Bailey Weldability of Ferritic Steels

1. Auflage 1994
ISBN: 978-1-84569-893-5
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 304 Seiten

Reihe: Woodhead Publishing Series in Welding and Other Joining Technologies

This book is chiefly concerned with the conventional fusion welding processes and their problems and will be of value to practical welding engineers, inspectors and metallurgists. The author also has inmind the needs of those concerned with design and specification, recognising the importance of dealing with problems at the design stage.

Autoren/Hrsg.

Bailey, Norman

Inhaltsverzeichnis

1;Front Cover;1
2;Weldability of Ferritic Steels;4
3;Copyright Page;5
4;Table of Contents;6
5;Introduction;9
6;Acknowledgements;12
7;Chapter 1. Factors influencing weldability;14
7.1;The welding process;14
7.2;Steel types;16
7.3;Weld cooling cycle;22
7.4;The heat-affected zone;27
7.5;The weld metal;31
7.6;Cutting and gouging;41
7.7;Residual stresses;42
7.8;Post-weld heat treatment;45
7.9;Weldability formulae;48
7.10;References;56
8;Chapter 2. Potential welding problem areas;58
8.1;Cracking;58
8.2;Welding faults;58
8.3;Inspection for defects;59
8.4;Joint integrity;59
8.5;Problems associated with PWHT;59
8.6;Service problems;60
8.7;Repair;60
8.8;Joining dissimilar steels;60
8.9;Joining dissimilar metals;63
8.10;References;66
9;Chapter 3. Solidification cracking;67
9.1;Description;67
9.2;Role of composition;79
9.3;Solidification cracking and the welding process;87
9.4;Control and avoidance of cracking;93
9.5;Liquation cracking;99
9.6;Copper pick-up;104
9.7;Ductility dip cracking;106
9.8;Detection and identification;106
9.9;References;114
10;Chapter 4. Lamellar tearing;115
10.1;Description;115
10.2;Role of inclusions;117
10.3;Susceptible joint types;122
10.4;Other factors;128
10.5;Control and avoidance of cracking;131
10.6;Control tests;132
10.7;Detection and identification;141
10.8;References;145
11;Chapter 5. Hydrogen cracking;146
11.1;Description;146
11.2;Hydrogen;147
11.3;Susceptibility to embrittlement;153
11.4;Stress to give cracking;157
11.5;Temperature of embrittlement;158
11.6;Techniques for avoiding cracking;159
11.7;Weld metal hydrogen cracking;168
11.8;Detection and identification;172
11.9;Other aspects of hydrogen in ferritic steels;175
11.10;References;181
12;Chapter 6. Reheat cracking;183
12.1;Description;183
12.2;Role of composition;185
12.3;Other influences;186
12.4;Control techniques;187
12.5;Detection and identification;191
12.6;References;192
13;Chapter 7. Faults of welding;193
13.1;Faults of shape;193
13.2;Inclusions;196
13.3;Cavities;198
13.4;References;199
14;Chapter 8. Inspection for defects;200
14.1;Inspection techniques;201
14.2;Inspectability;201
14.3;Consequences of metallurgical features;202
14.4;References;203
15;Chapter 9. Joint integrity;204
15.1;Strength and ductility;204
15.2;Toughness;206
15.3;Hardness;217
15.4;Creep resistance;217
15.5;Corrosion resistance;218
15.6;References;222
16;Chapter 10. Service problems;223
16.1;Fatigue and corrosion fatigue;223
16.2;Corrosion;224
16.3;Stress corrosion;224
16.4;Loss of toughness in service;225
16.5;High temperature service problems;231
16.6;Graphitisation;233
16.7;References;235
17;Chapter 11. Repair;237
17.1;Analysis prior to repair;237
17.2;Limitation of preheat;241
17.3;Need for post-weld heat treatment;241
17.4;Special circumstances;243
17.5;References;248
18;Further reading;249
19;Glossary;251
20;Index;284

Leseproben

2 Potential welding problem areas
Six potential problem areas which need to be considered in welding are summarised below and discussed in more detail in the chapters mentioned. Cracking (Chapters 3-6)
Although cracking comes under many names, there are basically only four types to be considered, of which two are restricted to limited types of steels. However, because welding itself is a complicated process, their understanding and control is not always straightforward. The four types are: 1 Cracking occurring at the late stages of solidification of the weld – solidification cracking of weld metal and liquation cracking, usually restricted to the HAZ in ferritic steels. 2 Lamellar tearing – restricted to plate steels. 3 Hydrogen cracking in both HAZ and weld metal. 4 Cracking during PWHT – restricted to a limited range of steels and weld metals and only when subjected to PWHT. Other types of cracking have not yet been firmly distinguished from other more likely types (e.g. the term ‘strain ageing cracking’ is likely to be a variant of hydrogen cracking in which strain ageing is unusually significant), or are insignificant in the context of welding ferritic steels (i.e. ductility dip cracking, which is discussed in Chapter 3). Welding faults (Chapter 7)
Although the control of welding faults is a matter of welding engineering, i.e. selection, control and maintenance of correct operating parameters, there are metallurgical aspects in the formation, presence and subsequent identification of welding imperfections and defects. These are discussed, as well as how measures taken to deal with metallurgical problems may influence welding faults. Inspection for defects (Chapter 8)
The correct selection and operation of NDE is strictly outside the scope of this book. Nevertheless, certain aspects are discussed because some details of inspection techniques are very much related to the types of defect likely to be present, whilst the requirements of the inspection imposed may impinge on the selection of welding methods. Also, a good metallurgical understanding of the types of defects likely to be present influences the type, scale and time of NDE required, as well as the interpretation of its results. Joint integrity (Chapter 9)
Joint integrity is concerned with the selection of consumables and welding parameters to meet the requirements imposed on weldments. The properties of most importance are weldment strength and toughness. The principles behind the selection of these features are discussed in the chapter. Other properties, such as resistance to creep, corrosion, oxidation and hydrogen content are more especially related to service conditions. Problems associated with PWHT (Chapter 6, 9 and 10)
The most common problem in heat treatment is that of distortion. Careful control of heating and cooling, with jigging in difficult cases, is important. Reheat or stress relief cracking is a problem restricted to a limited number of steels when they are subjected to conventional PWHT, and is discussed in Chapter 6. Temper embrittlement is a progressive embrittlement of steels when they are subjected to (often prolonged) heating within the temperature range 350–600 °C. The problem is confined to low alloy steels which contain relatively high levels of Mn, Si and the impurities P, Sn, Sb and As, Temper embrittlement is discussed as a service problem in Chapter 10, but it should be noted that steels with very susceptible compositions, and which are in thick sections, may cool so slowly through the sensitive temperature range after PWHT that they become embrittled. Strain ageing is a problem of as-welded joints and its effects are completely removed by PWHT. Service problems (Chapter 10)
In selecting steels, welding consumables and welding parameters, a fairly detailed knowledge of service conditions is needed, particularly where corrosion, high temperatures and/or the presence of high pressure hydrogen are involved. Repair (Chapter 11)
Much welding is carried out to modify or repair components that have been in service, and that need to be welded under conditions which are less favourable to achieving an ideal weld than the shop conditions where the component was originally welded. The possibility of such repair welding should be borne in mind at the original design stage. Difficulties may take the form of the need for positional welding, the inability to deploy the ideal welding process because of space limitations, restrictions on heating (either preheating temperatures or the inability to apply PWHT), because of nearby heat-sensitive material which cannot be moved or the location of the repair. In some cases there may be a requirement to weld pipework containing fluids (hot tapping) or to weld under water or in vacuum (e.g. in space). Joining dissimilar steels
The selection of welding consumables and procedures when joining dissimilar steels is normally a relatively simple matter. However, the selection of PWHT temperatures can pose problems. This section is concerned solely with joining of two ferritic steels; joints involving austenitic stainless steels or cast irons are discussed in the next section, along with joining ferritic steels to other metals. When joining two ferritic steels of different strength levels and different toughness requirements, simple considerations will show that only the strength of the weaker steel and the toughness of the less tough steel will be required at the joint. For example, when joining a 700 N/mm2 yield strength steel, requiring Charpy toughness at -50 °C, to a mild steel with toughness requirements at ambient temperature, the properties required at the joint itself should be those of the mild steel. Similarly, if a steel selected for its oxidation and creep resistance has to be joined to a simple C or C:Mn steel, the consumable does not require any alloying additions, other than those necessary to achieve the strength of the weaker steel. The selection of welding procedures, particularly preheat and interpass temperature and heat input, will normally depend on the requirements for the more highly alloyed of the two steels, bearing in mind the importance of carbon as an alloying element. Thus, in the previous example, preheat requirements and so forth should be in accordance with the needs of the high strength, alloyed steel. If, however, the high strength steel (assuming it to be of low carbon content, e.g. < 0.15%) is being joined to a simple medium carbon steel, then the requirements of the latter for preheat to avoid hydrogen cracking need to be taken into consideration, although some degree of compromise may be required. It will usually be helpful to use consumables giving ultra-low hydrogen levels. It may also be necessary to allow higher preheat and interpass temperatures than are normally acceptable for the tougher steel, on the grounds that full HAZ toughness of the higher strength steel will not be required at the dissimilar joint. In cases where a compromise is not possible, consideration should be given to the use of austenitic stainless steel or nickel alloy fillers, as discussed in the next section and in Chapter 5, because such fillers allow the use of low preheat temperatures (which are likely to be needed for the strong tough steels) for higher carbon steels. The PWHT of dissimilar joints requires some care. Firstly, there is a need to avoid over-tempering and weakening any quenched and tempered or similar steel. Secondly, there is a need to avoid impairing the HAZ toughness (and other properties) of either steel. Some guidance in these matters is given in the British pressure vessel standard BS5500.1 The basic principle is that one of the two steels must be the more important and the PWHT temperature range should be selected to ensure that its correct properties are achieved. Then the temperature is selected within that range to be as close as possible to what is required for the secondary steel. For example if a C:Mn steel (PWHT temperature 570–630 °C) is being welded to a major low alloy steel component (PWHT 620–680 °C) the aim PWHT temperature will be 630 °C, although particular care will be needed to achieve better than normal temperature control. If, on the other hand, a Cr:Mo steel (requiring PWHT at 700–750 °C) is welded to the C:Mn steel, then an alternative strategy is needed. This involves buttering the weld preparation with two layers of the weld metal selected for the joint (usually a C:Mn weld metal) and applying PWHT at 725 °C. The joint to the C:Mn steel is then completed and the whole is given a PWHT at 600 °C. The higher temperature will temper the Cr:Mo steel HAZ, and will not unduly harm the two layers of weld metal, which will have picked up some Cr and Mo by dilution from the parent steel and will also be partly melted out when the joint is completed. The subsequent 600 °C treatment will not harm the Cr:Mo steel and will be suitable for both the C:Mn steel and the weld metal. During this procedure, great care should be taken when the joint is being completed not to weld too close to the Cr:Mo steel after it has been buttered and given its higher temperature heat treatment. Low...