Lancaster Metallurgy of Welding

6. Auflage 1999
ISBN: 978-1-84569-486-9
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 464 Seiten

Reihe: Woodhead Publishing Series in Welding and Other Joining Technologies

A new edition of a well established and respected textbook from an author who is a recognised authority in this field. Joining techniques are one of the key technologies in materials engineering and this book provides comprehensive coverage of the subject. It is intended for undergraduate and graduate students of metallurgy, as well as those attending specialist welding courses.It is also a valuable source of reference for practising engineers and metallurgists concerned with joining processes. The text covers the metallurgical changes that take place during the welding process, the properties of welded joints, defects associated with welding and the behaviour of welded joints in service. There is a chapter devoted to joints between metals and ceramics, and on the use of structural adhesives. The various techniques used in microwelding and the joining of solid-state devices to printed circuit boards are briefly described.In addition to revising and updating the text throughout the author has made some specific alterations and additions to the book: - Brittle and ductile behaviour of solids, ductile francture, and the velocity of crack propogation are now included in the section on Fracture; - Friction stir welding in now included; - There is an additional chapter on adhesive bonding which includes bonding; - forces, polymer chemistry, types of adhesive, production technology, quality control and applications; - The section on heat flow has been expanded and includes worked examples; - A section on weld defects and the evaluation of non-destructive tests has been added; - A section on the welding metallurgy of aluminium-lithium alloys has been added; - A new section describes major structural failure in such catastrophes as the 'Alexander L Kielland' accident and the Kobe earthquake, and considers the role of welding in such failures.

Autoren/Hrsg.

Lancaster, J. F.

Inhaltsverzeichnis

1;Front Cover
;1
2;Metallurgy of welding;4
3;Copyright Page;5
4;Chapter 1. Introductory;6
4.1;1.1 Structural joints;6
4.2;1.2 The cohesion of crystalline solids;6
4.3;1.3 The plastic behaviour of crystalline solids;11
4.4;1.4 Solid-liquid interactions;20
4.5;1.5 Fracture;25
4.6;References;43
5;Chapter 2. Solid-phase welding;45
5.1;2.1 Physical aspects and metallurgy;45
5.2;2.2 Solid-phase welding processes;47
5.3;References;58
5.4;Further reading;58
6;Chapter 3. The use of adhesives for making structural joints;59
6.1;3.1 History;59
6.2;3.2 Bonding between adhesive and substrate;60
6.3;3.3 Polymers;66
6.4;3.4 The properties of adhesive polymers;71
6.5;3.5 Bonding procedures;77
6.6;3.6 Joint design and applications;85
6.7;References;88
6.8;Further reading;89
7;Chapter 4. Soldering and brazing;90
7.1;4.1 Physical aspects;90
7.2;4.2 Soldering;92
7.3;4.3 Brazing;99
7.4;References;109
7.5;Further reading;109
8;Chapter 5. The joining of ceramics: microjoining;110
8.1;5.1 Scope;110
8.2;5.2 The properties of ceramics;110
8.3;5.3 Glass-metal seals;112
8.4;5.4 Glass-ceramics;116
8.5;5.5 Brazing;119
8.6;5.6 Other techniques;121
8.7;5.7 Microjoining;125
8.8;References;131
8.9;Further reading;132
9;Chapter 6. Fusion welding processes and their thermal effects;133
9.1;6.1 The development of fusion welding;133
9.2;6.2 The nature of fusion welding;135
9.3;6.3 Types of fusion-welded joint;150
9.4;6.4 Heat flow in fusion welding;152
9.5;6.5 Weld defects;171
9.6;References;173
9.7;Further reading;173
10;Chapter 7. Metallurgical effects of the weld thermal cycle;174
10.1;7.1 Gas-metal equilibria;174
10.2;7.2 Gas-metal reactions in arc welding;181
10.3;7.3 The mechanism of gas absorption in welding;190
10.4;7.4 Porosity;192
10.5;7.5 Diffusion;195
10.6;7.6 Dilution and uniformity of the weld deposit;197
10.7;7.7 Weld pool solidification;198
10.8;7.8 Weld cracking;202
10.9;7.9 Metallurgical effects in the parent metal and solidified weld metal;206
10.10;References;214
10.11;Further reading;215
11;Chapter 8. Carbon and ferritic alloy steels;216
11.1;8.1 Scope;216
11.2;8.2 Metallurgy of the liquid weld metal;216
11.3;8.3 Transformation and microstructure of steel;230
11.4;8.4 The mechanical properties of the welded joint;244
11.5;8.5 Stress intensification, embrittlement and cracking of fusion welds below the solidus;253
11.6;8.6 Steelmaking;284
11.7;8.7 The welding of iron and steel products;292
11.8;References;312
11.9;Further reading;313
12;Chapter 9. Austenitic and high-alloy steels;315
12.1;9.1 Scope;315
12.2;9.2 The weld pool;315
12.3;9.3 Alloy constitution;321
12.4;9.4 Mechanical properties;329
12.5;9.5 Transformation, embrittlement and cracking;332
12.6;9.6 The use of austenitic Cr-Ni alloys for repair welding, cladding and transition joints;342
12.7;9.7 Corrosion-resistant steels: alloys and welding
procedures;350
12.8;9.8 Heat-resisting steels: alloys and welding
procedures;353
12.9;9.9 Hardenable high-alloy steels;354
12.10;References;355
12.11;Further reading;356
13;Chapter 10. Non-ferrous metals;358
13.1;10.1 Aluminium and its alloys;358
13.2;10.2 Magnesium and its alloys;376
13.3;10.3 Copper and its alloys;378
13.4;10.4 Nickel and its alloys;386
13.5;10.5 The reactive and refractory metals;393
13.6;10.6 The low-melting metals: lead and zinc;401
13.7;10.7 The precious metals: silver, gold and platinum;402
13.8;References;402
13.9;Further reading;403
14;Chapter 11. The behaviour of welds in service;404
14.1;11.1 General;404
14.2;11.2 The initiation and propagation of fast fractures;404
14.3;11.3 Slow crack propagation;414
14.4;11.4 Corrosion of welds;425
14.5;11.5 Assessing the reliability of welded structures;435
14.6;References;437
14.7;Further reading;437
15;Appendix 1: Symbols;439
16;Appendix 2: Conversion factors;442
17;Index;444

Leseproben

2 Solid-phase welding
2.1 Physical aspects and metallurgy
2.1.1 General
In solid-phase welding the object is to make a welded joint between two solid pieces of metal by bringing their surfaces into sufficiently close proximity for a metallic bond to be formed. In some processes, flash-butt welding for example, a liquid phase is formed between the two surfaces at an intermediate stage of the welding operation, but in the final stage the joint is upset to extrude the liquid and form the weld between the solid surfaces. Compared with fusion welding such a process has a number of advantages. In the first place, there is no cast metal in the joint, except in so far as it may be accidentally included. Secondly, the joint is made under compression, so that the risk of cracking is small; and thirdly the embrittling element hydrogen is absent. Unfortunately, solid-phase processes are only applicable to components of modest dimensions, and although the size of structures made in this way is increasing, they are never likely to be used for the construction of, say, an oil rig. 2.1.2 Bond formation
The primary requirement in making a solid-phase weld is to bring two clean surfaces close together. The barriers to obtaining perfect contact are twofold: the presence of non-metallic films, including chemisorbed gases, on the surface, and the physical difficulty of obtaining an exact fit over the whole of the two surfaces to be joined. Films of water, oil or grease are obvious hazards; being fluid they have good contact with the metal surfaces and are strongly bonded thereto, but since they have no strength, the strength of any joint so contaminated is greatly reduced. Oxide films are commonly brittle, and a joint consisting of the sandwich metal-oxide-metal, even if bonded, will in general be brittle and have low strength. Liquid films are removed by heating or, if welding is to be accomplished cold, by scratch-brushing. Oxide films are also removed by scratch-brushing, by lateral movement of the two surfaces, or by fluxing and melting. Obtaining good contact by direct pressure between two metal surfaces is difficult and, except in diffusion bonding, joints made in this way have very low strength. For effective mating in other solid-phase welding processes, lateral movement is necessary. When the surfaces are forced together with relatively light normal pressure, the proportion of surface that is brought within bonding range is small. However, the asperities of each surface penetrate the other and, if the two are moved laterally relative to each other, some of the asperities shear, and the clean metal surfaces so produced bond together. Repetition of this process naturally increases the bonded area. Alternatively, if the two surfaces are made to flow laterally in the same direction (conjoint flow), areas of unfilmed metal are formed in close proximity and therefore bonding takes place. Relative lateral flow occurs in ultrasonic welding and friction welding, and conjoint flow in butt and flash-butt welding, cold and hot pressure welding, and explosive welding. 2.1.3 Surface films
A metal is normally coated with a film of oxide, sulphide or carbonate, whose thickness lies within the range 10- 3 to 10- 1 µm. It may also have a layer of adsorbed gas, and it may be contaminated by oil, grease or other non-metallic substances. Oil films inhibit solid-phase welding either partially or completely when the temperature is such that they can persist. Oxide films hinder, but do not prevent, pressure welding. It is generally considered that, during deformation of the surface, the oxide film (or hardened surface layer produced by scratch-brushing) fractures and exposes areas of clean metal, which bond to the opposite surface wherever two clean areas come into contact. At elevated temperatures, oxide is dispersed by deformation, by solution or by agglomeration, or by a combination of these processes. Excessive amounts of dissolved oxygen cause brittle welds. Oxide inclusions, which may be present in massive form in welds made at elevated temperature, are also damaging to the mechanical properties. Oxide inclusions, dissolved oxygen and voids may be dispersed from the junction zone of carbon-steel pressure welds by soaking above 1000 °C for an adequate period, although if a proper welding technique is used no significant oxide contamination will be present. 2.1.4 Recrystallization
In welds made at room temperature, recrystallization of a surface zone occurs with low-melting point metals such as tin and lead, but most engineering metals must be welded at elevated temperature if recrystallization is to occur during the welding process. Recrystallization such that grains grow and coalesce across the original interface is not essential to welding, nor does it ensure that the best achievable properties have been obtained. Steel pressure welds made at temperatures above the upper critical temperature show continuity of grains across the interface but may still lack ductility due to oxide inclusions or other causes. However, increasing the welding temperature, which favours recrystallization, also favours the elimination of other defects, and improves the room-temperature ductility of the completed joint. 2.1.5 Diffusion
Gross macroscopic voids at the interface of pressure welds increase in size and diminish in number if the joint is soaked at elevated temperature, indicating that a process akin to that which causes the increase of density of metal powder compacts during sintering may be at work. Increasing temperature would favour such a mechanism, and it does in fact improve the ductility of pressure-welded joints. Diffusion may be important in removing contaminants from the weld zone, particularly oxygen in reactive metals such as titanium. For such material a postwelding solution treatment is required for optimum joint ductility. Diffusion is an essential feature of diffusion bonding, which is described in Section 2.2.5. 2.2 Solid-phase welding processes
2.2.1 Pressure welding at elevated temperature
Forge, hammer, butt and oxyacetylene pressure welding are all techniques designed to make solid-phase welds at elevated metal temperature. In butt welding and oxyacetylene welding, the metal is simply heated to a high temperature (1200–1250 °C in the case of carbon steel) while the joint is subject to axial compression. When the metal in the region of the interface reaches this temperature, it deforms under the axial load and there is a lateral spread which disrupts the surface films and permits welding to take place. Essential controls are the applied pressure and the amount of shortening of the parts being joined. Pressure may be constant or may be increased at the end of the welding cycle. In resistance butt welding, heating is accomplished by passing an electric current across the joint. This process is applied to welding bar and rod, to end-to-end welding of strip, and to the manufacture of longitudinally welded tube. The tube is formed from strip by a series of rolls, whence it passes between two copper rollers through which the welding current is applied, then through a pair of forging rolls, which force together the heated edges and make the weld. High-frequency resistance welding (Fig. 2.1) is the process most often used for the manufacture of ERW (electrically resistance welded) pipe. In principle it is similar to resistance butt welding but a high-frequency current is used as the power source. High-frequency current flows preferentially through the surface layers of a conductor, and in welding this effect minimizes the degree of upset and distortion. Downstream of the welding station knives remove the flash and further downstream a high-frequency coil reheats the weld to normalizing temperature. Flying saws cut the pipe to standard lengths (about 12.5 m). This technique is applied to line pipe (that is, pipe used for oil and gas transmission lines) in the medium diameter range and, being a relatively economical route, is used for a large tonnage of this product. 2.1 High-frequency resistance welding of pipe or tube (from Houldcroft, 1979). A relatively new method of pressure welding, suitable for butt joints in hollow sections and pipe, is magnetically impelled arc butt welding. The square-edged tube ends are separated by a small gap and an arc is struck between them. This arc is rotated around the tube or section by means of a radial magnetic field, and when the surfaces are heated sufficiently they are brought together under pressure to form a solid-phase weld. Liquid metal and impurities are extruded into the flash, which is removed by cutting tools. Suitable machines are commercially available and are used in the automotive industry for welding such items as car axles, drive shafts and shock absorbers, while a portable machine suitable for welding pipe up to 300 mm in diameter has been developed for the oil and gas pipeline industry. The productivity achievable with hot pressure welding techniques is high, but if defects are present they may penetrate the complete wall thickness. Such welds may also suffer from low impact properties, reputedly owing to the presence of non-metallic films. Ultrasonic testing will eliminate gross defects; otherwise fabricators rely on control of the welding variables to maintain quality. Flash welding is essentially different in that the two parts are first brought...