Materials Science Forum Vols. 539-543

Abstract: Ultra-fine grained steel (UFGS) with an average grain size of less than 1μm has been developed and is expected to demonstrate superior properties. However, its welded heat-affected zone, HAZ, substantially affecting the strength of a welded joint, will be easily softened after welding. Therefore, minimization of UFGS’s HAZ size during laser welding was carried out using the cooling conductor liquid nitrogen. It was found that a shielding gas with adequate flow rate for the liquid nitrogen depth was used to displace nitrogen on the area of laser beam irradiation to stabilize the weld bead. Also, because YAG laser system was mainly used because it has a lower laser induced plasma or plume temperature, which results in a decreased occurrence of pit and blowhole. HAZ size minimization strongly depends on the initial plate temperature. Reduced initial plate temperature shrinks the specific heated temperature range in which softening occurs. However, due possibly to decreasing thermal conductivity under room temperature, which prevents heat removal, the benefit of reducing the initial plate temperature is limited. The optimal initial temperature to minimize the HAZ size, in the present work, was found to be 123K.

Abstract: Insulated rail joint assemblies provide electrical insulation between two sections of rail for signalling purposes. In this work, rail steel was successfully bonded to PSZ ceramic using an active brazing technique. In order to increase the wettability of the PSZ ceramics, titanium coating was deposited on the ceramic surface using a filtered arc deposition system. A filler metal called BVAg-18 (60%Ag-30%Cu-10%Sn) was used and the joining was performed at a temperature of 750 °C. Bonding between partially stabilised zirconia and rail steel with BVAg-18 filler metal was not achieved using a standard brazing method. Bonding did occur with the BVAg-18 filler metal using the advanced brazing technique of active metal brazing, with best results obtained using a brazing temperature of 750oC and a dwell time of 10 minutes. The microstructure of the coating and joint interface were characterised by XRD, SEM and EDS.

Abstract: Brazing of Al to Cu using Al-Si-Mg-Bi brazing alloy has been carried out in a vacuum furnace. In the brazed interlayer, there were two kinds of intermetallic compounds. One of these
intermetallic compounds was θ phase and the other was δ(Cu3Al2). Tensile strength of the joint was only about 15MPa. Deformation behavior of Al/Cu brazing joint was brittle without deformation of the base metal. The specimen was fractured in the intermetallic compound which was mainly θ phase. In order to improve the tensile strength of Al/Cu dissimilar joint, Cu cladding Ag (thickness: 0.1mm) substituted for Cu. As the result, tensile strength of the joint was about 70MPa and the specimen was fractured in an Al base metal. In this joint, plate-like intermetallic compound, δ (Ag2Al) was formed in the brazed interlayer. The shape of δ(Ag2Al) was quite different from θ phase found in Al/Cu joint. It was considered that the shape of reaction layer remarkably affected to the strength of the Al/Ag-Cu clad dissimilar joint.

Abstract: Micro-resistance spot weldability of nickel free stainless steel sheet was studied to fabricate medical implants. Weld lobe was established and the effect of welding parameters on joint strength was investigated. Weld nugget represented a rapidly quenched austenitic cellular structure whose cell size is several μm with a little ferrite. The amount of δ-ferrite in weld nugget is smaller than that predicted by some Schaefller diagrams due to rapid solidification during micro-resistance spot welding.

Abstract: The grain growth behavior and mechanical properties in the friction stir weld zone after post weld heat treatment (PWHT) have been investigated. As PWHT temperature increased, a normal grain growth of as-welded equaxied grains ceased and abnormally grown grains with elongated shape coarsened. Huge elongated grains changed into smaller equaxied grains at 500°C. In case of lower heat input condition, abnormal grain growth initiated faster due to smaller initial grain size. The weld zone with bigger initial grains had advantages to maintain the thermal stability at high temperature. The hardness near the weld zone was almost recovered to the 95% of the unaffected base metal at 500 °C and the weld zone under lower heat input condition resulted in the homogeneous recovery through the whole weld zone.

Abstract: Hybrid laser-rotating arc welding (HLRAW) process was designed by combining the laser beam welding (LBW) process with the rotating gas metal arc welding (RGMAW) process. In this study, comparing with conventional HLAW, weld bead characteristics as a function of the various process parameters were evaluated for HLRAW. Moreover, welding phenomena were analyzed by high speed monitoring with laser illumination. The arc rotation enhances the weld pool motion, therefore it reduces the undercut formation which is one of most critical weld defects in the conventional laser-arc hybrid welding.

Abstract: Original position statistic distribution analysis (OPA) technique is a new kind of analysis method that can be used to detect the segregation, porosity and non-metal inclusion of iron and steel material. In the present paper, the technique has been applied to analyze two middle and low alloy welding plate samples. One sample is normal and there is no any defect in its welding seam, whereas the other sample has one obvious little defect in its welding seam. For each sample, both the welding seam and the mother material were scanned and analyzed, and the continuous content distribution of each element in scan area was thus accurately obtained. From the content distribution map, the weld shape and the transition zone were directly observed. By using the content curve from mother material to the welding seam, the content change range and its change gradient were calculated. These two parameters were suggested to evaluate the quality of the welding seam and the welding technology. For the sample with the little defect, in its defect region the contents of C and S presented higher, whereas the contents of metal elements such as Ni, Cr, Mn, Mo, Al presented lower. This result gave the reason why its mechanics property was relatively bad. It was suggested that the defect completely resulted from another kind of material.

Abstract: The objective of this investigation was to correlate the chemical composition of welding rods for gas tungsten arc welding with the fracture resistance and tensile properties of type 347 welds through the systematic tests and microstructural analyses. Five weld metals which differed in contents of carbon, nitrogen and niobium each other and a high δ-ferrite containing weld metal were deposited by the six different welding rods. J-R fracture resistance and tensile properties were evaluated for the type 347 welds. The microstructural examinations were performed to relate key microstructural features to mechanical properties. It was found that the contents of Nb(C,N) precipitates in type 347 welds were determined by the mixed function of carbon and nitrogen and niobium contents in welding rods. The strengths of type 347 welds were in direct proportion to the contents of Nb(C,N) and J-R fracture resistances were inversely proportional to the contents of Nb(C,N). It was concluded that the type 347 weld with high fracture resistance and adequate strength was obtainable by controlling the sum of carbon and nitrogen contents near 0.1wt% and a limitation of the carbon content below 0.04 wt% in welding rod.

Abstract: The recycling of scrap steels can be difficult due to the tramp elements that they can contain. During the steelmaking process, tramp elements such as Cu and Sn are difficult to be removed; and it is these elements that cause surface cracking of steels during hot rolling process (i.e. Cu and Sn liquid embrittlement).The paper consists of three different experiments into the suppression of surface cracking during the hot rolling process. For the oxidation in air, the surface cracking most severely occurred in the specimens which were oxidized around 1100°C in the tested range of 950-1200°C after a 1250°C heating. For the change in oxidation atmosphere from air to water vapor, the surface cracking occurred more severely although the mass gains were smaller in water vapor than in air oxidation. For the addition of Si and Ni in the water vapor conditions of 0%-30%H2O, the surface cracking was found to be suppressed effectively when the mass gain increased. The Cu and Sn enriched alloys at the scale/steel interface were closely observed by optical microscopy and scanning electron microscopy. The mechanism for suppression of the surface cracking was explained in terms of back diffusion of Cu and Sn into the steel and/or occlusion of Cu and Sn into the scale through the development of a rugged scale/steel interface.

Abstract: The interaction between softening and precipitation mechanisms in hot worked Nb microalloyed austenite is analysed with the help of a physically based model. The model is able to calculate the evolution over time of the dislocation density (stress), stored energy and precipitate pinning force, the recrystallized fraction, the average precipitate diameter and precipitate number density, as well as the concentrations of the precipitating Nb over time. It is assumed that nucleation of precipitates occurs heterogeneously at dislocations with recovery producing a continuous decrease in dislocation density. This results in a reduction of the available nucleation sites for precipitation as well as a decrease in the driving force for recrystallization along time. By comparing the model predictions and the experimental results the values of several physical parameters involved in the model are discussed.