Weld defects
Welds should be inspected to ensure there are no significant defects.
Cracks may occur due to contraction of the weld upon cooling either in the weld itself or in the parts being joined, although the use of correct welding procedures and appropriate electrodes will help ensure that the weld quality is sound and reliable. However, examination in the form of non-destructive testing (NDT) may be specified to confirm the strength of key welds and whilst both X-ray and ultrasonic methods are capable of revealing internal flaws, dye penetrants or magnetic particle methods will suffice for visual examination. Weld examination is costly and should be conducted on a strictly selective basis.
Distortions may develop locally due to the high temperatures associated with welding, but these can be controlled by appropriate procedures.
The application of heat during the welding process causes expansion, which is followed by contraction on cooling. These movements are rarely distributed evenly and this can cause both locked-in stresses in a large or complex assembly and distortion in a member which is free to move. This will depend upon a number of factors but the effects can be substantially minimized by sound details and suitable welding procedures. In addition, the fabricator will also use presetting, prebending and preheating techniques to ensure that members are true and free from twisting. The locked in stresses can be reduced by 'stress relief', which is a process of controlled heating and cooling of individual components in a furnace. However, this may not always be practical and for some designs the residual stresses may be a limiting factor. Welded fabrication can be painted with white contrast paint for magnetic particle inspection.
In exceptional conditions, special steels may need to be used.
It is inherent in the production process that rolled steel tends to have a layered structure. This is generally not a problem, as members tend to be stressed principally along their lengths, and for common building structures it can be ignored. However, some connections can lead to significant 'through thickness' stresses in plates, and under exceptional conditions, such as in the construction of large offshore oil platforms, these can create difficulties. These can often be avoided by suitable detailing, but if this is not possible, special steels with good properties in the 'short transverse' direction are available to special order (for example Corus's 'Hyzed' steels) are available to special order. However, for normal building construction, normal grade steels are adequate in this respect.
