RELATIONSHIPS BETWEEN DENSITY, PRESSURE, AND TEMPERATURE IN POROUS CUPRONICKEL ALLOYS

Castings of 70-30 cupronickel (NES 824) often contain an undesirable a mount of porosity, which leads to a high rejection rate for components made from this type of alloy. The possibility of removing such porosi ty by the application of hot isostatic pressing (hipping) to the casti ngs has been investigated. Part of this investigation involved the hip ping of artificial porosity specimens of controlled geometry and a com parison of the results obtained with the predictions of certain well k nown models for pressure assisted sintering. The results obtained were complicated by the effect of the specimen design on the rate of compa ction which is not taken into account in any of the models. However, w here the pores are truly internal the degree of agreement with both mo dels is reasonable. It is evident that for internal porosity the remov al of porosity in a typical industrial hipping operation occurs during the initial application of pressure and temperature rather than durin g the subsequent isothermal stage. This applies for up to 10% porosity in the form of a single hole but may not be true of large castings wi th uneven distributions of pores. (C) 1996 The Institute of Materials.