TWI Frequently asked questions
by Henryk Pisarski
Three basic strategies can be followed to avoid LBZs or, at least, minimise their impact on the risk of brittle fracture initiation (see FAQs 'What are LBZs?' and 'What is the significance of LBZs?'). The first strategy is to select steel (usually steel chemistry) which is not susceptible to LBZ formation. Pre-qualification testing of heat affected zones(HAZs) in steel prior to purchase is intended to provide a means of doing this. Standards such as API RP 2Z, appendices to EEMUA 158, BS EN 10225 etc, describe welding heat input conditions that need to be evaluated, the amount of HAZfracture toughness testing necessary and the metallurgical examinations that are required to validate the results.
For a given welding heat input/welding procedure and post-heat treatment condition, LBZ formation is primarily affected by steel chemistry and to a lesser effect, steel processing route. The weld metal usually has a secondary effecton the incidence of LBZ behaviour. Nevertheless, strength mismatch between the weld metal and parent plate can affect the assessment HAZ fracture toughness. It is generally held that weld metal strength overmatching is beneficial forthe assessment of HAZ fracture toughness. However, this may not be realised if the local HAZ strength undermatches the strength of the weld metal and parent material.
If LBZ behaviour is discovered after the steel has been purchased, two further strategies are possible. The first is to modify the welding procedure to either eliminate LBZs or to reduce their size. This can be done by appropriateweld bead placement to maximise HAZ refinement (e.g. modifying the angle of attack between electrodes and fusion face, modifying weld bead overlap and size and/or reducing welding heat input). Furthermore, LBZs can (in most but not allcases) be eliminated through appropriate post-weld heat treatment. In some situations, it has been found necessary to carry out experiments to establish the optimum heat treatment temperature/time necessary to obtain the desiredfracture toughness.
The second strategy is to avoid having significant crack-like defects where LBZs are present. This can be achieved by appropriate positioning of welds in the structure (e.g. locating welds away from regions where fatigue cracks arelikely to propagate) and by pre-service and in-service inspection to identify and, where necessary, repair the unacceptable crack-like indications. What constitutes a significant defect can be established by determining the fracturetoughness (in terms of CTOD or J) of the LBZ and using a fracture mechanics based defect assessment procedure, such as BS 7910 or API 579.
References
API RP 2Z - Recommended practice for preproduction qualification for steel plates for offshore structures, 4th Edition, 2005.
BS EN 10225: 2001 - Weldable structural steels for fixed offshore structures.
EEMUA 158 - Construction specification for fixed offshore structures in the North Sea, The Equipment Engineering and Materials Users Association, London, Revised 2005 (ISBN 0 85931 074 4).
