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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 65 (1979), No. 7

  • 「耐熱鋼・耐熱合金」特集号に寄せて

    pp. 723-724

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  • Development and Future Trend of Heat-Resisting-Steels and Alloys

    pp. 725-746

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  • The Role of Rare Earth Elements on the High-Temperature Oxidation of Heat-Resisting Alloys

    pp. 747-771

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  • Estimation of Oxidation Life of Austenitic Heat Resisting Steels for Automobile Exhaust Emission Control Equipment

    pp. 772-778

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    To select the materials suitable for exhaust emission control equipment of automobile, a method to estimate the oxidation life of the equipment has been investigated.
    Cyclic oxidation tests in air using the specimens of austenitic heat resisting steels have been carried out, and the standard oxidation-loss curves which express the relations among heating time, temperature and oxidation thickness loss of specimen were obtained.
    The oxidation thickness losses of the equipment through practical vehicle tests were compared with the ones calculated using the standard oxidation-loss curves. The oxidation losses through practical vehicle tests were about 2.7 times as much as the calculated values for SUS 304 and about 0.8 times for AISI 302B. Since these ratios are almost independent of driving condition, it is possible to estimate the oxidation life of the equipment by the standard oxidatioa-loss curves.
  • High Temperature Oxidation of Austenitic Stainless Steels in Reheating Furnace Atmosphere

    pp. 779-787

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    The high temperature oxidation of 16-25% Cr austenitic stainless steels including SUS 304 was studied at 1200-1300°C in 16% H2O-12%CO2-0-9% O2-N2 gases which were the main constituent of reheating furnace atmospheres.
    Oxidation of intermediate Cr steels including SUS 304, was much greater in O2 poor atmosphere than in O2 rich one. This was explained by the effect of H2O, CO2 and O2 on retardation and acceleration of internal oxidation.
    For lower Cr steels accompanying with heavy internal oxidation in either O2 rich or poor atmosphere, and high Cr steels being prevented internal oxidation in either atmosphere, the difference of oxidation rate with O2 contents in atmosphere was insignificant. In both cases, oxidation was rather slightly larger in O2 rich atmosphere than in O2 poor one.
  • High Temperature Corrosion of Austenitic Stainless Tubes in BFG, COG and Oil Mixed Fired Boilers

    pp. 788-797

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    Changes of appearances and metallurgical structures occurred by high temperature corrosion in austenitic stainless steel tubes during long time use for superheaters and reheaters of BFG, COG and oil mixed fired boilers were investigated. Corrosion of this type is occurred by molten deposit containing Na, V, Fe, S and SO2 in combustion gas, and is accelerated after long time operation over 20000 hours.
    In corroded tubes, sulfidization, carburization and Cr-depletion were always observed, and corrosion is progtessed by the cycles of (sulfidization→carburization→Cr-depletion→oxidization).
    On the basis of these results, the model of corrosion mechanism is presumed that scale on the tubes is changed and desolved into molten deposit at early stage and Cr in carburized zone under sulfidized layer is rapidly consumed through molten Ni-Ni3S2-eutectic by SO2, CO2 from molten deposit at progressed stage, and discontinuous oxidization of these Cr-depleted zones follows.
    x

    Readers Who Read This Article Also Read

    1. Heat Transfer Phenomena in Immersion Cooling of Heated Steel Plate and Influence of Water Stirring on Heat Transfer Tetsu-to-Hagané Vol.64(1978), No.1
    2. 第97回講演大会討論会講演概要 Tetsu-to-Hagané Vol.65(1979), No.2
    3. 第95回講演大会討論会講演概要 Tetsu-to-Hagané Vol.64(1978), No.2
  • Heat-resisting Alloys and Associated Failures in Petrochemical Applications

    pp. 798-810

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    x

    Readers Who Read This Article Also Read

    1. Heat Transfer Phenomena in Immersion Cooling of Heated Steel Plate and Influence of Water Stirring on Heat Transfer Tetsu-to-Hagané Vol.64(1978), No.1
    2. 第97回講演大会討論会講演概要 Tetsu-to-Hagané Vol.65(1979), No.2
    3. 第95回講演大会討論会講演概要 Tetsu-to-Hagané Vol.64(1978), No.2
  • Mechanism of Accelerated Oxidation of Heat Resistant Super Alloys Induced by Molten Salt Deposition

    pp. 811-819

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  • Recent Studies on Mechanisms of High Temperature Creep of Metals

    pp. 820-830

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  • Prediction of Long-time Creep-rupture Strength for High-temperature Materials

    pp. 831-842

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  • Creep Mechanism of Fe-4. lat. pct. Mo Alloy at High Temperatures

    pp. 843-850

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    Creep characteristics of a bcc solid solution were investigated under constant stresses of 6-40MPa at 1100-1200K in an argon atmosphere. The stress exponent for the steady-state creep rate was 3.2 and the apparent activation energy was 320kJ/mol.
    Creep curves were of inverse-or sigmoidal-primary types. After an increase in stress during the steadystate creep, inverse transient creep curve without instantaneous plastic srain was obtained. These results indicate that dislocations glide viscously.
    Internal stress was determined by stress-transient dip test applying a back-extrapolation technique. The average internal stress during the steady-state creep was obviously smaller than the applied stress. Hence, the role of dislocations which glide viscously cannot be neglected in discussion of the steadystate creep process in this alloy under the present creep condition.
    The creep characteristics were discussed in terms of the motion of dislocations which drag the Cottrell solute atmospheres, using the average effective stress and the density of dislocations within subgrains. The theoretically-expected values of the stress exponent and the apparent activation energy are in reasonable agreement with the exerimental values. The calculated value of the steady-state creep rate is about an order of magnitude smaller than the actually-observed one.
  • Effects of Metallurgical Variables on Creep Embritllement of Steels

    pp. 851-860

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    The effects of metallurgical variables on creep embrittlement of steels for use of boiler and chemical reactor vessels were investigated.
    The correlation between laboratory studies and failure cases of actual vessels was examined. The following results were obtained.
    (1) There existed the lowest creep ductility range for a steel at a certain Larson-Miller's parameter.
    (2) Most severe embrittlement ocurred in coarse grain region of HAZ (heat affected zone) of weldment.
    In order to improve the embrittlement of the HAZ, treatments as stress relief annealing of higher temper parameter or rapid heating similar to heat cycles in fine grain region of HAZ were most effective.
    (3) Detrimental effects due to impurities on the creep ductility ocurred only in synthetic HAZ specimens for 11/4Cr-1/2Mo steels. The effect of P on the creep embrittlement was smaller than that of Cu and Sb. The degree of the embrittlement corresponded to notch weakening as for the creep rupture strength.
    (4) Examination of a defect found by NDI at the welded joint in a reactor vessel showed that the cracks initiated at the root of the weldment and that the segregation of impurities on the grain boundaries in HAZ was extremely high.
  • Effects of Tempering and Stress Relieving Heat Treatments on Elevated Temperature Stress of 2.1/4Cr-1Mo Steel

    pp. 861-868

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    To clarify the effects of tempering and stress relieving heat treatments on elevated temperature strength of 2.1/4Cr-1Mo steel, teenile test and creep rupture test were carried out. Specimens were normalized and tempered at 640°-760°C or normalized and tempered at 680°C and subsequently stress relieved at 640°-720°C. Change in the substructure of specimens during creep rupture test was examined through electron microscope using extracted replicas. Results obtained are as follows.
    1) Elevated temperature tensile strength of stress relieved specimen decreases monotonously with the increase in the tempering parameter (T.P.) of the stress relief treatment. On the contrary, that of tempered specimen becomes independent of T.P. in the range of T.P. beyond about 19.5×103.
    2) Larson-Miller's master rupture curves for stress relieved specimens and for tempered (680°C) specimen change monotonously, but those for normalized specimen and for tempered (640°, 720° and 760°C) specimens show a tendency to shift to higher stress in the range of the Larson-Miller's parameter (PL.M.) below about 18.5×103.
    3) In high PL.M. range creep rupture stress decreases with the increase in the T.P. subjected to the specimen. At a critical PL.M. which is roughly equal to the T.P. of that specimen master rupture curve converges into the curve for normalized specimen.
    4) The change in the density of needle-like carbide during tempering or stress relieving heat treatments and also during creep rupture test roughly corresponds with the change in the master rupture strength of each specimen.
  • Estimation of Remaining Creep Life by Use of Tangential Strain on Outside Diameter of Boiler Tubes (2.25Cr-1Mo Steel)

    pp. 869-875

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    A remaining life estimation method fbr in-service boiler tubes (2.25Cr-1Mo steel) has been studied in terms of cumulative creep strains. With the life fraction rule presentation, values between 0.98 and 1.90 have been obtained by a numben of creep rupture tests under variable temperature conditions.
    The materials pre-crept to a same strain level at vanious strain rates between 0.002 and 2%/h showed the remaining life within a range of±0.2 of the value of the life fraction presentation. For a nelatively conservative estimate, correlations between tangential strains on the outside diameter of tube and remaining life have been established for a number of stress levels orresponding to hoop stresses.
    The present method enables to determine the remaining life nondestructively without operational histories.
  • Application of 9Cr-2Mo Steel for Fast Breeder Reactor Steam Generators

    pp. 876-885

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    9Cr-2Mo steel developed for boiler tubes was considered to be applicable to LMFBR steam generators. Therefore, we investigated the basic characteristics which will be required when this steel is applied to LMFBR steam generators. The results obtained are as follows.
    (1) Allowable stresses of 9Cr-2Mo steel on the base of ASME Case 1592 are reduced to be much higher at R.T. to 650°C than those of 2-1/4Cr-1Mo steel. High temperature fatigue properties are nearly equal with commercial 2-1/4Cr-1Mo and Type 304 steel, when the creep-fatigue interaction is considered.
    (2) 9Cr-2Mo steel is more insensitive to the weld cracking than stabilized high Cr steels.
    (3) This steel has almost the same carbon activity as Type 316 steel and the resistance to decarburization in sodium. The stress corrosion test in NaOH solution showed no cracking and better corrosion resistance than 2-1/4Cr-1Mo steel.
    In conclusion 9Cr-2Mo steel is considered to be a promising material for LMFBR steam generators.
  • Study on Creep Rupture Strength and Microstructure of 10Cr-2Mo-V-Nb Steel

    pp. 886-895

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    In order to develop a new ferritic heat-resisting steel that have high creep rupture strength, good weldability and oxidation resistance at the temperature of 600°C, we performed creep rupture tests, mechanical tests, microstructure observation by transmission electron microscope and X-ray diffraction analysis of electrolytic extraction residue of tested steel.
    Main resuhs are as follows:
    (1) It was found that the creep rupture strength of 0.05C-10Cr-2Mo-0.1V-0.05Nb steel was superior to that of SUS 304 steel at the temperature of 600°C.
    (2) This steel comprises two structure of delta ferrite and tempered martensite. Main precipitation at longer term are M23C6 and Fe2Mo at 600°C-700°C. The precitation of Fe2Mo type in delta ferrite would increse favorably the high temperature strength as well as the M23C6 and M6C carbides.
  • Effect of Grain Size on the High Temperature Creep Properties of 17Cr-14Ni Steel

    pp. 896-905

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    The effect of grain size, ranged from 22 to 404μm, on the creep behavior of carbon free 17Cr-14Ni steel was studied at the temperature of 850 and 900°C.
    The creep properties, mainly steady state-or minimum-creep rate, strongly depend on the grain size, that is, 1) steady state creep rate, ∈s, is independent of grain size for the grain sizes from 100 to 200μm, where ∈s reaches the minimum, 2) below about 100μm, ∈s increases with the decrease of grain size, whereas, 3) above about 200μm, ∈s increases with the increase of that.
    From the measurement of internal stress (σi) and the observation of the microstructures, it was concluded that under the constant applied stress the increase in ∈s with the decrease of grain size is attributable to the increase of effective stress, σeai, where σa is the applied stress.
    On the other hand, the recrystallization at the grain boundary triple points between large grains induced by the stress concentration seems to increase ∈s with the increase of grain size above 200μm.
  • The Effect of Strain-wave Form on the Low-cycle Fatigue Behavior of Type 304 and 321 Stainless Steel at Elevated Temperatures

    pp. 906-915

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    The influence of strain-wave form on the low-cycle fatigue life of type 304 and 321 stainless steels was studied at 650 and 700°C. Fully reversed strain-controlled low-cycle fatigue tests were conducted with a symmetrical triangular, a trapezoidal and an asymmetrical triangular wave form.
    The effect of creep-fatigue interaction was analyzed by the strain-range partitioning approach. The micro-structural change during the test was also observed and its effects on the fatigue life were discussed considering the mode of fatigue crack growth and the environmental effects.
    It was found that the creep-fatigue properties Δpp-Npp and Δcp-Ncp of 304 steel are the same as those of 321 steel, but the other effects such as the micro-structural change, environment and so on is greater in 321 steel than in 304 steel in such test as with trapezoidal strain-wave form of long hold time.
  • The Effect of Thermal Aging on the Low-cycle Fatigue Behavior of Type 304, and 321 Stainless Steel at Elevated Temperature

    pp. 916-922

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    The effect of thermal aging on the elevated temperature low-cycle fatigue life of 304 and 321 stainless steels was studied using the creep-fatigue analysis by the strain-range partitioning approach. The Δpp-Npp and Δcp-Ncp properties were determined based on the results of strain-controlled lowcycle fatigue tests with a symmetrical and an asymmetrical triangular wave form. The test with a trapezoidal strain-wave form was also conducted and the effec to fhold time was examined. Aging temperatures of 650 and 700°C and aging times of 1000 and 3000 hours were employed for 321 steel, and for 304 steel only one condition of 650°C and 1000 hours was examined.
    Thermal aging produced beneficial results relative to unaged material in the creep-fatigue properties Δpp-Npp and Δcp-Ncp of 321 steel and in the Δpp-Npp of 304 steel, whereas the Δcp-Ncp property of 304 steel was not influenced by thermal aging. It is suggested that the precipitation of the various carbides is responsible for the beneficial effect. On the other hand, the effect of hold time was more detrimental in aged material than in unaged one and the reduction in fatigue life was much greater than that predicted by creep effect.
  • Effect of Rising Mean Strain on Elevated-Temperature, Low-Cycle Fatigue Life of a SUS 316 Steel

    pp. 923-931

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    Fatigue tests were carried out on a Type 316 stainless steel at 600 and 700°C in order to clarify the effect of creep deformation increasing gradually during fatigue tests on elevated-temperature, low-cycle fatigue life. Strain wave forms with or without rising mean strain were used for truncated wave form with a 10-min hold in tension only and triangular wave form. The strain rate of ramps were 40%/min.
    For a given total strain range, fatigue life of the truncated wave form is smaller than that of the triangular wave form. The difference between them obtained at 600°C is larger than obtained at 700°C. Well-defined intergranular facets covered with small dimples were observed for specimens tested under truncated wave form at 600°C, but not at 700°C. These results could be explained by phenomenom of recovery process at 700°C, because of which stress applied grain boundaries might have decreased under truncated wave form.
    Rising mean strain per cycle under truncated wave form was creep strain, however no clear effect of the rising mean strain on low-cycle fatigue life was observed. Summation of the creep strain corresponding to relaxation stress and rising mean strain per cycle were almost constant for a given total strain range. It is considered that this is a reason why no effect of rising mean strain on fatigue life was observed.
  • Estimation of Creep Life of Boiler Tubes (18-8-Mo Stainless Steel) Remaining after Long Term Service

    pp. 932-938

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    For the failure analysis or in-service inspection of high tempemature parts of power plants, the estimation of the creep life remaining after a given service period and evaluation of such estimates are very important, especially when the parts have been accidentally overheated.
    It the cases, the life fraction rule is offer applied. To lay a foundation for this practice, creep rupture tests and metallographic studies were conducted on type 18-8-Mo stainless steel that had been used as superheater tube at 590-615°C for 27000 hours. It was found that a serive temperature with ±15°C, one of the indispensable conditions for reliable estimation of the remaining creep life is difficult to determine by means of metallographic observations and that for the estimation of remaining life by a comparison between the used tube and unused tube in creep rupture testing, both tubes must be heated equally. As a result, the estimation based on creep strain (tangential strain on outside periphery of the tube) was found to be the best method capable of non-destructive testing for a secular change of remaining life.
  • The Effect of the Grain Boundary Reaction on the Creep Rupture Properties of the Austenitic Heat Resisting Steel

    pp. 939-948

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    The effect of the grain boundary reaction on the creep rupture properties has been investigated for an austenitic engine valve steel (21-4N) at 600°-800°C.
    The creep ductility as well as the minimum creep rate increased with increasing amount of the reaction but its increase came to a halt above about 30 per cent of the reaction. The creep rupture strength was remarkably improved by very small amount of the reaction, and the steel with about 8 per cent of it had the maximum strength and the favorable ductility, while more than about 8 per cent of it rather decreased the rupture strength in spite of the increased ductility. A noticeable notch strengthening was observed on the steels with more than about 30 per cent of the reaction, but the steels with poor ductility were apt to be notch weakened.
    It was found by means of both optical and scanning electron microscopies that the increase in strength and ductility was resulted from the retardation of the grain boundary sliding and the occurrence of the ductile intergranular fracture owing to the reaction.
  • Improvement of High Temperature Strength of Austenitic Stainless Steel by Additions of Silicon and Nitrogen

    pp. 949-958

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    The effect of additions of Si, N and C on the creep rupture strength of Ni-Cr austenitic steel was studied, and the effect of heat treatment on the creep strength of 2%Si-13%Ni-24%Cr-0.25%N steel was also studied.
    The creep mpture strength at 1000°C depends on Si/N and the maximum value is obtained at Si/N≈10. Strengthening by additions of Si and N may be attributed mainly to the retardation of the Cr-nitride precipitation by addition of Si. Rupture elongation increases with Si content. Creep mpture strength at 1000°C increases with C content up to 0.10%, above which the strength decreases at longer time.
    Based on these data, 0.1%C-2%Si-13%Ni-24%Cr-0.25%N austenitic steel is designed for radiant tube use. In this steel further improvement of the strength is obtained by high temperature (1200-1250°C) solution treatment. In this condition, the creep rupture strength of this steel is included in the data band of HK40 surveyed by Nishino. The creep rupture strength at 1000°C decreases with ageing or cold worklng. Welded tube of this steel is put to the test for radiant tube, and it was found that this steel has excellent perfbrmance and durability.
  • Effects of Alloying Elements on Toughness of Austenitic Heat Resisting Steels Containing Nitrogen at Room Temperature after Aging

    pp. 959-968

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    The effects of C, N, Ni, Cr, Nb, Mo and B on the toughness after aging at 700/900°C has been investigated in 19/25% Cr-8/22% Ni steels. The toughness after aging has been evaluated by V-notch Charpy impact test at room temperature.
    The main results obtained are as follows.
    (1) The toughness after aging was remarkably reduced when carbides, M23C6, and nitrides, Cr2N, precipitated at grain boundary and twin boundary. Therefore the toughness strongly depended on the contents of C and N. Ni reduced the solubility of C and N, so that the addition of it in excess quantity promoted the precipitation of carbides and nitrides, and consequently reduced the toughness.
    (2) The addition of Nb or B in suitable quantity improved the toughness after aging, because those elements suppressed the precipitation of carbides and nitrides at twin boundary and simultaneously made the precipitates at grain bonudary to be fine and discontinuous. The addition of Mo within 1% slightly improved the toughness, but the addition of it over 1% reduced the toughness. The addition of much quantity of Mo promoted the precipitation of σ phase at grain boundary and consequently reduced the toughness remarkably.
  • The Characteristics of Sigma Phase Precipitated in HK40-alloy

    pp. 969-974

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    Concerning the sigma phase which has precipitated in HK 40 heat resistant cast steel, a study has been carried out with the object of clarifying its properties, precipitation mechanism and the effect on material strength.
    The results obtained are as follows.
    (1) The sigma phase precipitating in HK 40 alloy has two types in its shape and chemical composition, i. e. the massive type and the “needle type”. And it seems that the two types of sigma have different precipitation mechanism.
    (2) The massive type sigma phase precipitates in the P.F.Z. (precipitate free zone) around the primary carbide and grows up to about the same size as the primary carbide. On the other hand, the “needle type sigma phase” precipitates directly from the matrix.
    (3) Both types of sigma phase reduce the material strength, especially the needle type sigma phase affects it sevesely. But the effect of massive type sigma phase doesn't appear unless it precipitates more than 2 or 3% in volume.
    x

    Readers Who Read This Article Also Read

    1. Heat Transfer Phenomena in Immersion Cooling of Heated Steel Plate and Influence of Water Stirring on Heat Transfer Tetsu-to-Hagané Vol.64(1978), No.1
    2. 第97回講演大会討論会講演概要 Tetsu-to-Hagané Vol.65(1979), No.2
    3. 第95回講演大会討論会講演概要 Tetsu-to-Hagané Vol.64(1978), No.2
  • A Method for Estimating Residual Creep Lives of Cast HK-40 Reformer Tubings

    pp. 975-984

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    Results of short term creep rupture tests of creep damaged cast HK-40 reformer tubings, which were removed from ICI steam reformers after about 40000 to 70000 hours of service, were analyzed correlating to the various metallographic factors. It was found that area fractions of voids and fissures, which were characteristic of the third stage of creep degradation of HK-40 alloys, bore a good correlation with the creep rupture property expressed in the from of Larson-Miller parameters. Through analysis of the results, empirical equations for estimating the residual creep lives were proposed as follows:
    logσ=-0.7374+0.2994P0-0.01130P02
    P0=P+0.60+0.60V
    P=T×10-3 (logt+15)
    where,
    σ: scheduled operating stress (kg/mmmm2)
    T: scheduled operating temperature (°K)
    V: area fraction of voids (%)
    t: residual life (h)
    The only one inspection to be involved in the present estimation is the metallographic quantitative analysis of area fractions of microfractures as voids and microfissures. A criteria for the renewal of the existing reformer tubings was established to be an area fraction of voids and fissures of 1%, when the residual life would fall short of one year.
  • Effects of Ti, Nb and Mischmetal on Creep Rupture Strength of 25Cr-20Ni Casting Steel

    pp. 985-993

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    Centrifugally casting steel HK40 (0.4C-25Cr-20Ni) is widely used for reformer tube in the steam reforming process or cracking tube in the equipment which produces ethylen. A study has been made on the effects of complex addition of a small amount of the Ti, Nb and Mischmetal on the creep ruptures trength of HK40.
    By addition of Ti and Nb, (Ti, Nb) C is formed, and as a result of forming (Ti, Nb) C, precipitation of Cr carbides and their coarsening are retarded. Therefore, the strength and ductility of creep rupture are improved.
    Mischmetal is effective to improve the creep rupture strength, if it is added by alone. However the effect of Mischmetal can not be found in complex addition with Ti and Nb, because the strong effect of Ti and Nb hides the effect of Mischmetal.
  • On the Various Properties of Ni-Cr-Mo-W Heat Resistant Alloys for Nuclear Steel-making System

    pp. 994-1003

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    Ni-Cr-Mo-W system alloys were investigated in order to develop a heat resistant alloy which may satisfy various properties required for heat exchanger tube materials of nuclear steel-making system.
    In the first step, we investigated the effect of alloy elements on formability, high temperature strength, aging characteristics and weldability. And we selected some candidate alloy compositions in terms of creep rupture strength at 1000°C. Out of these alloys, Ni-0.07C-25Cr-5Mo-5W alloy turned out to be superior to other alloys when taking various properties.
    In the second step, the effects of C and Cr contents on creep rupture strength at 1000°C were examined on this alloy. The creep rupture strength was found to increase when C content was reduced from 0.07% to 0.03% and Cr content was raised from 25% to 27.5% and solution treatment temperature was raised. In terms of various properties, Ni-0.03C-27.5Cr-5Mo-5W system alloy was chosen, which also proved resistant to helium environment.
    In conclusion, we developed a candidate alloy with compositions of Ni-0.03C-27.5Cr-5Mo-5W which seemed to have high possibility of satisfying the creep rupture strength target, that is more than 1kg/mmmm2 at 1000°C and 5×104 hours in helium.
    x

    Readers Who Read This Article Also Read

    1. Heat Transfer Phenomena in Immersion Cooling of Heated Steel Plate and Influence of Water Stirring on Heat Transfer Tetsu-to-Hagané Vol.64(1978), No.1
    2. 第97回講演大会討論会講演概要 Tetsu-to-Hagané Vol.65(1979), No.2
    3. 第95回講演大会討論会講演概要 Tetsu-to-Hagané Vol.64(1978), No.2
  • Creep Rupture Properties and Weldability of 18Cr-15W-Ni Base Superalloy

    pp. 1004-1012

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    A study has been made of the effect of tungsten, chromium and molybdenum as solution strengthening elements for austenite matrix and yttrium and zirconium as grain-boundary affecting elements on creep rupture properties and weldability of Ni-base alloy which can be used for heat exchanger tube of high temperature gas cooled reactor. The optimum creep rupture strength at 1000°C was obtained in the composition of 18% Cr-15% W-0.5% Mo-Ni. Yttrium and zirconium were found to be beneficial elements to improve the creep rupture strength. The maximum creep rupture strength and good weldability of the alloy were obtained when the amounts of yttrium and zirconium were balanced to the amounts of sulfur and oxygen in the alloy. It suggests that these elements were considered to prevent the segregation of sulfur and oxygen at grain-bondaries.
  • Effects of Cr-W Balance and C Content on Structure and Creep Rupture Strength in Ni-Cr-W Alloy System

    pp. 1013-1021

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    In order to deterrnine the practically optimum composition in an Ni-Cr-W austenitic alloy system for an intermediate heat exchanger of a high temperature gas cooled reactor, the effects of Cr-W balance and carbon content on the struture and creep rupture strength in the alloy system have been investigated.
    Austenite single phase region determined by experiment is much smaller than that calculated by Barrows method in Ni-Cr-W system at 1000°C. Outside the austenite single phase region α-W and at least one of M6C and M23C6 carbides precipitate depending on the Cr-W balance. The precipitation of α-W and M23C6 carbide improves creep rupture strength, but that of M6C carbide rather degrades it, although solid solution strengthening is maximized at a Cr-W balance in the M6C predominant region. Among the alloys investigated in this study the 0.05% C-23% Cr-18% W-Ni alloy which is strengthened through the precipitation of M23C6 carbide and about 3% of α-W is the best from the point of view of creep rupture strength. For the carbon content of the above mentioned alloy the range from 0.040% to 0.065% is practically aptimum, because excessive carbon causes M6C carbide precipitatian and degrades creep rupture strength.
  • The Effect of Grain Size on Creep Strength of a 23Cr-18W-Ni Alloy

    pp. 1022-1030

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    In order to estimate the long term stability of the creep rupture strength in a 23Cr-18W-Ni alloy, which is developed for the use in a nuclear steel making process, the effect of grain size on creep behaviour has been studied on samples produced by the conbination of various kinds of heat treatment and cold work. Undissolved α-W particles were liable to prevent grain growth and this caused increase in creep rate at 1000°C under low stresses.
    The steady state creep rate, ε, with usual grain sizes and at 1000°C was found to be propor-tional to σ6 at high stresses and σ2.5/dd2 at low stresses, where σ is initial applied stress and d is average grain diameter. Grain boundary migration and/or recrystallization were observed in the specimens crept to secondary or tertiary stage at 1000°C. However, no evidence of grain boundary cracking or grain boundary sliding was obtained. These results suggest that the obvious and strong grain size dependence of the creep rate is related to recovery or recrystallization process near the grain boundary.
    When the grain size is larger than the critical value, the grain size dependence of the creep rate becomes ambiguous. This critical value increases with decreasing applied stress. This observation, in the engineering point of view, leads to the conclusion that the optimum creep strength around 1000°C at low stresses should be obtained when the grain size is about ASTM No. -1 ( grain dia-meter ≅400μ).
  • Microstructure and Creep Rupture Strength of Ni-Cr-W and Ni-Cr-W-Mo Alloys

    pp. 1031-1040

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    To obtain the basic knowledge needed for the development of alloys used for heat exchanger which is operated at near 1000°C in the nuclear steel making process, microstructure and creep rupture strength of Ni-Cr-W and Ni-Cr-W-Mo alloys which contain 12-20%Cr, 0-35%W and 0-25%Mo at 1000°C, have been investigated.
    The results are summerized as follows:
    (1) Solubility limit of Cr and W in Ni-Cr-W system is shown as Cr+ W≅39 (wt%). Above this limit of content of chromium plus tungsten, α-W is observed as second phase. When Mo is added to Ni-Cr-W alloy, Win the second phase, α-W, is gradually substituted by Ni, Mo and finally the second phase changes to μ-phase.
    (2) Creep strength of Ni-Cr-W solid solution alloy increases with increasing W content. The high creep strength is presumarably attributed to decrease of diffusion rate with increasing W content. The substitution of W by Mo decreases the creep strength.
    (3) Metallic phase α-W increases the creep and creep rupture strength. Intermetallic compound μ, however, increases creep strength, but does not increase creep rupture strength. This is caused by the preferential nucleation of cracks on the interface between matrix and μ-phase.
  • Cobalt-free Nickel-base Superalloys

    pp. 1041-1048

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    Cobalt-free nickel-base cast superalloys hava been developed. Cobalt is considered to be a be-neficial element to strengthen the alloys but should be eliminated in alloys to be used for direct cycle helium turbine driven by helium gas from HTGR ( high temp. gas reactor). The elimination of cobalt is required to avoid the formation of radioactive 60Co from the debris or scales of the alloys. Cobalt-free alloys are also desirable from another viewpoint, i. e. recently the shortage of the element has become a serious problem in industry.
    Cobalt-free Mar-M200 type alloys modified by the additions of 0.15-0.2wt% B and 1-1.5wt% Hf were found to have acreep rupture strength superior or comparable to that of the original Mar-M200 alloy bearing cobalt. The ductility in tensile test at 800°C, as cast or after prolonged heat-ing at 900°C ( the tensile test was done without removing the surface layer affected by the heatlng), was also improved by the additions of 0.15-0.2% B and 1-1.5% Hf.
    The morphology of grain boundaries became intricated by the additions of 0.15-0.2% B and 1-1.5% Hf, to such a degree that one can hardly distinguish grain boundaries by microscopes.
    The change in the grain boundary morphology was consldered, as suggested previously by one of the authors (M.Y.), to be the reason for the improvements in the creep rupture strength and tensile ductility.
  • A Series of Nickel-base Superalloys on γ-γ′ Tie Line of Alloy Inconel 713C

    pp. 1049-1058

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    Nickel-base superalloys used for blades in gas turbines are strengthened by γ′ (Ni3Al solution-hardened by alloying elements) precipitates. A general tendency is Observed that the alloy with the larger amount of γ′ has the higher creep strength. However, to clarify the relation between the amount of γ′ and creep strength of the alloy, it is necessary to avoid the change in solid solution hardening of γ and γ′, i. e. to keep their compositions constant.
    So, a series of alloys whose compositions are on γ-γ′ tie line of alloy Inconel 713C have been designed, The alloys were cast in vacuum to size to make creep rupture test pieces.
    Good agreement was obtained between the designed and measured amounts of γ′ after aging at 1000°C. γ and γ′ compositions were also nearly constant except Cr concentration which only a little affects γ and γ′ strengths.
    The following three points became clear by creep rupture test.
    1) The γ′ precipitation hardeninig nickel-base superalloys have the maximum creep strength at about 65mol%γ′.
    2) If γ′ amount is excess, eutectic γ′ forms to reduce creep strength.
    3) Grain boundary morphologies have effects on the rupture life.
  • Alloy Design for γ′ Precipitation Hardening Nickel-base Superalloys Containing Ti, Ta, and W

    pp. 1059-1068

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    The increase in the inlet gas temperature of a gas turbine is the most effective to raise the output efficiency of the turbine. For this purpose, blade materials having superior creep strength and hot corrosion resistance are required.
    In this study, γ′ precipitation hardening nickel-base superalloys being superior in the above two properties have been developed by a computer-aided alloy design. The outline of the computer-aided alloy design is as follows.
    At first, compositions of γ and γ′ which are in a state of equilibrium each other, both highly solid solution hardened, and have a small lattice mismatch are selected. Compositions of alloys with optimum amount of γ′ can be calculated by the application of the lever rule to the selected γ and γ′ compositions and the addition of carbides and boride compositions.
    The evaluation of the alloy was made by creep rupture tests and a crucible test using a molten Na2SO4+25% NaCl Salt mixture.
  • The Effect of the Microstructure and Heat Treatments on the Ductility of 50%Cr-Ni Castings

    pp. 1069-1078

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    A 50%Cr-Ni cast alloy has excellent corrosion resistance at high temperatures, but thicker castings show poorer ductility at room temperature. To find out the cause of this degradation the complex microstructure in castings with various thickness were analyzed qualitatively and quantitatively. And solution treatments to improve the ductility of castings and preconditionings to prevent them from being brittle in service conditions were respectively tried. Results are as follows:
    (1) The solidification of this alloy begins at lower temperatures than the eutectic line for Cr-Ni binary system and ends around 1295°C. The mean cooling rates, v, depend upon the casting thickness, d: v∝d-1.79.
    (2) The eutectic reaction in this alloy is retarded extremely by “halo effect. ” Dominant phases in cast structures are primary α1 (Cr-rich solid solution), which contains Widmanstätten-type γ2 (Ni-rich solid solution), and γ1, in which grain boundary reaction (GBR) advances during cooling down. In thicker castings with lower cooling rates, larger portion of γ1 is occupied by the colony of cellular GBR precipitates, α+γ.
    (3) The GBR colony acts as the site of initiation and propagation of cracks, so thicker castings with more colonies are less ductile.
    (4) Solution treatments at higher temperatures make α in GBR (α+γ) discontinuous and/or dispersed and improve the ductility of castings. But the application of this method to big ones is impractical.
    (5) The GBR progresses in service at high temperatures and impairs severely the ductility of castings. Preconditionings of the microstructure to leave discontinuous and separate α are effective to suppress fresh GBR and then to keep castings ductile.
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  • Progress in Manufacturing Procedure of Heat Resisting Steels and Alloys

    pp. 1079-1093

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  • Formability and Weldability of Zircomium Bearing Ferritic Stainless Steels

    pp. 1094-1103

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    The effect of zinconium alloying on the formability and the wedability of 17% chromium steels has been investigated by means of various tests.
    Alloying of zirconium whose content exceeds the value equivalent to the total amount of carbon and nitrogen (Zr≈7.4× (C+N) ) is found effective to improve the characteristics of the formability and the weldability of the steels. However, in the case of so much carbon and nitrogen contents, both the formability and the weldability of the steels containing zirconium are not improved, because of the precipitation of a large amount of zirconium carbides, zirconium nitrides or chromium carbides.
    When the Zr/ (C+N) ratio in the steels is too large, both the formability and the weldabilty are not excellent, because of the precipitation of a large amount of metallic compounds consisting of mainly iron and zirconium. The formability of the steels containing less than 0.01% (C+N) is not improved by alloying of zirconium, except of the bore expanding property.
    The range of optimum zirconium (Zr≈10× (C+N) ), carbon and nitrogen contents in the 17% chromium steels is found.
  • Influence of Alloying Elements and Prediction of Hot Deformation Resistance in Ni-base Alloys

    pp. 1104-1113

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    For the purpose of quick and precise estimation of hot deformation resistance in Ni-base alloy, hot torsional tests were carried out on solid sectioned specimens of 42 kinds of alloys. The results were analyzed by means of a multiple regression equation based on Ludwick's semi-logarithmic formula and Zener-Hollomon's relation. Variables of the equation are not only strain rate and temperature but also alloy contents of 11 elements, i. e. C, Si, Mn, Cr, Co, Fe, Mo, W, Al, Ti and Nb.
    Examinations of fitness of the equation were made on all of the Ni-base alloys, some 18-8 stainless steels and a low alloy steel. Consequently the fitness of the equation was fairly well except for an alloy containing a large amount of γ′ particles and the confidential limit for the estimation at 95% level was ±3.6kg/mmmm2. The equation also applied to prediction of deformation resistance under hot extrusion conditions and good correlation was found between the observed and calculated values.
    Among the elements, W, Mo, Cr and Co strengthened, in the order of magnitude, the Ni matrix, while Fe softened slightly. C and Nb also showed considerable strengthening effect, though its reliability was a little bit low. It was found that hot deformation resistance was strongly influenced by lattice constant, diffusion constant, Young's modulus and melting point but not by stacking fault energy. This leads to predict that Ta sholud show considerable strengthening effect.
  • Reheat Crackings in Nickel-Base Superalloys

    pp. 1114-1120

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    In this report, factors which infiuence the susceptibility to reheat crackings in Ni-base superalloys were clarified and the mechanism of reheat cracking was discussed.
    Decreasing S content in Ni-base superalloys is practically very useful to improve susceptibility to reheat of Ni-base superalloys. It is also important to select proper heat treatment conditions before welding to avoid reheat crackings in Ni-base superalloys.
  • 抄録

    pp. 1121-1125

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