The pulp industry has recently been required recovery boilers with higher steam temperature and pressure to improve plant efficiency. Given severe plant conditions, furnace water wall tubing tends to be suffered from serious smelt corrosion. In order to prevent the corrosion problems on the furnace water wall tubes in recovery boilers, a 25%Cr ferritic SMAW overlay material, which is included of Nb 1%, Al: 0.1% and Ti: 0.1%, has been developed in behalf of the 18%Cr overlay welding materials. On practical fabrications of the overlay welded panels in a factory, GMAW and PTAW, in addition to the SMAW welding materials, are needed to application to the automatic welding machines as one of the highly efficient welding methods. The 25%Cr GTAW welding material is also useful for a repair welding in order of good appearance of the overlay welded tubes. This study introduces the development of three kinds of 25%Cr ferritic welding materials, such as GMAW, PTAW and GTAW welding materials for furnace water wall in recovery boilers. The weldability, bend property and corrosion resistance of these overlay weld materials were examined. From these results, an overlay welded panel was fabricated by using of the 25%Cr overlay welding materials. Good corrosion resistance of the panel has been confirmed as the result of the field test in a recovery boiler in service for about two years. 25%Cr overlay welded tubes in the field test showed a good corrosion resistance.
The aim of this study was to investigate the influence of welding parameters such as arc current, welding speed and wire feeding speed on the grain refining in weld metal of type 310S austenitic stainless steel. Besides, the effect of the grain refinement on mechanical properties such as tensile strength, hardness and ultrasonic transmission was examined. Equiaxed grains of about 15 μm formed by hot-wire GTAW in weld metal and the fine grain formed over a wide range stably. When the wire current was low, the fine grains formed at the lower part of the weld metal. On the other hand, when the wire current was high, the grains formed at the upper part. The area ratio of fine grain in weld metal corresponding to the amount of fine grain in weld metal depended on the wire current. Besides, the ratio increased with decreasing in the arc current, increasing the welding speed and wire feeding speed. The maximum area ratio of fine grain in weld metal was about 50%. The hardness of the weld bead including the fine grain was higher than that without the fine grains. Besides, the formation of the fine grain showed higher strength and elongation. Moreover, a transmissivity of ultrasonic wave was improved significantly by including the fine grains in multi-pass weldment.
Welding process is an indispensable technology in industrial fields. However, it is very difficult to control its process and quality. In order to understand the phenomena and help to search an optimum condition, many numerical models have been constructed. In this study, a 3D weld pool numerical model which can be applied various joint geometries and welding positions is constructed. Convection in molten metal and deformation of free-surface are included in our model and a simplified heat source model is used. Our model can describe different phenomena when the welding position or the joint geometry is changed. However, since a simplified heat source model is used, we have to adjust parameters to reproduce experimental results by calculation. It is important to clarify the relationships between welding parameters and heat source properties in order to assess the calculation results quantitatively.
The properties of rotating tool spot joining joints with different rotating tools by the combination between the shoulder diameter and the probe length were investigated in this study. The shoulder diameter of rotating tool affects the tensile strength of the joint significantly. An oversize shoulder diameter decreased the joint strength. The probe length affects more the fracture mode than the joint strength. Interface fracture mode occurred with a rotating tool of short probe length. Plug fracture occurred with a rotating tool of long probe length. Rotary motion of restricted area in the joining interface by the rotating tool was confirmed by displacements of the joining interfaces. Rankin vortex was observed on the joining interface by velocity distribution of rotary motion of joining interface. Al oxide film layers were formed spirally on upper joining interfaces
The effect of joining speed on the joining strength of the dissimilar materials joint of the SUS304/ carbon fiber reinforced thermoplastic (CFRTP) was investigated. SUS304 plates (150mm×75mm×2.0mm) and CFRTP (consisted of PA6 matrix and 20wt% carbon fiber) plates (150mm×75mm×3.0mm) were joined by friction lap joining (FLJ) using rotational WC-Co tool. Different joining speeds in the range from 1.67mm/s - 16.7mm/s and the tool rotation speed of 8.33 s-1 were selected as processing conditions. Direct joining of SUS304/CFRTP was accomplished at all of joining speeds by FLJ. The tensile shear strength of the joint increased with increasing joining speed up to 3.33mm/s, however, it decreased from the joining speed of 3.33mm/s to 16.7mm/s. The interfacial fracture occurred at all of joining speeds. The fractured surface analysis of SUS304 revealed that teared fragments from the CFRTP base plate were bonded to SUS304. At the lower joining speed, the excessive heat input caused thermal decomposition of PA6 from the matrix of CFRTP resulted bubbles; the joint strength was decreased because of these bubbles. The shortage of the heat input at the higher joining speed caused insufficient wetting of molten PA6 to SUS304, which resulted in the decreased strength of the joint.
The aim of this study was to elucidate the grain refinement mechanism in weld metal of type 310S stainless steel by hot-wire GTA welding. Quenched microstructure of weld pool during welding was obtained by liquid-tin and water quenching method. Microstructure evolution of wire during heating was investigated using hot-wire heating device. The quenched microstructure showed that since the fine grain formed in conjunction with filler wire, the fine grain seemed to be formed depending on the fine grain formation of filler wire. According to the microstructure evolution of wire during heating, recrystallization, grain growth and local melting occurred in the heated wire in accordance with the temperature distribution of the wire. And the fine grain in the weld metal formed by inserting semi-molten state filler wire during feeding. The fine grain formed depending on the local melting phenomena at sub-boundary in the grain of filler wire by hot-wire condition. It was considered that the sub-boundary in the grain at which solute elements should enrich was formed during grain boundary migration in grain growth process, because the filler wire was rapidly heated with a heating rate of more than 1400°C/s. Therefore, the grain refinement phenomenon in weld metal of type 310S stainless steel seemed to be a phenomenon unique to the hot-wire GTA welding.
The aim of this study was to investigate the influence of wire feeding conditions such as wire feeding position and power supply distance on the fine grain formation in weld metal of type 310S stainless steel. When the wire feeding position was far from electrode of GTAW, area ratio of fine grain in weld metal increased. The maximum area ratio was obtained when the filler wire was fed to the back of weld pool and the wire melting depended on the joule heating. When the power supply distance was short, the wire wasn't fed to the back of weld pool. On the other hands, when the power supply distance was long, the wire wasn't also fed to the back of weld pool. Because, this phenomenon was related to the local melting region in the wire corresponding to the temperature gradient of wire. Therefore, the adequate feeding conditions was required to form the fine grain by hot-wire GTAW process.
The aim of this study was to investigate the influence of power supply distance on the grain refining in weld metal of type 430 stainless steel. The fine grain formed conjunction with filler wire according to microstructure obtained by water quenching method. From the result, local melting occurred in filler wire before fine grain. The fine grains formed at the lower part of the weld metal when the wire current was low. On the other hands, the fine grain formed at the upper part when the wire current was high. The amount of the fine grain increased with increasing the wire current. Moreover, the area ratio of fine grain in weld metal increased with decreasing the power supply distance and the size of fine grain formed with each power supply distance in weld metal increased with increasing the power supply distance. The effect of power supply distance on microstructure evolution in filler wire was investigated to obtain during heating of wire. Recrystallization and grain growth was observed in the heated wire. The grain size decreased with decreasing the power supply distance, because heating rate of wire increased with decreasing the power supply distance.