Toshihei Misawa

The long term growth of the protective rust layer formed on weathering steel by atmospheric corrosion during a quarter of a century

Abstract The rust layers formed on weathering and mild steels by atmospheric corrosion in an industrial region for a quarter of a century have been characterized using various analytical techniques. In particular, analysis of a local portion of the rust layers by means of Raman spectroscopy gives important information on the structure of the layers. It is elucidated that the inner stable and protective rust layer which covered the surface of weathering steel mainly consists of nano-particles of α-FeOOH containing a considerable amount of Cr. The relative change of the amount of rust constituents for low alloy steels supports well a newly proposed schematic progress of long term alteration in stable and protective rust layer formed on a weathering steel in an industrial environment, i.e. the γ-FeOOH, as an initial rust layer, is transformed into a final stable rust layer of α-FeOOH, probably via an amorphous oxyhydroxide substance, during the long term atmospheric corrosion of a weathering steel.

The mechanism of atmospheric rusting and the protective amorphous rust on low alloy steel

Abstract Atmospheric rusting of mild and low alloy steels was studied by means of infra-red and far infra-red spectrophotometries, X-ray and electron diffraction methods and scanning electron microscopy. The rusting process can be interpreted on the basis of a previously reported diagram for rust formation in aqueous solution. A large amount of amorphous matter in rust formed in semi-rural atmosphere was identified by infra-red and far infra-red spectra as amorphous ferric oxyhydroxide, FeO x (OH) 8–2x . The amorphous ferric oxyhydroxide rust on low alloy steel was dense and uniform, and contained a considerable amount of bound water. From these results it can be concluded that the amorphous ferric oxyhydroxide rust acts as a protective barrier against atmospheric rusting of the steels. Cu, P and Cr in low-alloy steels are inferred to favour the formation of crack-free, uniform rust layer and help to produce uniform amorphous ferric oxyhydroxide.

Small punch tests for evaluating ductile-brittle transition behavior of irradiated ferritic steels

Abstract Small punch (SP) tests using small and TEM disk miniaturized specimens were developed to evaluate the ductile-brittle transition temperature (DBTT) of HT-9 and JFMS ferritic steels as candidates for the structural materials of a fusion reactor. It was shown that the temperature dependence of SP fracture energies with scatter in miniaturized testing can give the reliable information on the DBTT by use of the statistical analysis based on the Weibull distribution. An empirical correlation between the DBTT measured by the Charpy-impact test and that by the SP test was obtained. It became possible to estimate the reliable DBTT and lower-bound SP fracture energy curve using the data partitioning method when there are few data points and/or scatter.

Fracture toughness evaluation of fusion reactor structural steels at low temperatures by small punch tests

Abstract A small punch (SP) test using miniaturized specimens has been performed for cryogenic austenitic steels at 4.2, 77 and 293 K to evaluate fracture toughness in a fusion material program. An SP testing cryostat for load versus deflection curve measurements has been successfully constructed. A universal relationship between valid fracture toughness, JIC, and equivalent fracture strain, \ ge qf , for austenitic steels at different test temperatures has been confirmed and empirical parameters for that relation have been determined using a linear regression model. A linear correlation between valid JIC and \ ge qf has been clarified for austenitic steels at low temperatures, where the regression coefficient is found to be 845 kJ/m2. Using the results of the SP test above room temperature, a new attempt at statistical analysis has been proposed to estimate the relative change in fracture toughness due to neutron irradiation.

Dependence of impact properties on irradiation temperature in reduced-activation martensitic steels

Ductile–brittle transition (DBT) behavior of 9%Cr-2%W reduced-activation martensitic (RAM) steels has been investigated following neutron irradiation in the fast flux test facility, materials open test facility (FFTF/MOTA) at different temperatures. Both the irradiations at 663 and 733 K cause an increase in DBT temperature, while the irradiation at 663 K induces the hardening and the softening at 733 K. Microstructural observation by transmission electron microscope (TEM) revealed that small dislocation loops existed in the specimen irradiated at 663 K and no such a loop, but relatively large M6C carbides and Laves phase were formed by the irradiation at 733 K. There appears to be a linear dependence between ΔDBTT and ΔσY in neutron irradiated RAM steels when irradiation induces the hardening. Irradiation embrittlement accompanied by the softening is considered to be due to reduction of cleavage fracture stress caused by the irradiation-induced recovery of the martensitic structure, namely decrease in dislocation density and formation of large precipitates.

Evaluation of toughness degradation by small punch (SP) tests for neutron-irradiated 214Cr-1Mo steel

Mechanical properties correlations between the small punch (SP) test and conventional tensile, Charpy impact and fracture toughness tests were investigated on neutron irradiated 214Cr-1Mo ferritic steel. Estimation of radiation induced changes on tensile strength, elastic-plastic fracture toughness (JIC) and ductile-to-brittle transition temperature (DBTT) are thought to be basically possible by mechanical properties correlations when based on sufficient pre-irradiation data.

Stress corrosion cracking and hydrogen embrittlement studies of austenitic and ferritic steels by small punch test

Abstract A small punch (SP) testing apparatus using miniaturized specimens in high temperature and high pressure aqueous solutions was successfully developed to evaluate the resistance to stress corrosion cracking (SCC) and corrosion of candidate structural steels in water-coolant environment under irradiation. Anodic polarization curves of Type 304 austenitic and HT-9 ferritic stainless steels were measured in borate buffered solutions at 523 K and SP fracture initiation energies of stress corrosion were obtained as a function of potential and strain rate. SP-SCC tests using small specimens showed that sensitized Type 304 steel and SCC susceptibility at the regions of corrosion potential and passive potential. Since the result of SCC susceptibility evaluation obtained in the presented work was in fair agreement with that demonstrated by popular slow strain rate test (SSRT) using uniaxial tensile specimens, the SP-SCC test was proposed as one of the SCC testing methods using irradiated miniaturized specimens. The effect of hydrogen charging on SP fracture behavior of HT-9 steel was observed and the SP test was confirmed to be useful for the evaluation of the susceptibility of candidate ferritic steels to hydrogen embrittlement.

Impact properties of intermetallic compounds

Abstract The impact fracture energies of γ-TiAl and Co 3 Ti intermetallic compounds were measured at temperatures between 293 and 1273 K using miniaturized U-notch specimens. For comparison, the impact fracture energy of a ferritic steel was also measured. The fracture energy of γ-TiAl gradually increases with increasing test temperature and the fracture mode changes from cleavage fracture at 293 K to intergranular fracture at elevated temperatures. The fracture energy of γ-TiAl is almost constant at around 1073 K and appears not to vary with its tensile properties, since the yield stress drastically decreases, while the tensile elongation drastically increases at that temperature. The increase in the fracture energy of γ-TiAl with temperature is considered to be as a result of the susceptibility to hydrogen-induced cleavage fracture decreasing at higher temperatures. A Co 3 Ti (Co-23.1at.%Ti) intermetallic compound breaks in a completely ductile manner when impact tested, while it undergoes severe intergranular embrittlement when deformed at a slow strain rate.

Designation of alloy composition of reduced-activation martensitic steel

An alloy composition of reduced-activation martensitic steel for fusion reactor is designed on the basis of the experimental results of postirradiation microstructure, mechanical properties, such as creep, fracture toughness and tensile properties, hydrogen effects and corrosion. At present, a desired composition of the steel is 0.1C-0.05Si-0.5Mn-9Cr-2W-0.25V-0.02Ti-0.05Ta- < 0.002S- < 0.002P by weight percent. Effects of the other minor elements such as Al, Zr and B are also inspected. An addition of 0.05 wt% Ta increases the high temperature strength but reduces the fracture toughness. Susceptibility to hydrogen-induced cracking is reduced by an addition of 0.03 wt% Al, though it results in a severe degradation of the fracture toughness. An addition of 30 wppm B together with the addition of 0.02 wt% Ti increases the fracture toughness. Void nucleation at grain boundaries, however, is enhanced by the B addition under the FFTF irradiation at 638 K in 10 dpa.

Manufacture and hydriding characteristics of unidirectionally solidified LaNi5Ni eutectic alloys with disintegration resistance

Abstract Disintegration of metal hydride powder due to repeated hydriding and dehydriding cycles has been a serious problem which has retarded the practical application of metal hydrides. In order to solve this problem, unidirectional solidification was applied to LaNi 5 Ni eutectic alloys of chemical compositions ranging from hypoeutectic to hypereutectic. Examination of the disintegration resistance and hydriding characteristics revealed that these unidirectionally solidified alloys possess better resistance to disintegration than the as-arc-melted alloys of similar compositions after 30 cycles of hydriding and dehydriding. Disintegration resistance is enhanced as the amount of ductile nickel phase increases because the development of microcracks is arrested and the volume expansion is relaxed by the nickel phase. These LaNi 5 Ni alloys had higher equilibrium pressures and lower hydrogen concentrations compared with those of LaNi 5 powder. The results obtained are discussed in terms of the effects of lattice strain in hydrided LaNi 5 and the amount of LaNi 5 .