Steel plate used during construction of an LNG tank had obviously endured fire exposure following an insulation blaze, but had the incident compromised the integrity of the structure?
This was the question facing the client responsible for the construction project when a fire, between the inner and outer skins, interrupted fabrication.
So it turned to TWI for a rapid assessment of the issues. The objective of the work was to assess whether the inner tank had suffered metallurgical damage.
TWI staff and equipment were despatched promptly to the scene of the incident. Due to time constraints the required solvents and etchants were sourced locally by the client. After a review of documentation and discussion with TWI's client, hardness measurements were made in selected areas of suspicion, using a portable hardness tester. Following grinding, polishing, finishing with diamond paste and etching in Nital, acetate replicas were prepared of the potentially fire-damaged regions, as well as in a control region of plate known to have been distant from the fire.
Horizontal and vertical seam welds near to these two regions were also surveyed with hardness traverses and acetate replicas spanning each weld and their associated heat affected zones. The HAZs would have greater sensitivity to any heating than would the parent steel. Examination under a portable optical microscope confirmed that the replicas were of acceptable quality to provide for later detailed study back in Great Abington.
As a result of the hardness tests alone, TWI was able to give the client some reassurance. The mean hardness measured in the most severely fire damaged plate was 260HV. In the control plate it was 258HV. So it could be concluded that any heating of the 9% Ni steel tank outer surface had not caused any reduction in hardness, and therefore strength.
Secondly, detailed examination of the replicas was made in the Metallurgical Laboratory at TWI, revealing that the microstructures of the parent steel and the heat affected zones in the potentially fire damaged plate and those in the undamaged plate were essentially the same. The HAZ microstructure of the horizontal seam weld joining suspect plates gave the strongest reassurance that the fire had not affected the plate microstructure. There was no evidence of any tempering of this HAZ in the fire damaged region. So it could safely be concluded that any heat to which the tank had been subjected was insufficient to cause appreciable microstructural modification.
As a result of both these hardness and microstructural observations, no appreciable differences between the fire damaged and the control region were found, and as a consequence TWI was able to recommend that no remedial action needed to be taken before the tank was put into service.
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