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From cutting edge research of new processes for the manufacture of equipment for the energy production industry: Walter Tosto S.p.A. demonstrates in this paper, its experience in the highly specialized robotic welding process TIG narrow gap, widely used in the construction of the European segments of the Vacuum Vessel for ITER, in material 316L (N)-IG (ITER Grade). Following a brief general introduction of the process of TIG welding, this paper analyses: the technological, metallurgical and constructive features of TIG narrow gap, and combined with WT's technical know-how, shows how it was possible to overcome every critical aspect of its implementation. [ABSTRACT FROM AUTHOR]

Enhanced cooling via forced convection using cryogenic liquid is an option for controlling grain growth in the heat afFected zone (HAZ) of ferritic stainless steel welds which improves joint strength. However, this technique seems to alter the martensite distribution in the high-temperature heat affected zone (HTHAZ) which is a critical constituent in rating the susceptibility to sensitization in ferritic stainless steel grades; any such information is not available in the literature. Thus, it is imperative to establish the influence of cryogenic cooling on sensitization dynamics in the HTHAZ. This paper discusses the influence of cryogenic cooling on sensitization in an AISI 430 ferritic stainless steel weld. It is established that cryogenic cooling increases the cooling rate in the HTHAZ and reduces the martensite volume percent by an average of 20%. This reduction in martensite content in the HTHAZ increases the level of ditched structure in cryogenically cooled welds and yields more ferrite-martensite ditched grain boundaries than in conventional welds. Although the cryotreated welds exhibit greater ditched boundary, the structure is still classified as nonsensitized, since no single grain boundary is completely surrounded by ditches. [ABSTRACT FROM AUTHOR]

The reheat cracking phenomenon (also known as stress relief cracking) is today a well known topic; since the first years of the 1950failure that can be brought to this kind of phenomenon appeared. Nevertheless today the causes and the factors that lead to this failure that interests different types of materials, are not totally cleared. This paper gives to the Reader an overview of the phenomenon, focusing the main causes, the materials that are susceptible, the preventive measures and, finally, the main tests for the characterization. [ABSTRACT FROM AUTHOR]

For dissimilar metal welds, between austenitic stainless steels and ferritic steels, an austenitic stainless steel filler material with an adequate composition is normally used. To minimize the risk of hot cracking, the weld zone should preferably be constituted by an austenitic microstructure with a small amount of ferrite δ. During solidification, the microstructures depend on filler material and base material compositions. Useful indications of the weld zone microstructures can be drawn from such diagrams, for example the Schaeffler diagram, which defines the fields of existence of the phases as a function of composition expressed in terms of chromium equivalent (Creq%) and nickel equivalent (Nieq%). Recently alternative diagrams to Schaeffler diagram have been proposed. An amendment was introduced in one of these diagrams to better characterize the boundary area between martensitic-austenitic structure and fully austenitic structure. In this paper, by means of the experimental test results of austenitic stainless steel and ferritic steel joints, welded with laser beam process, some specimens were obtained with a weld zone composition near to the boundary between martensitic and austenitic zone in the Schaeffler diagram. The content of the elements was calculated by energy dispersive microanalysis (EDS). The microstructures were examined by optical microscopy and scanning electron microscopy (SEM) and by Vickers microhardness tests. For some microstructures, which were fully austenitic, the position of the corresponding points were located in the martensitic-austenitic zone below the line defined by the original Schaeffler diagram, while they were in accordance with the new definition of the boundary zone of the modified Schaeffler diagram. [ABSTRACT FROM AUTHOR]

Modern superaustenitic stainless steels are quaternary alloys extensively used to resist severe aqueous corrosive service, with large contents of Cr, Ni, Mo and PREn index generally higher than 45. Fusion welding is the major joining method for fabricating components and vessels using these grades. In this paper, the behavior of these alloys and the consequence of their use infusion welded structures are illustrated. The typical microstructures of the base materials, the transformations followed during welding or heat treatment and their influence on the mechanical properties and corrosive resistance of the joints are presented. Fabrication practices, filler materials and the aspects common to the nickel alloys welding are discussed, as well. During the welded fabrication, the major difficult is to retain the fully austenitic microstructure with sufficient homogeneity throughout all the high temperature manufacturing operations, and to limit the tendency to the hot solidification cracking. The recent understanding of the segregation mechanisms during the solidification and the associated kinetic will allow a more effective control of the micro structural evolution, also by using high energy welding methods and newly formulated filler materials with non-austenitic primary solidification mode. [ABSTRACT FROM AUTHOR]

This paper has the target to summarize which are the fillers to be used in the pressure vessels cladding processes. We evaluate the most common requests in the customer's specifications and how to meet them while checking the results during construction. We gave great emphasis to the applications with austenitic stainless steels (series 300), without neglecting other applications such as Ni-based alloys. We evaluated the different processes to be used and the role of "b ferrite" which, besides being often a specific requirement of the customer, is also the key factor to control the cladding during the construction. We examined the various processes used in terms of thickness and dilution. The dilution is a key output, which depends on the process used and the parameters reported in the WPS. The accurate assessment of the dilution allows to choose the most suitable filler material and to avoid problems. The power source must be reliable and allow the repeatability of the process, as well as having a "duty cycle" adapted to production demands. It is important to underline that often being the surface to be plated several hundred square meters, the correct selection of the power sources reduce the cycle times and the delivery times. In fact, very often the cladding is on the production critical path. We presented some real cases, demonstrating how it is possible to meet the specification requirements with a careful analysis of a few experimental data. We dedicated a chapter to cladding repair defects subject. Cladding repairing defects need to be evaluated on a case by case basis, but in this presentation we propose some key guidelines that can be used to develop the repair specifications avoiding common problems. [ABSTRACT FROM AUTHOR]

This paper deals with a newly developed pretreatment for TIG arc welding to improve weld penetration. As a pre-treatment, a small oxidized region was produced on the surface of stainless steel plates by laser irradiation using oxygen gas prior to travelling TIG arc. Tlte effect of the pre-treatment (espe-cially named LA pre-treatment) on the TIG weld penetration was evaluated. It was found that the pre-treated region extremely smaller than the weld could make the TIG weld penetration deeper at lower welding speeds. [ABSTRACT FROM AUTHOR]