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The objective of the research, through both an experimental and a modelling approach, was to determine the parameters controlling the nitrogen level in a weld. A second objective was to study the relationship between the weld microstructure and the corrosion properties. More particularly, the potential interest of microelectrode techniques has been investigated. TIG welding has been investigated through both an experimental and a modelling approach. TIG and A TIG tests have confirmed tnat it is necessary to add nitrogen in the shielding gas in order to prevent nitrogen loss during welding. For duplex stainless steel, 2.5 % nitrogen in the shielding gas is sufficient, whereas for high nitrogen content austenitic stainless steels higher levels are necessary. It has also been shown that, for a given grade, the nitrogen content increases when the penetration increases. Penetration depends on the material composition, with a beneficial effect of surface active elements (0, S, etc.). The model developed was based on the nitrogen exchange between the plasma, the weld pool and the shielding gas. It was first developed to describe nitrogen evolution during a stationary arc situation. The results were in good agreement with experiments. The model was then adapted to the case of welding with an active flux. An attempt was made to describe the traveling arc situation. However, some improvements are still necessary. Pitting corrosion tests have confirmed the influence of nitrogen content on the corrosion sensitivity of TIG welds. Microelectrode techniques have been used to characterise the local corrosion behaviour of welds. It has been shown that the scanning vibrating electrode technique was of limited utility to study corrosion resistance of highly alloyed stainless steels. More promising results have been obtained with microcapillary technique which make local electrochemical measurements possible. Finally, MIG tests have been performed in order to study the influence of the shielding gas composition on nitrogen content in the weld and also on the formation of porosities. For superduplex stainless steel, it has been demonstrated that nitrogen must be added in the gas to prevent nitrogen loss. It has also been shown that the number of porosities in the weld depends on the C02 content in the gas and not on the nitrogen content.
This book describes the fundamental metallurgical principles that control microstructure and properties of welded stainless steels. It also serves as a practical "how to" guide that allows engineers to select the proper alloys, filler metals, heat treatments, and welding conditions to insure that failures are avoided during fabrication and service.
When considering the operational performance of stainless steel weldments the most important points to consider are corrosion resistance, weld metal mechanical properties and the integrity ofthe weldedjoint. Mechanical and corrosion resistance properties are greatly influenced by the metallurgical processes that occur during welding or during heat treatment of welded components. This book is aimed, there fore, at providing information on the metallurgical problems that may be encountered during stainless steel welding. In this way we aim to help overcome a certain degree of insecurity that is often encountered in welding shops engaged in the welding of stainless steels and is often the cause of welding problems which may in some instances lead to the premature failure of the welded component. The metallurgical processes that occur during the welding of stainless steel are of a highly intricate nature. The present book focuses in particular on the signif icance of constitution diagrams, on the processes occurring during the solidification of weld metal and on the recrystallization and precipitation phenomena which take place in the area of the welds. There are specific chapters covering the hot cracking resistance during welding and the practical welding of a number of different stainless steel grades. In addition, recommendations are given as to the most suitable procedures to be followed in order to obtain maximum corrosion resistance and mechanical properties from the weldments.
Two very successful conferences - in Glasgow and Beaune - were held on duplex stainless steels during the first half of the '90s. This book takes keynote papers from each, and develops and expands them to bring the topics right up to date. There is new material to cover grades, specifications and standards, and the book is fully cross-references and indexed. The first reference book to be published on the increasingly popular duplex stainless steels, it will be widely welcomed by metallurgists, design and materials engineers, oil and gas engineers and anyone involved in materials development and properties. The first reference book on this relatively new engineering material Based on keynote papers from major international contributors Covers grades, standards and specifications
This book reviews the behaviour of metals and alloys during welding. In the first part the heat flow in arc welding processes is discussed. The weld thermal cycle is explained in terms of heat input, and the geometry of weld and thicknesses to be welded. The real welding cycle is described in terms of thermal and strain cycles. The weld metal is characterized in terms of fusion stage, absorption of gases and stage of metal crystallization and structural transformation. The metallurgical background of cracking is described by a full set of crackability tests along with the evaluation of metals from the point of view of crackability. Post welding heat treatment is reviewed, and includes the relaxation of stresses induced by welding. Guidelines are given for the selection of steels for welded structures. Several chapters examine the weldability of particular steels, including high strength steels, stainless steels, high alloyed steels, cryogenic steels and other metals and alloys. The theories are quantified in the form of calculations or computing programmes. Readers will find sufficent data for software processing.
Nitrogen-alloyed austenitic stainless steels are becoming increasingly popular, mainly due to their excellent combination of strength and toughness. Nitrogen desorption to the atmosphere during the autogenous welding of these steels is often a major problem, resulting in porosity and nitrogen losses from the weld. In order to counteract this problem, the addition of nitrogen to the shielding gas has been proposed. This study deals with the absorption and desorption of nitrogen during the autogenous arc welding of a number of experimental stainless steels. These steels are similar in composition to type 310 stainless steel, but with varying levels of nitrogen and sulphur. The project investigated the influence of the base metal nitrogen content, the nitrogen partial pressure in the shielding gas and the weld surface active element concentration on the nitrogen content of autogenous welds. The results confirm that Sievert's law is not obeyed during welding. The weld nitrogen content increases with an increase in the shielding gas nitrogen content at low nitrogen partial pressures, but at higher partial pressures a dynamic equilibrium is created where the amount of nitrogen absorbed by the weld metal is balanced by the amount of nitrogen evolved from the weld pool. In alloys with low sulphur contents, this steady-state nitrogen content is not influenced to any significant extent by the base metal nitrogen content, but in high sulphur alloys, an increase in the initial nitrogen concentration results in higher weld nitrogen contents over the entire range of nitrogen partial pressures evaluated. A kinetic model can be used to describe nitrogen absorption and desorption during welding. The nitrogen desorption rate constant decreases with an increase in the sulphur concentration. This is consistent with a site blockage model, where surface active elements occupy a fraction of the available surface sites. The absorption rate constant is, however, not a strong function of the surface active element concentration. Alloys with higher base metal nitrogen contents require increased levels of supersaturation prior to the onset of nitrogen evolution as bubbles. These increased levels of supersaturation for the higher-nitrogen alloys is probably related to the higher rate of nitrogen removal as N2 the onset of bubble formation. Given that nitrogen bubble formation and detachment require nucleation and growth, it is assumed that a higher nitrogen removal rate would require a higher degree of supersaturation. Nitrogen losses from nitrogen-alloyed stainless steels can be expected during welding in pure argon shielding gas. Small amounts of nitrogen can be added to the shielding gas to counteract this effect, but this should be done with care to avoid bubble formation. Supersaturation before bubble formation does, however, extend the range of shielding gas compositions which can be used. Due to the lower desorption rates associated with higher surface active element concentrations, these elements have a beneficial influence during the welding of high nitrogen stainless steels. Although higher sulphur contents may not be viable in practice, small amounts of oxygen added to the shielding gas during welding will have a similar effect.