Materials Science Forum Vols. 580-582

Abstract: The occurrence of microcracks, especially ductility-dip crack in multipass weld metal during GTAW and laser overlay welding processes of Ni-base alloy 690 was predicted by the mechanical approach. The stress/strain analysis in multipass welds was conducted using the thermo elasto-plastic finite element method. The brittle temperature range for ductility-dip cracking (DTR) of the reheated weld metal was determined by the Varestraint test. Plastic strain in the weld metal accumulated with applying the weld thermal cycle in multipass welding. The plastic strain-temperature curve in the La free weld metal did not cross the DTR in the cooling stage of GTAW process, however, it crossed the DTR in the cooling stage of reheating process by subsequent welding. On the other hand, the plastic strain-temperature curves of any weld passes in the La added weld metal did not cross the DTR. Ductility-dip cracks occurred in the La free weld metal except for the final layer, however, any ductility-dip cracks did not occur in the La added weld metal during multipass welding. It could be understood that ductility-dip crack would occur during not only single-pass welding but also multipass welding when plastic strain intersected the DTR.

Abstract: In order to extend the life of petroleum pressure vessels operated in long term, it is demanded to establish the repair welding technique. To make clear the effect of weld thermal cycles during repair welding on the hydrogen content and weld cold cracking at the base metal of 2.25Cr- 1Mo steel / overlaying metal of austenitic stainless steels interface in the structural material of petroleum pressure vessels, the crack susceptibility was estimated by y-groove weld cracking test and repair welding test with varying overlay thickness and hydrogen exposure conditions. In addition, the hydrogen distribution in the material was calculated by the theoretical analysis using the diffusion equation based on activity. The crack susceptibility was raised with increase in the hydrogen content at the interface. It was concluded that the cracking could be prevented by controlling the repair welding process to reduce the hydrogen content at the interface.

Abstract: The characteristics of microstructures in friction stir (FS) weld of 304 austenitic stainless steel were examined. The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallized and recovered microstructures, respectively. The hardness of the SZ was higher than that of the base material and the maximum hardness was located in the TMAZ. The higher hardness in TMAZ was attributed to high density of dislocations and sub-boundaries. Electron microscopic observations revealed that ferrite and sigma phases were formed in austenite matrix in the SZ during friction stir welding (FSW).

Abstract: Facing the practical difficulties in reducing the diffusible hydrogen content of fluxcontaining welding consumables like flux-cored arc welding (FCAW) wires, the present study investigated the microstructural aspect to improve the hydrogen-induced cold crack (HICC) resistance of multipass weld metal of 600MPa strength. Two FCA welding wires were prepared by controlling the Ni content to give different weld microstructure, but to have similar levels of hardness and diffusible hydrogen content. HICC susceptibility of those two consumables was evaluated by 'G-BOP test' and also by 'multi-pass weld metal cold cracking test'. As a result of this study, it was demonstrated that microstructural modification with decreased proportion of grain boundary ferrite (GF) improved cold crack resistance of weld metal. The detrimental effect of GF against HICC has also been addressed based on the characteristics of weld metal cold cracking.

Abstract: Influence of heat input on the tensile strength and impact toughness of multipass weld metal made with AWS E81T1-Ni1 metal-cored wire was investigated. Welding parameters such as current, voltage and travel speed were varied independently to get different heat inputs. When it was increased by varying current, tensile strength of the weld metal increased even if more primary ferrite and wider columnar grains were observed. The increase is attributed to the higher recovery ratio of deoxidizing elements such as carbon, manganese and silicon due to the shorter reaction time in both wire tip and arc column. It also showed that impact toughness was influenced by the formation of reheated weld metal by subsequent passes and it decreased continuously with an increase of the amount of coarse grained region in the reheated weld metal.

Abstract: Microstructure formation of CP-Ti and TiB reinforced titanium were in-situ observed during the thermal cycle simulated for Tungsten Inert Gas (TIG) welding, by using laser scanning confocal microscopy. Under the in-situ observation of TiB reinforced titanium, heterogeneous nucleation of α-phase at inclusion was clearly detected and plate growth was shown in high timeresolution. Furthermore, it was observed that grain boundary of β -phase was pinned by the inclusions. Microstructure difference between pure and TiB reinforced titanium was explained based on those in-situ observations.

Abstract: Unidirectional solidification for low-carbon steel weld metal was characterized by using Time-Resolved X-Ray Diffraction (TRXRD) system. Solid-state phase transformation was also insitu observed in reciprocal lattice space. It was shown that TRXRD analysis had a potential as a comprehensive characterization technique for solidification and phase transformation process in welding. It made for the growth behavior of dendrites in unidirectional solidification and α γ δ − − phase transformation in steel weld metal to be characterized.

Abstract: In this study, the microstructure evolutions of the interface of the explosively welded cp- Ti/AISI 304 S.S composites due to heat treatment are presented. The composites were subjected to heat treatment process at various ranges of 650-950°C in argon atmosphere for 1hr. The investigations were carried out by using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The results reveal the presence of reaction layers in the interface and show that heat treatments cause to form different intermetallic phases at the interface. In addition, it is found that the width of the interfacial layer increases with temperature. Fractographical studies of the lug-shear test samples show the formation of river patterns and kirkendal pore structures on the Ti-side of the interface.