Yuzo Hosoi

Precipitation behavior of laves phase and its effect on toughness of 9Cr-2Mo Ferritic-martensitic steel

This study clarified the ralationship between the toughness of a 9Cr-2Mo dual phase steel and precipitates formed during aging, with special attention to the Laves phase (Fe 2 Mo). The ductile-brittle transition temperature (DBTT) is increased and the upper shelf energy decreased when the Laves phase begins to precipitate during aging. Electron microscopy and X-ray diffraction indicate that elimination of Si in the steel reduces the precipitation of the Laves phase and results in maintaining good toughness. It is also noted that the toughness of the steel is controlled by the total amount of precipitates (Laves + carbides) in the aging at 873 K for more than 3.6 × 10 3 ks. A time-temperature-precipitation diagram for the Laves phase is established and it clearly shows that the precipitation of the Laves phase is markedly retarded by the decrease of Si content. In Si-free steel, no Laves phase is observed in the temperature and time range investigated.

The effect of helium environment on the creep rupture properties of Inconel 617 at 1000°C

The effect of oxygen contents in helium and in vacuum on the creep rupture properties of Inconel 617 has been investigated at 1000°C under the stress of 3.5 Kg/mm2. The main results are as follows. 1) The creep rupture properties in 99.9999 pct and 99.995 pct helium and in high vacuum are almost the same as those in air. 2) The oxygen in helium causes a remarkable decrease of the creep rupture time and similar effect of oxygen is also found in the controlled pressure of air. 3) The effect of oxygen on the creep rupture properties is attributable to the decarburization which tends to decrease the high temperature strength.

Scanning Electron Microscopic Observation of Fracture Surface of Austenitic Stainless Steels in Stress Corrosion Cracking

Abstract Fracture surface of several austenitic stainless steels in stress corrosion cracking (SCC) in MgCl2 and CaCl2 solution was investigated fractographically by scanning electron microscopy. Effect of Mo, applied stress, and testing temperature on the fracture mode was studied. Type 304 steel and 310 steel fractured transgranularly in MgCl2 solution boiling at 143 C (289 F). Transgranular fracture was characterized by a fine parallel pleat pattern. This pattern seemed to be traces of dissolution of metals. Intergranular SCC occurred in Type 316 steel and 16Cr-15Ni-2~4Mo steels, and it was also observed in Type 304 steel fractured at temperatures lower than 125 C (257 F). Intergranular fracture tended to occur with increasing Mo content, applied stress, and lowering testing temperature.

Effect of thermo-mechanical treatment on toughness of 9Cr-W ferritic-martensitic steels during aging

Abstract The precipitation behavior and toughness during aging of thermo-mechanically-treated (TMT) 9Cr-2W, 9Cr-4W and 9Cr-2No ferritic-martensitic steels were investigated. All three steels showed an increase on toughness after TMT in the as tempered condition. This was the result of a fine-grained microstructure introduced by TMT. However, in 9Cr-2Mo and 9Cr-4W steels, precipitation of M 2 3C 6 , M 6 C and the Laves phase (Fe 2 W or Fe 2 Mo) was accelerated due to TMT and the beneficial effect of fine-grained microstructure on toughness was lost by aging at 875 K for 360 ks. In the case of 9Cr-2W steel, the accelerated precipitation was slight and good structural stability and toughness were maintained after 360 ks aging. A decrease in toughness, however, was observed after 360 ks aging, when the Laves phase was finally found to precipitate.

Change in microstructure and toughness of ferritic-martensitic stainless steels during long-term aging

A study has been made on the improvement of toughness of ferritic-martensitic (9Cr-2Mo) steel during long-term aging in the temperature range from 773 to 873 K. It was found that an increase in Mn and elimination of Si give a very beneficial effect on the toughness, whereas a relatively large loss of the toughness was found in conventional 9Cr-2Mo steel. P in the steel is also shown to be harmful to the touchness. Microstructural observations reveal that intermetallic compounds containing Si and P precipitate massively in ferrite grains, and relatively large carbides (M23C6 and M6C) are observed in martensite grains and at grain boundaries in the aged materials containing Si. On the other hand, in the aged Si-free high Mn steel, no such intermetallic compounds are detected. It is believed that intermetallic compounds as well as carbides play an important role in changes of toughness by aging and that the loss of toughness by aging may be prevented by the addition of Mn without Si. which probably suppresses the precipitation of intermetallic compounds and carbide growth.

Influence of tungsten, carbon and nitrogen on toughness and weldability of low activation austenitic high manganese stainless steels

The effect of alloying elements of tungsten, carbon and nitrogen on high temperature strength, toughness and weldability of Fe12Cr15Mn alloy has been investigated. The high temperature stregth of Fe12Cr15Mn0.2C0.1N at 873 K increases with the addition of 2–300W without affecting ductility. The toughness as estimated by Charpy tests, is also not influenced by the addition of 2–3%W, while the increase of carbon content decreases the absorbed energy. The transition temperature shifts to higher temperature by aging at 873 K for 3600 ks, but it is still lower than room temperature. The degradation of tougheness after aging is considered to be related to the precipitation of M23C6 on grain boundaries. The weldability evaluated by hot cracking susceptibility is not affected by alloying of tungsten and carbon in this alloy system. It is noted that the alloys studied show less hot cracking susceptibility than commercial AISI 316L stainless steel.

Microstructures of neutron-irradiated Fe12CrXMn (X = 15–30) ternary alloys

Abstract The FeCrMn alloy system is being studied as an alternative to the FeCrNi system because of the desire to reduce long-term radioactivation in fusion power devices. In this study four Fe12CrXMn (X = 15, 20, 25, 30 wt.%) alloys were irradiated in FFTF to 20 dpa at 643 K and 40 dpa at 679, 793 and 873 K to investigate the influence of manganese content on void swelling and phase stability. These results confirm and expand the results of earlier studies which indicate that the FeCrMn system is relatively unstable compared to that of the FeCrNi system, with alpha and sigma phases forming as a consequence of thermal aging or high temperature irradiation.

Effects of W and Mn on the mechanical properties, microstructure and aging behaviors of 10% Cr ferritic steels considered as reduced radio-activation materials

Abstract The effects of W and Mn alloying elements on the microstructure, mechanical properties and aging behavior of 10% Cr ferritic stainless steels were investigated. Tensile strength and hardness after tempering and/or aging treatment increased gradually with increasing Mn content, but the effect of W additions was very small. Mn and W, however, had large effects on the Charpy absorbed energy. The absorbed energy after tempering and/or aging treatment decreased with increasing Mn content. But as for the effect of W, Charpy absorbed energy had maximum peaks at 1% W addition for all 10Cr-0 to 5 Mn (mass%) steels. The fracture mode of the impact test at room temperature was complex. Martensitic materials showed intergranular brittle fracture after tempering but ductile fracture after aging treatment. Dual phase material which contains a ferritic phase and a martensitic phase indicated ductile fracture after tempering, but transgranular cleavage fracture occurred after aging. It was concluded that 1% W addition would be most appropriate to 10Cr-0 to 5Mn-0.08C ferritic steels from the points of view of hardness, tensile strength, Charpy absorbed energy and fracture mode.

Phase stability and high-temperature strengths of high manganese-chromium-iron base alloys as reduced radio-activation materials

Abstract The Fe-Cr-Mn alloy system has been proposed to replace the Fe-Cr-Ni alloy system in the construction of components for fusion energy devices. The present work is to investigate the potential properties of high Mn-Cr austenitic steels. Especially, the focus is placed on the microstructural stability and high-temperature mechanical properties of Fe-12% Cr-15% Mn-C-N alloys. By the combined addition of 0.2% C and 0.2% N to the Fe-12% Cr-15% Mn alloy, a good microstructural stability of austenitic phase and high-temperature tensile strengths comparable with those of Type 316 austenitic stainless steel are obtained.

Microstructure and mechanical properties of α-particle irradiated stainless steels

The effect of helium injected in stainless steels on the mechanical properties are investigated. The materials used are 100 μm thick foils of type 316 austenitic stainless steel, dual phase (9Cr-2Mo, ferritic and martensitic phase) stainless steel and duplex (22Cr-5Ni, ferritic and austenitic phase) stainless steel. The tensile strength (0.2% proof strength and ultimate strength in room temperature tensile test) of these materials increased slightly with the amount of injected alpha particles, but the elongation of these steels decreased with the alpha particle dose. Intergranular type fracture was observed only on the 316 stainless steel in which alpha particles had been injected in amounts up to 1.3 × 10 17 cm −2 at the depth of range. Transgranular fracture occurred on a post-injection annealed 316 stainless steel and on the other materials. The transmission electron microscope observation of helium bubble distribution established that the intergranular fracture is caused by a highly dense distribution of small helium bubbles at grain boundaries.