Qualification of P91 welds through Small Punch creep testing

Full text: As a result of its high temperature creep strength, the martensitic alloy P91 is a strong candidate for application as a pressure vessel and heat exchanger material of construction in Next Generation nuclear plant. This candidature is supported by lengthy and successful operation in other high temperature plant in particular in fossil fuel fired plant. The only potential area of concern which will have to be taken into account for the nuclear application concerns the behaviour of construction weldments. Any risk of Type IV cracking has to be avoided and hence a thorough knowledge concerning the integrity of the weld has to be demonstrated. This can be achieved on the macro scale through cross weld creep testing although the results of such tests are difficult to interpret and are not included in design codes. A novel approach is proposed which relies on the determination of the creep properties of the different zones of the weldment on a micro-scale based on the Small Punch Testing methodology. Although results from such tests are similarly debarred from design codes at present, the potential of the method should be evaluated through testing and analysis as it has a second prospective role in plant operation lifetime assessment once the plant is in operation. The test methodology has recently been incorporated into a CEN Code of Practice in order to improve the acceptability of the results. In this paper, results are presented from the Small Punch creep tests carried out exactly following the Code of Practice for the various constituent zones of the weldment and the potential weaknesses are identified within the heat affected zone. (author)