When low fracture toughness was identified during manufacture of some crucial oil pipeline, no fewer than three Industrial Members of TWI wanted their collective minds put at ease..the manufacturer, the installer and the oil giant concerned.
'There was no cracking' recalls TWI consultant Henryk Pisarski. 'But finding some very low fracture toughness values in the heat affected zones of the longitudinal welds prompted the pipe maker to carry out a very extensive programme of research. There was a lot at stake if the client refused to accept the order.'
Was the low toughness significant? It is invariably found in high heat input welds, like this one, a longitudinal submerged arc seam weld.
'By itself the presence of low toughness in the heat affected zone is not that surprising' says Pisarski. 'It's not normally very significant because defects tend not to occur there. Also the hoop stresses in service are relatively low because they're caused solely by pressure loading.'
The client was more concerned about the interaction between the longitudinal seam weld and the girth weld joining lengths of the 508mm diameter 18mm wall thickness X60 type pipe in question.
During manufacture samples of the pipe would be tested for leaks to a high hydraulic pressure. During installation however, the hoop stresses would have exceeded the in-service loads. But all the stresses would be parallel to the longitudinal weld, not across it. The only possible area of doubt would be where the longitudinal weld intersects the girth weld.
'As soon as you make a girth weld the intersection with the longitudinal weld becomes a multipass weld' explains Pisarski. 'And consequently you refine the coarsely graded microstructure of the longitudinal weld. There is always a tiny area either side of the weld where the grain coarsened heat affected zones meet.and that is what we were trying to test.'
It was TWI's role to make an independent judgement on the fracture toughness's fitness for service for the application, but it was in everyone's interest that it was deemed successful.
To demonstrate this clearly to all parties TWI had to demonstrate fitness for service at the intersection weld which involved considerable post test metallography. The microstructures that were potentially suspect were then reviewed by TWI's client, and the oil company who was the ultimate client. To every party's relief the pipe was deemed acceptable.
An aspect of the service TWI provided which particularly appealed to the client was that TWI was able to respond almost immediately. 'We met up in Paris at extremely short notice' remembers Pisarski. 'And with the help of e-mail to staff in Australia at the time, were able to give very timely contributions to the work.'
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