Masahisa Inagaki

Microstructure of Friction-Stir-Welded High-Nitrogen Stainless Steel

Friction stir welding (FSW) was applied to a 0.53% nitrogen stainless steel. The nitrogen content change and the microstructural evolution in the weld were investigated. The nitrogen content analysis revealed that the stir zone (SZ) showed roughly the same nitrogen content as the base material (BM). This result suggests that FSW is an effective welding process for keeping up nitrogen content of high-nitrogen steel weld. The microstructural observation showed that the weld had the BM, the SZ, the partially recrystallized zone (PRZ) and the heat affected zone (HAZ). The coarse grain structure of the BM changed to relatively fine microstructure of equiaxially recrystallized austenite grain structure in the SZ during FSW. The PRZ contained both the fine and coarse grain structures. The detailed TEM observations showed that the particles with about 11m in size were present in the advancing side of the SZ both along grain boundaries and grain interiors. The Cr nitride less than 100 nm was formed in the HAZ along grain boundaries, which indicates that a slight sensitization occurred in the HAZ during FSW.

Microstructures in Friction Stir Welded 304 Austenitic Stainless Steel

Friction stir welding was applied to 304 austenitic stainless steel. The microstructural evolution and hardness distribution in the weld were investigated. The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallised and recovered microstructures, respectively. The hardness of the SZ was higher than that of the base material and the maximum hardness was located in the TMAZ. The higher hardness in TMAZ was attributed to high density of dislocations and sub-grains. Electron microscopic observations revealed that ferrite and sigma phase were formed in austenite matrix in the SZ depending on the cooling rate during FSW.

A Systematic Survey of the Factors Affecting Zircaloy Nodular Corrosion

Out-of-reactor corrosion tests in 10.3-MPa steam at temperatures between 490 and 530°C have been carried out on Zircaloy-2 tube specimens of various fabrication histories including archive specimens of the same tube lots as were used in the commercial boiling water reactor (BWR) to get data for comparison between in- and out-of-reactor nodular corrosion performances. Test conditions, such as temperature and duration, as well as material parameters, such as chemical composition, texture, and precipitates, were examined to clarify the relation to the nodular corrosion susceptibility. The number density and size of nodules increased when tested at higher temperatures. Extending the test duration increased the nodule size, but not the number density. The archive tube lots showed different degrees of nodular corrosion in the out-of-reactor test and their relative rankings seemed to correlate with the visual ratings of in-reactor corrosion. A higher number density of precipitates tended to reduce the nodular corrosion susceptibility. It should be noted, however, that some lots constituted an exception to this, and the cause was sought in the tube manufacturing processes.

Corrosion Properties in Friction Stir Welded 304 Austenitic Stainless Steel

A 304 austenitic stainless steel was friction stir welded using polycrystalline cubic boron nitride (PCBN) tool. Relationship between corrosion properties and microstructure was examined in the weld using several corrosion tests and microstructural observation techniques. The stir zone (SZ) had better corrosion properties than the base material (BM). The corrosion tests showed that the intergranular corrosion was developed in the heat-affected zone (HAZ) compared to the BM and SZ although the grain boundaries were not severely corroded. TEM/EDS analysis revealed that Cr depletion zone near grain boundary in the HAZ was shallow and narrow. Friction stir welding (FSW) suppressed sensitisation in the HAZ, which could be explained by short duration at sensitisation temperatures during welding. On the other hand, many grain boundaries were deeply corroded in the AS, where the corrosion resistance was significantly degraded. The microstructural observation revealed that sigma phase was formed in the AS during FSW. Sigma formation produced the wide and deep Cr depletion zone with the minimum Cr content less than 12 wt % in the vicinity of the grain boundary in the AS, which severely deteriorated the corrosion resistance in the AS.

Microstructures and properties of friction stir welded 304 austenitic stainless steel

Abstract Friction stir welding (FSW) is a solid phase joining process which makes use of friction and was developed in 1991 by TWI (The Welding Institute) of Britain.1 This welding process received attention as a joining process to solve the problems associated with fusion welding of Al alloys and active research into the applications of this technique has been progressed by numerous research institutions, at home and abroad. As a result, this process was made fit for practical use in less than ten years of development and has been applied extensively in the manufacture of aerospace equipments,2 rolling stock3 and automobile components.4

Microstructural Evolution in 304 Austenitic Stainless Steel during Friction Stir Welding

The characteristics of microstructures in friction stir (FS) weld of 304 austenitic stainless steel were examined. The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallized and recovered microstructures, respectively. The hardness of the SZ was higher than that of the base material and the maximum hardness was located in the TMAZ. The higher hardness in TMAZ was attributed to high density of dislocations and sub-boundaries. Electron microscopic observations revealed that ferrite and sigma phases were formed in austenite matrix in the SZ during friction stir welding (FSW).

Photochemical Characterization of the Surface Oxide Films of Al‐Pd‐Si and Al‐Si

The effect of Pd addition to the Al alloy on the photoelectrochemical response is described. The oxide films are characterized by photoelectrochemical methods. The addition of Pd decreases the photocurrent during illumination. The properties of the surface oxide films on thin Al‐Pd‐Si and Al‐Si alloy films are discussed. The surface oxide film property is related to the increased corrosion resistance of Al‐Pd‐Si. A model of the aluminum oxide film structure on the Al‐Pd‐Si film is also proposed.

Effect of chemical composition on corrosion resistance of zircaloy fuel cladding tube for BWR.

Effects of Fe and Ni contents on nodular corrosion susceptibility and hydrogen pick-up of Zircaloy were investigated. Total number of 31 Zr alloys having different chemical compositions; five Zr-Sn-Fe-Cr alloys, eight Zr-Sn-Fe-Ni alloys and eighteen Zr-Sn-Fe-Ni-Cr alloys, were melted and processed to thin plates for the corrosion tests in the environments of a high temperature (510°C) steam and a high temperature (288°C) water.In addition, four 450kg ingots of Zr-Sn-Fe-Ni-Cr alloys were industrially melted and BWR fuel cladding tubes were manufactured through a current material processing sequence to study their producibility, tensile properties and corrosion resistance.Nodular corrosion susceptibility decreased with increasing Fe and Ni contents of Zircaloys. It was seen that the improved Zircaloys having Fe and Ni contents in the range of 0.30[Ni]+0.15[Fe]≥0.045 (w/0) showed no susceptibility to nodular corrosion.An increase of Fe content resulted in a decrease of hydrogen pick-up fraction in both steam and water environments. An increase of Fe and Ni content of Zircaloys in the range of Fe≤0.25w/0 and Ni≤0.1w/0 did not cause the changes in tensile properties and fabricabilities of fuel cladding tube. The fuel cladding tube of improved Zircaloy, containing more amount of Fe and Ni than the upper limit of Zircaloy-2 specification showed no susceptibility to nodular corrosion even in the 530°C steam test.