In this paper basic concept, brief history, state-of-the-art and future research and development items about high strength bolted connections in steel structures are mentioned. High strength bolted connections are generally used as site connections of steel building and bridge structures. Most predominant high strength bolted connections are high strength bolted grip joints, where high preload in bolts and high slip coefficient at connection surface are required. Tension type connections, where high preload in bolts are generally required, are also used in various type of structures. Tensile strength of usual high strength bolt (which are called F 10T) is 1, 000 N/mm2, and that of hot dip galvanized high strength bolt (F8T) is 800 N/mm2. But recently super high strength bolts, whose tensile strength is larger than 1, 400 N/mm2, were developed and already used in actual bulding structures. Nearly 150, 000 ton high strength bolts are yearly used in building strutuers and 15, 000 ton in bridge structures. Research and development items about high strength bolted connections expected in near future are strengthening of tensile strength such as 2, 000 N/mm2, size up of bolt daimeter, improvement of corrosion resistance and fire resistance efficiency, chemical treatment of nuts, heightening of slip coefficient of connection surface, improvement of tightening methods of bolts and so on.
For the purpose of reduction of construction cost, the minority steel plate girder bridges with large section main girder has been constructed in JH. In those bridges, the rationalization of field joints shall be required. The hybrid joints which are made welded flanges and bolted web, is considered one type of rational field joints. The loading experiments and FEM analysis using I-section girder, were carried out. As those research results, the performance of hybrid joints were cleared. With these results, the hybrid joints were applied in two expressway steel bridges.
Bolts are used in a wide variety of fields including the automobile industry, the civil engineering and construction industry, and the machine industry. Therefore, bolt strength class and fastening method are all highly variable according to use location and purpose. Consequently, the optimum steel grade for the application concerned must be selected from carbon steels to alloy steels. Bolts for steel structures are standardized by 'Sets of high strength hexagon bolt, hexagon nut and plain washers for friction grip joints (JIS B 1186 by Japanese Standards Association)' and 'Sets of torshear type high strength bolt, hexagon nut and plain washers for structural joints (JSS II 09 by Society of Steel Construction of Japan)'. But their bolts are not specified about used materials. However, it is necessary for the materials to possess a lot of characteristics, example for toughness and ductility, corrosion resistance, fire resistance, delayed fracture resistance, according to the use condition. In this paper, high strength bolt steels for steel structures are mainly introduced about conventional bolt, hot dip galvanized bolt, fire resistance bolt, weathering bolt, stainless bolt, super high strength bolt.
This review article on delayed fracture of bolts covers the following topics: general features of delayed fracture, case histories of in-service failures of high-strength bolts, existing demands for higher-strength bolts and their background, increasing needs for standardization of delayed fracture susceptibility evaluation method, and approaches so-far taken to prevent delayed fracture. The author emphasizes that, thanks to the recent intensive R&D activities, overcoming delayed fracture is now very close to the reality.
The evaluation methods of the delayed fracture properties of high strength steels and the methods which improve the delayed fracture resistance of steels have been reviewed. The evaluation method which is crucial for the development of steels with high strength has yet to be authorized. However, a new evaluating method which adopts the critical hydrogen concentration for fracture and the hydrogen concentration which is absorbed from the environment as parameters has been proposed. It was verified that evaluation using this method showed good agreement with exposure tests of bolts in the service environment, and this method is being applied not only to delayed fracture but to other hydrogen embrittlement problems. To improve the delayed fracture properties of steels, the hydrogen trapping phenomenon in alloy carbides such as VC, Mo2C and TiC, and the TMCP process have been researched extensively. By using these methods, the development of high strength steels with excellent hydrogen embrittlement resistance has progressed.
The literature on Microbiologically Influenced Corrosion (MIC) for the past 20-25 years was reviewed by standpoint on the microbiological processes happening on the material surface resulting in corrosion. The literatures showed early studies largely used electrochemical techniques to determine the corrosion due to microbes. However, the works in the recent past realized the importance of biological processes, which include bacterial adhesion and interaction of their metabolic end products with the materials leading to material degradation. Regarding the materials that were studied for the incidence of MIC, metals attracted the major attention during the early period, which over a period of time spilled over to other materials such as concrete, composites, plastic, art materials etc. This review mentions the need for an interdisciplinary approach between the material scientists and microbiologists to realistically study MIC on industrially important materials.
A velocimeter was proposed for the measurement of the surface velocity of molten metal flow. Cold model experiments were carried out to evaluate its accuracy. Wave motions were imposed on the molten Wood's metal surface by means of the Lorentz force. The propagation times of the waves in the upstream and downstream directions were detected with two level sensors. The surface velocity of molten Wood's metal flow was calculated from the propagation time difference and the distance between the two level sensors. The surface velocity could be accurately measured even if wave motions, i.e., meniscus level fluctuations caused by other external and internal oscillations existed on the molten metal surface. These disturbances can readily be removed using a narrow band pass filter because the frequency of the imposed wave motions is predetermined.
An automobile shredder residue (ASR) is an unavoidable by-product in the recycling of end-of-life vehicle (ELV). A new recycling process of ASR has been developed, in which ASR is put in coal-tar based oil and is separated by gravity into plastic floats and metal sediments. The fundamental experiments and bench scale test were carried out so as to confirm the separation behavior of ASR. The decomposition of polyurethane and dechlornation of polyvinyl chloride included in ASR proceeded rapidly under the conditions of 280°C and 15 min in coal-tar based oil, compared with in a gas phase owing to a large heat capacity and a high thermal conductivity of the liquid solvent. From the operation results of 1200 t/y pilot plant, the floats of 73% and sediments of 27% from ASR were recovered during 50 hours continuous operation. The floats composed of plastics free from copper and less of chlorine is suitable for reducing agent in blast furnace, and the sediments containing high contents of metals can be used for raw material in smelting. On the basis of above results, it is concluded that the ELV recycling of 95% or more can be achieved by using the proposal process.
This report proposes a new inclusion inspection method using ultra-sonic fatigue tests at 20 kHz. Fish-eye fractures occur in high-strength steels and the most harmful defect for fatigue in the risk volume of a specimen appears at the fracture site. In the other word, fatigue tests certainly specify the defect causing fatigue failure. The method was validated on the basis of fatigue test results for two low-alloy steels and a spring steel. Fatigue tests had a larger inspection volume and detected a larger inclusion than a conventional direct observation method on mirror surfaces. In case of a low-alloy steel in which about 10 μm Al2O3 inclusions existed together with smaller TiN inclusions, the TiN inclusion and matrix cracks also caused fish-eye fracture. Even in a such case, fatigue tests certainly specify the most harmful defect for fatigue. In case of Al2O3 inclusions, the inclusion size could be measured on the fracture surface. For TiN inclusions, however, the sizes were difficult to measure. For matrix cracks, the size could be measured in the spring steel but was difficult to measure in a low-alloy steel. Although distinction of inclusions based on ASTM standard and measurement of TiN inclusion sizes were basically possible in fatigue test, the procedures took time and were costly.
The pore structure is very critical and sensitive to react with CO2 in the case of cokes. But it is very difficult to detect the pore structure of cokes with nondestructive and high resolution because conventional porosimeter method can't detect macro pore (over 400 μm), and at the same time with destructive. We are at first demonstrating 3D-SPRITE imaging to detect the pore structure using SF4 gas, before and after CO2 reaction, (over 1 μm) with non destructive and the same pores, and then clarify the reaction mechanism. We found that the reaction rate of the cokes with CO2 depends on the pore size and the critical pore size is over 100 μm.
Microbiologically influenced corrosion was suspected in a corrosion failure of stainless steel (SUS316L) piping, carrying saline groundwater. Laboratory simulation studies were planned to find out the mechanism of the corrosion. The groundwater from the same plant was transferred to the laboratory aseptically and used for the experiment. Both SUS316L and SUS304L welds (base metal, heat affected zone, weld metal) were exposed to the test solution. After 40 d, corrosion sites were observed on the surface of coupons exposed to non-sterile groundwater supplemented with nutrients. In case of both sterile and non-sterile groundwater devoid of nutrient, the corrosion sits were not observed. Therefore, it is assumed that the corrosion occurrence requires bacteria, their activity and metabolism. The form of corrosion was pitting for base metal and heat affected zone, whereas, weld metal showed skeletal corrosion caused by preferential attack on γ-austenite. Generally, the pH showed no change during exposure test. The potential increased during exposure to nutrient contained non-sterile groundwater for all coupons. In the case of both sterile and non-sterile groundwater devoid of nutrient, the potential did not show an increase for all the coupons. The potential increase that is influenced by bacteria is assumed to be the reason for the easy occurrence of corrosion.
Microbiologically influenced corrosion was suspected in a corrosion failure of stainless steel (SUS316L) piping, carrying saline groundwater. Laboratory simulation studies were planned to find out the mechanism of the corrosion. Culturing of bacteria strains in the ground water revealed 7different species. Six of them were aerobic and one of them was anaerobic microbe that is Sulfate Reducing Bacteria (Desulfovibrio sp.). They were incubated separately in liquid medium and exposure studies were conducted. Formation of biofilm on SUS316L coupons was observed when they were exposed to the test solution with Comamonas sp., Sphingomonas sp., Agrobacterium sp., and Desulfovibrio sp. Pitting corrosion sites were observed on SUS316L coupons exposed in Desulfovibrio sp. inoculated solution. Corrosion potential of SUS316L coupons became noble when they exposed to the solution with Nutrient broth (NB). When SUS316L coupons exposed to the solution without NB and the sterile solution, the increasing of corrosion potential was not seen. The corrosion potential decreased when exposed to the aerobic microbes inoculated solution. However, in case of Desulfovibrio sp. inoculated solution, it stagnated in early stages, and increased after that. It was thought from the above result that Desulfovibrio sp. with aerobic microbes that are able to formed biofilm could influence this MIC.
In the use of stainless steels at seawater environment, pitting or crevice corrosion of stainless steels become a problem. It was reported that the potential of stainless steel in natural seawater environment rises more noble than in artificial seawater. On the other hand, it is well known that alloying chromium, molybdenum or nitrogen are effective in order to improve pitting and crevice corrosion resistance of stainless steel. The purpose of this study is to obtain a guideline for creation of high nitrogen-bearing stainless steel with excellent localized corrosion resistance. For that reason, it was investigated the effect of alloy elements on localized corrosion resistance for nitrogen bearing stainless steels. 17%Cr-14%Ni-2%Mo-0.2%N based stainless steels were used as test samples. Chromium, molybdenum and nitrogen contents were changed. Pitting potential in artificial seawater was measured. And crevice corrosion loss in simulated environment for seawater was estimated. As a result, chromium, molybdenum and nitrogen improve pitting and crevice corrosion resistance. It was shown that there was a synergistic effect between molybdenum and nitrogen. Crevice corrosion resistance of nitrogen bearing stainless steels was electrochemically investigated. It was cleared that crevice corrosion potential measured in potentiostatic after potentiodynamic method showed higher potential than in potentiostatic method. This behavior is probably caused by difference of surface statement. The guideline for creation of high nitrogenbearing stainless steel used in seawater environment was obtained.
In order to investigate the effect of the boron on microstructural change during the creep deformation of the high Cr ferritic heat resistant steel, the change of precipitate, microstructure and hardness during creep deformation was evaluated. Creep strength increases by suppressing creep deformation with increasing the boron content. The type of the precipitate doesn't change by the boron addition. M23C6 and MX precipitate at quality heat treatment, and then in addition, the Laves phase also precipitates during creep deformation. It is confirmed that the coarsening of M23C6 and Laves phase is suppressed and the refinement of MX is promoted by the addition of boron. In the boron free steel, the recovery of lath martensite structure such as the increase of the martensite lath width and the change into subgrain structure, progresses during creep deformation, and then homogeneous subgrain structure is observed in crept specimen. On the other hand, in 140 ppm boron containing steel, the lath martensite structure is kept within the grain, though subgrain is observed along prior austenite grain boundary in crept specimen. In accelerated creep region, the reducing of hardness of 140 ppm boron containing steel is smaller than that of boron free steel by the heterogeneous recovery at the gage portion. From the evaluation of the creep interrupted test material, it was confirmed that the addition of boron contributed to the creep strengthening by stabilizing the precipitate and suppressing the recovery of the lath martensite structure in the high temperature and long term condition.
Precipitation behavior of TaC in austenite of 8Cr2WV steel has been studied. Ta contents in the extracted residues were measured chemically for the specimens both usually normalized and step-normalized (directly cooled from a homogenized temperature to a normalizing temperature). In the step-normalized specimen precipitation of TaC is very slow due to lack of precipitation cites. On the contrary, in the usual normalized specimen precipitation of TaC approaches to an equilibrium value within a practical normalizing time, e.g. 0.5 h. The similar phenomenon is observed on NbC in 9CrlMoV steel. The reason for this phenomenon is that a lot of residual dislocations supply the precipitation cites and the induced strain by the inverse transformation assists the precipitation. The average particle size of TaC after the usual normalization increases as (time)1/13 which is much differ from the theoretical prediction of (time)1/5 for the dislocation coarsening. The TaC particles after the usual normalization are spherical or round angular and about 10 nm in diameter. Based on the above investigation solubility product of TaC in austenite of 8Cr2WV steel has been determined.
A radio-frequency glow discharge optical emission spectrometry is applied to perform rapid and quantitative analysis of tin, which is included in commercial steels as a typical tramp element. The measuring method to introduce a bias current into the glow discharge plasma is effective for enhancing the emission intensity and thus improving the detection sensitivity. Two atomic emission lines of tin: SnI 303.411 nm and SnI 317.504 nm, can be selected as the analytical line for the determination of tin in steels. By conducting the bias current of 52 mA at the r.f. power of 120 W, their emission intensities are 14 times larger than those obtained with the conventional plasma. In the case of SnI 303.411 nm, a calibration curve almost passing through the origin is obtained, leading to the limit of determination of less than 0.01 mass% Sn (several 10 ppm Sn). However, if the sample contains chromium, CrI 303.419 nm may interfere with the accurate estimation of the SnI intensity. On the other hand, major alloyed elements including chromium have no emission lines overlapping with SnI 317.504 nm, although a weak Fell line overlaps just with the tin line which gives a calibration curve not passing through the origin. When SnI 317.504 nm is employed as the analytical line, the limit of determination can be estimated to be about 0.01 mass% Sn.