Thermal insulation of wet shielded metal arc welds
Thesis (Nav. E.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1994, and Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Sciences & Engineering, 1994.
Three-Dimensional Simulation of Underwater Welding and Investigation of Effective Parameters The three-dimensional finite difference method was used to obtain temperature profiles , thermal history curves , and cooling times for single-pass underwater wet weldments
Ship salvage, harbor clearance, wreck removal, underwater pipelines, and conveyer equipment repair oftentimes require extensive underwater cutting and welding. Beginning in the mid-1930s, with the substitution of welding for lesser quality mechanical methods of joining, the overall cost and time spent on the job could be reduced considerably (Refs. 1–3). Underwater welding processes are classified as dry or wet based on their exposure to the ambient environment. Processes that are physically protected from the surrounding water are classified as dry, whereas in wet welding, the weld is directly exposed to the underwater environment (Ref. 4). Because of its lower costs, faster and more flexible operation, wet underwater welding offers more advantages than dry underwater welding (Refs. 2, 4). The term “underwater welding” as used in this paper refers to the wet welding technique where no mechanical barrier separates the welding arc from the surrounding water. While shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW) are both used for wet welding, SMAW is the most used process in wet applications (Refs. 1, 5, 6). Shielded metal arc welding offers the benefits of low cost and process simplicity, and has a considerable history of offshore application. However, for the joining of materials at depths exceeding 100 m, GTAW is often selected because of the quality of the welds produced (Ref. 5). Underwater SMAW is somewhat similar to SMAW performed in air (Ref. 7). In the SMAW process, heating with an electric arc is established between an electrode and the base plate while in wet SMAW, the arc is in the water between the electrode and the surface being welded (Refs. 1, 4, 7). In the case of gas tungsten arc welding, the arc is drawn between a water-cooled nonconsumable tungsten electrode and the plate. An inert gas shield is provided to protect the weld metal from the atmosphere, and filler metal may be added to the weld pool as required. Ignition of the arc is obtained by means of a high-frequency discharge across the root opening, since it is not advisable to strike an arc on the plate with the tungsten electrode. Normally, the inert gas shield used for welding aluminum and steel in countries like Great Britain is argon. Gas metal arc welding (GMAW) is, in effect, an extension of GTAW in which the electrode in this process is a consumable metal wire (Ref. 8). There are two major drawbacks during underwater welding: 1) rapid cooling of weld metal and heat-affected zone (HAZ) in comparison to welding in air, and 2) susceptibility to hydrogen embrittlement. In fact, the water acts as a large heat sink and draws off the heat of the electrode so that weld defects induced by the accelerated cooling usually appear in the HAZ in underwater welds. Also, since underwater welding induces an arc atmosphere that is high in water vapor content and in disso-
On Underwater Plasma Welding
Since plasma arc has been considered to be a suitable heat source for wet welding owing to the following factors, the feasibility of underwater welding by it is studied.(1) Plasma arc co-operating with plasma jet can be generated easily and kept in stable state in water, and is less sensitive to variations in torch-to-work spacing.(2) As energy transferred from plasma arc to work is highly concentrated, fusion efficiency of work is high.(3) Plasma arc is constricted by torch nozzle, so its directional property is excellent.(4) Plasma jet, kept operating all the diving time, acts as illuminating light source and makes it easy for the welder to work in water.As the first step to develop the process, preliminary experiments on some of the above-mentioned factors are made in simple welding environment, such as in city water and at water depth of 20 cm. And then butt welding and fillet lap welding under the conditions adopted according to information obtained in the preliminary experiments are done by using steel as base metal for a welded structure of 6 mm in thickness. Results of experiments show that underwater welding by plasma are (semi-transferred type plasma jet, i.e., plasma jet co-operates with plasma arc) can be done easily and stably and the weld has excellent properties in both tensile strength and notch toughness, though the weld metal includes blowholes to some extent. In conclusion, it is ascertained that this process can be put to practical use.