ISIJ International
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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559

ISIJ International Vol. 30 (1990), No. 8

  • Development of New Cryogenic Steels for the Superconducting Magnets of the Fusion Experimental Reactor

    pp. 567-578

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    DOI:10.2355/isijinternational.30.567

    The Japan Atomic Energy Research Institute (JAERI) has successfully developed new cryogenic steels for the superconducting magnets of the Fusion Experimental Reactor (FER) in collaboration with 4 steel companies. JAERI required a strength-toughness combination (σy>1 200 MPa, Klc>200 MPa√m, called JAERI box) that was beyond the capabilities of the existing austenitic stainless steels for cryogenic use. These requirements were determined from stress analyses, properties of the superconducting materials, crack propagation analyses, coil operation, etc. The new cryogenic steels were developed using tension, Charpy impact, and fracture toughness tests, step by step. The largest 4-K testing machine in the world, of maximum load 100 t, was installed at JAERI in order to evaluate the mechanical properties of large specimens and to investigate specimen size effects. The newly developed steels supplied from industrial heats (5-50 t) were named "Japanese Cryogenic Steels" (JCS), and their mechanical properties satisfied requirements mentioned the above. These steels have been already used in superconducting coils, such as JAERI's Demo Poloidal Coils (DPC).
    This paper reviews the development of the JCS for the superconducting magnets of the FER.
  • Effect of Phosphorus and Boron on Cryogenic Mechanical Properties of a Sensitized 17Cr-12.5Ni-2Mo-0.05Nb-0.2N Steel

    pp. 579-586

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    DOI:10.2355/isijinternational.30.579

    This research was undertaken to clarify effects of phosphorus and boron on cryogenic mechanical properties of a sensitized 17Cr-12.5Ni-2Mo-0.05Nb-0.2N steel. The steel was sensitized at 923-993 K for 270-720 ks and investigated at low temperature, especially at 4 K.
    An addition of either phosphorus or boron affected precipitation at grain boundaries during sensitization. Phosphorus increased the grain boundary precipitates. Boron suppressed the phosphorus effect. Both elements, however, did not show clear effect on tensile properties at 4 K of the sensitized steel. In a high phosphorus steel, Fe2Mo other than CrNbN, Cr2N, M23C6 and Nb(CN) precipitated and led to intergranular fracture. Therefore a deterioration of fracture toughness at 4 K for sensitized steel became much evident with an increase in phosphorus content. On the other hand, boron addition prevented intergranular failure and resulted in high fracture toughness for the sensitized one.
    Segregated phosphorus atoms seem to promote nucleation of precipitation and to weaken an interfacial binding of precipitates, especially Fe2Mo. On this assumption, effect of phosphorus and boron addition on mechanical properties at 4 K could be explained.
  • Development and Production of 18Mn-18Cr Non-magnetic Retaining Ring with High Yield Strength

    pp. 587-593

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    DOI:10.2355/isijinternational.30.587

    Retaining ring for electric generator is required to have non-magnetism, high strength, high ductility and stress corrosion cracking (SCC) resistance. Several kinds of materials for non-magnetic retaining ring have been developed and recently 18%Mn-18%Cr austenitic steel which has been developed to improve susceptibility to SCC of 18%Mn-5%Cr austenitic steel has begun to use. 18%Mn-18%Cr steel contained high nitrogen content has been investigated as the material with high strength, high ductility and SCC resistance. On the base of the fundamental studies of chemical composition and cold work on increasing yield strength of 18%Mn-18%Cr steel, a trial 18%Mn-18%Cr retaining ring with yield strength 1400 MPa was successfully manufactured and mechanical properties, fatigue strength, fracture toughness and SCC were investigated for the trial ring material.
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  • Development of Low Cost Non-magnetic Stainless Spring Steels

    pp. 594-599

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    DOI:10.2355/isijinternational.30.594

    In the present paper, the microstructure of recently developed non-magnetic austenitic spring steels of high strength is described and correlated to mechanical behaviour.
    It is possible to combine a high strength with a low magnetic permeability. This is achieved by a balanced alloying with strong austenite forming elements such as Ni, Mn and N.
    Further strengthening occurs as a result of precipitation hardening during tempering. The strong tempering response in Si-rich alloys is ascribed to enhanced precipitation, which is in part explicable by the ability of Si to increase the activity of C. In V-rich steels, the strong tempering effect is most likely caused by VN precipitation.
    A low magnetic permeability is achieved essentially by alloying with the strong austenite stabilizing elements Ni, Mn and N. Owing to the suppression of magnetic α-martensite a low magnetic permeability is maintained even after severe cold working.
  • Effect of Thermo-mechanical Control Process on Strengthening of a 22Mn-13Cr-5Ni Austenitic Stainless Steel Plate for Cryogenic Use

    pp. 600-607

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    DOI:10.2355/isijinternational.30.600

    The effect of thermo-mechanical control process (TMCP) on strengthening of a 22Mn-13Cr-5Ni austenitic stainless steel plate for cryogenic use has been investigated by using an intermittent hot compression test. Finish-compression temperature, cooling rate and start-cooling temperature are the factors controlling the recrystallized austenite grain size. The austenite grain becomes finer as the finish-compression temperature decreases and as the cooling rate and the start-cooling temperature increase in the recrystallization region, all of which are effective for strengthening the steel. On the basis of these results, a steel plate of thickness 70 mm was produced on a large scale under the optimum TMCP conditions and the mechanical properties of the steel plate were investigated. The TMCP steel attained a higher 0.2% yield strength than that of the solution heat-treated (ST) steel at -269°C, though the fracture toughness was slightly reduced.
  • Effects of Alloying Elements and Thermomechanical Treatments on Mechanical and Magnetic Properties of Cr-Ni Austenitic Stainless Steel

    pp. 608-614

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    DOI:10.2355/isijinternational.30.608

    Effects of alloying elements and thermomechanical treatment on mechanical and magnetic properties of Cr-Ni austenitic stainless steel have been investigated. N and Si increase hardness for annealed and cold rolled conditions. Aging treatment, following cold rolling, further increases hardness. Specially, Si exerts a substantial increase in hardness by aging treatment. On the other hand, Ni, Mn, Cr and N increase γ stability, while Si lowers γ stability against formation of strain induced martensite (α'). Nonmagnetism can be maintained by controlling γ stability, depending upon a degree of cold rolling. Based on the results, high strength nonmagnetic stainless steel can be obtained for Si and N containing Cr-Ni stainless steel with sufficiently high γ stability. Effects of Si on strain age hardening and α' transformation behaviors are discussed.
  • Effects of Alloying Elements on Hardening and Restoration Behavior of 15Cr-15Ni High Hardness Non-magnetic Stainless Steel

    pp. 615-624

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    DOI:10.2355/isijinternational.30.615

    In order to obtain informations for developing very hard non-magnetic stainless steel, the effects of various alloying elements on solid-solution hardening, cold-work hardening, strain age hardening, precipitation hardening and restoration behavior were investigated by using 15Cr-15Ni-1.5Mn steels melted in a vacuum furnace. These alloying elements were C, N, Si, P, Mo and B. Besides Vickers hardness tests, tensile tests, transmission electron microscopy, magnetic measurements, X-ray diffraction measurements, electrical resistivity measurements and autoradiography of B10 were carried out. By these experiments, the effects of alloying elements and their co-addition on hardness and strength were observed in detail and discussed at each step of solution treatment, cold-rolling and heating. Several effects were revealed especially concerning interaction of the elements.
  • X-ray Absorption Fine Structure Analysis of Interstitial (C, N)-Substitutional (Cr) Complexes in Austenitic Stainless Steels

    pp. 625-631

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    DOI:10.2355/isijinternational.30.625

    Local atomic environments around chromium atoms in 15Ni-15Cr austenitic steels with separate or combined addition of carbon, nitrogen and silicon are investigated by means of XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) measurements. A peak is observed on the rising slope of the chromium K-edge absorption spectrum. The peak is most distinct for the steels without alloying of carbon, nitrogen, or silicon. The peak is weaker in the steels with combined addition of silicon with carbon or nitrogen, and single addition of silicon than with separate addition of carbon and nitrogen. Height of the first peak in the Fourier transforms of EXAFS spectra of the solution-treated state lowers with addition of interstitial elements: carbon or nitrogen. These results suggest that interstitial atoms gather around chromium atoms to form a kind of Interstitial-Substitutional (I-S) complexes such as a shortrange ordered structure. Silicon probably promotes formation of the I-S complexes around chromium atoms. In the steels alloyed with carbon or nitrogen, the height of the first peak is elevated by cyclic or uniaxial tensile deformation probably because of the reduction in density of the I-S complexes. Plastic deformation produces no significant modification in the XANES spectra. The I-S complexes, which may form in Cr-Ni and Cr-Mn austenitic steels containing both or one of carbon and nitrogen, are considered to play an important role in solid solution hardening in the solution-treated state and also in work softening in the low cycle fatigue test owing to their breakaway during plastic deformation.
  • Effect of Si and Al Additions on the Low Temperature Toughness and Fracture Mode of Fe-27Mn Alloys

    pp. 632-638

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    DOI:10.2355/isijinternational.30.632

    The effect of Si and Al additions on the low temperature toughness and fracture mode of Fe-27Mn (in mass%) alloys was investigated in terms of the microstructure, heterogeneous deformation and stacking fault energy. The Fe-27Mn binary alloy has two constituent phases at 77 K; Epsilon martensite (ε) and metastable austenite (γ). It undergoes a ductile-to-brittle transition because of an intergranular fracture associated with heterogeneous deformation. The intergranular fracture is caused by stress concentration at grain boundaries on which large ε plates impinge. Silicon addition to the Fe-27Mn binary alloy is effective for refining ε plates and changes the fracture mode from intergranular to transgranular. The fracture is, however, a quasi-cleavage mode along ε plates so that Fe-27Mn-Si alloys also exhibit a ductile-to-brittle transition related to the formation of ε. The stress concentration at the intersection of ε plates results in the formation of microcracks along the ε plates leading to the quasi-cleavage fracture. Fracture modes of high manganese steels containing ε largely depend on the microstructure and the deformation behavior. Silicon addition probably affects not only the nucleation of ε, but also the cross slip behavior of partial dislocations at the intersection of ε plates through a decrease in the stacking fault energy of γ, thus leading to the change in fracture mode.
    Aluminum addition to high manganese steels is so effective for suppressing the γ-ε transformation that the low temperature embrittlement associated with the formation of ε does not occur with even a small Al addition. In an Fe-27Mn-2.5Al alloy, for example, toughness is high even at 77 K because the γ phase is so stable that the transformation to ε does not occur during deformation at 77 K.
  • Effect of Cold Rolling on Mechanical Properties and Fracture Mode of a 32Mn-7Cr Steel at Cryogenic Temperatures

    pp. 639-645

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    DOI:10.2355/isijinternational.30.639

    The effects of cold rolling on strength and toughness at 4 K were investigated for a high manganese steel, 32Mn-7Cr steel. The yield strength at 4 K was 1118 MPa for the unrolled material and was increased to 1379 and 1786 MPa for the 20 and 40% cold-rolled materials, respectively. However, the elongation and the reduction of area were decreased with an increase in reduction ratio. Especially, the elongation was significantly decreased. The Charpy absorbed energy and the fracture toughness were also decreased markedly. The decrease in toughness by cold rolling was due to the formation of subcrack and "flat brittle facet" in the fracture surface. The subcrack was caused by intergranular fracture, and the flat brittle facet is believed to be produced mainly by intergranular fracture and partly by twin cracking.
  • Suppression of Cryogenic Intergranular Fracture through Heat Treatments and Roles of Boron in High Manganese Non-magnetic Steels

    pp. 646-655

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    DOI:10.2355/isijinternational.30.646

    The effects of cooling rate and reheating conditions after solid solution treatment on mechanical properties, especially low temperature embrittlement, have been investigated by using Fe-32Mn-7Cr-(0.3-0.4)N, Fe-33Mn, Fe-33Mn-0.3C, Fe-44Mn, Fe-44Mn-0.3C, Fe-58Mn, and Fe-32Mn-7Cr-0.3N-(0.002-0.02)P-(0.0002-0.001)B steels melted in vacuum furnaces. It was found that intergranular fracture was suppressed and toughness increased in all these steels through cooling at an intermediate rate from solid solution treatment temperature or through reheating at around 723-773 K. After such toughening heat treatments no change in microstructures was revealed under an electron microscope and the decrease in tensile strength was very small if any. Autoradiography using α particle fission tracks showed that the grain boundary segregation of boron corresponded to the toughening. The reason for the high susceptibility to low temperature intergranular embrittlement of high Mn non-magnetic steels was also discussed together with the mechanism for the toughening resulting from the heat treatments.
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  • Cleavage-like Fracture at Low Temperatures in an 18Mn-18Cr-0.5N Austenitic Steel

    pp. 656-662

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    DOI:10.2355/isijinternational.30.656

    The low temperature embrittlement in an 18Mn-18Cr-0.5N retaining ring steel has been investigated by means of tension and Charpy impact tests. The effects of grain size, strain rate and pre-deformation were examined.
    The ductile to brittle transition (DBT) behaviour associated with the cleavage-like fracture has been found, where the fractured plane was regarded as {111}. The traces of intense planar plastic flow which may be related with twinning, slip band, or epsilon martensite were observed on the surface of plastically deformed specimen. When the pre-deformation was applied to the specimen at room temperature, the fracture stress at 77 K was slightly increased but the fracture mode was still cleavage-like. The DBT temperature (DBTT) is scarcely influenced by grain size, although the fractured surface unit is considered as austenite grain size. This is discussed based on the experimental data on the grain size dependences of flow stress and fracture stress. When the strain rate in tension test is increased up to 2.7×10-1/s, the fracture mode at 77 K has been found to change from cleavage-like to dimple. In this case, the plastic deformation occurs very locally and heavily, so as to induce the necking of the specimen.
  • Intergranular Fracture in Fe-Mn Austenitic Alloys at 77 K

    pp. 663-665

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    DOI:10.2355/isijinternational.30.663

  • Effect of Transformation Cycling on the ε Martensitic Transformation in Fe-Mn Alloys

    pp. 666-673

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    DOI:10.2355/isijinternational.30.666

    The effect of γ_??_ε cyclic transformation on the subsequent forward and reverse transformations was studied in low carbon Fe-17%Mn and Fe-25%Mn binary alloys. A remarkable decrease in Ms temperature by the cyclic transformation was observed in the Fe-25%Mn alloy. After 7 thermal cycles between 305 and 573 K, Ms decreased by over 60 K and the structure became fully austenitic at room temperature. On the other hand, As and Af temperatures increased with an increase in the number of thermal cycles. As the upper peak temperature of thermal cycling increased, the changes in Ms, As and Af became smaller. The decrease in Ms by the cyclic transformation was smaller in the Fe-17%Mn alloy than in the Fe-25%Mn alloy. A large number of dislocations were observed in the austenite of the Fe-25%Mn alloy after the thermal cycling between 273 and 573 K, although dislocations were scarcely observed before the thermal cycling. The hardness of austenite at 573 K was markedly increased by the thermal cycling between 273 and 573 K. A microstructure memory, which is a phenomenon that martensite forms in the same position where the martensite formed in the last forward transformaton, was not observed.
  • Effects of Alloying Additions on Fe-Mn-Si Shape Memory Alloys

    pp. 674-679

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    DOI:10.2355/isijinternational.30.674

    Fe-Mn-Si alloys are shape memory alloys which make use of the γ→ε stress-induced martensitic transformation. In this study, we report the effects of alloying additions on the shape memory effect (SME) of these alloys. It was found that the Ms temperature, the Neel temperature (TN) and the volume of stress-induced martensite govern the SME. Through the optimization of these factors we found that new alloy systems such as Fe-28Mn-6Si-5Cr, Fe-20Mn-5Si-8Cr-5Ni and Fe-16Mn-5Si-12Cr-5Ni alloys could exhibit good SME along with good corrosion resistance. And it was also found that the thermomechanical treatment which improved the SME in Fe-Mn-Si base system was also effective to improve the SME of these new systems.
  • Phase Equilibria in Fe-Mn-Al-C Alloys

    pp. 680-686

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    DOI:10.2355/isijinternational.30.680

    Phase constitutions of Fe-(20-30)wt%Mn-(0-10)wt%Al-C alloys have been investigated by electron probe microanalysis and transmission electron microscopy. The phase relations between austenite and perovskite carbide, (Fe, Mn)3AlC in the temperature range of 900-1 200°C have been carefully examined. An L12-type ordered structure, which was reported to be formed in rapidly solidified alloys as a metastable phase has not been detected in specimens aged at temperatures above 600°C.

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