Bailey / Coe / Gooch Welding Steels without Hydrogen Cracking

2. Auflage 1993
ISBN: 978-0-85709-309-7
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 160 Seiten

Reihe: Woodhead Publishing Series in Welding and Other Joining Technologies

A comprehensive guide to avoiding hydrogen cracking which serves as an essential problem-solver for anyone involved in the welding of ferritic steels. The authors provide a lucid and thorough explanation of the theoretical background to the subject but the main emphasis throughout is firmly on practice.

Autoren/Hrsg.

Bailey, Norman

Coe, F R

Gooch, T G

Hart, P H M

Jenkins, N.

Pargeter, R J

Leseproben

Chapter 2

Guidance on safe welding procedures by graphical methods

Publisher Summary

This chapter describes the construction of graphs and nomograms for the selection of suitable welding procedures for different steel-types. It describes the principles involved in using welding diagrams to derive welding procedures for the avoidance of HAZ cracking. The prediction method for low hardenability steels aims to avoid hard HAZ microstructures. This is achieved by selecting welding conditions that provide a sufficiently slow weld cooling rate. The cooling rate that is necessary depends on the composition of the steel and the amount of hydrogen introduced into the HAZ from the weld metal.

This chapter describes the principles involved in using welding diagrams to derive welding procedures for the avoidance of HAZ cracking. The diagrams are introduced schematically and are given in detail in Chapter 4. They have been constructed on the assumption that they must be capable of indicating welding procedures which will not lead to cracking, even in situations where the worst possible combinations of circumstances are encountered. For this reason there will be many occasions when circumstances will not be at their worst and a procedure less stringent than that indicated by the diagram will be successful. The particular diagram to be used depends largely on the type of steel to be welded. These different steel types are listed in detail in Chapter 4 but basically a division between those of low hardenability and those of high hardenability is used. A CE level of 0.60 (see Chapter 3) is used as the division between these two groups.

Low hardenability steels

The prediction method for low hardenability steels aims to avoid hard HAZ microstructures. This is achieved by selecting welding conditions which provide a sufficiently slow weld cooling rate. The cooling rate that is necessary depends on the composition of the steel and the amount of hydrogen introduced into the HAZ from the weld metal. Although the thickness of the sections to be joined does not influence the cooling rate needed to achieve a particular microstructure, it does play an important role (as the ‘combined thickness’) in defining the welding and preheat requirements to achieve that cooling rate.

All these parameters are linked in a diagram of the type shown schematically in Fig. 2.1. On the full diagram there are many lines referring to different preheat temperatures and combined thick-nesses, but for clarity only two of each are shown in Fig. 2.1. The broad arrow shows how, starting with the CE value, preheat temperatures and heat input levels can be selected for a particular combined thickness. The diagram can of course be traversed in the opposite direction, or from both sides to meet in the centre when restrictions have to be placed on bead size and preheat level.

2.1 Prediction diagrams bring together information on material composition (CE level), plate thickness, and joint geometry (combined thickness) so that preheat levels and heat inputs can be selected for successful welding.

The same diagram is shown in slightly greater detail in Fig. 2.2 so that the various steps required in its use can be illustrated, as follows:

2.2 Basic diagram shown in Fig. 2.1 amplified by adding further CE axes to deal with other steel compositions and hydrogen levels and then referring to tables linking electrode size and weld run length to heat input scale.

Step 1

Decide on CE axis appropriate to hydrogen level of process, joint type, etc (Chapter 3 and Table 3.2).

Step 2

Decide steel composition, calculate CE value (Chapter 3) and erect vertical into preheat portion of diagram.

Step 3

Decide combined thickness of joint in question (Chapter 3).

Step 4

Decide limitations on heat input, bead size, or electrode size which can be used (Chapter 3). These limitations may arise because of positional welding or because of a need to achieve minimum toughness levels in weld metal or HAZ.

Step 5

Trace horizontal line to obtain required preheat level.

It may also be necessary to decide limitations on the level of preheat and interpass temperature which can be used; for example, welding manually within an enclosed space may preclude high temperatures.

In Fig. 2.2 a heat input limitation has been shown and a vertical line from this scale meets the particular combined thickness chosen. From this point, horizontal movement meets the vertical from the CE scale to identify the line TT, which gives the minimum preheat temperature required. It should be noted that in making this construction the electrode size and run length are related to heat input by means of Tables 2.1–2.4. Bead sizes less than this maximum may be employed but only at the cost of increasing the preheat temperature and this can be checked in the diagram.

Table 2.1

95% < electrode efficiency = 110%

0.8

130

215

335

525

1.0

105

170

270

420

600

—

1.2

85

145

225

350

500

555

1.4

—

120

190

300

430

475

1.6

—

105

165

260

375

415

1.8

—

95

150

230

335

370

2.0

—

85

135

210

300

330

2.2

—

—

120

190

275

300

2.5

—

—

105

165

240

265

3.0

—

—

90

140

200

220

3.5

—

—

—

120

170

190

4.0

—

—

—

105

150

165

4.5

—

—

—

95

135

150

5.0

—

—

—

85

120

135

5.5

—

—

—

—

110

120

Table 2.2

110% < electrode efficiency =...