Stephen Liu

Recent developments in underwater wet welding

Abstract Developments in underwater wet welding processes over the past 25 years are reviewed. Shielded metal arc welding with rutile base coated electrodes is still by far the most common wet welding process in use. Research and development of wet welding electrodes has led to improvements in the control of hydrogen content, porosity, chemical composition, and microstructure of the weld metal. Additional work is required to develop welding consumables with improved control over diffusible hydrogen and porosity. Development of techniques such as temper bead welding has allowed successful wet welding repairs on steels having carbon equivalents greater than the traditional limit of 0.40. Alternative wet welding processes such as flux cored arc welding and friction welding are under development, but have yet to become widely accepted.

Development of closed‐loop control of robotic welding processes

Purpose – The purpose of this research is to develop closed‐loop control of robotic welding processes.Design/methodology/approach – The approach being developed is the creation of three‐dimensional models of the weld pool using stereo imagining. These models will be used in a model‐based feedback control system. Fusion of more than one sensor type in the controller is used.Findings – Three‐dimensional images can be produced from stereo images of GMAW‐p weld pools. This requires coordinating the image capture with the arc pulse to allow observation of the pool.Research limitations/implications – This is a work in progress. The imaging is not being done in real time at this point in time. Future work will address this issue. Also, how the image information is to be used to make corrections within the controller is future work.Practical implications – Closing the loop on GMAW welding will allow robotic automation of welding to proceed to a much broader degree of application.Originality/value – This paper dem...

The effect of solidification on the formation and growth of inclusions in low carbon steel welds

Weld metal transformations in low carbon steels are strongly dependent on the size and size distribution of non-metallic inclusions. Since inclusions are nucleants to proeutectoid phases, the presence of these second phase particles shift the continuous cooling transformation (CCT) curves to shorter times. Thus, the modelling of the formation and growth of inclusions is desirable to predict weld metal microstructure and properties. In this study, a new model is proposed considering solute redistribution during solidification. The model predicts the weld metal inclusion size distribution, considering diffusion-controlled deoxidation as the primary growth mechanism. An interesting feature of this model is that it predicts the change in the shape of the size distribution curve with the solute composition and the local solidification time.

Analysis of low transformation temperature welding (LTTW) consumables – distortion control and evolution of stresses

Abstract Low transformation temperature welding (LTTW) consumables have been reported to reduce the tensile residual stresses in weldments. Martensitic transformation induces compressive residual stresses and improves the fatigue resistance of welded joints. Several of these LTTW consumables have been developed at the Colorado School of Mines. This research work presents the comparisons of the experimentally and Sysweld calculated measurements for distortions and residual stresses for different plate thicknesses. In addition, residual stress evolution with time graphs were plotted to determine the amount of martensite required to promote compressive residual stresses and to calculate the time required to induce compressive residual stresses. The main aspect of this research is to analyse the behaviour of LTTW consumables in terms of distortion and residual stresses on various plate thicknesses.

Assessing metal transfer stability and spatter severity in flux cored arc welding

Abstract Five experimental basic type flux cored arc welding consumable wire electrodes were manufactured from the same base formulation. The composition of these electrodes was adjusted in an attempt to improve the operating performance. This involved additions of various ratios of alkali oxides, namely, lithium, magnesium, sodium, and potassium containing ingredients, in the flux formulations. The operating behaviours of these experimental electrodes and two reference products (i.e. one commercial basic T–5 and one commercial rutile T–1 electrode) were thoroughly investigated by recording welding arc signals using a high speed data acquisition system. By comparing these electrodes among themselves, the experimental electrodes were reported to exhibit extremely stable arcs, some showing electrical arc signals even smoother than those for the reference rutile grade electrode. Despite their improved metal transfer consistency, however, basic electrodes were characterised by somewhat higher spatter levels.

Precipitate stability in the heat affected zone of nitrogen-enhanced high strength low alloy steels

Abstract A high nitrogen, Ti–V microalloyed steel was found to produce coarse-grained heat affected zone (CGHAZ) with impact toughness superior to a similar Ti–V steel with lower nitrogen content. The CGHAZ microstructures of the high nitrogen steels were investigated to determine that precipitation in this region played an important role for the improvement of the mentioned toughness. The nitrogen-enhanced steel, with 130 parts per million (ppm) nitrogen, yielded a large number of fine nitride precipitates with sizes ranging from 30 to 900 A (3 to 90 nm). These cubic or rectangular-prism shaped, titanium-rich particles limited the austenite grain growth at high temperatures. The average austenite grain diameter of 50 μm at 1350°C was about three times smaller than that observed in the low nitrogen steel with 30 ppm nitrogen. The small grain size in the CGHAZ of the nitrogen-enhanced steel controlled the toughness. Instead of nitrides, the low nitrogen steel CGHAZ exhibited a large number of titanium or aluminum oxide particles. An important observation was that the particle number density of the nitrogen-enhanced steel decreased slowly, from 4.3×10 6 to 3.3×10 6 per mm 2 , with increasing holding time at 1350°C. Ostwald ripening alone was unable to explain the particle density change and precipitate growth. It was then determined that most TiN particles smaller than 420 A (42 nm) were located along the austenite grain boundaries. Thus, coarsening of these finer TiN precipitates at the grain boundaries would be determined by grain boundary diffusion and not lattice diffusion. In the case of welding, with thermal cycles characterized by high peak temperatures and short holding times, much of the titanium and nitrogen atoms would be expected to remain in solution, albeit in supersaturation. Hence, nitride particles larger than 420 A (42 nm) and located inside the austenite grains would receive solute atoms precipitating directly from the supersaturated matrix. Only after this supersaturated concentration reaches the equilibrium solubility that the effect of Ostwald ripening would become predominant in regulating the size distribution of the precipitates. In this paper, a combined diffusion model has been suggested to describe the growth mechanism of titanium nitride precipitates in the CGHAZ of a high nitrogen Ti–V microalloyed steel weld.

Welding of Materials for Energy Applications

Materials will play a critical role in power generation from both new and existing plants that rely on coal, nuclear, and oil/gas as energy supplies. High efficiency power plants are currently being designed that will require materials with improved mechanical properties and corrosion resistance under conditions of elevated temperature, stress, and aggressive gaseous environments. Most of these materials will require welding during initial fabrication and plant maintenance. The severe thermal and strain cycles associated with welding can produce large gradients in microstructure and composition within the heat-affected and fusion zones of the weld, and these gradients are commonly accompanied by deleterious changes to properties. Thus, successful use of materials in energy applications hinges on the ability to understand, predict, and control the processing–microstructure–property relations during welding. This article highlights some of the current challenges associated with fusion welding of materials for energy applications.

The influence of inclusion chemical composition on weld metal microstructure

The effects of nonmetallic inclusions on weld metal microstructures were investigated. The inclusions were extracted from niobium microalloyed steel weld metal specimens, and examined with light and electron microscopic techniques. An EDS (Energy Dispensive Spectroscopy) system was used to determine the chemical composition of the inclusions. Correlation between weld metal and inclusion composition was established. Aluminum, titanium, sulfur, and iron were the most important elements in the inclusions that affect the final weld metal microstructure. Mn/Si ratio was also found to affect the amount of oxygen and acicular ferrite in the weld. The state of deoxidation, as indicated by the amount of FeO present in the inclusions, actually determines the recovery of alloying elements and the amount of oxygen in the weld pool. It also determines the chemical composition of the nonmetallic inclusions. Inclusions with high aluminum content tend to cluster together forming larger particles while pure silica or silicate particles are small and well disseminated in the weld metal. This explains the different inclusion size distributions observed in the weld specimens of different oxygen concentration. Consequently, the prior austenite grain size will be different resulting in different amounts of acicular ferrite and grain boundary ferrite.

Solidification behaviour of laser welded type 21Cr–6Ni–9Mn stainless steel

Abstract The solidification mode and microstructures were characterised for various processing parameters for laser welding 21Cr–6Ni–9Mn stainless steel. Two heats with varying nitrogen content showed both primary ferrite and primary austenite solidification. Weld ferrite content varied from 1 to 11 vol.-%, and decreased as travel speed increased. Base metal nitrogen content affected both solidification mode and weld ferrite content. Nitrogen loss from the weld pool was found to range from 10 to 45%, and decreased with increasing travel speed. Solidification mode was dependent on both chemical composition and processing parameters. The solidification mode shifted from primary ferrite to primary austenite as travel speed increased due to increased undercooling. Solidification mode varied within welds with constant nitrogen content, and the change in solidification mode was attributed to changes in undercooling along the weld cross-section. It is proposed that the variation of solidification mode with undercooling is affected by both the solidification rate and thermal gradient.

Designing shielded metal arc consumables for underwater wet welding in offshore applications

The use of underwater wet welding for offshore repairs has been limited mainly because of porosity and low toughness in the resulting welds. With appropriate consumable design, however, it is possible to reduce porosity and to enhance weld metal toughness through microstructural refinement. New titanium and boron-based consumables have been developed with which high toughness acicular ferrite (AF) can be produced in underwater wet welds. Titanium, by means of oxide formation, promoted an increase in the amount of acicular ferrite in the weld metal, while boron additions decreased the amount of grain boundary ferrite (GBF), further improving the microstructure. Porosity reduction was possible through the addition of calcium carbonate at approximately 13 wt percent in the electrode coating. However, weld metal decarbonization also resulted with the addition of carbonate.