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1980 | Buch

Introductory Kapitel lesen Erstes Kapitel lesen

Metallurgy of Welding

verfasst von: J. F. Lancaster

Verlag: Springer Netherlands

Enthalten in: Professional Book Archive

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Über dieses Buch

This book is intended, like its predecessor (The metallurgy of welding, brazing and soldering), to provide a textbook for undergraduate and postgraduate students concerned with welding, and for candidates taking the Welding Institute examinations. At the same time, it may prove useful to practising engineers, metallurgists and welding engineers in that it offers a resume of information on welding metallurgy together with some material on the engineering problems associated with welding such as reliability and risk analysis. In certain areas there have been developments that necessitated complete re-writing of the previous text. Thanks to the author's colleagues in Study Group 212 of the International Institute of Welding, understanding of mass flow in fusion welding has been radically transformed. Knowledge of the metallurgy of carbon and ferritic alloy steel, as applied to welding, has continued to advance at a rapid pace, while the literature on fracture mechanics accumulates at an even greater rate. In other areas, the welding of non-ferrous metals for example, there is little change to report over the last decade, and the original text of the book is only slightly modified. In those fields where there has been significant advance, the subject has become more quantitative and the standard of math­ ematics required for a proper understanding has been raised.

Inhaltsverzeichnis

Frontmatter
1. Introductory
Abstract
The great majority of metal-joining processes have been invented in recent years but some, notably the forge welding of iron, brazing and soldering, have a very long history. Soldering and the hammer welding of gold appear to have been known during the Bronze Age, but they were mainly used for making ornaments. Welding first evolved as a technique of primary economic importance when the use of iron became widespread, it being required not only in order to make finished products, but also as part of the iron-making process itself. The method of smelting ironstone in ancient and medieval times was to heat a mixture of ore and charcoal in a blast of air, so regulating the heat that a proportion of the ore was reduced to iron without melting. The mass of iron particles and slag so formed was then removed from the furnace, cleaned, reheated, and then hammered in order to consolidate the metal. The blooms produced in this way were small, and, as a rule, a number had to be welded together in order to form a piece of workable size. Special welding furnaces were constructed for this purpose in some localities.
J. F. Lancaster
2. Processes and types of joint
Abstract
The methods of joining metals may be divided into two main categories: those which rely on macroscopic mechanical forces, on the one hand, and those that depend on interatomic or intermolecular forces on the other. The first category is typified by bolting. The joint strength is obtained from the shear strength of the bolts and the frictional forces that keep the nuts in place. In the second group of joining methods, with which this book is concerned, the surfaces to be joined are brought closely enough together for either the metallic bond or, alternatively, van der Waals or other intermolecular forces to become effective. Welding, brazing, soldering and adhesive jointing processes are the means by which the required degree of proximity may be achieved.
J. F. Lancaster
3. Mass and heat flow in welding
Abstract
The character of heat flow in welding determines the nature of the weld thermal cycle and hence, in the case of transformable alloys, the microstructure of the weld metal and heat-affected zone. Mass flow, which encompasses the flow of plasma in the arc column, the flow of metal from electrode to weld pool, and the flow of metal in the weld pool itself, has less influence on the metallurgy of the joint. Irregularities or instabilities in mass flow may, however, limit the usefulness and range of application of specific processes and may cause irregularities in the heat-flow pattern.
J. F. Lancaster
4. Metallurgical effects of the weld thermal cycle
Abstract
A number of reactions may take place in the liquid weld metal, firstly in the liquid drop at the electrode tip, secondly during transfer from electrode to weld pool, and thirdly in the weld pool itself. These reactions include:
(a)
Solution of gas, causing gas-metal reactions or reaction with elements dissolved in the liquid metal.
 
(b)
Evolution of gas, causing porosity and/or embrittlement.
 
(c)
Reaction with slag or flux.
 
J. F. Lancaster
5. Solid-phase welding
Abstract
In order to make a solid-phase weld between two pieces of metal, it is necessary to bring their clean surfaces sufficiently close together for a metallic bond to be formed between them. There are a number of techniques for accomplishing this end, but the essential operation in all cases is to press the two parts together either hot or cold, deforming the surfaces sufficiently to ensure partial or complete contact, and to expose fresh, unfilmed metal. Important factors which enter into the mechanics and metallurgy of this process are surface deformation, the dispersal of surface films, diffusion and re crystallisation. These factors will be discussed separately below.
J. F. Lancaster
6. Brazing, soldering and adhesive bonding
Abstract
In brazing, soldering and adhesive bonding the objective is the same as in solid-phase welding, namely, to produce a mechanically acceptable bond between two metal surfaces without fusing the bulk material. Whereas in solid-phase welding, however, the joint is made by bonding the two surfaces directly, in the processes now under consideration, a liquid is made to flow into and fill the space between the joint faces and then solidify. The liquid used necessarily has a lower solidification temperature than the metal or metals to be joined. Brazing filler metals comprise mainly copper, nickel, silver and aluminium/ zinc alloys, while the most commonly-used solders are lead/tin alloys. Adhesive joints between metals are made using a synthetic resin which solidifies by polymerisation. Although the physical phenomena associated with brazing, soldering and adhesive bonding are essentially the same, the three processes differ in their metallurgical effects, and in the means by which they are applied.
J. F. Lancaster
7. Carbon and ferritic-alloy steels
Abstract
It is the intention to discuss in this chapter those steels that have a body-centred cubic form at or above normal atmospheric temperature. Included are carbon steels with carbon contents up to 1·0%, carbon-manganese steels with manganese content up to 1·6%, and steels containing other alloying elements up to the martensitic type of 12% Cr steel and maraging nickel steel. Higher-alloy ferritic and ferritic/austenitic steels are also included, although they may not suffer an α to γ transformation. Other than this last group, the common feature about these alloys is that they may all, to a greater or lesser degree, be hardened as a result of passing through the weld thermal cycle and may therefore suffer a change in properties in the region of a fusion-welded joint. General metallurgical questions will be considered first, and a later section will deal with the individual alloys and alloy groups that are used in welded fabrication. Cast iron is included in the material groups since, from a welding viewpoint, it suffers the same type of transformation in the HAZ of fusion welds as steel, albeit in an extreme form.
J. F. Lancaster
8. Austenitic and high-alloy steels
Abstract
Austenitic steels considered in this chapter are the austenitic chromium nickel corrosion-resistant steels of the 18 Cr 10–12 Ni (commonly known as 18/8) type and the creep-resistant and scaling-resistant steels containing up to a nominal 25% Cr. Hardenable high-alloy steels are arbitrarily classified as those containing more than 20% of alloying elements but which are capable of being hardened by heat treatment. The corrosion and oxidation resistance of these steels results from the formation of a self-healing surface film of chromium oxide. They may be welded by any of the major processes, but chief consideration will be given to the phenomena associated with fusion welding, particularly SMA, GTA and GMA welding.
J. F. Lancaster
9. Non-ferrous metals
Abstract
Aluminium and aluminium alloys may be joined by arc-fusion welding, resistance welding, solid-phase welding, brazing and adhesive bonding. Low-melting aluminium-zinc solders have also been used to a limited extent. Fusion welding was first accomplished by the oxy-acetylene process but is now carried out by means of GTA and GMA welding and sometimes using coated electrodes. The power source for GTA welding is usually an a.c. transformer with high frequency re-ignition. When the electrode is positive a non-thermionic cathode (see Section 3.5.2.1) forms on the aluminium surface; the cathode spot wanders over the weld pool surrounding plate surface and removes most of the oxide. With d.c. electrode-positive most of the arc heat is absorbed by the tungsten electrode, and at normal welding currents this melts and is difficult to manage. With a.c. welding, a compromise is possible; during the electrode-positive half-cycle the oxide film is removed while during the electrode-negative half-cycle the workpiece is heated. However, d.c. electrode-positive may be used for special purposes. Welds may also be made with d.c. electrode-negative using helium as the shielding gas, but this too is for special applications. In general GTA welding is applicable to lighter sections than GMA welding.
J. F. Lancaster
10. The behaviour of welds in service
Abstract
One of the primary reasons for attempting to understand the complex physical and metallurgical processes that take place during the welding — particularly the fusion welding — of metals is to increase the reliability of welded joints by specifying the optimum materials, welding procedures and quality-control techniques. For example, in the boiler of a major power station there may be thousands of pipe welds, and the failure of only one of these welds may result in a plant shut-down. In structural work, deficiencies in a single weld may not have such serious effects; nevertheless, if a weld in a critical location fails, the structure as a whole may collapse or be condemned. By studying the reliability of welds in service it is possible to obtain a measure of the success or otherwise of the welding engineer in achieving sound joints.
J. F. Lancaster
Backmatter
Metadaten
Titel
Metallurgy of Welding
verfasst von
J. F. Lancaster
Copyright-Jahr
1980
Verlag
Springer Netherlands
Electronic ISBN
978-94-010-9506-8
Print ISBN
978-94-010-9508-2
DOI
https://doi.org/10.1007/978-94-010-9506-8