that Ni is a promising candidate as the substituting element for Pd in an Fe-Pd-based L10 ordered phase.
TL;DR: In this article, the microstructure and creep strength of simulated HAZ specimens and welded joints have been investigated for advanced 9-12 %Cr steels in order to make clear the mechanisms responsible for Type IV crack and to improve the creep strength.
Abstract: The microstructure and creep strength of simulated HAZ specimens and welded joints have been investigated for advanced 9–12 %Cr steels in order to make clear the mechanisms responsible for Type IV crack and to improve the creep strength of welded joints at high temperature. Creep and creep rupture tests were carried out at 650 °C (923 K) for up to about 104 h. Creep crack growth tests were also carried out using CT specimens. The creep rupture time of simulated HAZ specimens has its minimum after heating to AC3 temperature, which produces fine-grained microstructure. Reducing the width of HAZ by means of EB welding is effective for the extension of creep life but the brittle Type IV fracture appears even in the EB welded joints at low stress and long time conditions. The welded joint specimens were fractured in fine-grained HAZ and resulted in shorter creep life than those of base metal due to the formation of creep voids and cracks. Using a specially-designed FEM code for Type IV crack growth behaviour, the effect of creep ductility and void formation ahead of the crack tip on creep crack growth rate is successfully simulated. The Type IV fracture is suppressed by the addition of boron, which suppresses the formation of fine-grained region in HAZ.
TL;DR: In this paper, deformation evolution during long term service causes a softening and promotes dynamic recovery or recrystallisation during subsequent creep, which accelerates the onset of acceleration creep.
Abstract: Degradation in tensile and creep properties has been investigated for 2·25Cr–1Mo steel, after long term service at 577°C for 1·9 × 105 h. Creep tests were carried out at 550–690°C for up to about 10 000 h for the long term serviced material. The results are compared with those for virgin material tested for up to 100 000 h. The creep rupture time is shorter but creep ductility is larger for the long term serviced material than for the virgin material at high stress and short time conditions. The difference between the two materials becomes decreased with decreasing stress and increasing time. Microstructure evolution during long term service causes a softening and promotes dynamic recovery or recrystallisation during subsequent creep, which accelerates the onset of acceleration creep. This results in a higher minimum creep rate and a shorter rupture time for the long term serviced material than for the virgin material. The deviation from Monkman–Grant relationship is correlated with a decrease in...
TL;DR: In this paper, the steam oxidation behavior of 9Cr ferritic steel containing Pd was studied in comparison with that of modified 9Cr-1Mo steel (P91), and it was found that the oxidation of the 9Cr steel containing 3 mass% Pd, the surface of which was finished with 320-grit silicon carbide (SiC) paper, was suppressed by the formation of a protective oxide layer when it was exposed to steam at 650°C.
Abstract: The steam oxidation behavior of 9Cr ferritic steel containing Pd was studied in comparison with that of modified 9Cr-1Mo steel (P91). It was found that the oxidation of 9Cr steel containing 3 mass% Pd, the surface of which was finished with 320-grit silicon carbide (SiC) paper, was suppressed by the formation of a protective oxide layer when it was exposed to steam at 650°C. This experimental result is unusual in high-Cr ferritic steels, including mod.9Cr-1Mo. A similar suppression of oxidation was also observed in specimens of mod.9Cr-1Mo steel that were coated with Pd or Pt. When the steel containing Pd was exposed to steam at 650°C for 50 h, the thickness of the oxide layer became ∼0.3 μm. Analyses of the oxide layer by thin film x-ray diffraction, transmission electron microscopy (TEM), and electron diffraction suggest that the protective layer was made of (Fe, Cr)2O3 with a rhombohedral structure. A supply of Cr from the metal matrix because of the residual strain in the surface region of sp...
TL;DR: In this article, a two-layer coating of Ni−Cr+Al and a thick 50Ni−50Cr coating with the sealant with BS 5493 was applied for up to 3000 h of testing and the results showed that the two layer coating showed a similar performance despite the lower coating thickness compared to the thick coating.
Abstract: Plasma sprayed coatings of 50Ni–50Cr as undercoat coupled with an Al topcoat were investigated with the aim of filling the pores in the Ni–Cr coating with the Al topcoat at high temperatures. For comparison purposes, a thick 50Ni–50Cr coating (∼450 μm) with sealant according to BS 5493 was also applied. The results showed that both the two layer coating of Ni−Cr+Al and the thick coating with the sealant had excellent resistance against steam oxidation at 650°C for up to 3000 h of testing. It is interesting to note that the two layer coating showed a similar performance, despite the lower coating thickness compared to the thick coating. Both Ni and Cr diffused into the Al coating structure. Diffusion of Al into the Ni–Cr system (coating) was slower than that into the Fe–Cr system (substrate).
01 Dec 2005
TL;DR: In this article, the effect of microstructural change during ageing and creep on elastic modulus was investigated in ferritic steels and the elastic moduli of aged, crept and serviced (service-exposed) materials were investigated.
Abstract: In order to clarify the effect of microstructural change during ageing and creep on elastic modulus, the elastic moduli of aged, crept and serviced (service-exposed) materials were investigated. In ferritic steels, the elastic modulus obtained by ultrasonic pulse does not change after long-term ageing and creep although the formation of creep voids slightly decreases the elastic modulus. In SUS304 and SUS316, the elastic modulus evaluated by ultrasonic pulse increases during ageing due to precipitation of M23C6 carbides, Laves phase and σ phase though the elastic modulus decreases during creep because of the formation of creep voids on grain boundaries. The increased elastic modulus of aged material with a large amount of σ phase was regarded as that of composite consists of matrix and σ phase. For STBA24, the elastic modulus obtained by a high temperature tensile test decreases after long-term service. The tensile test at a higher strain rate was proposed to avoid underestimation of elastic modul...