TWI's expertise is being used in one of the world's largest bridges, now under construction in Hong Kong. The tower saddle units for the double decker road/rail Tsing Ma suspension bridge are being built by Davy International for main contractor Cleveland Bridge.
'The task that presented itself was to join a large carbon-manganese casting to a large carbon-manganese steel plate fabrication' Davy's Paul Madin said. 'They were too big to be made in one piece'.
'Manual metal arc was the natural choice of process because of the components' positional nature. It was also deemed most appropriate to fulfil the stringent minimum defects requirements. We had concerns, but not so much about the metallurgy of the job, that was straightforward. The preheat and restraint aspects however, were going to be significant so we called Abington early on' continued Madin.
Simply preheating the joint to 150°C created unacceptable gaps in the joint line. Davy was advised to take a balanced approach to preheating and welding. Heating elements were placed symmetrically about the joint to restore the intimate contact of the faces to be welded.
Four welders were then timed to work simultaneously, two on each side of the joint and two on the opposite side of the fabrication. But wasn't it necessary to restrain the fabrication during welding? 'No' said Madin. 'TWI advised us to let the saddles move as they wanted. If we had restrained them stresses would have been introduced which could cause cracking either during welding or cooling. After some six months we've had very good results indeed. The ultrasonics and magnetic NDT work has shown virtually zero defects.'