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Aged samples lend added credence to safety codes

 

Connect, no.78, July-August 1996

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Work at TWI could throw new light on the age-related properties of metals used in critical power station components.

Several austenitic stainless steel specimens have been stored in one of TWI's furnaces since the mid-eighties and are expected to provide a unique insight into the behaviour of such materials when held at high temperature year after year.

Cast austenitic stainless steel is used in the pump housings of nuclear power reactor pressure vessels. Failure of such a housing would be catastrophic. So the results of tests on these specimens could provide new safety assurances to users of nuclear power reactor pressure vessel codes.

A PWR pump housing operates at about 300 degrees C for most of its life. Austenitic stainless steel containing ferrite becomes embrittled at temperatures around 450 degrees C, but concern exists that some embrittlement may take place during the 40 year life of a reactor even at lower temperatures of around 300 degrees C.

'Work has been carried out in various parts of the world and in particular here at Abington in the mid-eighties, on samples that were aged for just over a year at 400 degrees C' Materials Technology manager Trevor Gooch told Connect. 'The results indicated that although some embrittlement did occur it was not of any great consequence at ferrite levels normally found in practice.'

During that work longer ageing studies were planned and samples were stored in a furnace. But funds for the extended work were not forthcoming, so the samples remained undisturbed until now. They have acquired just over 11 years ageing, at temperatures between 375 degrees C and 385 degrees C. Having been deliberately aged at high temperatures over such a long period, these specimens are believed to be unique.

The UK Health and Safety Executive has agreed to support work on the samples to determine their mechanical properties and whether or not they are sensitive to intercrystalline corrosion..

'The work has now been authorised' says Gooch. 'So we'll start by removing the specimens and machining them to make appropriate mechanical test specimens. We'll be measuring tensile characteristics but, most importantly, toughness. Our real interest is in the tearing resistance of these materials.

'I believe there'll be some degree of deterioration. Although during the eighties attention was focused upon the pump housing itself, people involved with welding suspected that the weld metal would be inferior because of the inevitable presence of inclusions.'

The previous work showed that the weld metal was indeed of appreciably lower toughness than the parent material even though both regions had suffered some degree of embrittlement.

'I suspect we'll find very much the same now' believes Gooch. 'The figures will be different, and the designers will no doubt use these to make the appropriate safety cases.'

'I believe it unlikely that the codes will be relaxed in the light of the new findings. The design codes will probably remain the same, as will the NDT requirements and the material property requirements' says Gooch. 'But this will undoubtedly leave the authorities happier that these components are safe. This will be a very reassuring demonstration'.

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contactus@twi.co.uk