The practical strength of line pipe steel has extended to X80 or greater. The line pipes of X80 grade have been commercialized successfully and now the development and evaluation of X100 and X120 grade line pipes are conducting. The effect of boron addition on the toughness and microstructure of seam weld metal was investigated in tensile strength range from 700 to 1100 MPa. Two types of weld metal were employed in this study. Type A weld metal was produced by boron added flux. The boron content in type A weld metal was around 30ppm. Type B weld metal was produced by boron free flux. The boron content in type B weld metal was 15ppm or less. In a tensile strength range less than 800MPa, type B weld metal had lower absorbed energy than type A because of grain boundary ferrite formation. In a tensile strength range of 800MPa or more, type B weld metal had greater absorbed energy than type A. In this tensile strength range, some of acicular ferrite is replaced with bainite or bainite/martensite and the absorbed energy decreases with increasing of tensile strength. And the type B weld metal had more amount of acicular ferrite and greater absorbed energy than type A in this tensile strength range.
Authors ultrasonically welded A6061 aluminum alloy sheet using two types of weld tips with the different contact face geometry, and investigated the effect of the weld tip on the performance and interface structure of the welds. One type of tip has a cylindrical contact face without knurl, which is called C-tip in this study. The other type of tip has flat contact face with knurl, which is called K-tip in this study. The following main results were obtained. The strength of the joints welded using C-tip was higher than that welded using K-tip and the fluctuation in joint strength with C-tip was smaller. The C-tip could stably produce the higher strength joint. Using K-tip, the knurl indentations were made on the workpiece surface due to the pyramidal projections on the weld tip, and the indentation size expanded with welding time, resulted in the deterioration of the joint property. On the other hand, the indentation made on a workpiece surface by using C-tip showed a distinctive shape like an ellipse elongated perpendicular to the ultrasonic vibration, and the indentation grew with welding time. Using K-tip, unbonded regions remained at the weld interface due to the concave on the weld tip face. In the cross-sectional structure parallel to the workpiece width of the joint welded using C-type of tip, the distinctive feature was observed that the faying surface of the anvil workpiece was mixed with that of the sonotrode workpiece by intense plastic deformation and a horn-like protuberance intruding into the sonotrode workpiece was formed at the periphery of the welded area.
It has been generally recognized that the fatigue crack growth rate in the Stage II (b) regime is little influenced by the microstructure and strength of steels. In order to retard fatigue crack growth by control of microstructure in heavy steel plate, this study investigates the effects of morphology and distribution of the martensite phase in a ferrite matrix on fatigue crack growth behavior. Steels with various martensitic morphology and distribution are prepared in the laboratory, and their fatigue crack growth rates are measured. As a result, steel with an elongated and banded martensitic structure shows the highest resistance to fatigue crack growth in the thickness direction. The fatigue crack growth rate of the improved steel is less than one tenth of the rate of conventional steel. Statistical analysis indicates that the fatigue growth rate conforms to the Weibull distribution and that there is a significant difference between both steels. Fatigue cracks of the improved steel propagate with frequent deflection and branching at ferrite/martensite interfaces. These crack paths lead to a decrease in the driving force of crack growth and enhancement of roughness-induced crack closure. This can be attributed to the remarkable improvement of the fatigue crack growth property.
The authors reported that the steel with an elongated and banded martensitic structure in a ferrite matrix has significantly high resistance to fatigue crack growth in the thickness direction. In this paper, the effects of area fraction, hardness, and aspect ratio of martensite with similar martensitic morphology and distribution on fatigue crack growth properties are investigated. As a result, the fatigue crack growth rate decreases with increasing hardness, area fraction, and aspect ratio of martensite. This corresponds to enhancement of crack deflection and branching. This crack path can be attributed to two mechanisms concerned with the increase of resistance and the decrease of driving force of crack growth. First, martensite constrains plastic deformation of ferrite at the crack tip. Second, transformation expansion of martensite changes the internal stress in the microstructure. The reasonability of these two mechanisms is confirmed experimentally.
It has been generally recognized that the fatigue life of welded joints is little influenced by the strength of steels due to the high stress concentration and the tensile residual stress near the weld toe. In this paper, improvement of the fatigue life of welded joints using steel with high resistance to fatigue crack growth (F/M steel) is investigated. F/M steel has a microstructure with an elongated and banded martensite phase distributed in a ferrite matrix and a fatigue crack growth rate of about one-harf to one-tenth in the thickness direction, compared to conventional steel. As a result, the fatigue life of an out-of-surface gusset welded joint increases with the decrease of the fatigue crack growth rate. The fatigue life of welded joints using F/M steel with the highest resistance to fatigue crack growth increases to about twice that of joints using conventional steel. Whereas the fatigue crack growth rate decreases significantly, the fatigue life of welded joints increases only slightly. This can be attributed to the stress ratio independent of the fatigue crack growth rate. In other words, the fatigue crack growth rate of F/M steel increases with the increase of the stress ratio, approaching that of conventional steel. In the case of welded joints, even if a fatigue test is carried out at a low stress ratio, the region near the weld toe is under a high stress ratio due to tensile residual stress. Therefore, improvement of the fatigue life of welded joints becomes comparatively small so that the effect of fatigue crack retardation of F/M steel decreases.
In this research, two-color pyrometry has been conducted to obtain the surface temperature of the weld pool, in which the weld pool is photographed by a high speed camera during arc welding. Two wave-lengths (950 and 980 nm) of light in the infrared range were selected from the thermal radiation light emitted from the weld pool at the instant when the arc was extinguished, by using an imaging spectroscope. The temperature is obtained from the intensity ratio of two waves of light. Consequently, in GTA welding, it is shown that the surface temperature distribution of a weld pool is affected by the sulfur content in the base metal. It is thought that this surface temperature distribution is determined by the balance between the driving forces of viscous drag from the cathode jet of plasma and Marangoni surface tension. In GMA welding, it is seen that the surface temperature distribution of a weld pool becomes uniform and the temperature is 1715–1845K, which is obviously lower than that of the metal droplet. For this reason, it is considered that the convection velocity on the weld pool surface and in the weld pool is high and, consequently, the heat input from the arc is transported to the back of the weld pool due to this convection. It is supposed that the edge of the pool below the arc has a high temperature compared with the central part of it due to the influence of high speed convection.
In case of using hot-dip galvanized steel beam-column joints, cracking of hot-dip galvanization coating occurs, frequently. Simulations of the phenomenon were carried out for different configurations of steel beam column joints. Also, residual stresses by means of welding were measured to consider their effect on the phenomenon. As results, it has been verified that the residual stresses and configuration of the toe of weld were main factors of the phenomenon. Non-Scallop with circular hole type was found to be effective to prevent cracking of hot-dip galvanized steel beam-column joints.
Nano-second pulsed Nd:YAG laser (1064nm) process on SnO2 system thin films on glass as transparent electrode for Flat Panel Displays(FPD) was investigated in this paper. Mechanism of laser ablation, which is an interaction between material and laser, was studied to approach low energy process for mass production application. It was found that carrier concentration is an important factor influencing the process from energy absorption mechanism of free electron vibration induced by laser. The process needed high carrier concentration for low energy process. Experiments showed threshold of the process was about 4.0x1025m-3 of carrier concentration. However, some showed exception of successful process with even low carrier concentration. Then an additional mechanism that YAG photon (1.16eV) excites electrons trapped at deep level to conductive band was inferred. Deep level was measured by cathode luminescence spectroscopy, and result showed existence of electron at less than 1.16eV. It is understood that SnO2 system thin films absorb laser energy through complex process of free electrons vibration and deep level electrons excitation. Considering the mechanisms, it is the first to achieve 20mJ/mm2 of low energy process. This result leads to high process speed and good prospect of the laser process for mass production application for FPD.
This paper deals with the improvement of weld metal toughness by formation of fully acicular ferrite microstructure in laser welding. The addition of 5-20% oxygen into the He shielding gas for laser welding of Ti added base plate formed Ti containing oxide inclusions in weld metal, which acted as the efficient nucleation sites of acicular ferrite. Although coarse grain boundary ferrite was formed along prior austenite grain boundaries, B addition effectively suppressed the formation of grain boundary ferrite. It was shown that the microstructure of laser weld metal was consisted of fully acicular ferrite when Al/O ratio was less than 1.13 and B content was more than 1/2.22(O-0.89Al-0.33Ti). The laser weld metal of fully acicular ferrite exhibited excellent toughness (vTrs<-100°C).
To form curved surface, in-plane strain is introduced into a plate by using several methods such as line heating, press working and laser heating in shipbuilding and sheet metal working in forming nose shape for the shinkan-sen. Laser forming could be a potential useful method for sheet metal forming as well as press working. Curved surface is classified based on its features of primary curvature radius into two typical shapes, so called bowl and saddle. So, when we research some technique to form curved surface, at least two types of bowl and saddle shapes should be investigated. For the bowl shape, some researchers reported that initial curvature radius has no effect on the in-plane strain induced by laser heating. In this report, the effect of initial curvature on the in-plane strain induced by laser heating is investigated for the saddle shape. As a result, shrinkage distributions caused by in-plane strain were varied with different initial curvatures. In this phenomenon, bending moment acts a key role in both heating and cooling processes.
Stress corrosion cracks (SCCs) occurring in the weld metal of a type 600 Ni base alloy during the creviced bent beam (CBB) test in pressurized pure water at 561 K have been characterized on the basis of orientation maps determined from electron back scattering pattern (EBSP) in order to distinguish the intergranular (IG) SCC from the interdendritic (ID) SCC. The specimen for the CBB test was cut from shielded metal arc weld metals of the Ni base alloy deposited using welding rods with C contents ranging from 0.03% to 0.11% and Nb contents from 1.23% to 4.39%. The CBB test was carried out on specimens in the as-welded state and those subjected to a post weld heat treatment (PWHT) consisting of tempering for 72 ks at 893 K and aging for 720 ks at 673 K. It turned out that most SCCs observed in all the weld metals except for those of high Nb content (0.076% C and 4.39% Nb) subjected to the PWHT were identified as IGSCC. Cracks identified on IGSCCs in the specimen of high Nb content receiving the PWHT were more than twice as deep as those regarded as transgranular (TG) and IDSCCs. Since the fractured surface of the IGSCC presented dendritic features, it was rather difficult to distinguish the IGSCC from the IDSCC based on the fracture morphology.
After the experience of Hanshin-Awaji Earthquake, demands for full penetration welding joint with large plate thickness have been increased to improve the performance of bridges under severe earthquake. At the same time, the full penetrated T-joints without significant defects are required from the fatigue strength point of view. In case of conventional methods, removable backing material or back gouging are employed. When the removable backing material is used, root openings are required and assembly process becomes complicated. The gouging produces large noise and fume which are not welcome for good working environment. To achieve defect free full penetration T-joint without environmental problem, high current pulsed MAG welding is introduced as an alternative method. By using this welding method, the plate with thickness from 15 to 30 mm can be welded without back gouging. It is reported that no welding defect occurs when the plate thickness is less than 17mm1). However, when the plate thickness is over 25 mm, the pear-shaped bead cracking were sometimes generated if appropriate welding conditions are not selected. Therefore, in order to apply this welding method for the construction of bridges, it is necessary to clarify the welding conditions which can prevent the formation of pear-shaped bead cracking. In this study, the influence of the welding conditions on the formation of pear-shaped bead cracking is examined through the experiments of the full penetration welding of T-joints. And the same problems are analyzed using Finite Element Method. The formation and growth of pear-shaped bead crack in T-joints welded by full penetration high current pulsed MAG welding can be simulated using temperature dependent interface element which is introduced in the thermal-elastic-plastic FEM analysis. Results of experiments show that the pear-shaped bead cracking is not formed when the heat input is greater than 2,500 J/mm or when the penetration bead is formed. However, both large heat input and large penetration bead (deep penetration compared to the penetration width) have negative influence on hot crack formation according to the commonly accepted knowledge. To understand these experimental results, FEM simulations are conducted and influences of heat inputs and size of penetration bead are investigated.
The objective of this research is to develop a 3D finite element method (FEM) for hot cracking using temperature dependent interface element. Proposed method can predict the formation and extension of hot cracking. It includes Iterative Substructure Method (ISM) which can reduce the computational time with keeping high accuracy. In this study, proposed method is applied to the problem of pear-shaped bead cracking under narrow gap welding. The applicability and the validity of the method are verified through serial computations. The computed results show that proposed method can simulate pear-shaped bead cracking and surface cracking which are distributed along the longitudinal cross section in welding line. Additionally, it is found that when the weldment is penetrated to the back of plate, pear-shaped bead cracking is difficult to form. The computational speed of the method using ISM increases by about 7 times faster than the method which doesn't use ISM.
The refinement of ferrite grain of ultra low carbon steel was observed in cyclic laser irradiation. The high heating and cooling rates were considered to cause the promoted nucleation and the suppressed grain growth. The process of refinement of grain during the laser irradiation was investigated by Monte Carlo simulation. In order to obtain the accurate simulation, the apparent activation energy of austenite grain growth was measured by isothermal heating experiments and used in the simulation of austenite grain growth. The results of the austenite grain growth simulations had a good agreement with the experimental results. Consequently, α → ← γ phase transformations which involved temporal and spatial heterogeneous nucleation were modeled with Monte Carlo method in order to simulate the process of the cyclic laser irradiation. The phase transformed fraction of ferrite and austenite with high heating and cooling rates were also obtained by simulation, and the curves of transformed fraction of ferrite and austenite showed sigmoidal variations with temperature. The results of the simulations showed the decrease of ferrite and austenite grain size with increase of times of the cyclic laser irradiation, which could explain the refinement of ferrite grain by the cyclic laser irradiation.
This paper discusses the efficient estimation method of the elasto-plastic-creep characteristics of BAg8 (72%Ag28%Cu) brazing filler metal for joining the cemented carbide of K20 (WC-Co) to the alloy tool steel of SKS3. For the estimation, so-called "tension-strain maintenance test" which consists of a tensile part and a stress relaxation part was employed. The advantage of this estimation method is that once this test has been conducted using only 1 specimen at a temperature, the elasto-plastic-creep characteristic of the specimen at the temperature can be estimated. Especially, the creep characteristics can be estimated without conducting long-term creep test. Therefore, by conducting tension-strain maintenance tests, the elasto-plastic-creep characteristics of BAg8 at 3 different temperatures were estimated. Based on the estimated results, the material constants for an elasto-plastic-creep constitutive model were determined at the 3 temperatures, and then they were expressed as a function of temperature to describe the deformation of BAg8 at arbitrary temperature. Using the constants for the elasto-plastic-creep constitutive model, some simulations of basic deformation, such as time-dependent pure tension and stress relaxation at different 2 temperatures were conducted. Then, by comparing the simulations with the experiments, it was found that the deformation behaviors in the experiments were almost predicted by the simulations. Moreover, the elasto-plastic-creep FEM analysis of the brazing process for joining K20 and SKS3 with BAg8 was conducted by employing the constitutive model with the material constants used for the above basic simulations. The residual stress distributions in the analyzed result were compared with the X-ray residual stress measurement results of an actual brazing joint. As a result, the residual stress distributions in the analysis approximated those in the actual brazing joint well. Namely, the elasto-plastic-creep characteristics of BAg8 could be estimated efficiently by the tension-strain maintenance tests using only 3 specimens.
The main purpose of this study is to ensure the structural integrity of power plants. The accidents of SCC (Stress Corrosion Cracking) at the weld region of the coolant pipe of nuclear power plant are reported at domestic and foreign countries. Since welding residual stress is one of the main factors of the phenomenon, understanding the welding residual stress distribution is important in the estimation of plant life cycle and maintenance. However, the residual stresses of the weld joint have complex distributions three-dimensionally. In this study, the inherent strain method combined with FEM (Finite Element Method) is applied to measure the welding residual stress distribution accurately. The pipe girth weld mock up was used in this study. As the mock up model was axial-symmetrical shape, the new theory of inherent strain method for the axial-symmetrical model was developed. The FEM program was also prepared based on the theory. Measuring the relaxed strains by cutting the model, the residual stresses distributions were analyzed by the FEM program. The inherent strain method gives the most probable values and the deviations of the residual stresses simultaneously. Around the middle of the weld center section, the axial stress value is small, but the circumferential stress is estimated to be large tension. Evaluating the analyzed results, the deviations are small enough for the most probable values. Moreover, the feature of the stress distributions fundamentally corresponds with the ones obtained by the thermal elastic-plastic analyses. Therefore, developed measuring method for axial-symmetrical model shows high reliability.