Tài liệu Welding practice

Welding Practice Welding Practice Brian D. Smith Tech Eng, Tech Weldl, MITO, CGIA Registered Welding Education Technician Acknowledgements The author expresses his thanks to the following organizations supply of information and illustrations: for their support and their Air Products PLC Arc Speed Services, Derbyshire Arco Ltd: safety signs (colour plates) The British Oxygen Company (BOC), Guildford and Derby: gas cylinder identification chart (colour plates and Figure 1.1) The British Standards Institute, Milton Keynes CENTRA, Manchester Chubb Fire Ltd: fire extinguishers (colour plates) East Midlands Further Education Council, Nottingham ESAB Group (UK) Ltd, Waltham Cross Gas Control Equipment, Skelmerdale The Health and Safety Executive Migatronic Welding Equipment Ltd, Loughborough C. S. Milne Ltd, Leicester Trueweld, Derby The Welding Institute, Abington, Cambridge Special thanks to Mr Len Gourd, BSe FWeldI, for his continuing support. Every possible effort has been made to trace copyright holders. Any rights not acknowledged here will be acknowledged in subsequent printings if notice is given to the publisher Butterworth-Heinemann An imprint of Elsevier Science Linacre House, Jordan Hill, Oxford 0X2 8DP 225 Wildwood Avenue, Woburn MA 01801-2041 First published 1996 Transferred to digital printing 2002 Copyright © 1996, Brian D. Smith. All rights reserved. The right of Brian D. Smith to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988 Nopartofthispublicationmaybereproducedinanymaterialfonn (includingphotocopyingorstoringinanymedium byelectronicmeansand whetheror not transientlyor incidentallytosomeother use ofthispublication)without the writtenpermissionofthecopyrightholderexceptinaccordancewiththeprovisionsoftheCopyright,Designsand Patents Act1988or under the termsofa licenceissuedby theCopyrightLicensingAgencyLtd,90TottenhamCourtRoad,London. EnglandWIT4LP.Applicationsforthecopyrightholder'swrittenpermissionto reproduceanypart ofthispublication shouldbeaddressedtothepublisher British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 340 61406 4 For information on all Butterworth-Heinemann visit our website at www.bh.com publications Printed and bound in Great Britain by Antony Rowe Ltd, Eastbourne Contents 1. Underpinning information 1 Arc welding safety Welding terminology Weld symbols (BS499 part 2 1980) Types of joints Features of a welded joint Distortion control Weld defects Inspection Welding procedure and welder qualifications 2. Manual metal arc welding 29 Manual metal arc welding equipment Striking the arc Stop/starts Electrode angles Weaving Electrode classification Electrode size and current capacity Electrode coatings Welding positions for plates Positional welding Plate positions Summary 3. Metal arc gas-shielded welding MAGS welding process Choice of shielding gas Electrode wires Wire feed system Metal transfer modes The self-adjusting arc Contact tip, nozzle settings Electrode extension Burn back Current and voltage settings British Standards for wires and fillers Features of the process Weaving Flux-cored arc welding Self-shielded flux-cored arc welding 47 vi CONTENTS Gas-shielded flux-cored arc welding MAGS positional welding 4. Tungsten arc gas-shielded welding 61 TAGS welding equipment Equipment selection Surge injector DC suppressor Contactor Arc striking Electrode types and diameters Gas nozzles Gas lens Torches Shielding gas AC/DC non-consumable electrode arcs 5. Oxyacetylene welding and cutting Standard welding/cutting equipment Safety precautions to be observed when using oxygen and acetylene cylinders Regulators Safety Flashback arrestors Hoses (BSS120) Purging Hose connectors Hose check valves '0' clips Welding torches Nozzles Equipment assembly (welding and cutting) Lighting-up procedure Flame settings Extinguishing the flame Welding techniques High- and low-pressure systems Filter glasses Plate edge preparation Oxyacetylene positional welding Bronze welding Fluxes Oxyfuel gas cutting Torches Fuel gases 77 CONTENTS Nozzles Hand cutting Quality of cut Supporting materials Cutting techniques 6. Questions 99 Health and safety MMA welding MAGS welding TAGS welding Gas welding and cutting Answers to questions Appendix 1: Welding processes Welding processes and their numerical 107 representation Appendix 2: Useful information 109 Index 113 vii Underpinning information Arc welding safety Under the Health and Safety at Work Act 1974 welders have a responsibility to take reasonable care for their safety and that of others by cooperating with safety requirements. Welding and the welding environment can create many hazards, but can be carried out quite safely if you play your part and observe some basic safety rules. There are a number of factors to be considered; a few of these are listed below. Personal protection Personal protection for the welder is most important, apart from the most obvious dangers of burns, stray sparks and falling objects, the arc gives off ultraviolet and infrared rays. These will affect the skin and eyes much in the same way as long periods of exposure to the sun. Suitable protection must be worn at all times to guard against these and other dangerous occurrences. Protective clothing comes in the form of gauntlets, leather coats, aprons, sleeves, spats and capes. The eyes should always be protected by a shade of filter glass suitable for the welding operation and amperage being used, or a clear glass for the chipping of slag. Table 1.1 details the recommended filter glasses. Protection for the feet in the form of safety boots with a toecap is an essential part of the welder's protective clothing. Boots protect the Table 1.1 Recommended filter glasses for welding Filter glasses for manual metal arc welding EWF up to 100 amps 8-9 EWF 100-300 amps 10-11 over 300 amps EWF 12-13-14 Filter glasses for oxyfuel gas welding: GWF Aluminium and alloys 3 GWF Brazing and bronzing welding 4 GWF Copper and alloys 5 GWF Thick plate and pipe 6 Filter glasses are usually protected on the outside by a separate, clear plastic cover lens. This will give protection from spatter particles and prolong the life of the filter lens. toes from damage by falling objects, while also giving the best protection against sparks entering the footwear. Some boots have a flap up the front, others may be one piece with high legs giving good protection to the foot and leg. Low shoes or trainers are not the ideal footwear for the welder. Many types of eye protection are available in the form of spectacles, goggles, face visors, clear glasses for use with welding helmets/ shields, etc. The most important aspect when choosing any form of eye protection is that it must conform to British Standard (BS)2092. Eye protection must be worn for all cutting, grinding, chipping and welding operations. Working at heights In your profession as a welder it is possible that you may have to work off scaffolding, 2 UNDERPINNING INFORMATION trestles, elevated platforms and so on. In most instances these elevated platforms will be erected or operated by qualified people eliminating the possible risk of collapse. As a welder working from such platforms, extra care must be taken to avoid even the slightest risk of electric shock. Although the effects of the shock may be dismissed, it could lead to loss of balance and falling with serious or fatal results. b er a 1ways Remem . . wor k mg a t h eig ht s.. t a k e ex t ra care w h en Responsibilities of the welder Under the Health and Safety at Work Act 1974, individuals are held responsible for their own safety as well as that of colleagues and those working in close proximity. Always ensure you have taken all reasonable, practical precautions to avoid the risk of accidents and fires, for example: a) good housekeeping; keep a tidy work area b) wear protective clothing on feet, body and eyes c) screening of workstation to protect passers-by d) use and position extraction equipment correctly e) carry out safety checks on equipment f) do not tamper with safety posters or signs. These are just a few points to consider before work commences. How many more can you think of? Welding fumes Welding fumes are virtually impossible to eliminate from the welding process. However, fumes can be rendered harmless by observing the following simple rules: .... a) the welder positlOnmg hIm/herself out of th e fume pa th b) using adequate ventilation either natural or mechanical - fans and extraction equipment, for instance c) correct positioning of source of ventilation some gases are heavier than air and will sink to the bottom of a confined area; in these cases extraction should be at a low level d) placing the source of extraction as close to the point of welding as possible without disturbing the gas shield e) siting the source of extraction to pull the fumes away from the welder f) avoid welding on contaminated surfaces, such as oil, grease, paint, galvanised metal, etc. R b f h' h' I d . emem ~r, urnes:, IC mc u e POlS?~oUS gases whIch asphyxIate are not always vIsIble. ... IS not possIble to use the above methWhere It ods, then breathmg apparatus may be required, supplied either from an airline or a personal body pack. If in doubt ask! Storage and handling of gas cylinders The most com~on .method of ~upplying gases use.d for weldmg IS from c~hnders. All gas cyhnders should be treated wIth respect, handled carefully and stored in well-ventilated conditions. Never allow cylinders to come into contact with heat, contaminants containing oil and grease, and observe the following handling and storage conditions: ... a) always keep cyhnders, flttmgs and connections fre~ from oil ~nd grease b) store cyhnde~s upnght .. c) transport cyhnders upnght. d) store flammable and non-flammable gases separately e) store full and empty cylinders separately f) avoid cylinders coming into contact with heat g) secure cylinders by 'chaining up' during storage, transport and use h) open cylinder valves slowly i) only use recommended leak-detection sprays j) report damaged cylinders to the supplier k) always check for leaks . h 0 ff gas supp Iyater I) SWltC f use. Gas cylinders find their way into many different environments from schools and colleges to heavy fabrication. Wherever gases are used the operator, supervisor and management should be aware of the dangers and familiar with the operating conditions, safety requirements, handling and storage conditions. Gas suppli- WELDING These are just a few points for you to think about. Always consider the working environment and ask yourself: what could cause a fire? - remove it and prevent it from happening. Electrical hazards The risk of electric shock is something which the welder should always be aware of, both to Injury resulting themselves and colleagues. from electric shock can be burns, loss of consciousness or death. If you come across someone who you suspect has received an electric shock, switch off the supply immediately and only remove the injured person from contact by means of suitarc For normal protection. ably insulated welding voltages, sufficient protection will be given by their clothing - provided it is dry. When freeing the injured person from the electrical source, it must be done with the use of a non-conducting material. Where staff trained in first aid are available, inform them immediately so they are able to administer the correct treatment. If no-one is available, and the situation is serious, attempt mouth-tomouth resuscitation while you send for help. Table 1.2 TERMINOLOGY 5 Remember that where there is a suspected risk of electric shock, due to working conditions or environment, adequate precautions must be taken. However minor the incident may appear, always ensure that the injured person is seen by a doctor. It is also advisable that those involved with electric arc welding be trained in the basic practice of mouth-to-mouth resuscitation. Welding terminology Welding, gas and arc, is a widely used process to join metals by fusion welding, brazing or bronze welding. The terminology tends to vary somewhat from county to county, and only contact, conversation and experience can create an awareness of the wide range of names used. This book has put together a list of terms based on the most widely used to describe the various features of the process. Table 1.2 gives terms relating to the gas and arc welding processes. Table 1.3 specifically relates to terms used in the thermal cutting of metals, and Table 1.4 deals with testing and examination. Terms relating to gas and arc welding Term Definition Actual throat thickness The perpendicular distance between two parallel lines joining the weld toes A disturbance of a DC welding arc caused by magnetic fields set up in the work The voltage between electrodes or between an electrode and the work during welding Retrogression of the flame into the blowpipe neck or body with rapid self-extinction A welding sequence in which short lengths of weld are deposited in a direction opposite to the direction of progress along the joint to produce a continuous or intermittent weld A piece of metal placed at a root and penetrated by weld metal. It may remain as part of the joint or removedby machining or other means A piece of metal or other material placed at a root and used to control the root penetration bead Fusing of the electrode wire to the current contact tube in any form of automatic or semi-automatic metal-arc welding process The linear rate of consumption of a consumable electrode in any consumable electrode process A localised collapse of the molten pool due to excess localised heating, resulting in a hole in the underlying weld run or parent metal A device for mixing and burning gasesto produce a flame for welding, brazing, bronze welding, cutting, heating and similar operations Arc blow Arc voltage Backfire Back-step sequence Backing strip (backing ring pipe) Backing bar (backing ring pipe) Burn back Burn off rate Burn through (melt through) Cutting/welding torch 6 UNDERPINNING Table 1.2 INFORMATION Continued CO2 flux cored welding (cord wire welding) CO2 welding Concave Cone fillet weld Continuous weld Convex fillet weld Covered filler metal Crack Crater pipe Deposited metal Dip transfer Dual shield welding Excess penetration bead Feather Fillet weld Filler metal (filler wire or filler rod) Flame snap-out Flashback arrester Flux Fusion penetration Fusion welding Fusion zone Gas economiser Gas envelope Gas pore (gas cavity) Gas regulator Globular transfer Metal arc welding in which a flux cored electrode is deposited under a shield of carbon dioxide Metal arc welding using a continuous bare wire electrode. The arc and molten pool being shielded by carbon dioxide shielding gas A fillet weld in which the weld face is concave (curved inwards) The inner part of a flame adjacent to the nozzle orifice known as the inner cone (oxyfuel gas welding) A weld along the entire length of a joint A fillet weld in which the weld face is convex (curved outwards) A filler metal having an outer covering of flux can be in the form of a continuous covering or contained in indentations along its length Discontinuity in the welded joints. Cracks may be longitudinal, transverse, edge, crater, centre line, fusion zone underbead, weld metal or parent metal A depression due to shrinkage at the end of a run (crater) where the source of heat was removed Metal after it becomes part of a weld or joint A method of metal-arc welding in which fused particles of the electrode wire in contact with the molten pool are detached from the electrode in rapid succession by the short circuit current, which develops every time the wire touches the molten pool Semi-automatic welding using a flux covered wire and a shielding gas Metal prottuding through the root of a weld made from one side only in excess of the stated limits The carbon-rich zone, visible in a flame, extending around and beyond the cone when there is an excess of fuel gas A fusion weld, other than a butt or edge weld, which is approximately triangular in cross-section Metal added during welding Retrogression of the flame beyond the blowpipe body into the hose, with possible subsequent explosion A safety device fitted in the oxygen and fuel gas system to prevent any flashback reaching the gas supply Material used during welding to prevent atmospheric oxidation and to reduce impurities or float them to the surface. Can also have a cleansing action on the surfaces to be joined The depth to which the parent metal has been melted into the fusion faces Joining together to form a union between metals in a molten state without the application of pressure An area of the parent metal at the fusion face which is melted to form part of the weld A device designed for temporarily cutting off the supply of gas to the welding equipment. A pilot jet may be fitted for relighting The gas surrounding the inner cone of an oxyfuel gas flame A cavity formed by entrapped gas during the solidification of molten metal A device for attachment to a gas cylinder or pipeline for reducing and regulating the cylinder or line pressure to the working pressure required Metal transfer which takes place as large globules transferred from the electrode to the weld area -' 8 UNDERPINNING Table INFORMATION Continued 1.2 Slag-trap Slot weld Spray transfer Stack cutting Staggered intermittent weld Striking voltage Submerged arc welding Sustained backfire TIG welding (inert gas tungsten Touch welding Toe Tungsten inclusion Two-stage regulator Undercut Weld Weld junction Weld metal Weld zone Welding procedure Welding sequence Welding technique Worm-hole A feature in a joint or joint preparation which may lead to the entrapment of slag A weld made between two overlapping components by depositing a fillet weld round the periphery of a hole in one component so as to join it to the other component Metal transfer which takes place as a steam of small droplets transfers from the electrode to the weld area The thermal cutting of a stack of plates usually clamped together An intermittent weld on each side of a joint arranged so that the welds lie opposite to one another The minimum voltage required to strike an arc Metal arc welding in which a bare wire or electrodes are used, the arc or arcs are covered by a flux, some of which fuses to form a removable slag on the weld. Some flux is recovered Retrogression of the flame into the blowpipe, the flame remaining slight. 'Popping' or 'squealing' with a small pointed flame coming from the nozzle or as a rapid series of minor explosions arc welding) Inert gas welding using a non-consumable electrode of pure or activated tungsten Metal arc welding using a covered electrode. The covering is kept in contact with the parent metal during welding The boundary between a weld face or root and the parent metal, or between a weld face and any underlying welds An inclusion of tungsten in the weld from the electrode in TIG welding process A gas regulator in which the cylinder or line pressure is reduced to the working pressure in two stages An irregular groove at a toe of a run in the parent metal at root, face or in previously deposited weld metal A joint between pieces of metal made liquid by heat, or by pressure, or by both. A filler metal mayor may not be added The boundary at the extent of melting and the heat-affected zone The metal melted during the making of a weld and retained in the weld joint The area of parent metal affected by the weld deposit A detailed list of actions to be followed during the production of a weld. The list will include materials, consumable, amps, volts, gases, joint details, etc. The order and direction in which welds are deposited in the joint The manner in which the operator controls the electrode or blowpipe during welding An elongated or tubular cavity formed by entrapped gas during solidification of molten metal WELDING TERMINOLOGY 9 WELD SYMBOLS (BS499 PART 2 1980) 11 12 UNDERPINNING INFORMATION