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Welding for Challenging Environments. Proceedings of the International Conference on Welding for Challenging Environments, Toronto, Ontario, Canada, 15–17 October 1985 PDF

353 Pages·1986·24.043 MB·English
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Pergamon Titles of Related Interest Gifkins STRENGTH OF METALS AND ALLOYS Harris MECHANICAL WORKING OF METALS International Institute of Welding THE PHYSICS OF WELDING NIku-Larl ADVANCES IN SURFACE TREATMENTS Nowackl THERMOELASTICITY, 2nd Revised Edition Osgood FATIGUE DESIGN, 2nd Edition Simpson FRACTURE PROBLEMS AND SOLUTIONS IN THE ENERGY INDUSTRY Welding Institute of Canada WELDING IN ENERGY-RELATED PROJECTS Related Journals (Free sample copies available upon request) ACTA METALLURGICA CANADIAN METALLURGICAL QUARTERLY CORROSION SCIENCE ENGINEERING FRACTURE MECHANICS INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES INTERNATIONAL JOURNAL OF PLASTICITY INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES METALS FORUM THE PHYSICS OF METALS AND METALLOGRAPHY WELDING IN THE WORLD ινΕυιικ<; FOR CHALLENGING ENvmoNMEivrs Proceedings of the International Conference on Welding for Cliallenging Environments, Toronto, Ontario, Canada, 15-17 October 1985 Edited by Welding Institute of C a n a da 0al(ville, Ontario, Canada P E R G A M ON P R E SS New York · Oxford · Beijing · Frankfurt · Sao Paulo Sydney · Tokyo · Toronto Pergamon Press Offices: U.S.A. Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, New York 10523, U.S.A. U.K. Pergamon Press, Headington Hill Hall, Oxford 0X3 OBW, England PEOPLE'S REPUBLIC Pergamon Press, Qianmen Hotel, Beijing, OF CHINA People's Republic of China FEDERAL REPUBLIC Pergamon Press GmbH, Hammerweg 6, OF GERMANY D-6242 Kronberg-Taunus, Federal Republic of Germany BRAZIL Pergamon Editora Ltda., Rua Ega de Queiros, 346, CEP 04011, Sao Paulo, Brazil AUSTRALIA Pergamon Press (Aust.) Pty. Ltd.. P.O. Box 544, Potts Point, NSW 2011, Australia JAPAN Pergamon Press, 8th Floor, Matsuoka Central Building, 1-7-1 Nishishinjuku, Shinjuku, Tokyo 160, Japan CANADA Pergamon Press Canada, Suite 104,150 Consumers Road, Willowdale, Ontario M2J 1P9, Canada Copyright © 1986 Pergamon Press Canada All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. First printing 1986 Library of Congress Cataloging in Publication Data International Conference on Welding for Challenging Environments (1985 : Toronto, Ont.) Welding for challenging environments. 1. Welding-Congresses. I. Welding Institute of Canada. II. Title. TS227.A1I56 1985 671.5'2 86-4958 ISBN 0-08-031866-5 In order to make this volume available as economically and as rapidly as possible, the authors' typescripts have been reproduced in their original forms. This method unfortunately has its typographical limitations but it is hoped that they in no way distract the reader. Printed In Great Britain by A. Wheaton & Co. Ltd., Exeter FOREWORD These proceedings of the Welding Institute of Canada's Third International Conference, Welding in Challenging Environments, held in Toronto on October 14/17, 1985, continues the Institute's commitment to make available, to the international welding community, the latest advances in specific technological fields and to provide a forum for international contact and discussion. The third Conference follows those held in 1980 on Pipeline and Energy Plant Piping in Calgary and in 1983 on Welding in Energy Related Projects, in Toronto. Proceedings of these conferences were also published by Pergamon Press Inc. We were pleased to arrange the present conference as an integral part of the 1985 Metals Congress of the American Society for Metals - the first time ASM has held their successful Metals Congress outside the USA. I would like to express our pleasure at the collaboration between the Welding Institute of Canada and the American Society for Metals that was achieved in this joint venture. The contributions which are presented in this publication combine the full texts of the papers presented with a record of the discussions, edited from tape recorded proceedings. They form a unique reference to the latest state of technological development, research and application of welded fabrications in challenging environments. The Welding Institute of Canada is proud to make the proceedings of this conference available on a widely distributed basis and trusts this publica­ tion will be a valued reference. NORMAN. F. EATON President Welding Institute of Canada ix RECENT DEVELOPMENTS IK PULSED GAS METAL ARC WELDING C.J. Allum GEC Industrial Controls Limited, Rugby, England. ABSTRACT Gas metal arc welding (GMAW) is currently attracting much interest on account of significant developments in process control over the past few years. These developments are largely associated with benefits gained from the application of modern solid state power devices to welding power supplies. In this paper pulsed current GMAW is considered with emphasis on interactions between pulse parameters, parameter selection, fusion characteristics and process control. KEYWORDS Pulsed current gas metal arc welding, metal transfer, power sources, process control. INTRODUCTION GMAW is arguably the most versatile of all welding processes being capable of joining in any position a wide range of materials, using manual or mechanised techniques on thin sheet or sections hundreds of millimeters thick. Process productivity is potentially high since filler metal is continuously deposited, with little slag, at high deposition rates (associated with efficient wire melting) and suitable for use in narrow gap weld preparations. Good toughness with very low deposited hydrogen levels are achievable meeting the requirements of many demanding applications. Furthermore GMAW allows close control of plate dilution and finds applications besides welding in cladding and brazing. GMAW has however, yet to achieve the potential outlined above. Historically, two process weaknesses (metal transfer and fusion characteristics) and a number of equipment related short comings have limited the application of GMAW. Metal may be transferred in a variety of modes. At very low currents short circuiting (dip transfer) is required and not all materials are weldable in this mode. The explosive nature of such transfers gives rise to spatter and intermittent arcing produces a susceptability to lack of fusion defects. At higher currents transfer becomes globular and non projected. Further increases in current result in a spray of small droplets, typically of wire diameter projected across the arc gap. 2 Welding for Challenging Environments Synergy is a control technique used in pulsed current MIG welding (Ref 1) where mean current is determined by wire feed speed such that stable wire melting and drop transfer occur. The outcome of this technique is simplified process operation with nominally one knob control. A wide range of methods exist for achieving the above characteristics but only two basic approaches are considered here (see Ref 8). One technique consists of driving the power supply in response to a wire feed speed control signal. This might for instance be used to increase pulse frequency proportionally to wire feed speed demand. Metal transfer can then be controlled by predetermined unit pulses of current (of specified Ip and Tp) while frequency control simply changes the time spacing between pulses with the effect of altering mean current. With this control scheme droplets of uniform size are detached at every mean current (i.e. W/F in constant) and mean current increases approximately in proportion to wire feed rate when low background currents are employed. Process control is then achieved directly from wire feed rate. For this type of control no arc length self adjustment exists i.e. when the torch is withdrawn from the work arc length increases with fixed wire extension. Arc length self adjustment may also be achieved by incorporating voltage control. Here a voltage error signal is generated (difference between reference voltage and measured voltage) which in effect modifies the wire feed to pulse frequency ratio to achieve the desired arc voltage. Features of conventional self adjusting GMAW are thereby regained. A second so called synergic technique relies entirely on voltage control to produce frequency modulation without any link between power supply and wire feed unit. Having set the required voltage and wire feed rate, the spacing between pre-determined pulses is modulated in self regulating manner i.e. arc current is self regulating. Process control is again of the one knob type and when a welding torch is withdrawn from the work both pulse frequency and mean current then fall in a self regulated manner so as to maintain a given arc voltage. By reducing process control to nominally one knob a range of further possibilities are presented. For instance thermal pulsing and backface control of full penetration. In the first case low frequency modulation of wire feed speed is used to produce overlapping weld beads which can have beneficial effects on fusion. The required changes in current changes are then automatically accomodated by the synergic type control technique. With backface penetration control (as practiced in TIG welding) a radiation signal from the underbead may be used to modulate top face heat input thereby controlling penetration (although this technique has yet to be developed for GMAW). All of the above techniques rely on steplessly variable control of the current waveform (especially pulse frequency). This may be achieved electronically with solid state devices (transistors and a range of thyristors) which are used in power circuits as current switches or variable resistors. Series regulator circuits use devices as variable resistors and have inherently high response with low ripple. However, at low arc voltages most of the process power is dissipated across power devices and these circuits, although essentially simple, are very inefficient with high cooling requirements. Switching circuits have much lower losses and air cooling is often appropriate. Devices are then either on or off and switched at a frequency characteristic of the circuit design/device capability (typically of order 20KHz). These circuits generate current ripple and have a slower response than achievable with series regulators. One praticularly energy efficient and physically small class of switching circuit are inverters of which there are a number of basic types. With these switching is on the primary side of the transformer and transformers can then be made much smaller than for secondary switched circuits. Welding for Challenging Environments 3 The first condition is concerned with drop detachment and reflects the observation that background parameters usually have little influence on the detachment event. It is often observed (see Fig 1) that a peak detachment can occur at high peak currents (Ip) of short duration (Tp) or lower peak currents of longer duration such that (1) is approximately obeyed where D, a detachment parameter, is constant and influenced by wire composition, wire diameter and shield gas type. Typical values for D are given in Table 1. In many situations preferred combinations of Ip and Tp exist and those identified by the writer are also shown in Table 1 where it is interesting to note that preferred peak currents are typically 1.5 times the so- called spray transition current (Is). The second condition is a simple statement of droplet volume (J2i) when one drop per pulse is detached where W is wire feed rate, F pulse frequency and A is wire cross- sectional area. In many situations W = K,I where 1 is mean current and Κ is a constant. Equation 2 then becomes:- ΐ - Ρ (3) F K.A Equations 1 and 3 allow pulse parameters to be specified provided Κ and D are known. The method is then:- 1. Select droplet volume and calculate I/F. _ 2. Select mean current for a given application. I/F then gives required pulse frequency. 3. Select Ip (at 1.5 Is say) and use D to calculate Tp. 4. Tb is now determined since frequency and pulse duration are known. 5. Background current is also determined and can be evaluated from the expression for mean current. In many commercial equipments, rules of this type are incorporated as control circuitry and the required parameter adjustments are automatically made thereby resulting in considerable simplifications in process operation (see process control). TABLE 1, Detachment Relationships Parameter Material/Gas Mild Steel Al S Steel (Pure) (308) Argon 5% C02 Argon Argon 2% dw (mm) 0.8 1.0* 1.2* 1.6* 1.2** 1,2*** D (A S) 160 310 430 640 130 315 F/I (Hz/IOOA) - 60 50-60 40 90 60-65 Ip (A) 200 280 350 400 200 280 Tp (ms) 4.0 4.0 3.5 4.0 3.3 4.0 4 Welding for Challenging Environments For many materials spray transfer occurs at currents too high to allow managable weld pool control in any position other than downhand welding. The principal method of overcoming transfer limitations outlined above was developed over two decades ago and uses pulses of current to detach small droplets while background current is held at a level compatible with managable weld pool size in out of position applications. Only recently with the advent of commercially available solid state power supplies has the potential of pulsed current welding been realisable. Pulse parameters can now be independently and steplessly varied to meet arc requirements where earlier equipments operated at fixed frequencies (related to mains frequency). The use of a range of process control techniques has also become practical (see Section 'Process Control'). Power control developments have undoubtedly removed many of the previous limitations concerning aspects of metal transfer behaviour and spatter. However, fusion characteristics appear to be largely determined by mean current and welding speed, technique and shield gas composition. As such, recent developments in power control have had relatively little impact on fusion which represents a significant challenge for future process developments. Indeed, shielding gases used are largely those previously developed for non-pulsed GMAW applications. METAL TRANSFER Current pulsing is applied in GMAW with the central objective of controlling the transfer of filler material. Using nominally square wave d.c. pulses it has been found possible to almost eliminate spatter and maintain stable arcs with small droplet transfers at currents down to about 50 amps (Ref 1). Satisfactory transfers are achieved by choosing suitable peak and background parameters which are usually identified by trial and error experimentation (Ref 2 and 3). Often more than one set of suitable parameters exist. The choice of parameters can in principal affect a wide range of droplet and arc characteristics including droplet volume, position of detachment within a pulse, number of droplets per pulse, droplet acceleration and momentum, arc stiffness and arc stability. In practice observation suggests that weld beads are tolerant to parameter changes (for a given mean current) and so general running characteristics (e.g. spatter, stability, stiffness, arc control) form the main criteria for parameter selection. An approach widely adopted is to choose parameters giving one droplet per pulse with detachment near the trailing pulse edge and transfer during the quiet background period. For given material parameters it is usually observed that the position within a pulse of drop detachment is determined by peak current (Ip) and duration (Tp) (Ref 2). Droplet volume is also influenced by background parameters (Ref 3). Clearly a wide range of parameter combinations exist in pulse current welding and often it is useful to employ a systematic approach to parameter selection. A semi-empirical approach to this problem is developed below. We start by considering conditions related to drop detachment and droplet volume;- Ip^ Tp = D (1) y W A = (2) Welding for Challenging Environments * Data from Reference 3 ** Data from Reference 4 *** Data from Reference 5 Also see Figures 1 and 2. WIRE MELTING BEHAVIOUR Wire melting behaviour has important implications for productivity and this can be affected by choice of pulse parameters. In pulsed MIG welding high current excursions have a significant influence on Ohmic heating and produce deposition rates higher than those achieved at the corresponding steady current. In view of high current peak excursions a none linear burn-off dependence on current might be expected. However, this is not generally observed (see Fig 2). To understand why linear behaviour is often observed consider the mechanisms by which wire melting occurs. Arc and resistive effects give:- W = '·< Ϊ + R (I^) ave. Where ^ is a coefficient related to arc heating and R is related to resistance offered by the electrically heated wire extension. The value of I averaged over a square pulse is approximately:- (I^) ave = (Ip^Tp + Ib^b) F = Ip^TpF for Ip^Tp»Ib2Tb = D F then W = i< + R D F (4) I ΐ The choice of f/F and D can clearly affect deposition rate per amp and maximum deposition at any mean current is obtained by the use of high frequency, high detachment parameter conditions (Ref 3). Long electrical extensions and small wire diameters also enhance deposition but this affect is not unique to pulse c_urrent welding. It should be noted that linear burn-off curves arc expected when I/F is constant. Deposition rate is little influenced by shield gas composition in GMAW. Welding polarity can however have a very significant effect where 50% higher deposition rates are observed (Ref 6) in a.c. welding of mild steel (compared with wire positive deposition rates) although a.c. synergic GMAW sets are not currently commercially available. FUSION CHARACTERISTICS The general characteristics of weld bead and plate fusion shape in GMAW (see Fig 3) are such that at least eight weld geometry characteristics may be identified which inturn depend on a variety of process parameters and many of these relationships have yet to be fully investigated. Emphasise here is therefore largely on qualitative effects although some quantitative results are discussed. 6 Welding for Challenging Environments The major process variables influencing plate fusion characteristics are mean current, welding speed and shield gas composition where the precise choice of pulse parameters is of secondary importance. For a given mean current and welding speed, shield gas composition has a pronounced effect on plate fusion shape (see Fig 3). Plate fusion generally exhibits a finger of penetration with weak secondary fusion extending towards the bead edges. Secondary fusion is much less pronounced on relatively low thermal conductivity materials (e.g. steels) than for aluminium. For steels secondary fusion is enhanced by the addition of C02 or helium to argon shields. However, non central finger penetration is often observed in helium rich shields and metal transfer characteristics deteriorate with the addition of high C02 levels. Significant work remains to identify Ar/He/C02/02/H2 gas mixes which optimise both transfer and fusion shape and thereby reduce susceptibility to fusion defects. Such work is considered particularly important for all-positional manual welding since dilution is often low (typically 20%) although at high currents/high speeds satisfactory dilution (30% and above) is achievable. Dilution (S) is found to be a useful measure of fusion and defined by:- Ap + Ad Where Ap and Ad are cross-section areas of plate fusion and deposited bead area respectively. For thick plates simple modelling considerations have been used to show that $ is a function of ϊν· (Ref 7, mean current χ welding speed) a result substantiated b]^ experiment and it is found that ^ behaves as shown in Fig 4. At high values of Iv, S reaches a saturation value C^m) and plate fusion^area then increases in proportion to heat input. However, at lower values of Iv, plate fusion is not a function of heat input. In this region a range of plate fusion areas corresponding to a given deposition area may exist depending on the structure of heat input). Such behaviour can be further illustrated by noting that the assymtotic shape of S = Mlv) becomes almost linear when 1/S is plotted against 1/iv (see Fig 5). Then; l ie (3) S ^m Iv Where C is a constant representing the line slope. Dilution now gives; Ap = S Ad 1-S Noting that vJ^d = Ka (where a is wire cross-sectional are a) and substituting for allows Ap to be expressed as:- Ap = Κ a (l/v) (6) ' 1 C + — L Sm Iv

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