Materials Science Forum Vols. 580-582

Abstract: Heterogeneous nucleation and growth of acicular ferrite in HSLA steel weld-metal were in-situ observed by using laser scanning confocal microscopy. The applied thermal cycle simulated cooling rate of large heat-input welding. The high time-resolved images made for heterogeneous nucleation at inclusion and hard impingement between ferrite plates to be clear. Furthermore, the crystal orientation relationships between observed substrate for the heterogeneous nucleation and nucleated solid were analyzed.

Abstract: H beam welds along rolling and transverse direction were investigated with nondestructive testing, mechanical testing and microstructural analysis. Crack and fracture occurred, during guided bend testing, in the specimens welded along rolling direction between flanges of the H beams with higher S content although no defect was detected prior to the bend test. The fracture seemed to be lamellar tearing because of step-like fracture propagation and terrace and wall fracture appearance of the ruptured bend test specimen. On top of this, lamellar tear was already created in the base metal area near the HAZ before the bend test in microstructural analysis on the welds. It seems that lamellar tearing occurred in the specimens welded along rolling direction between flanges of the H beams with higher S content as cracks were formed and propagated into a ferrite phase region in α-γ band structure in combination with low ductility in transverse direction due to MnS inclusions elongated along rolling direction.

Abstract: Laser welding of AISI 304 stainless steel was carried out by pulsed Nd:YAG laser machine under different welding conditions. Geometrical shape of the joints was circular butt. Studies were focused on laser power, welding velocity, beam diameter and pulse duration. Microstructure of welded joints was investigated by optical microscopy (OM) and scanning electron microscopy (SEM). OM and SEM results show no evidence of grain growth in heat affected zone (HAZ). In addition, columnar and equiaxed grains are observed in the weld metal. Various defects such as lack of fusion, underfill, porosity, hot crack and mismatch are observed as a result of improper welding conditions. The best welding conditions to obtain minimum defects are presented.

Abstract: In this study, the toughness of 11Cr ferritic stainless steel weld was evaluated by DBTT (Ductile-Brittle-Transition-Temperature) with the interstitial elements level. DBTT of the weld increased with increasing interstitial level due to the formation of martensite phase and solidsolution strengthening. Interstitial elements level should be limited by the adoption of back shielding gas during welding process because increased C+N level detrimentally affects the toughness of ferritic stainless weld. Adoption of Ar as back shielding gas lowered N content in the weld.

Abstract: This study was carried out on the development of the evaluation method for solidification cracking susceptibility of Inconel600/SUS347 dissimilar weld metals during laser welding. Some dissimilar weld metals which have different ratios of Inconel600/SUS347 were prepared by TIG welding and then were remelted on the U-type hot cracking tester by laser. Solidification cracking behavior during hot cracking test was observed by a high speed camera and the dynamic strain, close to the solidification crack, was evaluated. It appeared that local critical strain, for the initiation of solidification crack, was obtained by this strain measurement method. So the solidification cracking susceptibility could be directly evaluated based on the critical strain for different dissimilar joint. By using this method, it was discovered that solidification cracking occurred most easily when the ratio of Inconel600/SUS347 is 40%/60%, in the case of the Inconel600/SUS347 dissimilar laser welded joints.

Abstract: Micro flash butt welding of super duplex stainless steel with Zr-based metallic glass insert was carried out using the temperature controlling system. Zr55Cu30Ni5Al10 of Zr-based metallic glass with thickness of 0.05mm and Zr metal with thickness of 0.1mm and 0.5 mm were used as the insert materials, in order to improve weldability. The specimens were mounted on the dies using a Gleeble thermal simulator, and then, flash butt welding was made. After welding, Zrbased metallic glass insert became much thinner than Zr metal insert. The super-cooled liquid in the interface protruded outside due to the superplastic deformation. The formation of the protrusion discharged the oxide films on the butting surfaces and contact surface; resulting in metallurgical bonding through the fresh surfaces. The Fe-Zr metallic compound for Zr-based metallic glass insert was hardly observed. The micro flash butt welding with metallic glass insert was successfully accomplished for super duplex stainless steel.

Abstract: Chemical composition effect on the weld properties for low temperature steel was evaluated. The alloy elements of interest at the weld metal were Cr and Mo, which come from the steel plate and welding wire, respectively. Both side one run SAW process was carried out in a Ygroove butt joint. Microstructure of the weld metal is strongly dependent on the chemical composition of the steel plate and the welding wire, due to high dilution. The microstructure of the weld metal became fine acicular ferrite by increasing Cr and Mo content because of high hardenability effect. The weld metal having Cr and Mo possessed the highest impact toughness at low temperatures among the weld metals studied. Cr seems to have more effect than Mo on the toughness of the weld metal.

Abstract: The use of zinc coated steel in the automotive industry is rapidly increasing as a result of consumer demand for improved corrosion resistance of automobile bodies. Plasma arc welding of zinc coated steels is currently under investigation, with a strong emphasis on their weldability. In the present study, plasma arc welding has been performed on zinc coated steel with several welding parameters, such as current, travel speed and flow of plasma gas. Those parameters are interrelated in their effect on penetration, in order to produce sound welds. An optimum combination of parameters was sought, which would give full penetration of the weld, with the highest travel speed and no visual defects on every steel-sheet as a basis; hence, characterizing the process in the most interesting way and into a useful industrial applications. It is found that zinc coated steel sheets of 0.7 mm and 1 mm thickness can be welded by plasma arc welding. The microstructures of welds were almost similar. There is a slight difference in chemical composition of the welds because of zinc pick-up which varies from the top surface to the bottom welds. The presence of zinc in the welds did not have any effect on the mechanical properties.

Abstract: Microstructure and crystallographic orientation in the overlay weld metal have been investigated using Ni-base single crystal superalloy CMSX-4. Laser power, laser scanning speed and wire feeding speed were varied. The microstructure and crystal orientation were analyzed by microscopy and electron backscattered diffraction pattern. Microstructural observation revealed that the overlay weld metal grew epitaxially on the substrate and stray crystal tended to be formed at the conditions of high power and high Vf/Vw (Vf: Wire feeding speed, Vw: Laser scanning speed). In addition, solidification morphology, dendrite growth direction and single-crystallized condition were evaluated by combining solidification model to heat conduction model. These predicted results agreed qualitatively with the experimental ones. Based on the above results, the singlecrystallized cladding with ten passes could be achieved.

Abstract: The effects of strain rate in rolling on microstructures and mechanical properties of a nano-grained high purity copper processed by accumulative roll bonding (ARB) were studied. The rolling during ARB was conducted with two kinds of strain rates (2.6sec-1 and 37sec-1). The microstructural evolution of the copper with ARB proceeding was somewhat different in both methods. However, the variation of mechanical properties with ARB was very similar to each other.