Effect of a Preliminary Aging Treatment on the Oxidation Kinetic at High Temperature for a Cobalt - Based Alloy Strengthened by Tantalum Carbides

Abstract

Abstract:The purpose of this work is to examine if the behavior of refractory alloys in oxidation at high temperature can be influenced by the modification of the interdendritic carbides morphology due to a preliminary aging treatment at high temperature. A cobalt-based alloy strengthened by tantalum carbides was first exposed to 1200°C over a period of 100 hours, and characterized in high temperature oxidation, with samples cut in the middle of the alloy. Thermogravimetry runs were performed in air at 1000, 1100 and 1200°C during 50 hours, and the results were analyzed in order to specify the rates of transient oxidation, of parabolic oxidation and of chromia volatilization. The oxidized samples were metallographically examined. The fragmentation of the TaC carbides induced by the aging treatment tends to accelerate the oxidation kinetics in general, with higher values of parabolic and volatilization constants especially at 1200°C.

Influence of the morphologic evolution of the eutectic carbides at high temperature on the thermal expansion behavior of refractory cast alloys

“…It was for example seen recently for the same alloys [18] that such modifications of the primary carbides, which are present in the grain boundaries along which species involved in the oxidation phenomena (e.g. Cr) diffuse with easiness, can lead to changes in the kinetic of oxidation at high temperature: the longer the aging duration, the faster the mass gain by oxidation (especially at 1200 • C).…”

Kinetics of high temperature oxidation of chromium rich HfC reinforced cobalt based alloys

“…4-6)' and even 'changes in high temperature oxidation behaviour (Ref. 7)'. These evolutions necessarily affect sooner or later the interdendritric carbides reinforcing the concerned equi-axed superalloys but, fortunately, there exist carbides particularly resistant to such morphological phenomena.…”

“…They must be notably much more stable than chromium carbides, the volume fractions of which rapidly decrease at high temperature, and even than the tantalum carbides, which get fragmented 3 and then lead to significant loss of thermomechanical properties [4][5][6] and even changes in high temperature oxidation behaviour. 7 These evolutions necessarily affect sooner or later the interdendritic carbides reinforcing the concerned equiaxed superalloys, but, fortunately, there exist carbides particularly resistant to such morphological phenomena. This is notably the case of the HfC carbides, which were recently obtained in solidified cobalt based alloys 8 with the shape of script-like carbides forming an interdendritic eutectic compound with matrix as the more common tantalum carbides.…”

Oxidation Time Influence on the Structure of the Cobalt Base Clad Layer and Scale Morphology

“…Such a chemical composition is used for the wear resistant layers. Also chromium amount is high enough for allowing a chromia-forming behaviour for several tens or hundreds hours, a duration which depends on both the temperature and the corrosive environment [9]. The substrate was an A-R-H10S2M steel forging exhaust valve.…”