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Defects - hydrogen cracks in steels - prevention and best practice
Preheating of a jacket structure to prevent hydrogen
In this issue, techniques and practical guidance on the avoidance of hydrogen
cracks are described.
Preheating, interpass and post heating to prevent hydrogen crackingThere are three factors which combine to cause cracking in arc welding:
PreheatPreheat, which slows the cooling rate, allows some hydrogen to diffuse away and prevents a hard, crack-sensitive structure being formed. The recommended levels of preheat for carbon and carbon manganese steel are detailed in BS 5135. (Nb a draft European standard Pr EN 1011-2 is expected to be introduced in 2000). The preheat level may be as high as 200°C for example, when welding thick section steels with a high carbon equivalent (CE) value.
Interpass and post heatingAs cracking rarely occurs at temperatures above ambient, maintaining the temperature of the weldment during fabrication is equally important. For susceptible steels, it is usually appropriate to maintain the preheat temperature for a given period, typically between 2 to 3 hours, to enable the hydrogen to diffuse away from the weld area. In crack sensitive situations such as welding higher CE steels or under high restraint conditions, the temperature and heating period should be increased, typically 250-300°C for three to four hours.
Post weld heat treatment (PWHT) may be used immediately on completion of welding ie without allowing the preheat temperature to fall. However, in practice, as inspection can only be carried out at ambient temperature, there is the risk that 'rejectable,' defects will only be found after PWHT. Also, for highly hardenable steels, a second heat treatment may be required to temper the hard microstructure present after the first PWHT.
Under certain conditions, more stringent procedures are needed to avoid cracking than those derived from the nomograms for estimating preheat in BS 5135. Appendix E of this standard mentions the following conditions:
Use of austenitic and nickel alloy weld metal to prevent crackingIn situations where preheating is impractical, or does not prevent cracking, it will be necessary to use an austenitic consumable. Austenitic stainless steel and nickel electrodes will produce a weld metal which at ambient temperature, has a higher solubility for hydrogen than ferritic steel. Thus, any hydrogen formed during welding becomes locked in the weld metal with very little diffusing to the HAZ on cooling to ambient.
A commonly used austenitic MMA electrode is 23Cr:12Ni (eg from BS 2926:1984). However, as nickel alloys have a lower coefficient of thermal expansion than stainless steel, nickel austenitic electrodes are preferred when welding highly restrained joints to reduce the shrinkage strain. Figure 1 is a general guide on the levels of preheat when using austenitic electrodes. When welding steels with up to 0.2%C, a preheat would not normally be required. However, above 0.4%C a minimum temperature of 150°C will be needed to prevent HAZ cracking. The influence of hydrogen level and the degree of restraint are also illustrated in the figure.
Best practice in avoiding hydrogen cracking
Reduction in weld metal hydrogenThe most effective means of avoiding hydrogen cracking is to reduce the amount of hydrogen generated by the consumable, ie by using a low hydrogen process or low hydrogen electrodes.
Welding processes can be classified as very low, low, medium or high depending on the amount of weld metal hydrogen produced:
Figure 2 illustrates the relative amounts of weld metal hydrogen produced by the major welding processes. MMA, in particular, has the potential to generate a wide range of hydrogen levels. Thus, to achieve the lower values, it is essential that basic electrodes are used and they are baked in accordance with the manufacturer's recommendations. For the MIG process, cleaner wires will be required to achieve very low hydrogen levels.
General guidelinesThe following general guidelines are recommended for the various types of steel but requirements for specific steels should be checked according to BS 5135 or BS EN 1011:
More detailed guidance on the avoidance of hydrogen cracking is described in BS 5135.
Practical TechniquesThe following practical techniques are recommended to avoid hydrogen cracking:
Acceptance standardsAs hydrogen cracks are linear imperfections which have sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of BS EN 25817 (ISO 5817).
Detection and remedial actionAs hydrogen cracks are often very fine and may be sub-surface, they can be difficult to detect. Surface-breaking hydrogen cracks can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic or radiographic examination techniques. Ultrasonic examination is preferred as radiography is restricted to detecting relatively wide cracks parallel to the beam.
Most codes will specify that all cracks should be removed. A cracked component should be repaired by removing the cracks with a safety margin of approximately 5mm beyond the visible ends of the crack. The excavation is then re-welded.
To make sure that cracking does not re-occur, welding should be carried out with the correct procedure, ie preheat and an adequate heat input level for the material type and thickness. However, as the level of restraint will be greater and the interpass time shorter when welding within an excavation compared to welding the original joint, it is recommended that a higher level of preheat is used (typically by 50°C).
ReferencesBS 5135:1984 Arc Welding of Carbon and Carbon Manganese Steels
Pr EN 1011-1:1998 Welding - Recommendations for Welding of Metallic Materials
BS EN ISO 13916: 1997 Welding - Guidance on the Measurement of Preheating Temperature, Interpass Temperature and Preheat Maintenance Temperature
N Bailey et al, Welding steels without hydrogen cracking, Woodhead Publishing, 1993
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