HIGH - TEMPERATURE CREEP BEHAVIOUR OF CAST COBALT-BASE SUPERALLOYS

Two cast and heat-treated NbC and TaC - strengthened cobalt superalloys have been developed for an investment casting of spinner discs for glass wool industry. In this work constant load creep tests in tension were carried out in argon atmosphere at three testing temperature 900, 950 and 1000 degrees C and at the initial applied stresses ranging from 40 to 200 MPa. All the tests were continued until the final fracture. The results of creep testing were combined with microstructural and fractographic examinations by means of light and scanning electron microscopy. A mutual comparison of creep characteristics of the investigated superalloys under comparable creep loading conditions showed that NbC-strengthened superalloy exhibited longer creep life than TaC-strengthened one. Further, it was found that carbide precipitation is the primary strengthening mechanism in both cobalt-base superalloys under investigation and the amount, morphology and type of carbides have the decisive effect on the creep properties including creep damage and fracture processes. By contrast, CoNb superalloy exhibited a more brittle character of creep fracture mode than CoTa superalloy due to a premature fracture. This study was initiated to investigate in more details operating creep deformation processes and the effect of the creep microstructure and damage evolution on both investigated superalloys. The different behaviour and properties of studied superalloys were explained based on the received results of this study.