Welding trials permit reduced inspection times

A trial weld being inspected for evidence of hydrogen cracking

An Industrial Member has been able to reduce expenditure during an offshore welding operation following welding trials undertaken at TWI.

The problem facing the company was that weld inspection for hydrogen cracks must be performed with a sufficient delay after completion of welding to ensure that all cracking has finished. 72 hours had been specified to guarantee that this was so in this offshore work. However, to keep a barge standing by is very costly.

A safe minimum time for inspection cannot be predicted, given the present knowledge of hydrogen cracking. Nevertheless, it is possible to establish welding conditions which reduce the risk of hydrogen cracking so that the possibility can be discounted with confidence.

The problem was tackled in two stages. First, previously-established data were used to predict welding conditions more severe than those proposed, but which would still not be expected to result in hydrogen cracking. This prediction was then verified with laboratory tests. Then a full-scale trial weld was made, again using conditions more severe than those specified for the offshore welding operation. This involved completing the weld in one non-stop operation, requiring two shifts of welders working over a 24 hour period. After a 72 hour delay, the finished weld was examined for hydrogen cracks using ultrasonics and found to be crack free.

As a result of these trials, the welding procedure was shown to be sufficiently conservative to discount the risk of hydrogen cracking. With rigorous quality control and using additional CTOD data, the offshore welding could now be performed and inspected immediately with the confidence that cracking would not occur.

Clearly, this approach can be recommended only where there is a limited amount of welding which can be closely supervised. It is not a substitute for good NDT in general manufacturing. Nevertheless, the potential financial savings can be significant and the back-up work necessary to ensure the complete safety of the procedure becomes entirely cost effective.

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