Materials Science Forum Vols. 783-786

Abstract: The purpose of this study is to evaluate the effectiveness in the contribution of the interpass welding in the microstructures and properties of the dissimilar fusion zones produced with an alloy 625 and API 5L X65 steel pipes. Three multipass welded joints with v-groove, were made under the same welding parameters, therefore, changing only the interpass temperatures at: 150°C, 250°C and 450°C. The microstructural characterization was performed using the light microscopy and the scanning electron microscopy (SEM). The hardness test, charpy-V impact test and the transverse tensile test were conducted according to specific standards. The results have shown that all ruptures in the tensile tests occurred in the base metal. Both yield strengths and ultimate tensile strengths, have dropped to the 450°C interpass temperature. It was also observed a slight drop in the hardness with increasing interpass temperature. The Charpy-V impact test results showed no significant differences among the interpass temperatures. These said results indicated that the maximum interpass temperature for the alloy 625/X65 steel dissimilar welding is limited by the steel properties.

Abstract: The aim of this study is to evaluate the microstructure and properties, resulting from the dissimilar welding of Ni-based alloys like Hastelloy C276 and Inconel 625 with ASTM A516 Gr 60 steel, through the deposition of overlays by the TIG welding process with cold wire feed. The results showed that it is feasible to produce coatings with low dilution levels in a single layer. Regarding the microstructure, there was a strong segregation of alloying elements during solidification. The phenomenon of microsegregation is directly or indirectly associated with the mechanism of corrosion attack. The shear resistance tests showed that coalescence between coating and substrate, provided a high shear strength, even with the possible presence of defects and high hardness martensitic zones. The immersion test in FeCl3 showed that the alloy 625 had lower CPT when compared to alloy C276. However, in the CPT temperature for the alloy C276, the intensity of the attack for the alloy 625 was lower than the alloy C276.

Abstract: In this study, distributions of temperature and metal vapor during MIG welding of aluminum are obtained. Since a droplet forms and detaches at a tip of wire, and pass through the arc plasma during MIG welding, dynamic plasma diagnostics are demanded. This study aims to develop method for measuring dynamical variation of two-dimensional distribution of temperature and metal vapor concentration in the arc through optical measurement and to analyze behavior of the metal vapor in MIG welding. As the results, in MIG welding of aluminum, the arc plasma has double structure consisting of high temperature region apart from the arc axis and low temperature region near the arc axis due to influence of the aluminum vapor. Furthermore, the low temperature region near the arc axis occurs because the arc plasma is cooled especially through the intensive radiation loss caused by high concentration of the metal vapor.

Abstract: Hybrid laser-arc welding (HLAW) of butt joints in 3.18 mm thick aluminum alloy (AA) 6061-T6 sheets was investigated in the present study. Under optimized process conditions, high integrity welds with approximately 2% shrinkage and gas porosity were obtained. The weld bead geometry was determined to conform to the crown and root reinforcement specifications for welded aluminum construction - CSA W 59.2 M1191. Softening in the heat-affected zone (HAZ) and fusion zone (FZ) of the welds was observed, the former due to grain coarsening and the latter due to the dissolution of the hardening precipitates as well as the dilution from the application of the ER 5356 filler wire. Under optimized process conditions, the gap tolerance was determined to be 0.5 mm, beyond which the performance of the joints during bend testing was compromised.

Abstract: Precipitation-hardenable 6xxx series aluminum alloys are incorporated in many structural components with due consideration of their good combination of properties including a relatively high strength, outstanding extrudability and excellent corrosion resistance. Accordingly, AA6061 has been identified as a very good candidate material for structural lightweighting of transportation vehicles. However, the weldability of aluminum alloy (AA) 6061 by means of conventional technologies such as GMAW and GTAW methods is limited by sensitivity to solidification cracking. In this respect, friction stir welding (FSW) presents a tremendous potential for assembly of aluminum structures for the transportation industry due to the low heat involved that can mitigate crack formation and, thus, translate into improved mechanical performance of the assembly. In this work, FSW of 3.18 mm thick AA6061-T6 sheets in the lap joint configuration was investigated. This configuration is considered to be more challenging for assembly by FSW than the butt joint type due to the orientation of the interface with respect to the welding tools and the necessity to break the oxide layer on two aluminium alloy planar surfaces. Weld trials were performed to examine the influence of the FSW tool geometry and process parameters on the welding defects, microstructure, hardness and bend performance. Unacceptable material expulsion and/or significant thinning in one of the two overlapped sheets were produced under most conditions. A set of FSW tool geometries leading to a viable process operational window under which the risk of defects could be mitigated and/or eliminated was identified in this study.

Abstract: Diffusion bonding of Ti-6Al-4V titanium alloy in the coarse grained and ultrafine grained state was performed. The effect of initial structure and surface condition, as well as temperature and time, on the quality of joints was established. It is shown that, due to low-temperature superplasticity and high diffusion rate, samples with ultrafine grained structure demonstrate better bondability than coarse grained samples

Abstract: Temper bead welding (TBW) is one effective repair welding method for the large-scale nuclear power plants. Consistent Layer (CSL) technique is the theoretically most authoritative method among the five temper bead welding techniques. However in the actual operation, CSL technique is difficult to perform, and non-CSL techniques (Controlled Deposition technique, Half Bead technique, et al) are mainly used in the actual repair process. The thermal cycles in heat affect zone (HAZ) of non-CSL technique are more complicated than that of CSL techniques. Through simplifying the complicated thermal cycles to 4 types of thermal cycles, the neural network-based hardness prediction system for non-CSL techniques has been constructed. The hardness distribution in HAZ of non-CSL techniques was calculated based on the thermal cycles numerically obtained by finite element method (FEM). The predicted hardness was in good accordance with the experimental results. It follows that the thermal cycle simplification methods are effective for estimating the tempering effect during temper bead welding of non-CSL techniques.

Abstract: Stainless steel has been widely used in challenging environments typical to nuclear power plant structures, due its excellent corrosion resistance. Nickel filler metals containing high chromium concentration, including Alloy 82/182, are used for joining stainless steel to carbon steel components to achieve similar high resistance to stress corrosion cracking. However, the joint usually experience weld metal stress corrosion cracking (SCC), which affects the safety and structural integrity of light water nuclear reactor systems. A primary driving force for SCC is the high tensile residual stress in these welds. Due to large dimension of pressure vessel and limitations in the field, non-destructive residual stress measurement is difficult. As a result, finite element modeling has been the de facto method to evaluate the weld residual stresses. Recent studies on this subject from researchers worldwide report different residual stress value in the weldments [5]. The discrepancy is due to the fact that most of investigations ignore or underestimate the thermal recovery in the heat-affect zone or reheated region in the weld. In the current study, the effect of heat treatment on thermal recovery and microhardness is investigated for materials used in dissimilar metal joint. It is found that high equivalent plastic strains are predominately accumulated in the buttering layer, the root pass, and the heat affected zone, which experience multiple welding thermal cycles. The final cap passes, experiencing only one or two welding thermal cycles, exhibit less plastic strain accumulation. Moreover, the experimental residual plastic strains are compared with those predicted using an existing weld thermo-mechanical model with two different strain hardening rules. The importance of considering the dynamic strain hardening recovery due to high temperature exposure in welding is discussed for the accurate simulation of weld residual stresses and plastic strains. Finally, the experimental result reveals that the typical post-buttering heat treatment for residual stress relief may not be adequate to completely eliminate the residual plastic strains in the buttering layer.

Abstract: Ultrasonic impact treatment is a relatively new surface modification process that may be potentially applied to impart compressive residual stresses onto oilfield parts experiencing wear, fatigue, and possibly environmentally-assisted cracking. Through severe plastic deformation, ultrasonic impact treatment is herein investigated to surface harden two oilfield alloys, UNS N07718, a premium alloy with satisfactory oilfield performance but occasionally lacking surface hardness and abrasive wear resistance, and UNS G41400, a comparatively low-cost alloy restricted by its corrosion fatigue limit in oilfield rotating equipment. For comparison purposes, the two studied alloys were ultrasonic impact treated under identical conditions and carefully selected to exhibit similar yield strengths (900 MPa). Results from microstructural examinations, micro-hardness indentations, and residual stress measurements all indicate that ultrasonically treated surfaces exhibit superior properties that create opportunities for implementing this new surface modification process in selected oilfield applications.

Abstract: The effect of the CoAl precipitates on the deformation behavior of Fe-15.0Al-15.0Co (at.%) single crystals was examined. The spherical CoAl phase with the B2 structure was precipitated in the single crystals and was stable below 974 K. The bcc matrix and CoAl phase satisfied the cube-on-cube orientation relationship with a misfit strain of 0.25%. The single crystals showed a high yield stress up to 923 K while the stress dropped at 1023 K due to the dissolution of the CoAl phase into the matrix. Moreover, the activated sip system of the crystals containing the CoAl precipitates depended strongly on loading axis. At <149> orientation, {101} <111> slip favorable for the bcc matrix and the CoAl precipitates were sheared by a pair of 1/2<111> dislocations without forming Orowan loops. The CoAl single phase was known to hardly deform by <111> slip which resulted in high strength at <149> orientation. In contrast, {010} <001> or {hk0} <001> slip favorable for the CoAl precipitates was activated at <011> orientation, although the volume fraction of the CoAl phase was very small. <001> slip was generally impossible to take place in the bcc matrix, leading to the extreme hardening. Therefore, the difference in primary slip system between the bcc matrix and CoAl precipitates was responsible for the significant precipitation hardening.